Multiple Endocrine Neoplasia Type 2 (MEN 2)

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1. Clinical Overview

Definition and Significance

Multiple Endocrine Neoplasia Type 2 (MEN 2) is a rare autosomal dominant hereditary cancer syndrome caused by germline activating mutations in the RET proto-oncogene located on chromosome 10q11.2. [1] It represents one of the clearest examples of genotype-phenotype correlation in human genetics: the specific codon mutation directly predicts age of cancer onset, disease aggressiveness, and organ involvement. [2]

MEN 2 comprises three distinct clinical subtypes:

1. MEN 2A (80-85% of cases): Medullary thyroid carcinoma (MTC) in nearly 100% of patients, phaeochromocytoma in 50%, and primary hyperparathyroidism in 20-30%. [3]
2. MEN 2B (5% of cases): Highly aggressive MTC with onset in infancy, phaeochromocytoma in 50%, mucosal neuromas, marfanoid habitus, and intestinal ganglioneuromatosis. Notably, hyperparathyroidism does NOT occur in MEN 2B. [4]
3. Familial Medullary Thyroid Cancer (FMTC): MTC only, without other endocrine manifestations. Now considered a variant of MEN 2A rather than a separate entity. [1]

The condition affects approximately 1 in 30,000 individuals, with nearly complete penetrance for MTC by age 70. [5] Early genetic diagnosis and prophylactic thyroidectomy have transformed outcomes, reducing MTC-related mortality from over 50% to less than 10% in screened families. [6]

The Critical Cancers

1. Medullary Thyroid Carcinoma (MTC)

• Arises from parafollicular C-cells (calcitonin-producing neuroendocrine cells)
• Occurs in nearly 100% of untreated RET mutation carriers
• Metastasizes early to cervical lymph nodes, then to liver, lungs, and bone
• Critical limitation: Does NOT respond to radioiodine therapy (lacks sodium-iodide symporter) or conventional chemotherapy [7]
• Surgery is the only curative treatment
• Calcitonin serves as an excellent tumour marker, with levels correlating directly with tumour burden [8]

2. Phaeochromocytoma

• Catecholamine-secreting tumours of the adrenal medulla
• Occurs in 50% of MEN 2A and 2B patients
• Typically bilateral (70% in MEN 2 vs. 10% in sporadic cases) [9]
• Usually benign (malignancy less than 5% in MEN 2, unlike SDHx mutations)
• Life-threatening risk: Undiagnosed phaeochromocytoma can cause hypertensive crisis, myocardial infarction, stroke, or death during anaesthesia induction [10]
• Requires pre-operative alpha-adrenergic blockade before any surgical intervention

Comparative Features

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2. Epidemiology

Incidence and Prevalence

• Prevalence: Approximately 1 in 30,000 to 1 in 40,000 individuals worldwide [5]
• Annual incidence: 1-2 cases per million population [11]
• Hereditary MTC: Accounts for 20-25% of all medullary thyroid carcinoma cases (75-80% are sporadic) [12]
• MEN 2B frequency: Represents only 5% of MEN 2 cases but carries the worst prognosis [4]

Demographics

• Age: MTC onset varies dramatically by genotype:
  • "MEN 2B: First year of life (metastases documented at 6 months of age)"
  • "High-risk MEN 2A (codon 634): Early childhood (5-10 years)"
  • "Moderate-risk FMTC: Adolescence to adulthood (20-40 years) [2]"
• Sex distribution: Equal male-to-female ratio (autosomal dominant inheritance)
• Ethnic variation: No significant ethnic predisposition; reported in all populations studied

Risk Factors

• Primary risk factor: Germline RET mutation (necessary and sufficient)
• De novo mutations:
  • "MEN 2A: 5-10% of cases have no family history [13]"
  • "MEN 2B: 50% arise de novo, often in advanced paternal age [14]"
• Inheritance pattern: Autosomal dominant with 50% transmission risk to offspring
• Penetrance:
  • "MTC: Nearly 100% by age 70 (varies by codon) [15]"
  • "Phaeochromocytoma: 50% (age-dependent, higher in codon 634)"
  • "Hyperparathyroidism: 20-30% in MEN 2A only"

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3. Molecular Pathophysiology

The RET Proto-oncogene: Structure and Function

The RET (REarranged during Transfection) gene on chromosome 10q11.2 encodes a single-pass transmembrane receptor tyrosine kinase essential for neural crest development and kidney morphogenesis. [16]

Normal RET signaling:

1. Ligand binding: Glial cell line-derived neurotrophic factor (GDNF) family ligands bind to GFRα co-receptors
2. Receptor dimerization: Two RET monomers are brought together
3. Trans-autophosphorylation: Intracellular tyrosine kinase domains phosphorylate each other
4. Downstream signaling: Activates RAS/MAPK, PI3K/AKT, and PLCγ pathways, promoting cell survival, proliferation, and migration [17]
5. Signal termination: Receptor internalization and degradation

Mutation Mechanisms: Genotype-Phenotype Correlation

The location of RET mutations determines clinical phenotype through distinct molecular mechanisms:

MEN 2A: Extracellular Domain Mutations (Exons 10-11)

• Key codons: 609, 611, 618, 620, 630, 634 (most common, 60-85% of MEN 2A) [3]
• Mechanism: Mutations affect cysteine residues in the extracellular cysteine-rich domain
• Pathophysiology:
  • Loss of a cysteine creates an unpaired cysteine on adjacent receptors
  • Formation of aberrant intermolecular disulfide bonds
  • Ligand-independent constitutive dimerization
  • Moderate level of continuous downstream signaling [18]
• Clinical consequence: MTC onset in childhood-adolescence, moderate aggressiveness

MEN 2B: Intracellular Domain Mutations (Exon 16)

• Key codon: 918 (M918T mutation in > 95% of cases)
• Mechanism: Methionine-to-threonine substitution at codon 918 in the tyrosine kinase catalytic domain
• Pathophysiology:
  • Alters substrate specificity and ATP binding pocket conformation
  • Monomeric activation (dimerization not required)
  • Increased kinase activity and altered substrate recognition
  • Activation of novel signaling pathways (enhanced STAT3 signaling) [19]
• Clinical consequence: Extremely aggressive MTC from infancy, widespread metastases

FMTC: Variable Risk Mutations

• Codons: 768, 804, 891 (lower penetrance and later onset)
• Mechanism: Variable effects on receptor function, generally milder than 634 or 918
• Clinical consequence: MTC-only phenotype, later onset, lower aggressiveness [20]

The Hirschsprung Disease Connection

In contrast to gain-of-function mutations causing MEN 2, loss-of-function RET mutations cause Hirschsprung disease (congenital aganglionic megacolon). [21]

"Janus" mutations: Certain MEN 2A mutations (particularly codons 609, 611, 618, 620) can cause both phenotypes:

• Gain-of-function: Constitutive activation → cancer
• Loss-of-function: Impaired trafficking to cell membrane → reduced signaling during development → absent enteric neurons [22]
• Clinically, 3-7% of MEN 2A patients (especially codon 620) present with Hirschsprung disease in infancy

Exam Detail: Molecular signaling cascades in MEN 2:

1. RAS/RAF/MEK/ERK (MAPK) pathway: Primary driver of cell proliferation
2. PI3K/AKT/mTOR pathway: Promotes cell survival and inhibits apoptosis
3. PLCγ pathway: Calcium mobilization and PKC activation
4. STAT3 pathway (especially MEN 2B): Transcriptional activation of oncogenic programs

The M918T mutation in MEN 2B shows 10-fold higher transforming activity in cellular assays compared to codon 634 mutations, explaining the dramatic phenotypic differences. [19]

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4. American Thyroid Association Risk Stratification

The 2015 American Thyroid Association (ATA) guidelines revolutionized MEN 2 management by creating genotype-based risk categories that dictate timing of prophylactic thyroidectomy. [1]

ATA Risk Categories

Key Principles

1. HST mutations: Consider surgery in first year of life; metastatic disease documented at less than 1 year of age [23]
2. High-risk mutations: Surgery by age 5; balance cancer risk against surgical morbidity in young children
3. Moderate-risk mutations: Two strategies:
   • Early thyroidectomy (childhood): Eliminates cancer risk and surveillance burden
   • Biochemical surveillance: Annual serum calcitonin; operate when elevated or by late childhood [24]
4. Calcitonin thresholds:
   • less than 10 pg/mL: Normal
   • 10-40 pg/mL: C-cell hyperplasia likely

   • 40 pg/mL: Micro-MTC likely; surgery indicated [8]

Clinical Pearl: Pre-operative screening for phaeochromocytoma is MANDATORY before any thyroid surgery in MEN 2. Undiagnosed phaeochromocytoma can cause fatal intraoperative hypertensive crisis. Always measure plasma or 24-hour urinary metanephrines before scheduling thyroidectomy. If phaeochromocytoma is present, it must be resected FIRST (after alpha-blockade), followed by thyroidectomy. [25]

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5. Clinical Presentation

MEN 2 presentation varies dramatically by subtype and age at diagnosis.

MEN 2A Presentation

Medullary Thyroid Carcinoma:

• Often asymptomatic in gene carriers identified by screening
• Symptomatic presentation (in unscreened individuals):
  • Palpable thyroid nodule (firm, non-tender, may be fixed)
  • Cervical lymphadenopathy (hard, matted nodes in lateral neck compartments II-V)
  • "Advanced signs of local invasion:"
    • Hoarseness (recurrent laryngeal nerve invasion)
    • Dysphagia (oesophageal compression/invasion)
    • Stridor (tracheal compression)
  • "Paraneoplastic manifestations:"
    • Secretory diarrhoea (from high calcitonin or VIP secretion, occurs in 30% of metastatic MTC) [26]
    • Cushing syndrome (rare, from ectopic ACTH production)
    • Flushing (rare, from prostaglandins or calcitonin-gene-related peptide)

Phaeochromocytoma (50% of MEN 2A):

• Classic triad (occurs in only 24% of patients) [27]:
  1. Episodic severe headache (90%)
  2. Profuse sweating (65%)
  3. Palpitations/tachycardia (70%)
• Paroxysmal hypertension (15-minute attacks triggered by exercise, postural change, anaesthesia, tyramine-containing foods)
• Sustained hypertension (50% of cases)
• Hyperglycaemia (catecholamines inhibit insulin secretion)
• Catastrophic presentations:
  • Hypertensive crisis (BP > 220/120 mmHg)
  • Acute pulmonary oedema ("flash" pulmonary oedema)
  • Takotsubo cardiomyopathy
  • Myocardial infarction or stroke

Primary Hyperparathyroidism (20-30% of MEN 2A):

• Usually asymptomatic (detected on biochemical screening)
• Classic symptoms ("stones, bones, groans, psychiatric overtones"):
  • Renal stones (calcium oxalate)
  • Bone pain (osteoporosis, rarely osteitis fibrosa cystica)
  • "Gastrointestinal: Constipation, nausea, peptic ulcer disease, pancreatitis"
  • "Neuropsychiatric: Fatigue, depression, cognitive impairment"
• Milder than MEN 1: Hyperparathyroidism in MEN 2A is generally less severe than in MEN 1

MEN 2B Presentation

Distinguishing features present from infancy/early childhood:

1. Mucosal Neuromas (100% of MEN 2B):

• Oral cavity: Multiple painless, sessile, pedunculated nodules on:
  • Anterior dorsum of tongue (most common site)
  • Buccal mucosa
  • Lips (causing characteristic "bumpy" or "blubbery" appearance)
• Glistening, pink-white in colour
• May interfere with feeding in infants
• Diagnostic importance: Often the first clinical sign, visible in infancy before MTC is clinically apparent [28]

2. Ophthalmologic Features:

• Thickened corneal nerves (visible on slit-lamp examination in 100%)
• Everted eyelids with thickened margins

3. Marfanoid Habitus (75% of MEN 2B) [29]:

• Tall stature with decreased upper-to-lower segment ratio
• Arachnodactyly (long, thin "spider" fingers)
• Pectus excavatum or carinatum
• Joint hypermobility
• High-arched palate
• Scoliosis
• Key differentiation from Marfan syndrome:
  • NO lens dislocation (ectopia lentis)
  • NO aortic root dilation
  • FBN1 gene is normal

4. Gastrointestinal Ganglioneuromatosis:

• Diffuse or nodular neuronal proliferation throughout the GI tract
• Presents with:
  • Chronic constipation (may mimic Hirschsprung disease)
  • Megacolon
  • Abdominal distension
  • Failure to thrive in infants
• Can be the presenting feature leading to diagnosis

5. Medullary Thyroid Carcinoma:

• Extremely aggressive with earliest documented metastases at 2 months of age [23]
• Often metastatic at diagnosis if not identified by genetic screening
• Rapidly progressive despite treatment

MEN 2A Variant: Cutaneous Lichen Amyloidosis

A specific variant of MEN 2A (predominantly codon 634 mutations) presents with:

• Cutaneous lichen amyloidosis (CLA): Pruritic, hyperpigmented, scaly plaques
• Location: Characteristically in the interscapular region (upper back)
• Pathophysiology: Deposition of keratin-derived amyloid in papillary dermis
• Clinical significance: May be the first clinical sign, prompting genetic testing [30]

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6. Investigations

Genetic Testing

Germline RET sequencing: Gold standard for diagnosis

Indications for RET genetic testing [1]:

1. All patients with medullary thyroid carcinoma (even apparently "sporadic" cases; 5-10% have unsuspected germline mutations) [31]
2. All first-degree relatives of a confirmed MEN 2 patient (50% carrier risk)
3. Infants/children with mucosal neuromas or marfanoid features
4. Patients with phaeochromocytoma (especially bilateral or young onset)
5. Patients with Hirschsprung disease AND other MEN 2 features

Genetic counselling essentials:

• Pre-test counselling regarding implications for patient and family
• Discussion of cascade testing for relatives
• Implications for insurance and employment (varies by jurisdiction)
• Reproductive options (pre-implantation genetic diagnosis available)

Variants of uncertain significance (VUS):

• Novel RET variants not in published databases require functional analysis
• Manage conservatively: Annual calcitonin monitoring, individualized thyroidectomy decision [32]

Biochemical Screening and Monitoring

For Medullary Thyroid Carcinoma:

1. Serum Calcitonin:

   • Most sensitive marker for MTC (sensitivity > 95%) [8]
   • Normal: less than 10 pg/mL
   • C-cell hyperplasia: 10-40 pg/mL
   • MTC: Usually > 40 pg/mL
   • Correlation with tumour burden:
     • less than 150 pg/mL: Often localized to thyroid
     • 150-1000 pg/mL: High likelihood of lymph node metastases

     • 1000 pg/mL: High likelihood of distant metastases [33]

   • Calcitonin doubling time: Most important prognostic marker in recurrent disease
     • less than 6 months: Poor prognosis, 5-year survival ~25%
     • 6 months-2 years: Intermediate prognosis

     • 2 years: Favourable prognosis, 5-year survival ~95% [34]

2. Carcinoembryonic antigen (CEA):

   • Less sensitive than calcitonin but useful for prognosis
   • Elevated in ~50% of MTC cases
   • "Flip phenotype" (high CEA, normal/low calcitonin): Indicates poorly differentiated MTC with very poor prognosis [35]
   • Rising CEA in setting of stable calcitonin suggests aggressive dedifferentiation

Screening protocol in RET mutation carriers [1]:

• Annual calcitonin starting:
  • Age 3-5 years for moderate-risk mutations
  • Earlier for high/highest-risk if thyroidectomy delayed
• Continue until thyroidectomy performed

For Phaeochromocytoma:

1. Plasma free metanephrines (preferred):

   • Sensitivity 96-100%, specificity 85-89% [36]
   • Patient should be supine for 20-30 minutes before blood draw
   • Avoid medications that interfere (tricyclic antidepressants, levodopa)

2. 24-hour urinary fractionated metanephrines:

   • Alternative to plasma testing
   • Sensitivity 90-95%

3. Screening frequency in MEN 2 patients [1]:

   • Annual starting:
     • Age 8 years for high-risk mutations (codon 634)
     • Age 16 years for moderate-risk mutations
     • Age 11 years for MEN 2B
   • Continue lifelong (even after unilateral adrenalectomy, due to bilateral risk)

For Primary Hyperparathyroidism (MEN 2A only):

• Annual serum calcium (albumin-corrected or ionized)
• PTH if calcium elevated
• Start screening at age 8-16 years (earlier if codon 634) [1]

Imaging Studies

Neck Ultrasound:

• Pre-operative thyroid/nodal assessment
• Systematic evaluation of:
  • Thyroid parenchyma (echogenicity, nodules)
  • "Central compartment (level VI): Pretracheal, paratracheal, prelaryngeal nodes"
  • "Lateral compartments (levels II-V): Jugular chain nodes"
• Suspicious features: Hypoechoic, microcalcifications, irregular margins, increased vascularity
• FNA with calcitonin washout for suspicious lymph nodes > 1 cm

Cross-sectional imaging:

1. Contrast-enhanced CT neck/chest (if calcitonin > 150-400 pg/mL):

   • Lymph node mapping
   • Distant metastases (lung, liver, bone)

2. MRI adrenals (if metanephrines elevated):

   • T2-weighted: Phaeochromocytoma appears intensely bright ("lightbulb sign")
   • Better soft-tissue resolution than CT
   • Detects lesions > 0.5 cm

3. 123I-MIBG scintigraphy:

   • Functional imaging for phaeochromocytoma
   • Detects extra-adrenal paragangliomas
   • Lower sensitivity (77-90%) than anatomic imaging in MEN 2 [37]

4. 68Ga-DOTATATE PET/CT:

   • For staging metastatic or recurrent MTC (somatostatin receptor-positive)
   • Sensitivity 60-90% depending on tumour grade [38]

5. 18F-FDG PET/CT:

   • For aggressive or dedifferentiated MTC
   • High SUV uptake correlates with poor prognosis

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7. Management: Prophylactic and Therapeutic Thyroidectomy

Prophylactic Thyroidectomy: Principles and Timing

Rationale: Removing the thyroid gland during the C-cell hyperplasia stage (before invasive carcinoma develops) achieves 100% cure for MTC. [6]

Essential pre-operative evaluation [1]:

1. Exclude phaeochromocytoma (plasma/urinary metanephrines) – MANDATORY
2. Assess calcium/PTH (identify hyperparathyroidism)
3. Neck ultrasound (document baseline, identify adenopathy)
4. Baseline serum calcitonin and CEA
5. Vocal cord assessment (fibreoptic laryngoscopy) if any voice concerns

Surgical procedure: Total thyroidectomy

• Hemithyroidectomy is contraindicated: RET mutation affects all C-cells; leaving thyroid remnant guarantees recurrence
• Central compartment (level VI) lymph node dissection:
  • "NOT routinely performed in prophylactic surgery if:"
    • Calcitonin normal or minimally elevated (less than 40 pg/mL)
    • No palpable or ultrasound-detected nodes [1]
  • "Indicated if:"
    • Calcitonin > 40 pg/mL (MTC likely present)
    • Ultrasound-detected lymphadenopathy
    • Palpable thyroid tumour
    • MEN 2B (high risk of early nodal metastases)

Parathyroid management:

• Attempt to preserve all 4 parathyroids with intact blood supply
• If parathyroid appears devascularized → autotransplantation:
  • Confirm parathyroid histology on frozen section
  • Mince into 1-mm fragments
  • Implant 15-20 fragments into sternocleidomastoid or brachioradialis muscle [39]
  • Mark site with non-absorbable suture/clip for future identification

Timing by genotype [1]:

1. HST (MEN 2B: M918T, A883F):

   • Within first year of life, ideally by 6-12 months
   • Earlier if calcitonin already elevated or nodes detected
   • Requires expert paediatric endocrine surgeon and anaesthesia
   • Central node dissection usually required (high metastatic rate)

2. High risk (C634, C630):

   • Before age 5 years
   • May delay to age 5-7 if:
     • Excellent family compliance with annual monitoring
     • Calcitonin remains undetectable
     • No ultrasound abnormalities
   • Rationale for delay: Larger neck structures reduce surgical morbidity (hypoparathyroidism, RLN injury)

3. Moderate risk (C609, C611, C618, C620, V804, etc.):

   • Two evidence-based approaches [24]:

   Option A - Early childhood thyroidectomy:

   • Eliminates MTC risk entirely
   • Removes anxiety/burden of lifelong surveillance
   • Avoids risk of non-compliance with monitoring

   Option B - Biochemical surveillance:

   • Annual calcitonin monitoring
   • Thyroidectomy when:
     • Calcitonin rises above normal range, OR
     • Patient/family preference, OR
     • By late adolescence (to avoid surveillance into adulthood)
   • Advantage: Some carriers may avoid or defer surgery for decades

Therapeutic Thyroidectomy (Established MTC)

Surgical principles [40]:

1. Total thyroidectomy (always)

2. Central compartment (level VI) dissection:

   • Mandatory in all therapeutic cases (even if nodes appear normal)
   • 50-70% of patients have occult nodal metastases [41]
   • Compartment-oriented approach: En bloc resection of all fibro-fatty tissue from:
     • Superior: Hyoid bone
     • Inferior: Innominate artery
     • Lateral: Carotid arteries bilaterally
     • Includes: Pretracheal, paratracheal, pre-laryngeal (Delphian) nodes

3. Lateral neck dissection (levels II-V):

   • Indications:
     • Ultrasound or CT-confirmed lateral adenopathy
     • Calcitonin > 200-400 pg/mL (even if imaging negative, occult nodes likely) [33]
     • Palpable lateral nodes
   • Technique: Modified radical neck dissection (preserving internal jugular vein, sternocleidomastoid, spinal accessory nerve)
   • Bilateral if bilateral imaging-detected disease or very high calcitonin

4. Surgery for local invasion:

   • If MTC invades strap muscles → resect muscles
   • If tracheal/oesophageal invasion → shave/resect involved segment (requires multidisciplinary team)
   • RLN invasion: Sacrifice nerve only if grossly invaded by tumour; attempt preservation if disease can be shaved off

Post-operative management:

1. Levothyroxine replacement:

   • Target TSH 0.5-2.0 mU/L (normal range) [1]
   • NOT suppressed like in differentiated thyroid cancer
   • Rationale: C-cells lack TSH receptors; suppression provides no benefit and causes harm (osteoporosis, atrial fibrillation)
   • Typical dose: 1.6 mcg/kg/day (adjust based on TSH)

2. Calcium supplementation:

   • Monitor calcium closely first 48-72 hours (check every 6-12 hours)
   • Transient hypocalcaemia common (30-60% in children) [42]
   • Supplement with:
     • Calcium carbonate 1-3 g/day (divided doses with food)
     • Calcitriol 0.25-0.5 mcg twice daily if severe or symptomatic
   • Most resolve by 6-12 weeks
   • Permanent hypoparathyroidism: 0-3% in prophylactic surgery, 10-25% in therapeutic surgery with central dissection [42]

3. Post-operative surveillance:

   • Calcitonin and CEA at 2-3 months post-op, then every 6 months for 2 years, then annually
   • Stimulated calcitonin test (calcium or pentagastrin stimulation) no longer recommended (withdrawn due to adverse reactions) [43]
   • Undetectable calcitonin (less than 2 pg/mL): Biochemical cure
   • Detectable but stable low calcitonin (2-10 pg/mL): May indicate microscopic residual disease; monitor
   • Persistent or rising calcitonin: Residual/recurrent disease; requires imaging and potential re-operation

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8. Management: Phaeochromocytoma

Pre-operative Medical Preparation (Roizen Criteria)

Phaeochromocytoma surgery without adequate preparation has 20-80% mortality. [44] All patients must achieve Roizen criteria before surgery:

1. Blood pressure less than 160/90 mmHg for 24 hours
2. No orthostatic hypotension (SBP drop > 80/45 mmHg on standing)
3. No ST-T wave changes on ECG (no ischaemia)
4. No more than 1 premature ventricular contraction per 5 minutes

Alpha-adrenergic blockade (ALWAYS FIRST):

Phenoxybenzamine (non-selective, irreversible α-blocker):

• Dose: Start 10 mg PO twice daily, increase by 10-20 mg every 2-3 days
• Target dose: 1-2 mg/kg/day (usually 20-100 mg/day in divided doses)
• Duration: Minimum 10-14 days before surgery [45]
• Mechanism: Causes vasodilation, allows contracted plasma volume to re-expand
• Side effects: Postural hypotension (warn patient), nasal congestion, fatigue
• Advise high-salt, high-fluid diet to expand volume

Alternative: Doxazosin or prazosin (selective α1-blockers, shorter-acting)

• Dose: Doxazosin 2-32 mg daily
• Advantage: Shorter half-life (fewer post-operative hypotensive issues)
• Disadvantage: Competitive inhibition (may be overcome by catecholamine surge)

Beta-adrenergic blockade (ONLY AFTER adequate α-blockade, typically after 3-5 days):

Critical rule: NEVER give β-blockers before α-blockade

• Rationale: Blocking β2-mediated vasodilation leaves α1-mediated vasoconstriction unopposed → hypertensive crisis, pulmonary oedema, stroke [46]

Propranolol or atenolol:

• Indication: Persistent tachycardia > 100 bpm despite α-blockade
• Dose: Propranolol 10-40 mg TID, or atenolol 25-50 mg daily
• Goal: Heart rate 60-80 bpm

Calcium channel blockers (adjunct):

• Amlodipine 5-10 mg daily
• Can be added for persistent hypertension despite maximal α/β-blockade

Pre-operative volume expansion:

• High-salt diet (> 200 mEq/day)
• IV crystalloid (1-2L) on day before surgery
• Goal: Prevent severe post-resection hypotension

Surgical Strategy: Cortical-Sparing Adrenalectomy

Key issue in MEN 2: 50-70% of phaeochromocytomas are eventually bilateral. [47] Total bilateral adrenalectomy causes:

• Addison disease: Requires lifelong glucocorticoid and mineralocorticoid replacement
• Risk of Addisonian crisis: Leading cause of death in MEN 2 patients is adrenal crisis (intercurrent illness → inability to take oral steroids → shock → death) [48]

Cortical-sparing adrenalectomy (CSA):

• Goal: Remove medullary tumour while preserving adrenal cortex to maintain endogenous cortisol/aldosterone production
• Technique: Careful sub-capsular dissection to preserve cortical rim
• Amount needed: Leaving ≥30% of one adrenal cortex is usually sufficient for steroid independence [49]
• Outcomes:
  • "Steroid independence: 60-90% of patients (varies by amount of cortex preserved) [50]"
  • "Recurrence risk: 10-40% over 10-20 years (acceptable trade-off vs. lifelong steroids) [47]"
  • "Reoperation: Feasible if recurrence occurs"

Initial approach for MEN 2 phaeochromocytoma [25]:

1. Unilateral disease: Cortical-sparing adrenalectomy on affected side
2. Bilateral disease:
   • Option A: Bilateral cortical-sparing (if technically feasible)
   • Option B: Total adrenalectomy on more-involved side + cortical-sparing on less-involved side
   • Option C: Staged procedures (3-6 months apart) if bilateral cortical-sparing not feasible simultaneously
3. Recurrence in remnant: Total adrenalectomy of affected side (if contralateral adrenal intact)
4. Bilateral recurrence after CSA: Total bilateral adrenalectomy (last resort)

Surgical approaches:

1. Laparoscopic transperitoneal:

   • Standard approach for tumours less than 6 cm
   • Good visualization, familiar anatomy
   • Disadvantage: Pneumoperitoneum may cause catecholamine release

2. Posterior retroperitoneoscopic adrenalectomy (PRA):

   • Patient prone
   • Direct retroperitoneal access
   • Advantages: No bowel manipulation, less pain, faster recovery, ideal for bilateral staged procedures
   • Preferred approach for MEN 2 phaeochromocytoma in many centres [51]

3. Robotic-assisted:

   • Improved visualization and precision for cortical-sparing
   • Growing evidence for safety and efficacy [52]

Intra-operative anaesthetic management:

• Arterial line for continuous BP monitoring
• Central venous access
• Avoid tumour manipulation until vascular control achieved
• Hypertensive crisis: IV phentolamine 2-5 mg bolus, or sodium nitroprusside infusion
• Post-resection hypotension (common): IV crystalloid, decrease anaesthetic depth, discontinue vasodilators
• May require vasopressors (noradrenaline) for 24-48 hours post-op

Post-operative Management

If cortical-sparing successful (unilateral or sufficient bilateral cortex preserved):

• No steroids needed if:
  • Normal or elevated cortisol on post-op day 1-2
  • Patient clinically well
• Monitor for adrenal insufficiency signs (hypotension, nausea, hypoglycaemia)
• ACTH stimulation test at 6 weeks to confirm adequacy

If total bilateral adrenalectomy:

• Hydrocortisone replacement: 15-25 mg/day in 2-3 divided doses (e.g., 10 mg AM, 5 mg noon, 5 mg evening)
• Fludrocortisone replacement: 0.05-0.2 mg/day (for mineralocorticoid)
• Sick-day rules education:
  • Double or triple hydrocortisone dose during illness, surgery, major stress
  • Seek medical attention if unable to take oral medications (need IV hydrocortisone)
• Emergency steroid card/MedicAlert bracelet

Long-term surveillance:

• Annual metanephrines (even after unilateral adrenalectomy → risk of contralateral development)
• Annual adrenal MRI (after cortical-sparing → monitor for recurrence in remnant)

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9. Management: Primary Hyperparathyroidism (MEN 2A Only)

Frequency: 20-30% of MEN 2A patients (does NOT occur in MEN 2B or FMTC) [3]

Natural history:

• Typically multi-gland hyperplasia (vs. single adenoma in sporadic HPT)
• Milder than MEN 1 (slower progression, lower complication rate)
• Median age at diagnosis: 30-40 years

Surgical indications [53]:

• Symptomatic hypercalcaemia
• Asymptomatic but meeting criteria:
  • Serum calcium > 1 mg/dL above upper limit of normal
  • Creatinine clearance less than 60 mL/min
  • Bone mineral density T-score < -2.5 at any site
  • Age less than 50 years
  • Nephrolithiasis or nephrocalcinosis

Surgical options:

1. Subtotal parathyroidectomy (3.5 glands):

   • Remove 3 parathyroids completely
   • Leave well-vascularized remnant of 4th gland (~50 mg, size of pea)
   • Mark remnant with clip for future identification
   • Recurrence risk: 15-20% over 10-20 years [54]

2. Total parathyroidectomy with autotransplantation:

   • Remove all 4 parathyroids
   • Confirm parathyroid tissue on frozen section
   • Select healthiest-appearing gland, mince into 1-mm pieces
   • Implant 15-20 fragments into forearm (brachioradialis) or SCM muscle pocket
   • Rationale: If recurrence, can re-operate on forearm under local anaesthesia (vs. hazardous re-exploration of scarred central neck near RLN) [39]
   • Graft function: Takes 2-4 weeks; requires temporary calcium/calcitriol supplementation
   • Success rate: 90-95% achieve eucalcaemia

Medical management (if surgery declined/unsuitable):

Cinacalcet (calcimimetic):

• Mechanism: Allosteric activator of calcium-sensing receptor (CaSR) → suppresses PTH
• Dose: 30-90 mg PO twice daily
• Efficacy: Normalizes calcium in 60-75%, reduces PTH [55]
• Limitations: Does not prevent parathyroid growth; stop if surgery planned (can interfere with intra-operative PTH monitoring)

---

10. Advanced/Metastatic MTC: Systemic Therapy

Indications for Systemic Therapy

Surgery is the only curative treatment for MTC. Systemic therapy is reserved for:

• Unresectable locoregional disease
• Distant metastatic disease (lung, liver, bone)
• Symptomatic disease requiring tumour debulking
• Rapidly progressive disease (calcitonin doubling time less than 2 years) [56]

Observation may be appropriate for:

• Asymptomatic metastatic disease with very slow progression (calcitonin doubling time > 2 years)
• Low metastatic burden
• Elderly patients with significant comorbidities

Selective RET Inhibitors (First-Line)

Selpercatinib (LOXO-292, Retevmo):

• Mechanism: Highly selective RET kinase inhibitor
• Efficacy (LIBRETTO-001 trial) [57]:
  • "Overall response rate (ORR): 69% in treatment-naïve patients, 69% in previously treated"
  • "Median duration of response: > 2 years"
  • "CNS activity: 85% intracranial response rate (crosses blood-brain barrier)"
• Dose: 160 mg PO twice daily (patients less than 50 kg: 120 mg twice daily)
• Advantages over multi-kinase inhibitors:
  • Superior efficacy
  • Better tolerability (RET-specific, less off-target toxicity)
  • CNS penetration for brain metastases
• Side effects:
  • "Hypertension (20-30%): Monitor BP, treat with standard antihypertensives"
  • "Hepatotoxicity (10-15%): Monitor LFTs, dose reduce if AST/ALT > 3× ULN"
  • Haemorrhage (10%)
  • QT prolongation (rare)
  • Hypersensitivity reactions (rare)

Pralsetinib (BLU-667, Gavreto):

• Mechanism: Selective RET inhibitor (similar to selpercatinib)
• Efficacy (ARROW trial) [58]:
  • "ORR: 71% in treatment-naïve, 60% in previously treated MTC"
  • "Median PFS: Not reached at 2 years"
• Dose: 400 mg PO once daily
• Side effects: Similar to selpercatinib (hypertension, hepatotoxicity, neutropenia)

Selpercatinib vs. pralsetinib: No head-to-head trials; both effective. Choice based on dosing preference, drug availability, side effect profile.

Multi-Kinase Inhibitors (Second-Line)

Used if selective RET inhibitors unavailable, intolerant, or progressive disease on RET inhibitor.

Vandetanib (Caprelsa):

• Mechanism: Inhibits RET, VEGFR2, EGFR
• Efficacy (ZETA trial) [59]:
  • "Progression-free survival: 30.5 months vs. 19.3 months (placebo)"
  • "ORR: 45%"
  • "Overall survival: No significant difference (slow natural history of MTC)"
• Dose: 300 mg PO once daily
• FDA approval: 2011 (first systemic therapy approved for MTC)
• Side effects:
  • "QT prolongation (8-14%): Requires baseline ECG, ECGs at weeks 2, 4, 8, 12, then every 3 months; contraindicated if QTc > 450 ms"
  • "Diarrhoea (50-60%): Manage with loperamide, dose reduction"
  • "Rash (40-50%): Acneiform; topical clindamycin, oral doxycycline"
  • Hypertension (30%)
  • "Photosensitivity: Strict sun protection"
• REMS program: Prescribers and pharmacies must be certified due to QT risk

Cabozantinib (Cometriq):

• Mechanism: Inhibits RET, VEGFR2, MET
• Efficacy (EXAM trial) [60]:
  • "Progression-free survival: 11.2 months vs. 4.0 months (placebo)"
  • "ORR: 28%"
• Dose: 140 mg PO once daily
• Side effects:
  • "Hand-foot skin reaction (HFSR): 50%, dose-limiting; prophylaxis with urea cream, aggressive emollients"
  • Diarrhoea (60%)
  • Hypertension (35%)
  • Fatigue (40%)
  • Fistula/GI perforation (rare but serious)

Choice of MKI: Vandetanib or cabozantinib; similar efficacy. Choose based on:

• Comorbidities (avoid vandetanib if cardiac disease/QT risk)
• Side effect profile (avoid cabozantinib if poor performance status/HFSR intolerance)

Other Systemic Therapies

Chemotherapy: MTC is largely chemotherapy-resistant. Dacarbazine, 5-FU, cyclophosphamide have very low response rates (less than 10%) and are rarely used. [61]

Peptide receptor radionuclide therapy (PRRT):

• 177Lu-DOTATATE for somatostatin receptor-positive MTC
• Case series show partial responses in 20-30% [62]
• Investigational; not standard of care

External beam radiation therapy (EBRT):

• Palliative role:
  • Symptomatic bone metastases (pain relief)
  • Brain metastases (if not amenable to surgery/stereotactic radiosurgery)
  • Unresectable neck disease causing dysphagia/airway compromise
• Adjuvant role (controversial):
  • May consider for R1 resection (microscopic positive margins)
  • Gross residual disease (R2)
  • Evidence limited; not routinely recommended [63]

---

11. Special Considerations

Pregnancy and MEN 2

Pre-conception counselling:

• Genetic counselling: 50% risk of transmission to offspring
• Pre-implantation genetic diagnosis (PGD): IVF with embryo selection for RET-negative embryos (eliminates risk) [64]
• Thyroidectomy: Ideally completed before conception
• Phaeochromocytoma screening: ESSENTIAL before pregnancy
  • "Undiagnosed phaeo in pregnancy: Maternal mortality 10-50%, fetal mortality 15-45% [65]"
  • "If phaeo detected: Resect before conception or during 2nd trimester"

Management during pregnancy:

If RET-positive, thyroid intact:

• Defer thyroidectomy until post-partum
• Monitor calcitonin each trimester (physiologic rise in pregnancy is minimal)

If phaeochromocytoma diagnosed during pregnancy [66]:

• 1st trimester: Alpha-blockade + surgery in 2nd trimester (safest window)
• 2nd trimester: Alpha-blockade + surgery during 2nd trimester
• 3rd trimester:
  • "Option A: Alpha-blockade, planned C-section at 36-37 weeks + adrenalectomy"
  • "Option B: Alpha-blockade, elective C-section + adrenalectomy in same operation"
• Labour/delivery: Contraindicated (tumour manipulation → catecholamine crisis)

Levothyroxine in pregnancy (if post-thyroidectomy):

• Requirements increase 30-50% in pregnancy
• Monitor TSH each trimester
• Target TSH less than 2.5 mU/L in 1st trimester, less than 3.0 in 2nd-3rd trimesters

Paediatric Considerations

MEN 2 is one of few hereditary cancer syndromes requiring childhood surgery.

Psychosocial support:

• Genetic diagnosis in childhood has profound psychological impact
• Involve child psychologist, age-appropriate counselling
• Address guilt in parents (transmitted mutation)
• Prepare child/family for surgery, lifelong medication

Surgical challenges in young children:

• Technical difficulty: Small structures (thyroid, parathyroids, RLN, trachea)
• Higher risk of hypoparathyroidism and RLN injury in children less than 2 years
• Requires paediatric endocrine surgeon with high-volume experience [67]

Long-term issues:

• Lifelong levothyroxine compliance
• Transition from paediatric to adult endocrinology
• Reproductive counselling in adolescence/adulthood

Hirschsprung Disease + MEN 2A

Janus mutations (C609, C611, C618, C620): 3-7% of these MEN 2A patients have Hirschsprung disease. [22]

Clinical scenario:

• Infant presents with intestinal obstruction (failure to pass meconium)
• Rectal biopsy confirms aganglionic segment
• Surgical pull-through performed for Hirschsprung disease
• RET testing should be performed:
  • If family history of MTC or MEN 2
  • If associated anomalies (congenital central hypoventilation syndrome, neuroblastoma)
  • If atypical Hirschsprung (long-segment or total colonic)

If RET mutation confirmed:

• Early thyroidectomy still required (Hirschsprung does not protect against MTC)
• Annual metanephrine/calcium screening as per standard MEN 2 protocol

---

12. Prognosis

MTC Prognosis

Overall survival for hereditary MTC (MEN 2):

• Stage I (confined to thyroid, no nodes): 10-year survival > 95% [68]
• Stage II (local nodes): 10-year survival 75-85%
• Stage III (extensive nodes, extrathyroidal extension): 10-year survival 50-70%
• Stage IV (distant metastases): 10-year survival 20-40%

Prophylactic thyroidectomy outcomes:

• Biochemical cure (undetectable calcitonin): > 95% if surgery performed before MTC development [6]
• vs. Symptomatic presentation: 10-year survival 60-70% (due to advanced stage at diagnosis)

Prognostic factors [34,69]:

• Calcitonin doubling time (most important in recurrent disease):
  • less than 6 months: 5-year survival 25%, 10-year survival 8%
  • 6 months-2 years: 5-year survival 92%, 10-year survival 37%

  • 2 years: 5-year survival 95%, 10-year survival 95%

• Stage at diagnosis
• Age (worse prognosis if less than 40 or > 60 years)
• Extent of initial surgery (R0 resection vs. R1/R2)
• Genotype:
  • "MEN 2B: Worst prognosis (aggressive, early metastases)"
  • "Codon 634: Intermediate"
  • "FMTC (codon 804, 768): Best prognosis"

Phaeochromocytoma Prognosis

• Malignancy rate: less than 5% in MEN 2 (vs. 10-15% in sporadic, 30-40% in SDHx mutations) [70]
• Post-resection outcomes:
  • "Cure rate: > 95% (if benign)"
  • "Normalization of blood pressure: 70-90%"
  • "Recurrence in remnant (after cortical-sparing): 10-40% over 10-20 years [47]"
• Contralateral phaeochromocytoma (after unilateral resection): 50% develop over lifetime → requires annual surveillance [71]

Life Expectancy

With modern genetic screening and prophylactic surgery:

• Life expectancy approaches normal if:
  • MTC prevented/cured by early thyroidectomy
  • Phaeochromocytomas detected and managed appropriately
  • Excellent long-term compliance with surveillance
• Causes of death in screened MEN 2 families:
  • Metastatic MTC (if thyroidectomy delayed or refused)
  • Cardiovascular events (phaeochromocytoma-related)
  • Addisonian crisis (post-bilateral adrenalectomy)
  • Non-MEN 2 causes (comparable to general population)

---

13. Exam Perspectives and Viva Preparation

Viva Point: Opening statement for MEN 2 viva:

"Multiple Endocrine Neoplasia Type 2 is an autosomal dominant hereditary cancer syndrome caused by germline activating mutations in the RET proto-oncogene on chromosome 10. It is characterized by medullary thyroid carcinoma in nearly 100% of cases, phaeochromocytoma in 50%, and in the MEN 2A subtype, primary hyperparathyroidism in 20-30%. The hallmark of modern management is genotype-based risk stratification, which dictates the timing of prophylactic thyroidectomy, ranging from infancy in MEN 2B to individualized approaches in lower-risk genotypes. Pre-operative exclusion of phaeochromocytoma is mandatory before any surgical intervention."

Key statistics to know:

• Prevalence: 1 in 30,000
• MTC penetrance: Nearly 100% by age 70
• MEN 2B frequency: 5% of MEN 2 (worst prognosis)
• Phaeochromocytoma: 50% in MEN 2A/2B, usually bilateral (70%)
• Codon 634: Accounts for 60-85% of MEN 2A
• M918T (codon 918): > 95% of MEN 2B

Common Exam Questions

1. "How do you differentiate MEN 2A from MEN 2B clinically?"

Model answer: "The key differentiating features are:

• Physical examination: MEN 2B patients have characteristic mucosal neuromas on the tongue and lips, marfanoid habitus, and thickened corneal nerves on slit-lamp examination. MEN 2A patients typically have no distinctive physical features.
• Age of onset: MEN 2B presents with aggressive MTC in infancy, whereas MEN 2A onset is in childhood to adulthood.
• Hyperparathyroidism: Occurs in 20-30% of MEN 2A but is absent in MEN 2B.
• Genotype: MEN 2A is most commonly associated with codon 634 mutations in exon 11, while MEN 2B is caused by M918T mutations in exon 16 in over 95% of cases.
• Prognosis: MEN 2B is far more aggressive with earlier metastases and worse outcomes."

2. "What is your approach to a 4-year-old child with a confirmed codon 634 RET mutation?"

Model answer: "This is a high-risk mutation requiring prophylactic thyroidectomy before age 5 according to ATA guidelines. My approach would be:

Pre-operative assessment:

1. Plasma or 24-hour urinary metanephrines to exclude phaeochromocytoma (mandatory)
2. Serum calcitonin and CEA (baseline, assess if MTC already present)
3. Neck ultrasound (thyroid and nodal assessment)
4. Serum calcium and PTH
5. Multidisciplinary discussion with paediatric endocrine surgeon, endocrinologist, anaesthesia

Surgical planning:

• Total thyroidectomy with attempted preservation of all 4 parathyroids
• Central compartment dissection NOT routine if calcitonin less than 40 pg/mL and no adenopathy
• Autotransplant any devascularized parathyroids to sternocleidomastoid

Post-operative:

• Lifelong levothyroxine replacement targeting TSH 0.5-2.0 mU/L
• Calcium monitoring (risk of transient hypoparathyroidism)
• Calcitonin/CEA at 3 months post-op to confirm biochemical cure
• Annual screening for phaeochromocytoma and hyperparathyroidism starting age 8
• Genetic counselling and family cascade screening"

3. "A patient with known MEN 2A presents with paroxysmal headaches and sweating. How do you investigate?"

Model answer: "This suggests phaeochromocytoma. My investigation approach would be:

Biochemical confirmation:

• Plasma free metanephrines (first-line, sensitivity 96-100%)
• Or 24-hour urinary fractionated metanephrines
• If elevated: confirms biochemical diagnosis

Anatomic localization:

• MRI adrenals (T2-weighted shows bright 'lightbulb' sign)
• Consider CT if MRI contraindicated
• Given MEN 2, bilateral disease is likely in 70%

Additional work-up:

• ECG (look for LVH, arrhythmias, ischaemia)
• Echocardiogram if symptomatic or abnormal ECG (assess for catecholamine cardiomyopathy)
• Genetics review to confirm RET mutation status

Pre-operative preparation (before ANY surgery, including thyroidectomy if not yet done):

• Alpha-blockade with phenoxybenzamine for minimum 10-14 days
• Achieve Roizen criteria (BP less than 160/90, no orthostatic hypotension, no arrhythmias)
• Add beta-blockade ONLY AFTER adequate alpha-blockade if tachycardic
• High-salt diet, volume expansion

Surgical management:

• Cortical-sparing adrenalectomy preferred (preserve steroid independence)
• Posterior retroperitoneoscopic or laparoscopic approach
• If bilateral, consider staged procedures or bilateral cortical-sparing
• Expert anaesthesia with arterial line, central access"

4. "What are the indications for systemic therapy in metastatic MTC?"

Model answer: "Systemic therapy is indicated for:

• Symptomatic metastatic disease requiring tumour debulking
• Rapidly progressive disease with calcitonin doubling time less than 2 years
• Unresectable locoregional disease
• High metastatic burden threatening organ function

First-line therapy is now selective RET inhibitors (selpercatinib or pralsetinib) based on LIBRETTO-001 and ARROW trials, showing overall response rates of 69-71% with superior tolerability and CNS penetration compared to older multi-kinase inhibitors.

Second-line options include vandetanib or cabozantinib if RET inhibitors unavailable or progressive disease.

Observation with close monitoring is reasonable for asymptomatic, slowly progressive disease (doubling time > 2 years), particularly in elderly patients with comorbidities, as MTC can have indolent behaviour even with metastases."

Common Mistakes (How to Fail)

❌ Stating that MTC responds to radioiodine therapy (C-cells lack NIS; RAI is useless)

❌ Recommending TSH suppression post-thyroidectomy for MTC (C-cells lack TSH receptors; suppression causes harm without benefit)

❌ Giving beta-blockers before alpha-blockade in phaeochromocytoma (unopposed alpha → hypertensive crisis)

❌ Performing thyroidectomy without screening for phaeochromocytoma (can cause intraoperative death)

❌ Recommending hemithyroidectomy for MEN 2 (all C-cells affected; guaranteed recurrence)

❌ Confusing MEN 2B with Marfan syndrome (no lens dislocation or aortic root dilation in MEN 2B)

❌ Stating that hyperparathyroidism occurs in MEN 2B (only in MEN 2A)

---

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Last updated: 2026-01-10

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