Radiation Safety

Overview

Ionising radiation is used in the ICU for portable chest imaging, computed tomography (CT), fluoroscopy during line placement and other procedures, and occasionally interventional radiology. The risks for an individual study are small but accumulate over a patient's admission and over a clinician's career, so radiation protection follows a defined set of units, principles, and limits. The governing goal is to obtain the diagnostic information needed at the lowest dose that is reasonably achievable - the ALARA principle (As Low As Reasonably Achievable).[1][1]

ALARA rests on two complementary justifications: any imaging study must first be justified (it must do more good than harm) and then optimised (the dose must be kept as low as possible while still answering the clinical question). For staff, protection is achieved through the three cardinal measures - time, distance, and shielding.[1]

Dose units

Three quantities are examinable, and candidates must keep them distinct:[1]

• Absorbed dose (the energy deposited per unit mass of tissue) is measured in Gray (Gy), where 1 Gy = 1 joule per kilogram. Absorbed dose describes the physical energy deposition and is independent of radiation type or tissue.
• Equivalent dose, measured in Sievert (Sv), multiplies the absorbed dose by a radiation weighting factor (wR) that accounts for the differing biological damage of radiation types. X-rays, gamma rays, and beta particles have a weighting factor of 1; alpha particles 20; neutrons 5-20 depending on energy. For the diagnostic X-rays used in the ICU, wR = 1, so absorbed dose in Gy and equivalent dose in Sv are numerically equal.
• Effective dose, also in Sievert, further multiplies the equivalent dose by a tissue weighting factor (wT) that reflects the variable radiosensitivity of organs (bone marrow, colon, lung, stomach, breast, and gonads are among the most sensitive). Effective dose gives a single whole-body risk estimate that allows doses from different examinations and to different body regions to be compared - it is the quantity used to express the dose of a CT chest or a chest X-ray.[1]

[1]

Because diagnostic X-rays have wR = 1, the Gray and the Sievert are interchangeable for X-rays (1 Gy = 1 Sv). The distinction matters only for particulate radiation. Practical submultiples: 1 mSv = 1000 microSv; background radiation is about 2-3 mSv per year (so 1 mSv is about 4-5 months of natural background). [1]

Typical doses

Knowing the approximate effective dose of common examinations is examinable and gives a sense of scale:[1][3]

• A chest X-ray is about 0.02 mSv (roughly 3 days of natural background radiation).[3]
• A CT chest is about 7 mSv - approximately 350 chest X-rays (commonly quoted as ~200-400, about 2 years of background).[2][3]
• A CT abdomen-pelvis is about 10-20 mSv (about 500-1000 chest X-rays, or 3-7 years of background).[3][4]
• A CT head is about 2 mSv; a CT pulmonary angiogram (CTPA) about 5-15 mSv.[3]
• Fluoroscopy dose depends on screening time and is measured as a dose-area product (DAP); bedside procedures add a few mSv of staff scatter per minute of screening.[1]

[1]

The ALARA principle

ALARA - As Low As Reasonably Achievable - is the foundational doctrine of radiation protection.[1] It is not a number but a philosophy: the dose from any imaging study should be no higher than is needed to obtain the diagnostic information. ALARA derives from the fact that stochastic effects (cancer, hereditary harm) have no threshold - every exposure, however small, adds some risk - so dose must be justified and then minimised.

ALARA has two pillars:[1]

1. Justification - no examination should be performed unless it does more good than harm. The clinician ordering the study bears this responsibility: is the question answerable by ultrasound, MRI, or a study already done? The ICU patient accumulates scans quickly, and unnecessary CTs add real risk.
2. Optimisation - once a study is justified, the dose should be kept as low as reasonably achievable through protocol selection (low-dose CT, reduced milliampere-seconds, limiting the scan range, shielding, collimation), and for staff through the three protections (time, distance, shielding). [1]

For staff, ALARA translates into the three cardinal protections described below. For the patient, ALARA is achieved by the radiographer/CT protocol (collimation, tube current modulation, pitch, limiting scan length) and by the clinician asking whether the scan is needed at all. [1]

The three protections - time, distance, and shielding

Radiation protection for staff (and for the patient where possible) rests on three cardinal measures, all of which reduce exposure. They are easiest to remember as a triad.[1][1]

1. Time - minimise exposure time

Minimise the time spent near the source. Dose is proportional to exposure time: doubling the screening time doubles the dose. Practical steps:[1]

• Use pulsed fluoroscopy at the lowest acceptable frame rate (e.g., 3-7.5 pulses per second rather than continuous 25-30 fps) - this alone can cut dose by half or more.
• Pre-position the patient and equipment before screening; use the last-image-hold function to review anatomy without re-screening.
• Plan tube angulation and tabletop position with the beam off.
• Take the fewest exposures that answer the question. [1]

2. Distance - maximise distance from the source

Dose falls with the inverse square of distance from the source. This is the inverse-square law and is the most powerful single protective measure.[1]

• Doubling the distance from the source quarters the dose (1/2^2 = 1/4).
• Trebling the distance reduces it to one-ninth.
• Halving the distance quadruples the dose. [1]

During any exposure - a portable chest X-ray on the unit, or fluoroscopy in theatre - staff should step back at least 2 metres from the patient and the X-ray tube. Scatter falls off rapidly, so a few steps dramatically reduce staff dose. Where possible, leave the room during portable imaging, standing behind the lead-lined control panel or a mobile lead screen.[1]

[1]

3. Shielding - interpose a barrier

Place a radiation-attenuating barrier (most commonly lead) between the source and the person.[1]

• Lead aprons - worn for any fluoroscopy or proximity to the beam; a minimum of 0.5 mm lead (Pb) equivalent over the front (often 0.25 mm front and 0.25 mm back for a wraparound apron). The apron covers the radiosensitive bone marrow of the spine, pelvis, and sternum.[1]
• Thyroid shields - a separate collar of 0.5 mm Pb equivalent protecting the thyroid, highly radiosensitive.
• Lead glasses - protect the lens of the eye (the deterministic threshold for cataracts is low, ~0.5 Gy).
• Lead gloves - when hands are near the beam (rare; usually hands should be OUT of the beam entirely).
• Mobile lead screens and lead-lined walls/doors - structural shielding for fixed X-ray rooms.
• For the patient: gonadal shielding (where it does not obscure the region of interest), collimation of the beam to the area of interest (the single most important patient-dose reducer for plain film), and the lowest-dose protocol that answers the question.

Shielding materials and the half-value layer (HVL)

The effectiveness of a shield is described by the half-value layer (HVL): the thickness of a material that halves the intensity of the radiation beam. For diagnostic X-rays (around 100 kVp):[1]

• The HVL of lead is about 0.1-0.3 mm - hence a 0.5 mm lead apron attenuates the beam by far more than half (about 90-95 per cent at typical diagnostic energies).
• The HVL of concrete is about 40-50 mm, of barium plaster a few millimetres, and of water/tissue about 30-40 mm (which is why the patient is their own shield, and why scatter rather than the primary beam is what reaches staff). [1]

Lead equivalence is the practical descriptor of aprons: a 0.5 mm Pb-equivalent apron attenuates around 90-99 per cent of scatter at diagnostic energies, and is the minimum standard. Heavier 1.0 mm Pb-equivalent aprons offer more attenuation but are heavier and less well tolerated - the gain is small for the comfort cost. [1]

CT radiation dose

CT is the largest single source of medical radiation exposure for many ICU patients, and its use has risen steeply. A single CT chest delivers about 7 mSv - roughly 350 chest X-rays (commonly quoted as ~200-400, depending on technique), or about two years of natural background.[2][3] A CTPA for suspected pulmonary embolism delivers 5-15 mSv, and an abdomen-pelvis CT 10-20 mSv. Cumulative CT exposure over an admission is not trivial: in the largest dose-response studies, CT in childhood and adolescence was associated with a measurable increase in subsequent leukaemia and brain-tumour risk, with a dose-response relationship.[5][6]

CT dose is described by three quantities the candidate should know:[1]

• CT dose index (CTDIvol) - the absorbed dose within a single rotation, measured in mGy; a measure of scanner output for a given protocol.
• Dose-length product (DLP) - CTDIvol multiplied by the scan length (mGy.cm); a measure of total dose delivered for the examination.
• Effective dose (mSv) - estimated from the DLP using a region-specific conversion factor (k, in mSv/mGy.cm; roughly 0.014 for chest, 0.015 for abdomen, 0.002 for head). This is the figure quoted as "the dose" of a CT. [1]

Dose reduction in CT (all part of ALARA for the patient) is achieved by:[1]

• Automatic tube current modulation (ATCM) - the scanner reduces mA as it rotates through thin body regions.
• Reducing scan length to the region of interest only.
• Increasing pitch (table travel per rotation) where image quality allows.
• Lower kVp protocols (e.g., 80 or 100 kVp) for CTPA in smaller patients - the iodine contrast is higher relative to soft tissue at lower kVp, allowing lower dose with better vessel opacification.
• Iterative reconstruction algorithms that allow lower mA while preserving image quality.
• Avoiding non-indicated multiphase scans (non-contrast plus arterial plus venous plus delayed) - each phase roughly doubles dose. [1]

Stochastic versus deterministic effects

The biological effects of ionising radiation fall into two categories whose distinction is central to the exam and to dose limits.[1][1]

Stochastic effects - no threshold

Stochastic (probabilistic) effects are those in which the probability of harm rises with dose, but with no threshold - any exposure, however small, carries some risk. The two stochastic effects are cancer (somatic) and hereditary (genetic, affecting future offspring).[1][1]

Because there is no threshold, every exposure adds some risk, which is why ALARA applies even to small doses. The risk is roughly linear with cumulative dose. The lifetime excess cancer risk from a single 10 mSv CT is estimated at about 1 in 1000 to 1 in 2000 in adults, higher in children.[7][8]

Deterministic effects - with a threshold

Deterministic effects (now called tissue reactions by the ICRP) are those in which the severity of harm rises with dose above a threshold - below the threshold there is no clinically meaningful harm. Deterministic effects are the concern after high-dose fluoroscopy, prolonged interventional procedures, or accidental exposure.[1]

Key thresholds (ICRP 118):[1]

• Skin erythema - threshold around 2 Gy (early transient erythema may occur at ~2 Gy; main erythema at ~6 Gy; moist desquamation at ~15 Gy).
• Temporary epilation - around 3 Gy; permanent epilation above ~7 Gy.
• Cataracts (lens of eye) - threshold around 0.5 Gy (ICRP 118 lowered this substantially from the earlier ~0.5-2 Gy, with a single acute dose of ~0.5 Gy sufficient).
• Temporary sterility - testes ~0.15 Gy, ovaries ~0.6-1.5 Gy; permanent sterility at ~3.5-6 Gy (testes) and ~2.5-6 Gy (ovaries).
• Bone marrow depression - ~0.5 Gy; fatal marrow syndrome at whole-body doses above ~3-5 Gy.
• Depression of intellectual development in the fetus above ~100 mGy (8-25 weeks gestation is the window of maximal sensitivity). [1]

These thresholds underpin the dose limits: occupational limits keep the whole-body, lens, skin, and extremity doses well below the deterministic thresholds even over a working lifetime.[1]

[1] [1]

Dose limits

Dose limits translate the principles into numbers that staff and regulators can enforce. The ICRP 103 (2007) limits are widely adopted and are the exam answer:[1]

[1]

Key points to remember:[1]

• The 20 mSv/year occupational whole-body limit is averaged over 5 years - a worker may receive up to 50 mSv in one year provided the 5-year average does not exceed 20 mSv/year.
• The 1 mSv/year public limit is far below occupational limits, reflecting that the public does not benefit from the work and includes children and pregnant women.
• The 500 mSv/year skin/extremity limit is much higher than the whole-body limit because skin and extremities are less radiosensitive than the bone marrow and gonads (which dominate the whole-body effective dose).
• The lens limit (150 mSv/year, tightened to 20 mSv/year) reflects the lower deterministic threshold for cataracts established in ICRP 118.
• The pregnant worker fetal limit (1 mSv for the remainder of pregnancy) is comparable to the public limit and is intended to keep fetal risk negligible. [1]

Personnel monitoring and dosimetry

Staff who work with ionising radiation wear a personal dosimeter to record cumulative dose. The dosimeter is worn under the lead apron at the level of greatest whole-body exposure (chest or waist), to estimate the dose to the bone marrow. A second dosimeter may be worn outside the apron at the collar or on the temple to estimate dose to the thyroid and lens.[1][1]

Three technologies are in common use:[1]

• Film badge - photographic film in a holder with filters; the darkening indicates dose and energy. Cheap but relatively insensitive at low doses and must be developed monthly.
• Thermoluminescent dosimeter (TLD) - lithium fluoride crystals store energy from radiation and release it as light when heated; more sensitive and accurate than film.
• Optically stimulated luminescence (OSL) - aluminium oxide crystals stimulated by a laser; highly sensitive, accurate, and the modern standard for personal dosimetry (e.g., the Landauer InLight badge).
• Electronic personal dosimeter (EPD) - real-time digital readout, useful in fluoroscopy where immediate feedback on dose rate encourages dose-reducing behaviour. [1]

Dosimeters are read at set intervals (monthly or quarterly) and the cumulative dose is logged against the worker's record; exceeding investigation levels triggers review of working practice. [1]

[1]

Radiation and pregnancy

Pregnancy is the single setting in which radiation decisions most often cause anxiety and confusion; the principles are clear.[1][1]

For the pregnant patient

• Minimise CT. Where possible, answer the clinical question with ultrasound or MRI (neither uses ionising radiation).[1]
• If CT is necessary, shield the abdomen with lead where it does not obscure the region of interest, use a low-dose protocol (e.g., reduced mA, single phase, limited scan range), and image only the region needed. CTPA for suspected PE in pregnancy is a common justified indication; the fetal dose from a CTPA (around 0.1-0.5 mGy) is far below the deterministic threshold and the small stochastic risk is justified by the diagnostic need.[1]
• Fetal deterministic thresholds: above ~100 mGy (10 rad) there is a measurable risk of intellectual disability and growth restriction, with the 8-25 week window of maximal CNS sensitivity. Most diagnostic CT delivers fetal doses well below 50 mGy, so deterministic effects are not a concern for diagnostic studies - the relevant risk is the small stochastic risk of childhood cancer.[1]
• The traditional "10-day rule" (defer non-urgent irradiation until after the next period) and "28-day rule" have been superseded by the principle that any study that is clinically justified should proceed, and any that is not should be deferred regardless of cycle timing.[1]
• After an in-utero exposure of less than 100 mGy (the vast majority of diagnostic studies), the risk is not considered grounds for recommending termination of pregnancy.[1]

For the pregnant worker

• Once pregnancy is declared, the fetal dose is limited to 1 mSv for the remainder of the pregnancy.[1][1]
• The worker continues to use lead aprons, thyroid shields, and distance from scatter; a dosimeter is worn under the apron at waist level (over the fetus) to monitor fetal dose directly.
• In practice, most fluoroscopy and ICU imaging work can continue safely within these limits; the pregnant worker is not barred from the ICU or theatre, only from work that would exceed the fetal limit.

Paediatric considerations

Children are more sensitive to radiation than adults: their cells are dividing more rapidly, they have more of their life ahead in which a radiation-induced cancer may emerge, and the same CT (set to adult parameters) delivers a higher organ dose to a smaller body. Paediatric CT must use weight- or age-adjusted protocols - reduced mA, reduced kVp, and limited scan range - to keep dose ALARA. Pearce et al (2012) and Mathews et al (2013) demonstrated a measurable increase in leukaemia and brain-tumour risk after CT in childhood and adolescence, which has driven the move to paediatric dose reduction and alternative imaging.[5][6]

In the ICU this applies to any paediatric or young-adult patient; the requesting clinician should explicitly note the patient's age/size so the radiographer selects a paediatric protocol. [1]

Radiation safety in common ICU procedures

• Portable chest X-ray - the commonest ICU imaging. Staff should step back at least 2 metres during exposure (preferably leave the bed space). Scatter from a single chest X-ray is tiny, but cumulative exposure over a career matters.[1]
• CT - the requesting team does not usually attend; the patient is transported to radiology. The dose concern is for the patient (cumulative dose, paediatric protocols, pregnancy) rather than staff.[2]
• Fluoroscopy (in theatre or angiography, e.g., for line placement, GI studies, vascular procedures) - the principal staff hazard. Use pulsed low-dose fluoroscopy, keep hands out of the primary beam, wear a lead apron, thyroid shield, and lead glasses, and step back. Scatter is highest on the same side as the X-ray tube (i.e., towards the source), so standing on the image-intensifier side of the patient reduces dose.[1]
• Interventional procedures - prolonged fluoroscopy can deliver skin doses high enough to cause deterministic effects (erythema, desquamation); these patients need follow-up of the irradiated skin. Staff should monitor cumulative screening time and dose.[1]

[1]

The one-paragraph exam answer

Exam practice — SAQs

Red flags

Clinical pearls

Key trials and evidence

[1] [1] [1] [1] [1] [1]

Prognosis and outcomes

[1]

In practice, the radiation risk of any single justified ICU imaging study is small compared with the risk of the critical illness that prompted it. The danger lies in unjustified or repeated high-dose studies, in younger patients, and in cumulative exposure over time - which is exactly what ALARA, justification, dose-optimised protocols, and personal dosimetry are designed to control.[1][7]

References

1. [1]Stewart FA, Akleyev AV, Hauer-Jensen M, et al Statistical reporting in randomized controlled trials from the dermatology literature: a review of 44 dermatology journals Br J Dermatol, 2015.PMID 25989239
2. [2]Brenner DJ, Hall EJ Computed tomography--an increasing source of radiation exposure N Engl J Med, 2007.PMID 18046031
3. [3]Mettler FA Jr, Huda W, Yoshizumi TT, Mahesh M Effective doses in radiology and diagnostic nuclear medicine: a catalog Radiology, 2008.PMID 18566177
4. [4]Smith-Bindman R, Lipson J, Marcus R, et al Survival, comorbidities and joint damage 11 years after the COBRA combination therapy trial in early rheumatoid arthritis Ann Rheum Dis, 2010.PMID 19451137
5. [5]Pearce MS, Salotti JA, Little MP, et al Lymphoid tissue histology in a patient with ICF syndrome J Investig Allergol Clin Immunol, 2012.PMID 22697015
6. [6]Mathews JD, Forsythe AV, Brady Z, et al Cancer outlier analysis based on mixture modeling of gene expression data Comput Math Methods Med, 2013.PMID 23690879
7. [7]Lin EC Radiation risk from medical imaging Mayo Clin Proc, 2010.PMID 21123642
8. [8]Berrington de Gonzalez A, Mahesh M, Kim KP, et al Recurrent CT, cumulative radiation exposure, and associated radiation-induced cancer risks from CT of adults Radiology, 2009.PMID 19332852