4.1 ALARA Principles, Structural Shielding & Dose Limits

4.1 ALARA, Shielding and the Correct Regulatory Scope

Radiation-protection questions require two decisions: identify the exposure category, then apply the rule that actually governs the source and situation. A number remembered from an older textbook can be wrong for the lens of the eye, pregnancy or the regulator named in the question.

As low as reasonably achievable (ALARA) means optimizing protection while achieving the medical task. A justified examination still needs adequate image quality; simply reducing exposure until the image is unusable can lead to a repeat. For workers and the public, dose limits are ceilings, not exposure targets. The medical exposure of the patient is managed through justification and optimization rather than by applying an occupational annual dose limit.

Time, distance and shielding: use the assumptions

Time: At a constant dose rate, dose equals rate multiplied by time. If a hypothetical work location has a steady equivalent-dose rate of 2 microSv/hour, spending 30 minutes there gives 1 microSv: convert 30 minutes to 0.5 hour before multiplying. A varying rate requires accounting for its variation. This arithmetic is not a prediction of a patient's diagnostic dose.

Distance: For a point-source approximation without intervening attenuation or changes in geometry:

$$I_2 = I_1\left(\frac{d_1}{d_2}\right)^2$$

Doubling distance gives one-quarter of the initial intensity; tripling it gives one-ninth, approximately 11.1%. A patient is an extended scatter source, so an actual staff dose need not follow that idealization exactly. Use distance together with positioning, barriers and the facility's approved operating procedures. Two metres is approximately 6.6 feet; it is not an exact conversion to six feet or a universal safe-distance guarantee.

Shielding: Material, thickness and beam quality determine attenuation. A half-value layer (HVL) reduces the specified beam quantity to one-half under the stated conditions. Two identical HVLs leave one-quarter if the same attenuation model applies. A tenth-value layer (TVL) leaves one-tenth. With a constant exponential attenuation coefficient, one TVL is approximately 3.32 HVLs; a broad diagnostic spectrum can change as it passes through material, so that ratio is not a universal design shortcut.

Identify the regulator before applying a limit

The final PNRI Code of Regulations, CPR Part 3, Revision 02, published in the Official Gazette on March 8, 2021, governs the practices and licensed materials within its scope. Section 2(b)(1) expressly separates X-rays and other electrically generated radiation and related facilities under the Department of Health–Food and Drug Administration (DOH–FDA). PNRI training or dosimetry services do not make PNRI Part 3 the licensing rule for every diagnostic X-ray room.

When an examination case expressly asks about a PNRI-licensed radioactive-material practice, use the following current provisions. For a machine-X-ray facility, determine the applicable DOH–FDA requirements and facility authorization rather than silently transferring the PNRI table.

The adult effective-dose and lens limits are separate checks. Effective doses of 24, 22, 18, 16 and 20 mSv total 100 mSv, averaging 20 mSv/year; none exceeds the 50 mSv single-year ceiling. That satisfies those two effective-dose numerical tests, but says nothing about a worker's separately assessed lens or extremity doses, and does not establish that protection is optimized. Do not replace the five-year test with a standalone 50 mSv annual allowance.

For the young-apprentice category, access to a controlled area is supervised and only for the specified training or study purposes. Section 23.2 also prohibits occupational exposure of persons under 16. Age and purpose matter; the lower limits do not authorize unsupervised radiation work.

After pregnancy notification, the employer must adapt working conditions to provide the required embryo/fetal protection. Section 24 does not make notification a reason for automatic exclusion from work. The 1 mSv provision concerns the embryo/fetus throughout pregnancy, not a new annual whole-body limit for the worker and not an invented uniform monthly allocation.

Public exposure, carers and visitors are different cases

Section 44(a)(2) permits a higher effective dose in a single year in special circumstances while retaining a five-year average no greater than 1 mSv/year. It does not reproduce the old text's explicit universal 5 mSv single-year ceiling, and it is not permission to disregard authorization or optimization.

Two particular PNRI medical situations also require careful reading:

• Under section 40(b), release after radionuclide therapy requires the medical physicist or Radiation Protection Officer to establish that the dose likely to reach public and family members will not exceed 3 mSv. Section 40(c) additionally requires protective instructions, including written instructions, when another person's total effective dose equivalent is likely to exceed 1 mSv.
• Under section 44(c), a visitor to a patient who cannot be released under section 40 may receive more than 1 mSv only when the dose does not exceed 3 mSv and the authorized user determines beforehand that the visit is appropriate.

These are specific release and visitor provisions. They do not create a universal 3 mSv allowance for all carers, visitors or diagnostic procedures. Carer/comforter protection also involves the applicable dose constraints and information requirements. Absorbed dose is expressed in gray; equivalent and effective dose use sievert. A table headed “absorbed dose in mSv” confuses different quantities.

Structural barriers: a design task, not a thickness slogan

A primary barrier intercepts the intended useful beam. Secondary shielding addresses scattered and leakage radiation. Their adequacy depends on the intended beam directions and installation. A wall, door, viewing window or joint can undermine a design if its protection does not match the approved specification.

Assess the source spectrum and output, workload, geometry, use of each barrier and occupancy of adjacent areas. Workload describes expected radiation production over the design period; a use factor describes the fraction directed toward a particular primary barrier; an occupancy factor describes occupation of the protected location. Do not assign one fixed use or occupancy factor merely because an area is called a floor, corridor or car park. The calculation's units and assumptions must match its method.

For example, changing a neighbouring store into an occupied office can change shielding needs even when the machine stays put. Check the proposed use against the approved assessment before operating under the changed conditions. A blanket 1/16-inch primary wall, 1/32-inch secondary wall, seven-foot height or one particular glass thickness cannot establish adequacy for every room. Facility design and review must follow the applicable national requirements and actual installation.

Sources: PNRI final CPR Part 3, Revision 02, especially sections 1–2, 23–25, 35–38, 40 and 44–47; IAEA SSG-46, X-ray room design, paragraphs 3.9–3.16; IAEA Diagnostic Radiology Physics, chapter 24.7; ICRP Publication 105.