4.1 ALARA Principles, Structural Shielding & Dose Limits

4.1 ALARA Principles, Structural Shielding & Dose Limits

Radiation protection is a foundational pillar of radiologic technology practice. Because ionizing radiation has the potential to induce somatic and genetic biological damage, radiographers must strictly adhere to established safety guidelines and regulatory mandates. The overarching philosophy governing diagnostic imaging is ALARA (As Low As Reasonably Achievable). This concept assumes that any dose of radiation, no matter how small, carries some degree of biological risk, adhering to the Linear Non-Threshold (LNT) dose-response model.

---

The ALARA Philosophy and Cardinal Rules of Protection

ALARA is not merely a recommendation; it is a regulatory requirement and professional obligation designed to minimize stochastic radiation risks (such as radiation-induced carcinogenesis and hereditary mutations) while keeping deterministic effects (such as skin erythema and cataractogenesis) below threshold levels. Radiographers achieve ALARA through the application of the three Cardinal Rules of Radiation Protection:

1. Time

Radiation dose to personnel and patients is directly proportional to the duration of exposure. Minimizing exposure duration directly reduces total dose: $\ ext{Dose} = \ ext{Dose Rate} \ imes \ ext{Time}$ In diagnostic radiography, exposure duration for static radiographies is controlled by fractional second exposure times. In fluoroscopy, radiographers minimize time by utilizing intermittent pulsed fluoroscopy and monitoring the mandatory 5-minute cumulative timing device (300 seconds), which emits an audible alert when 5 minutes of total beam-on time have elapsed.

2. Distance

Increasing distance from the radiation source is the most effective means of occupational dose reduction. Radiation emitted from a point source spreads out isotropically, following the Inverse Square Law. The intensity ($I$) of radiation is inversely proportional to the square of the distance ($d$) from the source: $\frac{I_1}{I_2} = \left(\frac{d_2}{d_1}\right)^2$

• Practical Implication: Doubling the distance from an x-ray tube or scatter source (e.g., moving from 1 meter to 2 meters) reduces exposure intensity to one-fourth ($25%$) of the original value. Tripling the distance decreases exposure to one-ninth ($\approx 11.1%$). During mobile radiography and fluoroscopy, radiographers should stand at least 2 meters (6 feet) away from the x-ray tube and patient.

3. Shielding

When time cannot be reduced further and distance is constrained, attenuation materials must be placed between the radiation source and exposed individuals. Lead ($\text{Pb}$) is the standard shielding material in diagnostic radiology due to its high atomic number ($Z = 82$) and high mass density. Protective equipment and structural barriers are rated in terms of Lead Equivalent ($\text{Pb eq}$) thickness.

• Half-Value Layer (HVL): The thickness of a specified absorbing material required to reduce the radiation intensity to $50%$ of its original value.
• Tenth-Value Layer (TVL): The thickness required to reduce radiation intensity to $10%$ of its original value ($1 \ ext{ TVL} \approx 3.3 \ ext{ HVL}$).

---

Occupational, Public, and Embryo/Fetus Radiation Dose Limits

The limits that govern Philippine practice are those in CPR Part 3, "Standards for Protection Against Radiation", promulgated by the Philippine Nuclear Research Institute (PNRI) and published in the Official Gazette (Volume 100, No. 36, 6 September 2004). CPR Part 3 adopts the International Basic Safety Standards framework. Many widely used radiography textbooks are written to the US NCRP Report No. 116 framework instead, and its figures differ in three places. Learn the Philippine values as the governing set, and recognise the NCRP figures so you are not caught out by an item written from a US text.

Occupational dose limits (PNRI CPR Part 3, Section 13.2)

Each licensee must control occupational exposure so that none of the following is exceeded:

• Effective dose of 20 mSv per year averaged over five consecutive years. This is the primary occupational limit — not 50 mSv.
• Effective dose of 50 mSv in any single year. A permitted ceiling for one year only, provided the five-year average still satisfies 20 mSv per year.
• Equivalent dose to the lens of the eye: 150 mSv in a year.
• Equivalent dose to the extremities (hands and feet) or the skin: 500 mSv in a year.

Worked check: a technologist recording 24 mSv, 22 mSv, 18 mSv, 16 mSv and 20 mSv over five years averages exactly 20 mSv per year and has never exceeded 50 mSv in a single year, so the limits are satisfied — but ALARA still requires investigation of why the doses are that high.

Apprentices, trainees and students (Section 13.3)

• No person under 16 years of age may be subjected to occupational exposure.
• No person under 18 years may work in a controlled area except under supervision and only for training purposes.
• For apprentices and students aged 16 to 18: effective dose 6 mSv in a year; lens of the eye 50 mSv in a year; extremities or skin 150 mSv in a year.

Pregnant workers (Section 13.4)

A female worker notifies the licensee of her pregnancy so that her working conditions may be modified. The licensee must ensure that the embryo or fetus is afforded the same broad level of protection as a member of the public, and must avoid substantial variation above a uniform monthly exposure rate so as not to exceed 1 mSv in a year. Declaration is the trigger; an undeclared pregnancy cannot be managed.

Members of the public (Section 15.3)

• Effective dose 1 mSv in a year.
• In special circumstances, up to 5 mSv in a single year, provided the average over five consecutive years does not exceed 1 mSv per year.
• Lens of the eye: 15 mSv in a year.
• Skin: 50 mSv in a year.

Comforters and visitors of patients (Section 15.4)

Doses to a person who knowingly and voluntarily helps to comfort or support a patient are constrained so that the absorbed dose is unlikely to exceed 5 mSv during the period of the patient's diagnosis or treatment. Note that this is a dose constraint, not a dose limit, and that medical exposure of the patient is expressly outside the dose-limit system — a patient's diagnostic dose is controlled by justification and optimisation, never by a numerical limit.

Consolidated table

The three highest-yield differences. (1) The Philippine primary occupational limit is 20 mSv/yr averaged over five years, with 50 mSv only as a single-year ceiling. (2) The cumulative 10 mSv x age rule is an NCRP construct and does not appear in CPR Part 3. (3) The Philippine embryo/fetus provision is 1 mSv in a year with a uniform monthly rate, not the NCRP's 0.5 mSv per month and 5 mSv per gestation.

---

Structural Shielding Design Principles

Radiographic rooms must be enclosed by engineered structural protective barriers to safeguard surrounding facility areas from primary and secondary radiation exposure.

Primary Protective Barriers

A primary protective barrier is designed to attenuate the primary (useful) beam leaving the x-ray tube port before it reaches unshielded areas.

• Location: Any wall toward which the primary beam can be directed (e.g., chest bucky wall, floor under table).
• Lead Equivalent Requirement: Must contain $1/16\ ext{ inch}$ (approximately $1.6\ ext{ mm}$) lead equivalent.
• Height Requirement: Must extend $7\ ext{ feet}$ ($2.1\ ext{ meters}$) vertically upward from the floor when the x-ray tube target is located within 5 to 7 feet of the barrier wall.

Secondary Protective Barriers

A secondary protective barrier is designed to attenuate secondary radiation, which consists of scatter radiation from the patient and leakage radiation from the x-ray tube housing. It is never hit by the unattenuated primary beam.

• Location: Walls parallel to the primary beam path, control booth walls, and ceiling structures.
• Lead Equivalent Requirement: Must contain $1/32\ ext{ inch}$ (approximately $0.8\ ext{ mm}$) lead equivalent.
• Overlap Requirement: Secondary barriers must overlap primary barriers by at least $1/2\ ext{ inch}$ ($1.25\ ext{ cm}$) at all structural junctions.
• Control Booth Barrier: The radiographer control console wall is classified as a secondary barrier. The room layout must ensure that x-ray photons undergo at least two scatter interactions before entering the control booth area. The observation window must contain $1.5\ ext{ mm Pb eq}$ glass.

Structural Shielding Calculation Parameters

Medical physicists calculate required barrier thickness based on three key exposure factors:

1. Workload ($W$): Represents the volume of radiation output from an x-ray unit, measured in milliampere-minutes per week ($\text{mA-min/wk}$).
2. Use Factor ($U$): The fractional portion of beam-on time during which the primary beam is directed toward a specific barrier wall ($U = 1$ for floor; $U = 1/4$ for chest wall; $U = 1/16$ for side walls unlikely to receive primary beam).
3. Occupancy Factor ($T$): The fraction of work time during which an area beyond the barrier wall is occupied by individuals (Full Occupancy $T = 1$ for offices/nursing stations; Partial Occupancy $T = 1/2 \ ext{ to } 1/5$ for corridors/restrooms; Frequent Occupancy $T = 1/8 \ ext{ to } 1/16$ for stairways/parking lots).