屏東縣醫事放射師公會九十七年度繼續教育 ( 二 ) 主題：輻防安全與法規 日期 : June 15,2008 時間 : 14:10-15:00 地點 : 屏東基督教醫院六樓集會室

NCRP No.151 主辦 : 屏東縣醫事放射師公會、財團法人屏東基督教醫院 講員 : 張寶樹 (Pao-Shu Chang, PhD) 高雄醫學大學醫學放射技術學系、輻防班，附設醫院放腫科、輻防室、輻委會 醫學物理師 ( 醫物甄字第 019 號 ) ，高級輻射防護專業人員 ( 輻專高字第 003 號 )( 已換發輻專師字 00095 號 ) ，非醫用可發生游離輻射設備高級操作執照 ( 非醫 人字第 10753 號 ) E-mail:[email protected] Cell phone: 0939272822 Office phone:07-3121101 轉 2355 、 7127 高醫輻防班網頁 : http://rptic.dlearn.kmu.edu.tw/

NCRP REPORT No. 151 Structural Shielding Design and Evaluation for Megavoltage X- and Gamma-Ray Radiotherapy Facilities Recommendations of the NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS December 31, 2005

Contents 1. Introduction 2. Calculational Methods 3. Workload, Use Factor, and Absorbed-Dose Rate Considerations 4. Structural Details 5. Special Considerations 6. Shielding Evaluation (Surveys) 7. Examples Appendix A. Supporting Data (Figures) Appendix B. Supporting Data (Tables) Appendix C. Neutron Monitoring for Radiotherapy Facilities

1. Introduction The quantity recommended in this Report for shielding design calculations when neutrons, as well as photons, are present is dose equivalent (H)( 等效劑量 ). The units of dose equivalent are J kg–1 with the special name sievert (Sv).

Quantities and Units The quantity kerma measured in air (Ka) is recommended for shielding design calculations in low linear-energy-transfer (LET) environments.

Fig. 1.1. Fluence-to-dose equivalent conversion coefficients.

Low-energy Accelerator and Highenergy Accelerator For purposes of shielding design for x-ray beams, the terms low-energy accelerator (defined as ≤10 MV accelerating voltage) and high-energy accelerator (defined as >10 MV accelerating voltage) will be employed.

Controlled Area A controlled area is a limited-access area in which the occupational exposure of personnel to radiation or radioactive material is under the supervision of an individual in charge of radiation protection.

Controlled Area In radiotherapy facilities, these areas are usually in the immediate areas where radiation is used, such as treatment rooms and control consoles, or other areas that require control of access, occupancy and working conditions for radiation protection purposes.

Uncontrolled Area Uncontrolled areas for radiation protection purposes are all other areas in the hospital or clinic and the surrounding environs.

Shielding Design Goals and Effective Dose In NCRP Report No.151, shielding design goals (P) are levels of dose equivalent (H) used in the design calculations and evaluation of barriers constructed for the protection of workers or members of the public.

Recommendation for Controlled and Uncontrolled Areas Recommendation for Controlled Areas: Shielding design goal (P) (in dose equivalent): 0.1 mSv week–1 (5 mSv y–1) Recommendation for Uncontrolled Areas: Shielding design goal (P) (in dose equivalent): 0.02 mSv week–1 (1 mSv y–1)

Shielding Design Assumptions Attenuation of the primary beam by the patient is neglected. The patient typically attenuates the primary beam by 30 % or more. The calculations of recommended barrier thickness often assume perpendicular incidence of the radiation.

Shielding Design Assumptions (i) Leakage radiation from radiotherapy equipment is assumed to be at the maximum value recommended by IEC (2002) for the radiotherapy device, although in practice the leakage radiation is often less than this value.

Shielding Design Assumptions (ii) The recommended occupancy factors for uncontrolled areas are conservatively high. For example, very few people spend 100 % of their time in their office. The minimum distance to the occupied area from a shielded wall is assumed to be 0.3 m.

Shielding Design Assumptions (iii) The “two-source rule” is applied whenever separate radiation components are combined to arrive at a barrier thickness. The two-source rule is even more conservatively safe when applied to dualenergy machines, even though the individual energies cannot be used simultaneously.

Workload The workload (W) for radiotherapy equipment covered in NCRP No.151 is the time integral of the absorbed-dose rate determined at the depth of the maximum absorbed dose, 1 m from the source.

Workload The units for W are Gy week–1 and conversion to a workload W2 at a distance d2 different than 1 m would be ： W2 = W (1 m)2 / (d2)2.

Use Factor The use factor (U) is the fraction of a primary-beam workload that is directed toward a given primary barrier. The value for U will depend on the type of radiation installation.

Occupancy Factor The occupancy factor (T) for an area is the average fraction of time that the maximally exposed individual is present while the beam is on.

Primary Radiation and Secondary Radiation In radiotherapeutic applications, the radiation consists of primary and secondary radiations.

Primary Radiation Primary radiation, also called the useful beam, is radiation emitted directly from the equipment that is used for patient therapy.

Primary Barrier A primary barrier is a wall, ceiling, floor or other structure that will intercept radiation emitted directly from the source. It needs to attenuate the useful beam and also any secondary radiation that impinges on it to the appropriate shielding design goal.

Secondary Radiation Secondary radiation consists of radiation scattered from or produced by interactions with the patient and other objects as well as the leakage radiation from the protective housing of the source.

Secondary Barrier A secondary barrier is a wall, ceiling floor or other structure that will intercept the secondary radiation. It needs to attenuate the secondary radiation to the appropriate shielding design goal.

Fig. 1.2. Schematic of radiation sources (primary, leakage and patient-scattered) and the primary and secondary barriers.

Basic Principles Exposure of individuals to primary and secondary radiations can be reduced by one or a combination of the following methods: increasing the distance between the individual and the sources of the radiation, limiting the exposure time, and interposing protective shielding between the individual and the radiation sources.

Qualified Expert The term qualified expert used in NCRP No.151 is defined as a medical physicist or a health physicist who is competent to design radiation shielding in radiotherapy facilities, and who is certified by the American Board of Radiology, American Board of Medical Physics, American Board of Health Physics, or Canadian College of Physicists in Medicine.

Types of Radiotherapy Installations Many modern radiation therapy facilities now utilize TBI, IMRT, stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT).

TBI In total-body photon irradiations, the maximum field size directed onto a specific primary barrier is often used with beam-on times of 15 min or more. The use factor for that barrier can be much larger than would be the case for routine fields delivered to the patient from multiple directions.

IMRT Intensity modulated radiation therapy (IMRT) can be accomplished with different technologies, but the net result is that the standard therapeutic absorbed dose is delivered from many directions around the patient with as much as 10 times the normal beam-on time. The fluence on the primary barriers is quite similar to the conventional treatment regimen but the leakage radiation on the secondary barriers may be much larger.

SRS and SRT With SRS and SRT, high individual absorbed doses are delivered to patients and therefore both the primary and secondary barrier workloads can be greater than in the standard case. Likewise, multiple, oblique angles are used and this can skew assumptions about the use factors for the barriers if they were not explicitly considered in the design.

Strategic Shielding Planning Strategic shielding planning for a radiotherapy facility incorporates a knowledge of basic planning and shielding principles. The strategic planning concept involves the use of shielding options dictated by a knowledge of the sources of radiation in a facility, the occupancy and usage of adjacent areas, and whether specific walls, floors and ceilings must be considered primary or secondary barriers.

Construction Inspection Thickness and density of concrete. Thickness of metal shielding and polyethylene used for neutron shielding. Thickness of metal behind recesses in the concrete (e.g., laser boxes). HVAC shielding baffle if used. Location and size of conduit or pipe used for electrical cable of any type. Verification that the shielding design has been followed.

Documentation Requirements Shielding design report. Construction, or as-built, documents. Post-construction survey reports. Information regarding remedies. More recent reevaluations of the room shielding relative to changes (e.g., in utilization) which have been made or are still under consideration

2. Calculational Methods Shielding design for medical radiation therapy facilities has been based on simple empirical equations developed by Mutscheller (1925; 1926) and later refined by NCRP (1976; 1977).

Fig. 2.1. Basic shielding schematic of an individual at Location O protected from radiation source S at d distance away by a shield at B.

Fig. 2.2. Production of radiation types in a linear accelerator. Radiations to the right of the line have significant production cross sections in accelerators with photon energies above ~10 MeV.

Primary Barriers The transmission factor of the primary barrier (Bpri) that will reduce the radiation field to an acceptable level is ：

Primary Barriers The transmission factor of the primary barrier (Bpri) that will reduce the radiation field to an acceptable level is ：

Primary Barriers P = shielding design goal (expressed as dose equivalent) beyond the barrier and is usually given for a weekly time frame (Sv week–1) dpri = distance from the x-ray target to the point protected (meters) W = workload or photon absorbed dose

Primary Barriers U = use factor or fraction of the workload that the primary beam is directed at the barrier in question T = occupancy factor for the protected location or fraction of the workweek that a person is present beyond the barrier. This location is usually assumed to be 0.3 m beyond the barrier in question (see Table B.1 in Appendix B for recommended occupancy values)

Number of TVLs The required number (n) of TVLs is given by: n = log (Bpri)

Barrier Thickness The barrier thickness (tbarrier) is given by: tbarrier = TVL1+ (n – 1) TVLe The first (TVL1) and equilibrium (TVLe) tenth-value layers of the desired material are used to account for the spectral changes in the radiation as it penetrates the barrier.

Fig. 2.3. Relationship between the slant thickness (ts = t/cos θ) of radiation incident on a barrier with angle of obliquity ( θ) and thickness of the barrier (t). Also shown is a scattered photon with a path length
Total Transmission Factor For the primary photon beam, the total transmission factor is the product of the transmission factors of each of the individual materials in the barrier (e.g., BT = Bconc BPb Bsteel, for concrete, lead and steel, respectively).

Fig. 2.4a. Width of primary barrier protruding into the room.

Fig. 2.4b. Arrangement for the primary barrier when the inside wall is continuous.

Fig. 2.4c. Arrangement for the primary barrier when lead or steel is used to maintain a uniform wall thickness.

Fig. 2.4d. Sketch showing angulation of the plane of gantry rotation at 45 degrees to the walls. Note the asymmetry of the extremities of the primary beam on the outside of the wall (A, B) compared with the central axis of the beam.

Neutron dose-equivalent per week The following empirical equation was used to estimate the neutron dose-equivalent per week beyond the laminated barrier when the collimator is opened to maximum size (Figure 2.5).

Neutron dose-equivalent per week Hn = neutron dose equivalent per week (μSv week– 1 ) Do = x-ray absorbed dose per week at isocenter (cGy week–1) R = neutron production coefficient (in neutron microsievert per x-ray centigray per beam area in m2) (i.e., μSv cGy–1 m–2) Fmax = maximum field area at isocenter (m2) tm = metal slab thickness (meters)

Neutron dose-equivalent per week t1 = first concrete slab thickness (meters) t2 = second concrete slab thickness (meters) TVLx = tenth-value layer in concrete for the primary x-ray beam (meters) (Table B.2) TVLn = tenth-value layer in concrete for neutrons (meters) (can be extracted from Figure A.2) 0.3 = distance from the outer surface of the barrier to thepoint of occupancy (meters)

Fig. 2.5. Laminated barrier with metal of thickness tm between concrete thicknesses of t1 and t2.

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