Units S H I E L D I N H = D Q. H: Dose equivalent (Sv) D: Dose (Gy) Q: Quality Factor. 1Sv = 1 J/Kg. 1Gy = 1 J/Kg

Transcription

1 S H I E L D I N G H = D Q Units H: Dose equivalent (Sv) D: Dose (Gy) Q: Quality Factor 1Sv = 1 J/Kg 1Gy = 1 J/Kg if dose is expressed in units of cgy (rad) then dose equivalent is expressed in units of rem. Other common unit for H is msv. When solving shielding problems be consistent in using units.

2 Linac Orientation RADIATION PROTECTION AND SAFETY IN RADIOTHERAPY (a) tsec RADIATION PROTECTION AND SAFETY IN RADIOTHERAPY (a) tsec (b) t pri Linac Orientation High density concrete (b) FIG Typical floor plan for an isocentric high energy linac bunker. (a) The machine gantry rotation axis is parallel to the maze entry corridor; the primary barriers are parts of the floor and ceiling, as ell as parts of the east and est alls. (b) The machine gantry rotation axis is perpendicular to the maze entry corridor; the primary barriers are parts High density of the floor and ceiling and parts of the north and south alls. Normal density concrete concrete (2.35 g/cm3) is used in all alls except for the south all, hich is made of high density concrete (5 g/cm3). The door to the treatment room maze is a neutron shielded door. FIG Typical floor plan for an isocentric high energy linac bunker. (a) The machine gantry rotation axis is parallel to the maze entry corridor; the primary barriers are parts of the floor and ceiling, as ell as parts of the east and est alls. (b) The machine gantry 601

3 radiation scattered from or produced by interactions ith the patient and other objects as ell as the leakage radiation from the protective housing of the source. A secondary barrier is a all, ceiling floor or other structure that ill intercept the secondary radiation. It needs to attenuate the secondary radiation to the appropriate shielding design goal. A full discussion of primary and secondary barriers is given in Section 2. Barriers Fig Schematic of radiation sources (primary, leakage and patient-scattered) and the primary and secondary barriers. NCRP All rights reserved. Licensed to Mohammad Salehpour Donloaded 04/04/06 Single user license only, copying and netorking prohibited. Shielding Parameters Workload, W (cgy m2 eek-1) Use factor, U Occupancy factor, T Leakage Radiation

4 Workload W Definition Output produced by therapy unit per eek at 1 m in cgy Example: If a unit treats 25 patients per day ith an average dose of 200 cgy per fraction, then W = cgy m 2 eek -1 Use Factor U Definition Fraction of the operating time during hich the radiation is directed toard a particular barrier Typical Use Factors are: Floor: 1 Walls: 1/4 Ceiling: 1/4-1/2

5 Occupancy Factor T Definition Fraction of the operating time during hich the area of interest is occupied by the individual Typical values of T are: Full occupancy: 1 Partial occupancy: 1/4 Occasional occupancy: 1/8-1/16 Shielding Equations No. of tenth-value layers Primary Radiation Barrier WUT P d2 P= B B= d2 WUT ( ) N = log10 B 1 Barrier thickness t pri = T1 + ( N 1) Te T1: 1st TVL Te: subsequent TVL P: Permissible dose equivalent. (e.g. 5 rem/year for controlled area & 0.1 rem/year for non-controlled area NCRP91, 0.5 rem/year for controlled area NCRP151) B: Transmission factor to reduce dose to P in the area of interest

6 Example RADIATION PROTECTION AND SAFETY IN RADIOTHERAPY tsec 2 S A * d Evaluate the thickness of concrete needed for the primary shield shon here () at the point (A). This unit treats 40 patients per day ith an average dose of 250 cgy per fraction utilizing a 20 MV beam. The distance from source (S) to the point (A) is 4.4 m. a) Radiation Therapy Supervisor s office b) Hospital corridor Example High density concrete W = 40 pt/day x 250 cgy m2/pt x 5 day/eek W = cgy m2/eek U = 1/4 Typical floor plan for an isocentric high energy linac bunker. (a) The machine tion axis is parallel to the maze entry corridor; the primary barriers are parts a) T as= ell 1, aspparts = (5ofrem/year) / 50alls. (eeks/year) and ceiling, the east and est (b) The machine gantry b) T = 1/4,toPthe= maze (0.1 entry rem/year) 50 primary (eeks/year) is is perpendicular corridor;/ the barriers are parts r and ceiling and parts of the north and south alls. Normal density concrete ) is used in all alls except for the south all, hich is made B = [0.1x(4.4)2]/[50000x1x1/4] = 1.55x10of-4high density g/cm3).a) The door to the treatment room maze is a neutron shielded door. N = log10(1/b) = 3.81 = 48 + (2.81) * cm b) B = [0.002x(4.4)2]/[50000x1/4x1/4] = 1.2x10-5 N = 4.92 = 48 + (3.92) * cm Note: We used the recommendations of NCRP 91 in this example. 601 Dose-Equivalent index TVL for X-rays in concrete (NCRP report 51, 1977)

7 Example W = 40 pt/day x 250 cgy m2/pt x 5 day/eek W = cgy m2/eek U = 1/4 a) T = 1, P = (5 rem/year) / 50 (eeks/year) b) T = 1/4, P = (0.1 rem/year) / 50 (eeks/year) a) B = [0.1x(4.4)2]/[50000x1x1/4] = 1.55x10-4 From the graph 173 cm b) B = [0.002x(4.4)2]/[50000x1/4x1/4] = 1.2x10-5 From the graph 222 cm Note: We used the recommendations of NCRP 91 in this example. Dose-Equivalent index TVL for X-rays in concrete (NCRP report 51, 1977) NCRP 151 Table B.2

8 Barrier Width 28 / 2. CALCULATIONAL METHODS Fig. 2.4a. Width of primary barrier protruding into the room. 28 / 2. CALCULATIONAL METHODS Barrier Width Fig. 2.4a. Width of primary barrier protruding into the room. Fig. 2.4b. Arrangement for the primary barrier hen the inside all is continuous. Fig. 2.4b. Arrangement for the primary barrier hen the inside all is continuous. NCRP All rights reserved. Licensed to Mohammad Salehpour

9 Secondary Barrier Scatter B s = P αwt i400 F id 2 i d 2 α: fractional 1 m for a f.s. 400cm 2 scatterer F: area of the scatterer d: distance from scatterer to area of interest d : distance from source to scatterer Scattering Angle α (6MV X-ray) 15 9x x x x x x10-3 NCRP No. 51, 1977 Secondary Barrier Leakage B L = Pid WT Workload (WL): Wpri d: distance from source to area of interest

10 look up in NCRP 151 Distances 2.3 SECONDARY BARRIERS / 33 Fig Room layout shoing distances associated ith patientscattered (dsca, dsec) and leakage radiations (dl). As noted, the scattered-radiation energy is significantly degraded (beyond 20 degree scattered radiation) from that of the primary beam and thus separate data are used to compute its transmission through the barrier. Tables B.5a and B.5b give TVL values in concrete and lead, respectively, for radiations scattered from the patient at different scattering angles and beam energies. For other materials, the TVL for the patient-scattered radiation can be estimated by using the mean energy of the scattered radiation from Table B.6 (Appendix B) and the TVL values from Figures A.1a and A.1b (Appendix A). The barrier transmission of leakage radiation alone (BL) is given by Equation 2.8. Secondary Barrier 2 P dl B L = W T Scatter Bs = (2.8) In Equation 2.8, the factor 10 3 arises from the assumption that leakage radiation from the accelerator head is 0.1 % of the useful beam. The use factor again is taken as one, and dl is measured from the isocenter if it can be assumed that the accelerator gantry angles used are, on average, symmetric. If this is not the situation, then the distance to the individual barriers should be taken from 2 2 of the 2 accelerator head to each barrier and the closest approach Leakage P 400 i id i d α WT F BL = Pid 0.001WT 1 2 NCRP All rights reserved. Licensed to Mohammad Salehpour If the thickness of the to barriers differ by For Megavoltage installations, the leakage Donloaded 04/04/06 barrier usually far exceeds that required for only, copying and at netorking least 3 HVLs (1 TVL) of primary beam, Single user license prohibited. the scattered radiation, since the leakage the thicker of the to ould be adequate. If radiation is more penetrating than the the difference is less than 3 HVLs, then 1 scattered radiation. HVL should be added to the larger one.

11 Neutrons a: transmission factor (1 for Pb) Neutron fluence Q: Neutron source strength per unit dose of x-ray Φ total = Φ dir + Φ sc + Φ th aq 5.4aQ 1.26Q Φ dir = ;Φ sc = ;Φ th = 4π d12 S S d: distance from target to point of interest S: Surface area of treatment room H0: neutron dose eq. at d0 d1: distance from isocenter to centerline of maze Neutron H d2: length of maze RADIATION PROTECTION AND SAFETY IN RADIOTHERAPY T/T0 is the ratio of outer maze area to the inner maze entrance H = (H0 )(T / T0 )(d 0 / d1 )2 10 d2 / 5 (a) D = KΦ total 10 d2 / TVD 2 K: ratio of captured gamma to total n tsec (0.77x10-10) TVD2: tenth value distance (6.2 m) Example (b) d1 d2 High density concrete FIG Typical floor plan for an isocentric high energy linac bunker. (a) The machine gantry rotation axis is parallel to the maze entry corridor; the primary barriers are parts of the floor and ceiling, as ell as parts of the east and est alls. (b) The machine gantry