Room: AAPM ePoster Library
To improve the accuracy of photon skyshine calculations for medical linear accelerator facilities. A widely used formula for the prediction of photon skyshine has been shown to be very inaccurate by comparison with numerous measurements. Discrepancies of up to an order of magnitude have been observed. Circumstances are delineated under which photon skyshine may be ignored.
A scaling formula with a single fitting parameter is derived by considering singly scattered photons. This formula accurately accounts for measurements and provides physical insight into the skyshine phenomenon.
The location of the maximum dose rate depends on the ratio of the roof height above isocenter to the distance from the isocenter to the outer surface of the sidewall. For nominal linac room dimensions, the maximum dose occurs at a distance from the outer wall of approximately two times the height of the roof above the isocenter. The skyshine dose rate is proportional to the field area and not the solid angle subtended by the beam raised to the power 1.3, as predicted by the standard formula. For lightly shielded roofs (concrete thickness less than about 0.5 m), the photon skyshine for 6 MV exceeds that for 18 MV by about a factor of two. At intermediate distances the skyshine declines as one over the distance and not one over the distance squared.
The formula quoted for photon skyshine in NCRP Report #151 is not even qualitatively correct. A scaling formula has been derived, assuming singly scattered photons, that accurately fits measured photon skyshine data. If a roof is shielded so as to avoid designation as a “high radiation area,” (less than 1 mSv in any one hour) photon skyshine will be approximately two orders of magnitude lower than the recommended maximum weekly dose value of 20 µSv.