An equivalent shape approximation for photon doses in lung

An equivalent shape for an inhomogeneous body exposed to a photon field of given dimensions is proposed. The shape is defined by the equivalent lengths of lines radiating from the point of interest and terminating at the beam edges or the body surface. The expected scatter contributions at the focal point of this water-equivalent condensed body are compared with those at the point of interest in the inhomogeneous body. It is shown that the contributions are expected to be approximately equal, provided no major changes have been made in the primary photon fluences. In practice there will be some changes in the primary fluences. The effects of such changes are examined experimentally by comparing the measured dose at a point in an inhomogeneous phantom with the dose in the corresponding condensed body. The measurements were made for three field arrangements and for four qualities of photon beam. The method is applicable to cases in which full electron equilibrium has not been reached.