Analysing collimator structure effects in head-scatter calculations for IMRT class fields using scatter raytracing

The frequent blocking of the irradiated volume in intensity modulated radia tion therapy (IMRT) makes the head-scatter fraction of the incident photon fluence more significant than that in conventional therapy with open fields . On the other hand, certain collimator configurations block scatter photon s directed to a given observation point while allowing primary photons to b e transmitted. The 'anomalous blocking' makes the primary field a poor indi cator of the scatter fluence. Since large MU-to-cGy ratios in IMRT can magn ify head-scatter uncertainties, it becomes necessary to accurately model bo th the effective scatter source and the collimator structure that lit-nits the scatter reaching the irradiated volume. First we obtain a dual-source m odel, using a Taylor series expansion to derive the effective scatter sourc e distribution from the data measured for the Elekta SL20 linac equipped wi th a multi-leaf collimator (MLC). Then, using a raytracing algorithm, we ca lculate the transmission of scatter rays from the effective scatter source plane to points in the patient plane. The method can account for the anomal ous blocking of scatter by the MLC leaves and the backup diaphragms. For a variety of collimator settings tested, the calculations agree with measurem ents to an accuracy of 0.002 Psi (10 x 10) where Psi (10 x 10) is the total (primary + scatter) photon fluence of an open 10 x 10 cm(2) field for the same MU delivered. Although the significance of collimator structure in IMR T depends strongly on fields shapes employed for the delivery, potential Cu mulative errors on the order of a few per cent can be avoided in fluence ca lculations if the proposed method is used.