Analytical model for photon peripheral dose estimation in radiotherapy treatments

© 2015 IOP Publishing Ltd. This work aims to generate a simple analytical model that allows estimation of peripheral photon equivalent dose to organs of individual patients, valid for any isocentric technique. Photon radiation scattered in the LINAC head has been simulated as a virtual source of radiation emitting isotropically so that, before reaching a point inside the patient, it decreases with the square law and with attenuation duetoair and tissue. Leakage has been simulated as a constant background dose along the patient. Firstly, a dose-to-points basic model was proposed and parameterized byfitting it toabsorbed doses measured with TLD-700 in ahumanoid phantom. Secondly, this model was generalizedto any other situation involving intensity-modulated beams of any size and shape. Validation of this general model, usable beyond 10 cm from thefield edge, was carried outbycomparing estimation with TLD-100 doses for VMAT and IMRT treatments aswellaswith experimental data and models existing in the bibliography. Finally, an equivalent dose-to-organs model has been proposed by rescaling individual anatomical dimensions ontoamathematical phantom in order to make an estimation of organ length for dose calculation. The parameterized extended model, accounting for intensity-modulated beams of any shape, predicts measurements withamaximum relative uncertainty of±25%. This general model, easy to apply in a clinical routine thanks to the ready availability of input parameters, has been proposed and validated for estimation of photon equivalent doses to peripheral organs. Finally, as a first step, ithas been implemented into a piece of software termed PERIPHOCAL (PERIpheral PHOton CALculation), which iseasily transferred toacommercial treatment planning system (TPS).