Abstract ID: 188 Monte Carlo simulation of “out-of-field” distributions in a pediatric phantom in external radiotherapy – Couch top impact

Abstract

Introduction The scattered radiation from external devices in radiotherapy may cause altered dose distribution among other complications. Although some dose corrections are made for blocking trays, ion chambers, etc. the dose perturbation by the treatment couch top scatter is often overlooked. This out-of-field scattered radiation may induce secondary cancer, especially in pediatric patients, due to biological and geometric factors. Since the out-of-field dosimetric accuracy of treatment planning systems (TPS) is known to decrease, and many of them do not provide the capability of implementing the actual treatment couch top, a PENELOPE based MC code named PRIMO was used to study the couch impact on the out-of-field dose estimation in pediatric irradiation. Material and methods A treatment plan was designed within PRIMO for a hypothetical brain tumor, using two lateral and one cranio-caudal 6MV beam configuration. The plan was created on a CT dataset of 5-year anthropomorphic pediatric phantom. The original dataset was modified with MatlabTM to: keep the original HU values inside the body contour; introduce a real treatment couch HU values and; define the pixel values outside the body and couch as air. Two simulations were performed: with and without the presence of the treatment couch. The PRIMO output data for both simulations were reshaped, manipulated and written as two single multi-frames DICOM files. The dose distribution outside the body contour in both plans was eliminated, and both dose distributions inside the body along the axial plan were compared using 3D gamma function (2%, 2 mm) on PTW-Verisoft. Results Within the treatment field, the results showed good agreement between the two dose distributions (over than 99% of points passed 3D gamma test), indicating that no significant attenuation occurred to the lateral beam crossing the treatment couch. The agreement decreased gradually far away from the isocenter to reach 93% at 20 cm away from isocenter, showing a cranio-caudal back-scattered contribution to dose from the treatment couch. Conclusion For out-of-field scattered dose estimation it is advisable to include the treatment couch in dose calculations.