Poster - Thur Eve - 36: Out-of-Field dose in craniospinal irradiation

The risk of radiotherapy induced secondary cancer depends on the integral dose delivered to the patient where the dose delivered within the radiation field is accounted for, as well as dose to out-of-field organs from scattered and leakage radiation. While commercial treatment planning systems allow accurate determination of in-field dose, they are generally not capable of accurate out-of-field dose prediction. Secondary cancer risk is especially an issue in craniospinal treatments where involved patients are often children or young adults. In this work we therefore propose a mathematical model that accurately predicts out-of-field dose for patients treated by craniospinal irradiation at the American University of Beirut Medical Center. An anthropomorphic phantom was imaged, planned and treated, with thermoluminescent dosimeters inserted in the phantom at in-field and out-of-field locations. The measurements showed that our treatment planning system calculated accurately (within 2%) dose inside the field, but did not perform well at points just outside the field edge and consistently underestimated the dose at points further away from the field edge. From the out-of-field measured data, a model was developed that predicts out-of-field dose at a point in the patient based on the distance of that point to the treatment field edge. The developed model is of the double-gaussian type; it contains parameters that can be tuned to make it applicable in other centers where linac geometry and treatment techniques may differ.