On-line compensation of dose changes introduced by tumor motion during scanned particle therapy

Abstract

Tumor irradiations using scanned particle beams provide superior target conformity and dose homogeneity for stationary tumors. In case of intrafractional motion interference between beam scanning and tumor motion causes deteriorations of the deposited dose distributions necessitating dedicated motion mitigation techniques. Different techniques are currently investigated at GSI. The most favorable among them in terms of target conformity and sparing of organs at risk and normal tissues is beam tracking, i.e. adapting the Bragg peak positions on-line according to the tumor motion in all three dimensions. Adaptation of Bragg peak positions only does not mitigate possible dose changes along the beam’s path. Consideration of the respective dose changes has been shown to be beneficial for a future clinical implementation of beam tracking but has to be performed on-line, i.e. during treatment, because of tumor trajectory variations between different respiratory cycles. Functionality to account for these dose changes caused by tumor motion has been implemented in the experimental branch of the therapy control system at GSI. Basic functionality of the on-line dose compensation was tested experimentally with a series of measurements in 2D with radiographic films and in 3D with an array of ionization chambers. In both cases a reference irradiation could be reproduced using the dose compensation functionality. In case of the ionization chamber measurement severe over- and under-dosages of up to 25% compared to reference irradiation for 3D beam tracking without on-line dose compensation could be reduced to below 3% by additionally employing the dose compensation functionality. It has been shown that the fluence of every rasterpoint can be individually adapted during irradiation.