Utilization of real-time motion tracking methods in radiotherapy for prostate cancer

Abstract

Newly found high fractionation sensitivity of prostate cancer has led to increasing use of stereotactic body radiotherapy (SBRT) in the treatment of prostate cancer. SBRT reduces treatment costs and increases treatment capacity and patient comfort, but due to high fraction doses and few treatment fractions, it requires higher accuracy of the treatment delivery than standard prostate radiotherapy (RT). In addition to standard procedure of interfraction motion correction, also the intrafraction motion correction of the prostate needs to be considered in the localization of the SBRT treatment. For this demand, electromagnetic (EM) localization methods capable of intrafraction motion tracking and treatment adaptation have been developed. In addition to EM methods, rectal immobilization devices (RR), developed for rectal dose sparing, have been suggested to reduce the intrafraction prostate motion. The aim of this thesis was to create a treatment protocol for prostate SBRT utilizing EM methods and RR, in which regard various features of these methods were investigated. These include: the immobilizing effect of the RR (Rectafix, Mimator AB), localization accuracy of two EM systems (RayPilot, Micropos Medical AB, and Calypso, Varian Medical Systems) and dosimetric benefit of continuous motion monitoring based motion correction and beam gating strategy. Results of this thesis indicate that the use of RR in standard clinical setting may increase the intrafraction prostate motion, thus reducing the accuracy of treatment delivery. Localization accuracy of the RayPilot was found being compromised due to the positional instability of the intraprostatic transmitters, whereas the accuracy of the Calypso was comparable to kV imaging of intraprostatic fiducial markers. Dosimetric and motion analyses revealed, that single pre-treatment CBCT-guided treatment localization can lead to clinically relevant target dose deficits, whereas additional pre-treatment motion correction is adequate for most of the patients. However, continuous motion monitoring based correction strategy with beam gating is required to ensure high target dose coverage and to minimize the risk organ doses for all fractions. Currently implemented treatment protocol for prostate SBRT in Tampere University Hospital is based on the findings of this thesis and utilizes continuous motion monitoring based motion correction and beam gating with Calypso.