The effects of motion on the dose distribution of proton radiotherapy for prostate cancer

Proton radiotherapy of the prostate basal or whole seminal vesicles using scattering delivery systems is an effective treatment of prostate cancer that has been evaluated in prospective trials. Meanwhile, the use of pencil beam scanning (PBS) can further reduce the dose in the beam entrance channels and reduce the dose to the normal tissues. However, PBS dose distributions can be affected by intra- and interfractional motion. In this treatment planning study, the effects of intra- and interfractional organ motion on PBS dose distributions are investigated using repeated CT scans at close and distant time intervals. The minimum dose (Dmin) and the dose to 2% and 98% of the volumes (D2% and D98%), as well as EUD in the clinical target volumes (CTV), is used as measure of robustness. In all patients, D98% was larger than 96% and D2% was less than 106% of the prescribed dose. The combined information from Dmin, D98% and EUD led to the conclusion that there are no relevant cold spots observed in any of the verification plans. Moreover, it was found that results of single field optimization are more robust than results from multiple field optimizations.