Dosimetric Impact of Intrafraction Motion in Online-Adaptive Intensity Modulated Proton Therapy for Cervical Cancer
A method was recently developed for online-adaptive Intensity Modulated Proton Therapy (IMPT) in cervical cancer patients. The advantage of this approach, relying on the use of tight margins, is challenged by the intra-fraction target motion. The purpose of this study was to evaluate the dosimetric impact of intra-fraction motion on the target due to changes in bladder filling in cervical cancer patients treated with online-adaptive IMPT.
In ten patients selected to have large uterus motion induced by bladder filling, the intra-fraction anatomical changes were simulated for several pre-fraction durations for online (automated) contouring and planning. For each scenario, the coverage of the primary target was evaluated with margins of 2.5 and 5 mm.
Using a 5 mm PTV margin, median accumulated D98% was above 42.75 GyRBE1.1 (95% of the prescribed dose) in case of a pre-fraction duration of 5 and 10 minutes. For a pre-fraction duration of 15 minutes, this parameter deteriorated to 42.6 GyRBE1.1. When margins were reduced to 2.5 mm, only a 5-minute duration resulted in median target D98% above 42.75 GyRBE1.1. In addition, smaller bladders were found to be associated with larger dose degradations compared to larger bladders.
This study indicates that intra-fraction anatomical changes can have a substantial dosimetric impact on target coverage in an online-adaptive IMPT scenario, for patients subject to large uterus motion. A margin of 5 mm was sufficient to compensate for the intra-fraction motion due to bladder filling for up to 10 minutes of pre-fraction time. However, compensation of the uncertainties that were disregarded in this study, by using margins or robust optimization, is also required. Furthermore, a large bladder volume restrains intra-fraction target motion and is recommended for treating patients in this scenario. Assuming that online-adaptive IMPT remains beneficial as long as narrow margins are used (5 mm or below), this study demonstrates its feasibility with regards to intra-fraction motion.