Dosimetric Impact of Intrafraction Motion in Online-Adaptive Intensity Modulated Proton Therapy for Cervical Cancer

Forfattere Berger T, Godart J, Jagt T, Vittrup AS, Fokdal LU, Lindegaard JC, Jensen NBK, Zolnay A, Reijtenbagh D, Trnkova P, Tanderup K, Hoogeman M.
Kilde Int J Radiat Oncol Biol Phys . 2021 Apr 1;109(5):1580-1587 Publiceringsdato 20 nov 2020


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.