Intra-adaptational changes in online adaptive radiotherapy: from the ideal to the real dose.

[BACKGROUND AND PURPOSE] Online adaptive radiotherapy has demonstrated dosimetric benefits by accounting for interfractional organ variations. However, this study investigates the dosimetric impact of intra-adaptational anatomical changes that take place during the adaptation process.

[METHODS] Our retrospective analysis was conducted on 155 fractions from 8 prostate cancer patients treated with adaptive radiotherapy using the Varian Ethos system (Varian, Palo Alto, California, USA). Various dose-volume metrics for the targets and organs at risk were assessed for (1) the non-adapted (an original plan on a pretreatment cone-beam CT [CBCT], acquired at the beginning of a treatment session), (2) the adapted (an adapted plan on a pretreatment CBCT), and (3) the delivered dose distributions (an adapted plan on a pre-irradiation CBCT acquired for patient position verification with recontoured organs).

[RESULTS] For the target metrics, we quantitatively proved that the delivered dose distribution was still beneficial in comparison to the non-adapted one, despite the anatomical changes during the adaptation process. The bladder dose-volume metrics strongly depended on the bladder volume variations across the planning CT and both CBCTs, frequently showing improvement during the adaptation process as the bladder continued to fill. In contrast, no clear trend was observed for the rectum or posterior rectum wall metrics. In only a small fraction of sessions (up to 5% for most metrics) were the metric objectives not achieved with the delivered dose while they were achieved with the adapted one. Physiological reasons for these occurrences stemmed from meteorism occurring between pretreatment and pre-irradiation CBCTs.

[CONCLUSION] This study confirms that the dosimetric advantages of online adaptive radiotherapy persist in clinical practice, despite anatomical changes due to the time delay needed for the adaptation process.