Planning target volume margin assessment for online adaptive MR-guided dose-escalation in rectal cancer on a 1.5 T MR-Linac

Abstract

This study assessed the margins needed to cover tumor intrafraction motion during an MRguided radiotherapy (MRgRT) dose-escalation strategy in intermediate risk rectal cancer. Methods: Fifteen patients with rectal cancer were treated with neoadjuvant short-course radiotherapy, 5x5 Gy, according to an online adaptive workflow on a 1.5 T MR-linac. Per patient, 26 3D T2 weighted MRIs were made; one reference scan preceding treatment and five scans per treatment fraction. The primary tumor was delineated on each scan as gross tumor volume (GTV). Target coverage margins were assessed by isotropically expanding the reference GTV until more than 95% of the voxels of the sequential GTVs were covered. A margin with a coverage probability threshold of 90% was defined as adequate. Intra-and interfraction margins to cope with the movement of the GTV in the period between scans were calculated to indicate the target volume margins. Furthermore, the margin needed to cover GTV movement was calculated for different time intervals. Results: The required margins to cover inter-and intrafraction GTV motion were 17 mm and 6 mm, respectively. Analysis based on time intervals between scans showed smaller margins were needed for adequate GTV coverage as time intervals became shorter, with a 4 mm margin required for a procedure of 15 min or less. Conclusion:The shorter the treatment time, the smaller the margins needed to cover for the GTV movement during an online adaptive MRgRT dose-escalation strategy for intermediate risk rectal cancer. When time intervals between replanning and the end of dose delivery could be reduced to 15 min, a 4 mm margin would allow adequate target coverage.

Mesorectal motion evaluation in rectal cancer MR-guided radiotherapy: an exploratory study to quantify treatment margins

“…Differently from previous studies, which supposed a possible impact of tumor location and stage on tumor motion itself, we found no correlation between these factors and MM, relying on the better morphological detail provided by MR images [26][27][28]. Considering these and other differences in the methodology applied in the previous studies, our results are not directly comparable with others.…”

“…MRgRT is currently considered as one of the best strategies to carry out dose escalation protocols in LARC, in particular with the application of online adaptive (OA) MRgRT protocols, in order to increase the dose-dependent therapeutic efficacy [28][29][30].…”

Impact of endorectal filling on interobserver variability of MRI based rectal primary tumor delineation

“…With the advent of MRI-Linac hybrid devices it has become possible to considerably reduce planning target volume margins as treatment plans can be adapted to the anatomy of the day based on daily MRI scans. Several studies have shown that planning target volumes required to cover the remaining intrafractional motion with or without rectal filling can be reduced to 4 mm using an online adaptive workflow [2] , [6] , [9] . The present study demonstrates that the addition of rectal ultrasound gel results in more accurate target volume definition in rectal cancer.…”

“…In addition to being able to use the superior soft tissue contrast of the MRI during treatment, the MRI-Linac also allows the treatment plan to be adapted daily to the anatomy of the day. These advantages allow the safety margins for rectal cancer dose escalation to be reduced to a few millimeters instead of 1 cm or more (Bonomo, Lo Russo et al 2021, Eijkelenkamp, Boekhoff et al 2021. At the same time, with such small margins accurate target volume definition becomes more crucial.…”

ICRU REPORT 97: MRI-Guided Radiation Therapy Using MRI-Linear Accelerators

“…Intrafractional motion during the online ART process remains a major concern because the adaptive plan may no longer be relevant to the anatomy if substantial anatomic changes such as bulk or peristaltic motions occur during the planning process. For example, it was reported that a 6-mm planning margin was required to compensate for the intrafraction GTV motion occurring during the online ART process for rectal tumors treated on a 1.5 T MRI-linac (Eijkelenkamp et al , 2021). The margin can be reduced to 4 mm, if the procedure time can be controlled within 15 min.…”