Assessment of delivered dose in prostate cancer patients treated with ultra-hypofractionated radiotherapy on 1.5-Tesla MR-Linac.

Abstract

Objective:

To quantitatively characterize the dosimetric effects of long on-couch time in prostate cancer patients treated with adaptive ultra-hypofractionated radiotherapy (UHF-RT) on 1.5-Tesla magnetic resonance (MR)-linac.

Materials and methods:

Seventeen patients consecutively treated with UHF-RT on a 1.5-T MR-linac were recruited. A 36.25 Gy dose in five fractions was delivered every other day with a boost of 40 Gy to the whole prostate. We collected data for the following stages: pre-MR, position verification-MR (PV-MR) in the Adapt-To-Shape (ATS) workflow, and 3D-MR during the beam-on phase (Bn-MR) and at the end of RT (post-MR). The target and organ-at-risk contours in the PV-MR, Bn-MR, and post-MR stages were projected from the pre-MR data by deformable image registration and manually adapted by the physician, followed by dose recalculation for the ATS plan.

Results:

Overall, 290 MR scans were collected (85 pre-MR, 85 PV-MR, 49 Bn-MR and 71 post-MR scans). With a median on-couch time of 49 minutes, the mean planning target volume (PTV)-V95% of all scans was 97.83 ± 0.13%. The corresponding mean clinical target volume (CTV)-V100% was 99.93 ± 0.30%, 99.32 ± 1.20%, 98.59 ± 1.84%, and 98.69 ± 1.85%. With excellent prostate-V100% dose coverage, the main reason for lower CTV-V100% was slight underdosing of seminal vesicles (SVs). The median V29 Gy change in the rectal wall was -1% (-20%-17%). The V29 Gy of the rectal wall increased by >15% was observed in one scan. A slight increase in the high dose of bladder wall was noted due to gradual bladder growth during the workflow.

Conclusions:

This 3D-MR-based dosimetry analysis demonstrated clinically acceptable estimated dose coverage of target volumes during the beam-on period with adaptive ATS workflow on 1.5-T MR-linac, albeit with a relatively long on-couch time. The 3-mm CTV-PTV margin was adequate for prostate irradiation but occasionally insufficient for SVs. More attention should be paid to restricting high-dose RT to the rectal wall when optimizing the ATS plan.

• Authors: Lin-Rui Gao, Yuan Tian, Ming-Shuai Wang, Wen-Long Xia, Shi-Rui Qin, Yong-Wen Song, Shu-Lian Wang, Yu Tang, Hui Fang, Yuan Tang, Shu-Nan Qi, Ling-Ling Yan, Yue-Ping Liu, Hao Jing, Bo Chen, Nian-Zeng Xing, Ye-Xiong Li, Ning-Ning Lu
• Journal: Frontiers in oncology (Front Oncol) Vol. 13 Pg. 1039901 ( 2023) ISSN: 2234-943X [Print] Switzerland
• PMID: 36741014 (Publication Type: Journal Article)
• Copyright: Copyright © 2023 Gao, Tian, Wang, Xia, Qin, Song, Wang, Tang, Fang, Tang, Qi, Yan, Liu, Jing, Chen, Xing, Li and Lu.