Session Item

In stereotactic body radiation therapy (SBRT) of prostate cancer, the management of the inter- and intra- fractionation motions is needed. MR-guided radiotherapy can administrate daily adaptive radiotherapy and motion monitoring during treatment, while can be both time and resource intensive such as manual delineation. Contours can be propagated from planning MRI to session MRI via a deformable image registration (DIR). This study evaluates the validity of treatment planning using contours propagated by DIR against contours manually delineated in daily session on the Elekta Unity MR-Linac.

Fifty fractions in total were analysed from 10 patients of prostate cancer who were treated with SBRT (37.5 Gy in 5 fractions). All patients underwent placement of hydrogel spacer. Clinical target volume (CTV) encompassed prostate and the proximal 1cm of the seminal vesicle and planning target volume (PTV) was expanded CTV isotropically by 5 mm. Organ at risk was bladder and rectum (sigmoid or intestine was not involved). Contours (bladder, rectum, prostate, seminal vesicle, CTV, and PTV) of DIR and manual were evaluated by DICE coefficient. Dose constrains of manual contours were evaluated by treatment plan using contours propagated by DIR on session MRI.

Median of the DICE coefficient were 0.94 (0.31-0.99) for bladder, 0.97 (0.78-0.99) for rectum, 0.97 (0.89-0.99) for prostate, 0.89 (0.6-0.97) for seminal vesicle, 0.96 (0.89-0.98) for CTV, and 0.96 (0.91-0.97) for PTV, respectively. D max of bladder wall were met optimal in 98% and failed in 2%. D 1cc and D 53% of bladder wall were met optimal in all sessions. D max of rectum wall were met optimal in 98% and failed in 2%. D 1cc and D 53% of rectum wall were met optimal in all sessions. D 95% of prescription PTV were met optimal in 84% and met mandatory in 16%. D 98% of prescription PTV were met optimal in 98% and met mandatory in 2%. D 1cc of prescription PTV were met optimal in all sessions. The case of failed dose constrains in D max of bladder wall was exceeded by 0.012 Gy and the case of failed dose constrains in D max of rectum wall was exceeded by 0.012 Gy.

In the setting of CTV encompassed prostate and proximal 1cm of the seminal vesicle with hydrogel spacer and sigmoid or intestine was not involved organ at risk, the application of treatment plan using contours propagated by DIR was deemed acceptable.