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Relation of Daily Dosimetric Variations to the Accumulated Dose Distribution in Online MR-Linac Guided Prostate SBRT

C McCann*, M Davidson, B Keller, J Detsky, A Loblaw, J Stewart, M Campbell, A Kim, K Wong, M Wronski, D Vesprini, M Ruschin, Sunnybrook Odette Cancer Centre, Radiation Therapy Program, Toronto, ON CA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: The 1.5T MR-Linac (MRL) enables online replanning to account for anatomical changes. The purpose of this study was to evaluate how daily plans relate to the accumulated dose in prostate SBRT.

Methods: Daily adaptation on the MRL includes a localization T2-weighted MR for contouring. The clinical target volume (CTV) was defined as prostate plus proximal 1cm of seminal vesicles and expanded by 4mm for the planning target volume (PTV). A treatment plan is generated for each fraction and evaluated based on dosimetric criteria levels: (1) “Ideal”: derived from SBRT outcomes; (2) Acceptable- derived from institutional experience; (3) Out of Tolerance. For 13 fractions in 3 patients, we measured the overall proportion of organ-at-risk (OAR) and target criteria falling within each tolerance level. We analyzed 65 OAR parameters (13 fractions x 5 constraints) and 26 target dose constraints (13x2). Deformable image registration was used to warp all doses to a reference image on which dose was accumulated and subsequently categorized according to the 3 tolerance levels.

Results: 13 fractions in 3 patients treated on MRL were analyzed. 74% (48/65) OAR criteria were Ideal, 18.5% (12/65) were acceptable and 7.5% (5/65) were out-of-tolerance, all which were bladder constraints. On the dose accumulation distributions, none of the OAR criteria, including bladder, were out of tolerance, with 93% (14/15) being ideal, and 7% (1/15) acceptable. For the target criteria 46% (12/26) were ideal, 35% (9/26) acceptable and 19% (5/26) out of tolerance. On the dose accumulated plans, none of the target criteria were out-of-tolerance.

Conclusions: The cumulative dose distribution in online adaptive prostate SBRT satisfies most dosimetric criteria despite daily variations, likely due to hot spots and areas of under-coverage averaging out. Future work involves re-assessing the tolerance bands to optimize the tradeoff between online planning flexibility with clinically acceptable cumulative doses.

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