Room: AAPM ePoster Library
Purpose: To quantify the dosimetric advantage of performing daily online re-planning over rigid shift corrections in prostate SBRT (40Gy in 5 fractions) using a 1.5-T MRI-Linac (MRL).
Methods: Fourteen fractions were analyzed over 4 prostate SBRT patients who were clinically treated on MRL using a daily online re-planning workflow involving physician re-contouring. The clinical target volume (CTV) was the prostate plus proximal 1cm seminal vesicles. The planning target volume (PTV) was CTV plus 4mm with prescribed dose of 36.25Gy. Four main dosimetric criteria included: (1) >99% of CTV receiving 40Gy (i.e. V40Gy> 99%); (2) Bladder V39.5Gy<2cm³; (3) Rectum Dmax<40.6Gy; and (4) Rectum V38Gy<1cm³. For each fraction, a daily T2-weighted MRI was used for contouring, and a plan was adapted to the shape (ATS) of the new anatomy. Retrospectively, a new plan for each fraction was generated based on a rigid match of anterior rectum and adapting to the position (ATP) of the new isocentre. The resulting dosimetric criteria for each workflow were compared using the “true” (i.e. physician-drawn) daily contours.
Results: Of 56 analyzed criteria (14 fractions at 4 criteria/fraction), there were 28 (50%) versus 8 (14.2%) violations using ATP and ATS, respectively. All ATP plans yielded CTV V40Gy <99% (range: 91.5%-98.7%) whereas ATS always yielded CTV V40Gy>99%. For rectum Dmax, ATP yielded 3 violations for which ATS passed and additional 5 violations occurred with both workflows. For bladder V39.5Gy, ATP yielded 3 violations for which ATS passed and 1 violation occurred with both workflows. There were no criteria for which ATS failed and ATP passed. The CTV Dmin was >36.25Gy in 14 and 11 plans for ATS and ATP, respectively.
Conclusion: With daily MR-guided online re-planning, greater dose can be safely allocated to the true target, which could pave the way for dose escalation or sub-volume boosting.