Overlap guided adaptive fractionation

<i>Objective.</i>Online-adaptive, magnetic-resonance-(MR)-guided radiotherapy on a hybrid MR-linear accelerators enables stereotactic body radiotherapy (SBRT) of abdominal/pelvic tumors with large interfractional motion. However, overlaps between planning target volume (PTV) and dose-limiting organs at risk (OARs) often force compromises in PTV-coverage. Overlap-guided adaptive fractionation (AF) leverages daily variations in PTV/OAR overlap to improve PTV-coverage by administering variable fraction doses based on measured overlap volume. This study aims to assess the potential benefits of overlap-guided AF.<i>Approach.</i>We analyzed 58 patients with abdominal/pelvic tumors having received five-fraction MR-guided SBRT (>6 Gy/fraction), in whom PTV-overlap with at least one dose-limiting OAR (bowel, duodenum, stomach) occurred in⩾1 fraction. Dose-limiting OARs were constrained to 1cc⩽6 Gy per fraction, rendering overlapping PTV volumes underdosed. AF aims to reduce this underdosage by delivering higher doses to the PTV on days with less overlap volume, lower doses on days with more. PTV-coverage-gain compared to uniform fractionation was quantified by the area above the PTV dose-volume-histogram-curve and expressed in ccGy (1ccGy = 1cc receiving 1 Gy more). The optimal dose for each fraction was determined through dynamic programming by formulating AF as a Markov decision process.<i>Main results.</i>PTV/OAR overlap volume variation (standard deviation) varied substantially between patients (0.02-5.76cc). Algorithm-based calculations showed that 55 of 58 patients benefited in PTV-coverage from AF. Mean cohort benefit was 2.93ccGy (range -4.44 (disadvantage) to 22.42ccGy). Higher PTV/OAR overlap variation correlated with larger AF benefit.<i>Significance.</i>Overlap-guided AF for abdominal/pelvic SBRT is a promising strategy to improve PTV-coverage without compromising OAR sparing. Since the benefit of AF depends on PTV/OAR overlap variation-which is low in many patients-the mean cohort advantage is modest. However, well-selected patients with marked PTV/OAR overlap variation derive a relevant dosimetric benefit. Prospective studies are needed to evaluate AF feasibility and quantify clinical benefits.