S2127
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
Results: AF improved PTV coverage in 55/58 patients. The average benefit was 2.9 ccGy (range –4.4 to +22.4 ccGy), with higher PTV–OAR overlap variability correlating with greater gains (Figure 2). For illustration, one patient with substantial inter- fractional overlap variation [5.9, 3.3, 3.9, 4.2, 8.4 cc] received adaptive doses of 6, 10, 10, 8, 6 Gy, respectively, achieving a 14.2 ccGy benefit. In this case, high doses were delivered in fractions with small overlaps and reduced in fractions with large overlaps, compensating for unfavorable anatomy (Figure 2).
PTV-OAR overlap may vary between fractions. Adaptive fractionation (AF) exploits this inter-fraction motion by adjusting daily fraction doses according to the tumor-OAR configuration1. We developed an AF algorithm that adjusts the dose according to daily PTV- OAR overlap, aiming to maximize tumor coverage within overlapping regions while maintaining OAR constraints. Material/Methods: PTV underdosage was quantified as the area above the PTV dose-volume-histogram (DVH) up to the prescribed dose (Figure 1), expressed in ccGy, where 1 ccGy corresponds to a loss of 1 Gy to 1 cc of PTV. This underdosage was approximated by the cost function : ct = ot * (dt – dmin), where otis the daily overlap, dt the daily dose and dmin the OAR dose constraint (Figure 1). Based on daily MR scans acquired on the MR-linac, the overlap was measured for each fraction. A dynamic programming algorithm determined the optimal fraction dose by minimizing the expected cumulative cost over all fractions. The optimal dose was derived from patient-specific probability distributions of overlap volumes, increasing the dose for small overlaps and decreasing it for large overlaps (Figure 1). The daily dose was constrained between 6 Gy (dmin) and 10 Gy, ensuring delivery of the prescribed total dose while maintaining per-fraction OAR constraints. The algorithm was retrospectively applied to 58 five-fraction abdominal SBRT patients treated on the MR-linac and compared with uniform fractionation.
Conclusion: While the average cohort benefit is modest due to small overlap variations, well-selected patients with large interfractional motion and overlap variation, can experience significant dosimetric gains. These findings support further investigation into patient selection and integration of overlap-guided adaptive fractionation into an MR-guided adaptive radiotherapy workflow. References: Pérez Haas, Y., Ludwig, R., Dal Bello, R., Tanadini-Lang, S., & Unkelbach, J. (2023). Adaptive fractionation at the MR-linac. Physics in Medicine & Biology, 68(3), 035003. https://doi.org/10.1088/1361-6560/acafd4 Keywords: Adaptive Fractionation, MR Guided RT Inter-fractional Spatial and Dosimetric Variations of Target Volumes and OARs in Rectal Cancer Patients Undergoing Radiotherapy on the CT-Linac Qianxi Ni 1 , Zerun Cai 2 1 Radiation Oncology, The Affiliated Cancer Hospital of Xiangya School of Medicine, Central South University/Hunan Cancer Hospital, Changsha, China. 2 School of Nuclear Science and Technology, University of South China, Hengyang, China Digital Poster 1505
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