Advanced dose accumulation methods: A comparative study in online adaptive radiotherapy for abdominal-pelvic lymph node oligometastases.

PURPOSE: To investigate whether advanced registration methods for dose accumulation lead to meaningful differences in online adaptive radiotherapy (OART) to aid in-treatment dose adaptation (ITDA). In ITDA, fraction dose is adjusted when anatomy is favorable to shorten treatment or increase target dose. METHODS AND MATERIALS: Twenty-five patients with abdominal-pelvic lymph node oligometastases were retrospectively included. 5 x 9 Gy OART was simulated. Gastrointestinal organ (GIO) segments were propagated and doses accumulated using five techniques: DVH summation, rigid registration, image-based and hybrid deformable registration, and Thin Plate Spline Robust Point Matching (TPS-RPM). Metrics included: Dice, Hausdorff distance (HD-99 %), mean surface distance (MSD), and accumulated GIO V20Gy3, D0.5cc, and D2.0cc, and respective robustness. The most accurate method was compared to DVH summation for ITDA. RESULTS: Geometric accuracy increased significantly with registration complexity. TPS-RPM achieved the highest accuracy (median Dice 0.98, HD-99 % 1.4 mm, MSD 0.25 mm) with lowest variability, and yielded significantly lower accumulated GIO doses compared to DVH summation (D0.5cc 25.5 Gy vs 26.9 Gy and V20Gy3 5.4 Gy vs 5.9 Gy). Nonetheless, ITDA outcomes were identical or similar between the approaches. Fractions were reduced by 17 %. PTV V100 % and PTV D95 % increased significantly. Only PTV D95 % differed. CONCLUSION: Although advanced registration methods improve geometric accuracy and predict lower accumulated GIO doses, differences in fraction reduction or target dose were absent or minimal with ITDA compared to DVH summation. DVH summation therefore remains practical and efficient for this application, avoiding unnecessary complexity without compromising outcomes.