S494
Clinical - Gynaecological
ESTRO 2026
40 Gy in 20 fractions with SIB to 53/55Gy to gross lymph nodes. Spatial biological dose summation was performed on the same CT with multi-TPS workflow on RayStation, separately for α / β of 3 and 10 for entire-body dose conversion.3-Fraction IGABT was planned on the last fractional brachytherapy image (best-case response). Optimization
in sigmoid and bowel(Figure1).
goals aimed D98 HR-CTV>75-80Gy (EMBRACE-III RAPID). EBRT dose was extracted from EQD2-summed plan
separately for HR-CTV and organs of interest (OOI). Mutual small volumes were directly combined to assume worst-case scenario (EBRT_D0.1/2cc+Brachy_D0.1/2cc). Dose accumulation was performed on both conventional summation (CS, with LQ model) and DIR spatial dose summation. DIR sum was performed on commercial software MIMs, using 1st brachytherapy CT as reference. The registration employed a two- step approach: all planning images were first aligned using rigid registration, followed by Hybrid DIR which utilized both intensity- based and contour-based (Key contours: Bladder, Rectum, HR-CTV) metrics to drive the transformation. The accuracy of the registration was evaluated using quantitative Similarity Indices: Hausdorff-Distance (HD), Mean-Distance-to-Agreement (MDA), Dice- Similarity-Coefficient (DSC), and Jaccard Index. Finally, the dose from each CT was transferred for registration on 1st Brachytherapy CT for dose Results: Ten patients (7 T2b and 3 T3b stages) harbored brachy HR-CTV of 39.4 (IQR,27.2- 46.7) cc. clinical target achieved median D50/90/98% of HR-CTV at 126/88/79 Gy10. Goals for OOI per EMBRACE II EBRT protocol were achieved with D2cc/0.1cc for bladder were 76/92Gy3, rectum 64/74Gy3, sigmoid 50/58Gy3, and bowel 51/57Gy3. The trend of higher doses but still within OOI limits with DIR summation was observed (Figure2), however, uncertainty signified the challenges in several scenario as shown with quantitative Similarity Indices, especially
Conclusion: Multi-TPS dose summation of Brachy-like SBRT + VMAT 40Gy/20F + IGABT achieves favourable HR-CTV coverage while adhering to international OOI criteria or further multi-
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