S735
Clinical – Lower GI
ESTRO 2026
assurance (RTTQA) with full planning data were analysed. Data was available on 58 patients. The submitted elective planning target volume (PTV) contours were edited to create two further volumes, one excluding external iliac nodes (PTV-ext) and one excluding both external iliac and inguinal nodes (PTV- ext/ing). Figure 1 demonstrates the three volumes. Three plans were retrospectively created for each case treating gross tumour to 50.4Gy and prophylactic nodes to 40Gy all in 28fractions. To avoid planner bias, plans were created using RapidPlan (Varian knowledge-based planning solution) m
Conclusion: Exclusion of external iliac nodes results in a modest reduction in dose to bowel bag, however additional exclusion of inguinal nodes reduces dose to femoral heads, genitalia and a further reduction in bowel bag. With appropriate diagnostic imaging and robust RTTQA processes it may be appropriate to risk adapt elective nodal volumes within the context of a clinical trial to further reduce dose to OARs. References: [1] Sebag-Montefiore D et al. Radiati Oncol 2025;S1246-1248;[2] Nilsson MP et al. Acta Oncol 2023;62:897-906 Keywords: anal, volumes, toxicity, dose Predicting Progression in Patients with Locally Advanced Rectal Cancer Using Inflammatory Biomarkers Hilal Alkis, Hatice Kubra Gunduz, Ece Ercan Radiation Oncology, Marmara University, Istanbul, Turkey Purpose/Objective: Systemic inflammatory markers are associated with response and prognosis of locally advanced rectal cancer. The study aims to investigate the inflammatory biomarkers, albumin-to-alkaline phosphatase ratio (ALB/ALP) and colon inflammatory index (CII) score, as predictors of recurrence and survival in patients with Digital Poster 4095
odel to determine consistent optimal OAR sparing. We report elective PTV volumes and OAR constraint metrics applied to bladder, femoral heads, genitalia and bowel space. A paired T-test was used to compare PTV to the reduced dose volumes. Results: Median elective planning volumes in the 3 cohorts were 1339cc (range 891-2172), 1084 (range 778-1540) and 623 (range 449-947) for PTV, PTV-ext and PTV- ext/ing respectively. Bladder dose metrics were unaffected by reduced elective planning volumes. Femoral heads and genitalia were unaffected by PTV- ext but a significant reduction in D50, D35 and D5 using PTV-ext/ing. The median dose to 400cc of bowel bag in PTV, PTV-ext and PTV-ext/ing respectively, was 26.1Gy (range 2.0 to 36.4), 22.0 (range 1.7 to 34.9) and 20.1 (range 1.5 to 33.4); and to 250cc was 34.6 (range 4.0 to 38.9), 29.5 (range 3.8 to 37.5), 26.8 (range 3.9 to 37.3). Table 1 details the mean reductions for each constraint metric.
locally advanced rectal cancer who received neoadjuvant/adjuvant radiotherapy (RT). Material/Methods:
A total of 77 locally advanced rectal cancer (T3-T4 and/or N+) patients treated with RT were included in the study. Female/Male ratio was 30/47. T3 and T4 tumors were detected in 52 (67%) and 25 (33%) patients, respectively. Fifty-nine patients (77%) were
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