ESTRO 2023 - Abstract Book

S285

Sunday 14 May 2023

ESTRO 2023

18 years (1 – 82), with 42% paediatric (<16Y), 17% teenage and young adult (16-24Y) and 41% adults (25Y+).

For patients who developed acute RIST of G2+, the median volume receiving 10Gy, 20Gy and 30Gy were significantly higher (P<0.0001) than in those without acute RIST, (Figure 1). Similarly, median Dmax dose was significantly higher in the G2+ RIST group compared to G0 (60Gy vs 25Gy; P<0.0001). Using the ROC curve, we observed threshold volumes (Specificity, Sensitivity) for V10Gy, V20Gy and V30Gy to be 68.2 (0.80,0.89), 44.4 (0.89,0.87) and 22.6 (0.94,0.91) respectively, V30Gy ROC curve is shown in Figure 3. Fitting each in univariate logistic regression models, we observed patients with V10Gy > 68.2, V20Gy > 44.4 and V30Gy > 22.6 have 32, 34 and 46 more odds of G2+ RIST, compared to patients with volumes below the respective thresholds.

Conclusion The volume of irradiated skin and Dmax are risk factors for developing acute RIST in patients treated with PBS PBT. Further analyses, including multivariable considerations, are ongoing to include a larger cohort and those with Grade 1 acute RIST to potentially identify a dose volume threshold predictive of acute RIST. MO-0384 Dosimetric impact of bowel filling during proton therapy for paediatric abdominal neuroblastoma A. Ghica 1 , D. Botnariuc 1,2 , M. Hussein 2 , V. Rompokos 3 , D. D'Souza 3 , M. Gaze 3 , J. Gains 3 , P. Lim 3 , C. Veiga 1 1 University College London, Centre for Medical Image Computing, London, United Kingdom; 2 National Physical Laboratory, Medical Radiation Science, Teddington, United Kingdom; 3 University College London Hospitals NHS Foundation Trust, Radiotherapy, London, United Kingdom Purpose or Objective Day-to-day variations in the gastrointestinal (GI) tract contents may affect the robustness of proton therapy (PT) plans, leading to target coverage loss and/ or overdosage of organs-at-risk (OARs). We investigated the impact of simulated GI filling variations on the dosimetry of paediatric abdominal neuroblastoma PT. Materials and Methods Data from a historical cohort of 20 high-risk abdominal neuroblastoma paediatric patients replanned for pencil beam scanning PT (21Gy[RBE] in 14 fractions) was used in this study. To infer the impact of bowel filling variations on dosimetry, the Hounsfield units (HU) within the GI tract were replaced on the planning CT to six homogeneous scenarios: HU = {50, 0, -100, -500, -700, -1000}. These aimed to represent bowel contents from solid to gas. Dose distributions were calculated on the nominal and simulated scenarios, and key dose metrics were derived for structures used for plan optimisation (CTV, PTV, kidneys, liver, vertebra). Then each structure-specific dose metric per simulated scenario (P HU ) was normalised to the respective value calculated on the nominal plan (P nm ) as (P HU -P nm ) × 100/P nm . The Pearson correlation coefficient