ESTRO 2023 - Abstract Book

S1129

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ESTRO 2023

69 patients were included for analysis. Acute toxicity was registered at baseline, mid- and end of treatment (EOT), and 2 weeks after EOT. Late toxicity was registered at one year after EOT, both using CTCAE v. 4. Bowels were retrospectively delineated on the planning CT as 1) Bowel cavity; including the entire peritoneal cavity from lower limit of L4 to last visible bowel loop as per RTOG, 2) bowel bag; part of the peritoneal cavity including bowel loops and 3) individual bowel loops (Figure 1). V15Gy, V30Gy and V45Gy were determined for the different bowel delineations and compared to toxicity scores. Difference between toxicity and dosimetric values was evaluated using Mann-Whitney test.

Results Peak acute grade ≥ 2 diarrhea was seen in 28 (40.6%) of 69 patients and late toxicity was seen in 6 (9.4%). One-year CTCAE was available for 93%. Median [IQR] volume was 1517.3 cm3 [1169.7;2025.6] for bowel cavity, 1125.1 cm3 [864.8;1431.3] for bowel bag and 813.6 cm3 [634.9;1008.0] for bowel loops. Figure 2 shows boxplots of V15Gy, V30Gy and V45Gy for bowel cavity, bowel bag and bowel loops, for patients with grade 0-1 and ≥ 2 acute and late diarrhea. For all evaluated parameters vi found larger bowel volumes with higher toxicity grade, however none reached significance. The most pronounced difference was seen for bowel cavity V15 for both acute and late toxicity. Median [IQR] V15Gy for patients with no acute diarrhea was 747.4 cm3 [480.3;1006.3]and 883.9 [673.6;1118.2] for patients with grade ≥ 2 (p=0.095). Median [IQR] V15Gy for patients with no late diarrhea was 802.4 cm3 [504.6;1015.9] and 834.1 cm3 [662.5;1457.3] for patients with grade ≥ 2 (p=0.053).

Conclusion Acute and late diarrhea is still significant with modern radiotherapy for anal cancer. There was a trend for higher toxicity with higher irradiated volumes, but insignificant for all delineation methods, however V15Gy showed most promise for both acute and late diarrhea.

PO-1400 Initial experience of rectum SIB radiotherapy on the Unity MR-Linac

H. Stankiewicz 1 , J. Chick 1 , M. Ingle 2 , A. Mitchell 1 , H. Barnes 3 , T. Herbert 3 , S. Nill 1 , U. Oelfke 1 , B. Ng-Cheng-Hin 2 , S. Bhide 2 , A. Dunlop 1 1 The Royal Marsden NHS Foundation Trust and The Institute of Cancer Research, The Joint Department of Physics, London, United Kingdom; 2 The Royal Marsden NHS Foundation Trust and The Institute of Cancer Research, Radiotherapy, London, United Kingdom; 3 The Royal Marsden NHS Foundation Trust, Radiotherapy, London, United Kingdom Purpose or Objective A 2-phase rectum simultaneous integrated boost (SIB) treating 52.5/45Gy to the primary and elective targets in 25 fractions (#) was implemented. This enables initial boosting of the primary target using MRgART on the Unity MR-linac (MRL, Elekta AB, Stockholm) with 16.5/9Gy/5# to the primary and elective targets and subsequent elective non-adaptive treatment on