S411
Clinical - Gynaecological
ESTRO 2026
Volume (CTV) and Planning Target Volume (PTV) were mapped from the treatment planning CT (tp-CT) to the pv-CT and forward dose calculation was performed. To assess the volume of theVC not covered by the CTV or PTV, the Region of Interest (ROI), VC outside of CTV (VC-CTV) and outside of PTV (VC-PTV) were created. Volume differences ( Δ ) in comparison to the planning CT images and dose parameters like the Mean Dose (DMean), Dose Volume in % (D99, D98, D95) and highest dose of defined volumes (e.g., D0.03cm ³ ) for each ROI were evaluated. For further statistical analyses, patients were divided into 2 groups depending on the use of a RB (17 with RB and 6 without RB). Univariate analyses were performed using independent t-tests on aggregated subject means. Spearman’s correlation was employed to evaluate associations between ROI volumes and VC dose coverage. Results: 139 pv-CTs were analysed. The use of a rectal balloon significantly reduced VC displacements, resulting in fewer pv-CTs with the VC located outside the CTV (40% vs. 91%, p = 0.0252) and PTV (28% vs. 68%, p = 0.0362). Target coverage remained stable across all fractions and there was no significant difference between groups (Figure 1 and 2). Mean doses to rectum and bladder were comparable between groups and within clinically acceptable limits. The applied PTV margins of 5 mm (7 mm in beam direction) ensured robust dose coverage despite interfractional anatomical variations.
Conclusion: Although rectal balloon application effectively reduced interfractional VC motion, there was no significant Δ in target coverage or organs- at-risk doses. The use of consistent bladder filling and standardized PTV margins contributed to stable dosimetry outcomes, confirming the robustness of the used IMPT treatment protocol in postoperative pelvic proton therapy. However use of rectal spacers may be beneficial, especially in patients with known gastrointestinal comorbidities or repeated trapped air in the rectum. Keywords: Particle therapy, female pelvis, rectal spacer
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