S2715
RTT - Patient preparation, immobilisation, and verification protocols
ESTRO 2026
1 Radiotherapy, HagaZiekenhuis, The Hague, Netherlands. 2 Radiotherapy, dr Bernard Verbeeten Institute, Tilburg, Netherlands
Results: Setup accuracy for both mask types remained within 1.5 mm and 1°, consistent with clinical tolerance. For new masks, mean translational Σ / σ were approximately 1.2–1.5 mm, and rotational Σ / σ ranged 0.4–0.6°. For reused masks, translational Σ / σ were around 1.0–1.8 mm (highest deviation in the lateral axis, 1.56 mm) and rotational values 0.5–0.7°, showing no significant differences (p = 0.485, 0.589). Reused masks reduced fabrication cost by 41.9%, simulation cost by 38.7%, and plastic waste by 41.9%, equivalent to 13 fewer masks being produced. Treatment precision, workflow continuity, and patient safety remained consistent between both groups. Conclusion: Reusing Encompass masks in HyperArc SRT maintains geometric precision and treatment accuracy while substantially lowering costs and medical plastic waste. The non-significant differences in Σ and σ confirm that standard PTV margins remain appropriate, ensuring clinical consistency and safety. This practice demonstrates clinical non-inferiority and measurable benefits in both cost and sustainability. By reducing patient expenses and institutional waste, mask reuse supports broader access to advanced radiotherapy — a human-centered, green oncology approach that integrates precision, affordability, and sustainability in modern cancer care. This single-center retrospective study may be subject to selection bias and warrants multicenter validation. References: 1. van Herk M. Errors and margins in radiotherapy. Semin Radiat Oncol. 2004;14(1):52–64.2. Komiyama R, et al. Intra-fractional patient motion when using the Qfix Encompass immobilization system during HyperArc treatment of patients with brain metastases. J Appl Clin Med Phys. 2021;22(10):196–204.3. Lightowlers S, et al. A national framework for moving towards more environmentally sustainable radiotherapy. Radiother Oncol. 2025;188:110215.4. Chumsuwan N, et al. Comparative study of setup errors between new and reused thermoplastic masks in irradiated head and neck cancer patients. ASEAN J Radiol. 2023;24(2):122–136. Keywords: Encompass mask reuse,Value-based RT,green oncology Digital Poster 1834 The effect of pre-treatment enema usage on the rectum filling for prostate patients during radiotherapy treatment Nina van Rooijen 1 , Saskia Treur-van der Does 1 , Chin L. Ong 1 , Jules Lansu 1,2 , Eric Franken 1
Purpose/Objective: During radiotherapy treatment of prostate cancer, the amount of rectal filling influences the administered dose, and consequently the target coverage and rectal toxicity. Aiming to reduce both rectal volume and variation during ultra-hypofractionated radiotherapy in prostate cancer, some institutes administer enemas prior to each fraction. This study retrospectively compares the effectiveness of an enema for this purpose. Material/Methods: Patients with or without an enema prior to radiotherapy of the prostate were compared. Treatment consisted of 5x7.25Gy in 2 weeks and 20x3.1Gy in 5 weeks for patients with or without an enema, respectively. Data were collected from the planning CT and 5 CBCTs in both treatment groups. For patients without an enema, the first CBCT of each treatment week was used (i.e. at radiobiological comparable time-points to those with an enema). Furthermore, for patients with an enema (planning CT and CBCTs), the diagnostic PSMA PET-CT scan (without an enema) was used for a baseline measurement to facilitate an intra-patient comparison.To assess rectal filling, the rectum from the cranial border of the seminal vesicles to the proximal border of the anal canal was contoured. Subsequently, the average rectum cross-section area (ARCSA) was calculated by dividing the contoured rectal volume by the rectal length [1]. Results: For each of the patients with (n=10) or without (n=10) an enema (Microlax), the ARCSA is presented in figure 1 at the planning CT and 5 CBCTs. With respect to rectal volume, no significant differences were observed between patients with or without an enema. However, the variability of the rectal volume was significantly lower in patients with an enema (p=0.03), in comparison to patients without an enema. For patients with an enema, the mean ARCSA of alle enema scans (i.e. the planning CT and the 5 CBCTs), was compared to the ARCSA without an enema at diagnostic PSMA PET-CT in figure 2.
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