S2748
RTT - Patient preparation, immobilisation, and verification protocols
ESTRO 2026
driven adjustments and explore adaptive criteria to further optimize resource allocation. References: [1] Fiagan, Y. A., Bossuyt, E., Nevens, D., Machiels, M., Chiairi, I., Joye, I., ... & Verellen, D. (2023). The use of in-vivo dosimetry to identify head and neck cancer patients needing adaptive radiotherapy. Radiotherapy and Oncology, 184, 109676. [2] Dogan, N., Mijnheer, B. J., Padgett, K., Nalichowski, A., Wu, C., Nyflot, M. J., ... & Greer, P. B. (2023). AAPM Task Group Report 307: use of EPIDs for patient - specific IMRT and VMAT QA. Medical physics, 50(8), e865-e903. Keywords: invivo dosimetry-EPID Digital Poster Highlight 4373 Tattoo-free positioning accuracy in SBRT lung radiotherapy without surface guidance Kaylee Bon, Myrthe Maas, Lars de Groot, Sam Emmerig, Gordon Lim, Zeno Gouw, Jan-Jakob Sonke, Folkert Koetsveld Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, Netherlands Purpose/Objective: Traditionally small permanent tattoos are used as reference points in radiotherapy, but these have disadvantages including psychological distress, cultural or religious objections and reduced visibility on darker skin. (1,2) However, removing tattoos raises concerns about setup inaccuracies. Translation errors are corrected using daily cone-beam CT, but rotational deviations cannot be fully compensated. Surface Guided Radiotherapy (SGRT) and online adaptive radiotherapy (OART) could eliminate tattoos, but cost and complexity limit widespread use. (3,4,5) With current daily imaging for lung Stereotactic Body Radiotherapy (SBRT) registration is performed on the vertebrae followed by tumor-mask registration. Final couch corrections are based on the latter. This raises the question whether tattoos remain necessary, as spinal rotations have minimal dosimetric impact in small SBRT lung tumors. This study quantifies clinically relevant vertebral rotations to evaluate whether standardized tattoo-free workflow could apply to all lung cancer patients, including those with larger or multiple tumors, nodal metastases or critical OARs which are currently excluded. Material/Methods: A retrospective analysis with IRB approval was performed on 63 patients with NSCLC or lung metastasis treated with SBRT: 31 with and 32 without tattoos. All patients were positioned using a thorax-
were performed during the first three fractions of all courses to detect systematic errors. Subsequent monitoring was site-specific: HN treatments on alternate days; pelvic/gynecologic and thoracic/abdominal courses on the 7th, 14th and 21st fractions. EPID-measured doses were compared to TPS predictions using gamma analysis, with acceptance criteria of 3%/7 mm and a pass threshold of P γ > 95% [2]. Action levels included replanning for persistent anatomical change and adjustment of bladder/bowel preparation. Results: From January 2024 to October 2025, 14386 IVD tests were performed in 927 patients (HN 3376 tests/ 270 patients; Pelvis 9891 tests/ 577 patients; Thorax/Abdomen 1119 tests/ 80 patients). Pass rates were 92.3% (HN), 94.2% (Pelvis), and 95.5% (Thorax/Abdomen) (Figure 1).
HN showed the highest out-of-tolerance (OOT) rate (7.7%), mainly due to progressive patient weight loss with contour shrinkage, affecting VMAT modulation in regions such as parotids, oral cavity, and neck. Pelvic OOTs were associated with variable bladder/rectal filling and intraluminal gas, whereas thoracic/abdominal OOTs were less frequent and linked to bowel filling and anatomical variations. Alternate-day HN monitoring enhanced the detection of progressive anatomical changes and typically triggered CT-based replanning, restoring gamma agreement in subsequent checks; in contrast, pelvic OOTs were largely corrected through reinforced preparation protocols. Overall, the program served as a trigger for treatment adaptation, with the clearest clinical impact in HN patients. Conclusion: An EPID-based IVD program is feasible in a high- volume center achieving high pass rates and detecting clinically actionable anatomical changes. The uniform 3%/7 mm, P γ > 95% criterion, combined with site- tailored scheduling, effectively identifies replanning candidates, particularly in HN, and supports practical improvements in pelvic preparation. Future work will quantify the dosimetric and clinical impact of IVD-
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