ESTRO 2025 - Abstract Book

S4314

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

3. Chen WC, Perlow HK, Choudhury A, Nguyen MP, Mirchia K, Youngblood MW, et al. Radiotherapy for meningiomas. Journal of neuro-oncology. 2022;160(2):505-15. 4. Rogers CL, Perry A, Pugh S, Vogelbaum MA, Brachman D, McMillan W, et al. Pathology concordance levels for meningioma classification and grading in NRG Oncology RTOG Trial 0539. Neuro Oncol. 2016;18(4):565-74.

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Digital Poster Clinical Target Volume (CTV) to Planning Target Volume (PTV) margin expansions for gynecological patients undergoing extended field radiotherapy.

Rayna Quey, Jeannie Lin, Richard Yeo, Jeffrey Tuan Radiation Oncology, NCCS, Singapore, Singapore

Purpose/Objective: Extended field radiotherapy (RT) targets both the pelvic and para-aortic nodal (PAN) regions in the treatment of gynaecological malignancies (1). Standard CTV to PTV margin expansions are 1-1.5 cm for the primary tumour and 0.7 cm for the PAN region (2,3), as currently practiced in our institution. However, studies indicate patients undergoing this approach face an elevated risk of Grade 2 or higher haematological and gastrointestinal toxicities (4), highlighting the need for careful consideration of treatment parameters and their potential side effects. The study’s aim is to investigate the setup errors of gynecological malignancies treated with extended field RT, and to assess whether the current institutional margin expansions could be safely reduced to minimize toxicities. Material/Methods: 16 patients with uterine and cervical primary malignancies of Stage 3C1 to 4B, were randomly selected from January 2023 to December 2023, all immobilized using the All-In-One (AIO) system. They received a total of 45-50.4Gy in 25 28 fractions using Volumetric Modulated Arc Therapy. Imaging protocol included daily kilovoltage (kV) imaging to isocentre, and weekly stitched cone-beam computer tomography (CBCT), with each patient undergoing 5 stitched CBCT. Only stitched CBCT data were analyzed because it encapsulates the entire pelvic and nodal treatment volume in a single image, which allows a more accurate depiction of the isocentre shifts. 240 datasets were analyzed. Systematic (Σ) and random (σ) errors were computed using Van Herk’s formula across three planes (vertical, longitudinal, lateral), and PTV margin expansion was calculated. PTV margin = 2.5Σ + 0.7σ

Results: