S322
Clinical - Breast
ESTRO 2026
1 Radiotherapy, Royal Marsden Hospital, London, United Kingdom. 2 Radiotherapy, Institute of Cancer Research, London, United Kingdom Purpose/Objective: The MRL offers new possibilities for MR-guided radiotherapy for breast cancer. We report our two- year clinical experience on the feasibility, toxicity and patient acceptability of delivering adjuvant PBI on the MRL. Material/Methods: All breast cancer patients treated within the Prospective Evaluation of Radiotherapy Using Magnetic Resonance Image-Guided Treatment (PERMIT) trial (NCT03727698) were identified. All were WHO PS 0-1, with completely resected, early stage, low risk breast cancer. Non-contrast planning CT scans were acquired supine with a bolus sheet at the patient’s chin. Contouring was as per IMPORT LOW. The CTV-PTV margin was reduced from 10 to 7 mm in the last 8 patients following institutional validation (1). An Adapt to Position (ATP) workflow using IMRT delivering either 40 Gy/15# or 26 Gy/5#. MRL timing sheets and clinician and patient reported toxicities were collected using CTCAE v5.0 and EORTC questionnaires. Results: Twenty female patients were treated between January 2020 and August 2023. The median (range) age was 63 (51-80) years. 18/20 patients received adjuvant endocrine therapy. Total 130# were delivered - the first three patients received 40Gy/15#/3weeks, the rest received 26 Gy/5#/1week as per FAST FORWARD. The mean (SD) total time on couch was 21:32 (03:31) minutes. The mean (SD) treatment delivery time for 2.67 Gy was 3:04 (1:24) minutes, and 4:54 (0:41) minutes for 5.2 Gy. Table shows MRL timingsClinician reported two patients with G2 breast pain at baseline, and one with G1 dermatitis radiation at 2-6 weeks post RT. No toxicities >G2, and all resolved to G0-1 over time. A few patients scored 4 “very much” breast and arm symptoms at end of treatment and 6 months which resolved over time with majority scoring 1 “not at all”.
Histograms show clinician reported toxicities for breast pain and dermatitis radiation over time. Waffle plot summarize patient reported breast and arm toxicities over time using Likert score 1 "not at all" (light orange), 2 "a little" (dark orange), 3 "quite a bit" (red), or 4 "very much" (pink). All completed two years follow up with no local or distal recurrences. Conclusion: Delivery of PBI on the MRL using ATP is feasible and safe with high patient satisfaction and tolerability. Both patient and clinician-reported side-effects were low and improved with time. References: 1. Tan IZ, Mitchell A, McNair H, Dunlop A, Herbert T, Nartey J, et al. A Multicenter Study of Clinical to Planning Target Volume Margins for Adjuvant Partial Breast Irradiation Delivered on the 1.5T MR-Linear Accelerator. Int J Radiat Oncol. 2023 Oct 1;117(2):e725. Keywords: Partial breast radiotherapy, MR guided, MR linac A Fixed Beam Proton PBS Breast Treatment; Pseudo ARC in Head First Prone Position. Sion Koren, Aaron Allen, Nir Honig, Sharon Rigler, Hila Rosenfeld, Nir Peled, Rafael Pfeffer Radiation Oncology, Helmsley Cancer Center, Shaare Zedek Medical Center, Jerusalem, Israel Purpose/Objective: Prone breast treatment is a known approach for large breasts. It can also reduce the heart and lung dosage. Proton beam therapy for breast treatments is another option to reduce normal tissue toxicity.P-Cure LTD. (Shilat, Israel) is an FDA approved Proton therapy fixed beam PBS system, that offers upright treatments with adaptive capabilities and image guidance. In addition, a mount flat table top enables to conduct treatment delivery in this patient support device. We sought to pilot prone breast treatments with the P- Digital Poster 4292
Cure system and compare to photon prone treatments, showing this approach feasibility.
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