S2190
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
coverage. OARs were Duodenum_PRV, LargeBowel_PRV, SmallBowel_PRV, SpinalCord_PRV, ChestWall, Aorta, Liver, Contralateral Kidney, Skin, and Stomach_PRV. Proximal PRVs were cropped back from the GTV with no margin. Either two full VMAT arcs or two half VMAT arcs were chosen as providing the best OAR sparing. Dose constraints are based on TROG 15.03 [1], RTOG 0915 [2], and Timmerman [3]. Results: For most OAR dose constraints, the adapted plans showed better OAR sparing than the scheduled plans: Case 1 = 11/16; Case 2 = 8/9; case 3 = 10/16. In Case 1, LargeBowel_PRV constraints (D0.03cc ≤ 27.90Gy, D1.50cc ≤ 23.40Gy) failed in the scheduled plan (30.86 Gy and 27.15 Gy) and passed in the adapted plan (25.04 Gy and 20.99 Gy). In Case 2, the SpinalCord_PRV constraint (D0.03cc ≤ 4.00Gy) failed in the scheduled plan (5.63 Gy) and passed in the adapted plan (2.88 Gy). In Case 3, the LargeBowel_PRV constraint (D0.03cc ≤ 27.9Gy) failed in the scheduled plan (29.22 Gy) and passed in the adapted plan (24.61 Gy). For all other OAR constraints, both the scheduled and adapted plans passed. See figures 1 and 2 for example dose distributions and DVHs from a scheduled and adapted plan.
sufficient to account for intrafraction motion with acceptable delivered dose. In this cohort, these margins could potentially be reduced further to 6mm and 3mm respectively, provided this was validated with further dosimetric analysis. References: [1] Harris EER, Latifi K, Rusthoven C, Javedan K, Forster K. Assessment of Organ Motion in Postoperative Endometrial and Cervical Cancer Patients Treated With Intensity-Modulated Radiation Therapy. Int J Radiat Oncol 2011;81:e645–50. https://doi.org/10.1016/j.ijrobp.2011.01.054.[2] Wang G, Wang Z, Guo Y, Zhang Y, Qiu J, Hu K, et al. Evaluation of PTV margins with daily iterative online adaptive radiotherapy for postoperative treatment of endometrial and cervical cancer: a prospective single- arm phase 2 study. Radiat Oncol 2024;19:2. https://doi.org/10.1186/s13014-023-02394-2. Keywords: Margins, Dosimetry, Endometrial Digital Poster 4602 Feasibility of on-line CBCT-based adaptive SABR for renal cell cancer Christopher G Thomas 1,2 , Hannah Dahn 3,2 , Derek Wilke 3,2 1 Medical Physics, Nova Scotia Health, Halifax, Canada. 2 Radiation Oncology, Dalhousie University, Halifax, Canada. 3 Radiation Oncology, Nova Scotia Health, Halifax, Canada Purpose/Objective: Stereotactic ablative radiotherapy (SABR) is a viable non-invasive treatment for non-surgical candidates with primary renal cell cancer [1]. Our aim is to assess the feasibility of using on-line CBCT-based SABR for renal cell cancer. Material/Methods: Three previously treated patients were anonymized for this study. A board-certified radiation oncologist contoured artificial tumours within the kidneys for each patient (two on the right, one on the left). A PTV margin of 5 mm was applied to GTVs. GTVs were < 4 cm in diameter and prescribed 26 Gy in one fraction as per FASTRACK II. Adaptive kidney SABR treatments were simulated using HyperSight CBCT images on an Ethos v2.0 Emulator (Varian Medical Systems, Inc.). The Emulator simulates treatment fractions by adapting contours from the reference plan to the CBCT images of the day and calculating an adapted treatment plan. Clinicians have the choice of the scheduled plan (the reference plan calculated on the acquired CBCT) or the adapted plan (the reference plan adapted to meet dose constraints on the acquired CBCT).Dose optimization is realized using RT Intents to prioritize sparing of OARs over PTV
Conclusion: On-line CBCT-based adaptive SABR planning for renal cell cancer is feasible and provides better sparing than non-adapted planning. References: 1. Siva et al. “Stereotactic ablative body radiotherapy
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