ESTRO 2023 - Abstract Book

S204

Saturday 13 May

ESTRO 2023

Median PFS was 16.5 months (95% CI 9.8 - 21.5), with a 4-year PFS of 21% (95% CI 14 - 32). Twenty-one patients (23%) remained long-term disease-free, 33/90 (37%) patients were oligoprogressive ( ≤ 3 new lesions) at first recurrence and 36/90 (40%) developed a polymetastatic relapse. Interestingly, median PFS was lower in the group treated with 5 fractions (8 months) compared to both other groups (18 months, p = 0.056) (Figure 2). Median OS was not reached (95% CI 53 months – NA) and 4-year OS was 68% (95% CI 59 – 78).

Conclusion At long-term follow-up, no new safety signals were observed and SBRT remained entirely safe, whatever the fractionation. Also, local control remained excellent and did not differ between schedules. The overall PFS rate (21% at 4 years) was consistent with the literature. However, while neither intended nor powered for such an analysis, the results might suggest that 3 or single fraction schedules could lead to superior oncological outcomes. Of note, only 40% of patients went on to develop polymetastatic disease, confirming the existence of a true oligometastatic state. OC-0268 Should OARs be prioritized in SABR for oligometastases? A secondary analysis of the SABR-5 trial R.E. Cereno 1 , B. Mou 1 , S. Baker 2 , N. Chng 3 , G. Arbour 4 , A. Bergman 5 , M. Liu 5 , D. Schellenberg 2 , Q. Matthews 3 , V. Huang 6 , A. Mestrovic 7 , D. Hyde 8 , A. Alexander 9 , H. Carolan 5 , F. Hsu 10 , S. Atrchian 1 , I. Mohamed 1 , A. Lin 1 , T. Berrang 9 , A. Bang 5 , W. Jiang 2 , H. Pai 9 , S. Tyldesley 5 , R. Olson 11 1 University of British Columbia / BC Cancer - Kelowna, Radiation Oncology, Vancouver / Kelowna, Canada; 2 University of British Columbia / BC Cancer - Surrey, Radiation Oncology, Vancouver / Surrey, Canada; 3 BC Cancer - Prince George, Radiation Oncology, Prince George, Canada; 4 University of British Columbia, Data Science Institute, Vancouver, Canada; 5 University of British Columbia / BC Cancer - Vancouver, Radiation Oncology, Vancouver, Canada; 6 BC Cancer - Surrey, Radiation Oncology, Surrey, Canada; 7 BC Cancer - Victoria, Radiation Oncology, Victoria, Canada; 8 BC Cancer - Kelowna, Radiation Oncology, Kelowna, Canada; 9 University of British Columbia / BC Cancer - Victoria, Radiation Oncology, Vancouver / Victoria, Canada; 10 University of British Columbia / BC Cancer - Abbotsford, Radiation Oncology, Vancouver / Abbotsford, Canada; 11 University of British Columbia / BC Cancer - Prince George, Radiation Oncology, Vancouver / Prince George, Canada Purpose or Objective Despite potential improvement in survival after stereotactic ablative radiotherapy (SABR) for oligometastasis, concerns over its safety remain. To mitigate the risk of toxic effects, target coverage is sacrificed to prioritize organs-at-risk (OAR) during SABR planning. This study evaluated the effect of this practice on dosimetry, local recurrence (LR), and progression- free survival (PFS) in patients treated from a population-based phase II trial. Materials and Methods The single-arm phase II SABR-5 trial treated patients with up to 5 oligometastases between November 2016 and July 2020. The protocol-specific planning objective was to cover 95% of the planning treatment volume (PTV) with 100% of the prescribed dose, however PTV coverage must be reduced as needed to meet OAR constraints. This trade-off was measured using the coverage compromise index (CCI), computed as minimum dose received by the hottest 99% of the PTV (D99)