S2959
Late-Breaking
ESTRO 2026
and cumulative incidence at 6 months, 12 months and 24 months are reported using Common Terminology Criteria for Adverse Events (CTCAE v 5.0). Median International Prostate Symptom Score (IPSS) scores at baseline and 24 months scores are also reported. EPIC-26, IIEF-5 and EQ-5D scores were recorded. All participants completed the 24m toxicity assessment. Results Between 27 th July 2021 and 29 th November 2023, 47 patients were assigned to either 2F SBRT (n=23) or 5F SBRT (n=24); one patient withdrew from 2F SBRT before treatment commenced. The 46 participants who started study treatment received all fractions of treatment on the MR-Linac. Median age was 74 years, and 8/46 (17%) patients identified as Black and Minority Ethnic. 10/46 (22%) patients had NCCN high risk tumours. At baseline, 1/22 (5%) patients receiving 2F SBRT and 0/24 patients allocated 5F SBRT recorded ≥ grade 2 GU AEs. At 24 months, 4/22 (18%) 2F SBRT and 3/24 (13%) 5F SBRT patients reported grade 2 GU AE; 1/22 (5%) of 2F SBRT and 0/24 (0%) 5F SBRT reported G2 GI AE. Cumulative incidences are noted in Table 1. There were no ≥ grade 3 GU or GI AEs. Table 1. Cumulative grade 2+CTCAE GU, GI and sexual AEs
patients’ scores remaining in the mild-moderate symptom range (both groups). Quality of life indices
are pending. Conclusions
The results reflect an increase in GU AE in the medium-term post-SBRT seen elsewhere in the literature [3], and very low rates of GI AE with similar rates seen in 2F SBRT and 5F SBRT. References [1] As N van, Griffin C, Tree A, Patel J, Ostler P, Voet H van der, et al. Phase 3 Trial of Stereotactic Body Radiotherapy in Localized Prostate Cancer. New England Journal of Medicine 2024;391:1413–25. https://doi.org/10.1056/NEJMoa2403365. [2] Brand DH, Tree AC, Ostler P, van der Voet H, Loblaw A, Chu W, et al. Intensity-modulated fractionated radiotherapy versus stereotactic body radiotherapy for prostate cancer (PACE-B): acute toxicity findings from an international, randomised, open-label, phase 3, non-inferiority trial. Lancet Oncol 2019;20:1531–43. https://doi.org/10.1016/S1470- 2045(19)30569-8. [3] Tree AC, Ostler P, van der Voet H, Chu W, Loblaw A, Ford D, et al. Intensity-modulated radiotherapy versus stereotactic body radiotherapy for prostate cancer (PACE-B): 2-year toxicity results from an open-label, randomised, phase 3, non-inferiority trial. Lancet Oncol 2022;23:1308–20. https://doi.org/10.1016/S1470-2045(22)00517-4. [4] Wolfe S, Diven MA, Marciscano AE, Zhou XK, Kishan AU, Steinberg ML, et al. A randomized phase II trial of MR-guided prostate stereotactic body radiotherapy administered in 5 or 2 fractions for localized prostate cancer (FORT). BMC Cancer 2023;23:923. https://doi.org/10.1186/s12885-023- 11430-z. [5] Winkel D, Bol GH, Kroon PS, van Asselen B, Hackett SS, Werensteijn-Honingh AM, et al. Adaptive radiotherapy: The Elekta Unity MR-linac concept. Clinical and Translational Radiation Oncology 2019;18:54–9. https://doi.org/10.1016/j.ctro.2019.04.001. [6] Brand DH, Brüningk SC, Wilkins A, Naismith O, Gao A, Syndikus I, et al. The Fraction Size Sensitivity of Late Genitourinary Toxicity: Analysis of Alpha/Beta ( α / β ) Ratios in the CHHiP Trial. International Journal of Radiation Oncology*Biology*Physics 2023;115:327–36. https://doi.org/10.1016/j.ijrobp.2022.08.030. Keywords: SBRT, MR-Linac, prostate cancer, 2-fraction
Toxicity Domain
2F SBRT (%)
5F SBRT (%)
Time Period
GU Grade 2+
At 6 months 0/21 (0.0) 0/23 (0.0)
Up to 12 months Up to 24 months
4/24 (19.1) 6/24 (25.0)
1/22 (4.6)
4/22 (18.2)
GI Grade 2+
At 6 months 0/21 (0.0) 1/22 (4.6)
Up to 12 months Up to 24 months
0/22 (0.0) 1/22 (4.6)
1/22 (4.6) 1/24 (4.2)
8/18 (44.4) 9/22 (39.9) 10/22 (45.5)
11/19 (57.9) 14/23 (63.6) 19/24 (73.9)
ED Grade 2+
At 6 months
Up to12 months
Up to 24 months
Median IPSS total scores showed minimal change from baseline to 24 months in both treatment groups (5F SBRT: 2.5 to 4.0; 2F SBRT: 4.5 to 5.0), with over 95% of
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