ESTRO 2025 - Abstract Book

S3733

Physics - Radiomics, functional and biological imaging and outcome prediction

ESTRO 2025

Material/Methods: Prostate cancer patients undergoing stereotactic ablative radiotherapy (SABR) over five fractions on an Elekta Unity MR-Linac were recruited to MOMENTUM (ClinicalTrials.gov ID: NCT04075305). A T2*-weighted (T2*W) sequence (multi-echo gradient-echo, TR = 1367 ms, TE = 4.6 – 55.2 ms, 12 echoes, resolution = 1.2 x 1.2 x 5 mm 3 ) was run at treatment planning (initial baseline, BL0), immediately before beam-on at treatment visit 1 (repeat baseline, BL1), and immediately before beam-on at treatment visits 3 and 5 (TV3 and TV5). R2* maps were generated from fitting to all TEs, and whole-prostate ROIs were defined on TE = 36.8 ms images separately for each time point. Median R2* and prostate volumes were calculated. Within-subject coefficient of variation (wCV) was calculated from BL0 and BL1 data to evaluate repeatability. Repeated measures (rm)-ANOVA was used to evaluate changes over the course of treatment, and post-hoc tests with Bonferroni corrections were used to compare individual time points. Results: From February 2023 - July 2024, complete BOLD datasets were acquired for 14 patients. Baseline R2* = 17.6 ± 2.1 s -1 (mean ± std. dev.), with wCV = 2.5%. There were significant differences in R2* across time points (rm-ANOVA, p=0.001, Fig 1a), with post-hoc tests showing significant R2* decreases from baseline to TV3 (p<0.04) and TV5 (p<0.0001). Baseline prostate volume = 29.5 ± 11.9 cm 3 , with wCV = 9.6%. There were significant differences in volume across time points (rm-ANOVA, p<0.00001, Fig 1b), with post-hoc tests showing significant volume increases from baseline to TV3 (p<0.004) and TV5 (p<0.04). Conclusion: The observed R2* decrease (consistent with hypoxia decrease) occurs after only two SABR fractions [2], earlier than previous R2* decrease measured towards the end of 20-fraction treatments [5].

The observed prostate volume increase is consistent with previous reports of prostate swelling in 5-fraction treatments [6,7].

Whole-prostate R2* measurements on an MR-Linac are repeatable and sensitive to early SABR-induced changes. This work contributes to the validation of R2* as a quantitative imaging biomarker for prostate radiotherapy.

Keywords: Prostate, MR-Linac, Hypoxia

References: [1] O’Connor et al., Nat. Rev. Clin. Oncol. 2017;14:169–186. [2] Hoskin et al., Int. J. Radiat. Oncol. Biol. Phys. 2007;68:1065–1071. [3] Milosevic et al., Clin. Cancer Res. 2012;18:2108-2114.

[4] Ezekiel et al., Radiother Oncol. 2024;194:S3873. [5] Bano et al., Proc. Intl. Soc. Mag. Reson. Med. 2022. [6] Alexander et al., Clin. Oncol. 2022;34:e383-e391. [7] Nicosia et al., Radiother. Oncol. 2024;190:110043.

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