ESTRO 2022 - Abstract Book

S56

Abstract book

ESTRO 2022

Conclusion Our cohort showed that SABR for oligometastases offers high rates of local control with minimal toxicity. Our outcomes for LC, OS and PFS are consistent with the UK NHS CtE registry study 2 and SABR COMET 3 . In our series, we have shown that SABR delayed subsequent treatment by a median of 12 months. Ongoing trial results are awaited to further determine the benefit of adding SABR to the standard-of-care therapy for oligometastases. References: 1- UK SABR Consortium. Stereotactic ablative body radiation therapy (SABR): a resource. Version 6.1. January, 2019. https://www.sabr.rg.uk/wp-content/uploads/2019/04/SABRconsortium-guidelines 2019-v6.1.0.pdf (accessed May 1, 2020). 2- Anastasia Chalkidou, Thomas Macmillan, Mariusz T Grzeda, et al. Stereotactic ablative body radiotherapy in patients with oligometastatic cancers: a prospective, registry-based, single-arm, observational, evaluation study. Lancet 2021; 22: 98- 106. 3- Palma DA, Olson R, Harrow S, et al. Stereotactic ablative radiotherapy versus standard of care palliative treatment in patients with oligometastatic cancers (SABR-COMET): a randomised, phase 2, open-label trial. Lancet 2019; 393: 2051–58.

PD-0079 Volumetric responses with stereotactic radiosurgery and immunotherapy in melanoma brain metastases

M. Shanker 1 , H. Foley 1 , S. Crowley 1 , E. Thomson 1 , C. Bradhurst 1 , M. Huo 1 , V. Atkinson 2 , M. Foote 1 , M. Pinkham 1

1 Princess Alexandra Hospital, Radiation Oncology, Brisbane, Australia; 2 Princess Alexandra Hospital, Medical Oncology, Brisbane, Australia Purpose or Objective This study examines the MRI volumetric tumour response over time of melanoma brain metastases (MBM) following Gamma Knife (GK) stereotactic radiosurgery (SRS) and aims to synthesize a predictive model of volumetric change following treatment. Materials and Methods A retrospective analysis was performed of patients who received single-fraction Cobalt-based SRS for MBM at a single- institution in Australia. Predictive factors relating to patient and treatment factors including systemic therapy type and timing were collected. Treatment volume of each lesion was delineated on a T1-weighted Gadolinium contrast enhanced MRI at baseline and at each follow-up scan. Cubic spline interpolation was used to impute missing data across time intervals and standardised to 3-monthly intervals. The primary outcome was relative volumetric change in T1-contrast enhancement at 3, 6, 9, 12- and 15-months relative to baseline. A repeated measures ANOVA was used to assess for differences in mean volumetric change between interpolated 3-month intervals. A linear mixed multivariate regression analysis was performed to assess the relationship between explanatory variables and volumetric change with a two-tailed significance of α =0.05. Results 101 patients with 425 MBM were treated with SRS in the study period. Median follow-up was 29.2 months (IQR 19.7-39.8). Median dose was 20Gy (IQR 18-20). Median baseline volume and lesion diameter were 0.24cc (IQR 0.06-1.02) and 7.7mm (IQR 4.8-12.4) respectively. 53% of patients were BRAF mutant with 65% having failed BRAF inhibitors at time of SRS. 34.4% of patients received BRAF inhibitors concurrently (± 48 hours) and 77% of patients received concurrent immunotherapy (4 weeks prior to 4 weeks post-SRS). Normalised to baseline, median interpolated lesion size at 3, 6, 9, 12- and 15-month was 73.6%, 55.4%, 49.7%, 54.2% and 42.5% respectively. There was a statistically significant decrease in lesion size in the 0-3 and 3-6 month interval (p<0.0001). Lesion regression stabilized after 6-months. 89.7% of treated lesions had durable local control on MRI at last follow-up. 5% of patients experienced symptomatic radionecrosis. Every 1mm increase in maximal lesion diameter was associated with a 1.5% (95% CI 0.04-2.9, p=0.04) and 2% (95% CI 0.4- 3.7%, p=0.016) greater reduction in volume at 3- and 6-months. Patients receiving concurrent immunotherapy had a significantly greater regression in tumor volume at 3-months (37% superior [95% CI 6.0-68.1%, p=0.02] and 6-months (48% superior [95% CI 7.4-89.5%, p=0.02]. Comparing changes to baseline imaging, on multivariate analysis 3-month MRI imaging was predictive of ultimate treatment response (complete or partial response) [HR=2.611, 95% CI 1.235-5.52, p=0.012] whereas 6-, 9- and 12- month MRI imaging was not predictive of ultimate best response to treatment. Conclusion This study demonstrates a significantly greater volumetric regression with concurrent immunotherapy and SRS in MBM in the initial 6 months following treatment.