2060 - MRI-Guided Response-Adaptive Hypofractionated Radiotherapy for Soft Tissue Sarcomas
Presenter(s)
B. J. Eckelmann1, G. C. Blitzer2, Z. S. Morris3, and J. Crosby4; 1Department of Human Oncology, University of Wisconsin Hospitals and Clinics, Madison, WI, 2Department of Human Oncology, University of Wisconsin Hospital and Clinics, Madison, WI, 3University of Wisconsin, Madison, WI, 4Department of Human Oncology, University of Wisconsin School of Medicine and Public Health, Madison, WI
Purpose/Objective(s): Radiation (RT) can provide local control (LC) in both palliative and definitive settings for soft tissue sarcoma (STS). Conventionally fractionated RT for STS provides modest rates of LC. In an effort to improve upon this LC, our institution regularly performs dose escalation with hypofractionation in 5-6 treatments using MRI-guidance. When the initial treatment plan is determined to provide inadequate coverage of the gross tumor volume (GTV), we have used a treatment break after 3-5 fractions to allow for potential radiation response, in an effort to maximize GTV coverage on the final 1-3 treatment fractions. Here, we evaluate whether patients who undergo a treatment break show a reduction in GTV size or an improvement in GTV coverage.
Materials/Methods: Patients were retrospectively identified. Eligibility criteria included receiving treatment with MRI-guidance, receiving 5-6 fractions, and histologically confirmed STS. Clinicopathologic features, patient characteristics, RT dose and fractionation, LC, toxicity, and treatment break were collected. Toxicity was graded on the Common Terminology Criteria for Adverse Events (CTCAE) v5.0 scale.
Results: Twenty patients who did not undergo a treatment break (cohort A) and 11 patients who did undergo a treatment break (cohort B) were identified. Median follow-up was 11 months in cohort A and 12.5 months in cohort B. Twenty-seven lesions in cohort A and 20 in cohort B were treated. Median prescribed dose and fractionation was 54 Gy in 6 fractions for both cohorts. Median GTV volume was 45.75 cm3 in cohort A and 460.63 cm3 in cohort B. Rates of acute toxicity and late toxicity were higher in cohort B (90% and 73%, respectively), the majority of which were grades 1 and 2. There was a reduction in the GTV size from simulation to final treatment fraction in cohort B (-3.9%) compared to cohort A (+7.3%) (p=0.007). There was an improvement in GTV coverage from simulation to fraction five in cohort B (+10%) compared to cohort A (0%) (p=0.04). There were seven (7/27, 26%) local recurrences in cohort A and four local recurrences in cohort B (4/20, 20%). Cumulative incidence of local recurrence with death as a competing risk was not different between the two cohorts (p=0.9).
Conclusion: MRI-guided response-adapted hypofractionated radiotherapy is a novel treatment method that may enable greater target coverage in patients with unresectable STS. Utilizing spatially fractionated dosing and a treatment break with response-adapted planning can improve GTV coverage by allowing tumor size reduction during a course of radiotherapy. This approach may be useful for larger lesions which are not well-covered during initial treatment plan and merits prospective investigation.