ESTRO 2021 Abstract Book

S618

ESTRO 2021

stenosis/atelectasis (LS; 5% guideline). The TCP at 2 years was calculated using the Jeong et al model. Using open source software based on CERR, relative dose distributions were scaled by a factor varying between 0.5 and 1.5, and fractionation resulting in highest TCP where clinical constraints are not violated was selected (optimal plan - “Planopt”). For Planopt, we computed the predicted TCP and NTCP for RP2+, RP3+, E2+, LS, and internally validated model general pulmonary toxicity (from Tekatli et al) for grades 3+ and 5 (GP3+ and GP5 respectively); we compared these values to the observed local control rates at 2 years and complications resulting from the actual treatments. Results 10 and 15 fractions were most commonly selected in Planopt. The actuarial rate of local control at 2 years using the original prescription was 72% (95%CI 57-88%). The median Planopt TCP was 87% (range 6-93%). In total, 39 patients had Planopt with TCP > 80% while respecting all constraints, while 14 patients had “Planopt” TCP of < 50% (Fig). Among all Planopt, 35% were constrained by either the preset limit or guideline of RP3+ NTCP (Fig). The Planopt NTCPs for both RP2+ and RP3+ were reduced by nearly half compared to patients’ actual toxicity (Table). There was also reduction in GP3+ and GP5 (Table), with minimal change in E2+ and LS rates.

Conclusion Individualizing treatments based on NTCP and TCP-driven simulations halved the predicted relative to observed rates of RP3+. Future work includes further refinement of the normal tissue limits and guidelines to achieve an optimized balance between predicted normal tissue complications and TCP (therapeutic ratio), specifically for patients with Planopt TCP < 50%.

PD-0786 Spatiotemporal fractionation schemes for the treatment of multiple brain metastases N. Torelli 1 , J. Unkelbach 1 1 University Hospital Zurich, Department of Radiation Oncology, Zurich, Switzerland Purpose or Objective The optimal fractionation scheme for the treatment of intracranial metastatic diseases is still unclear, with many ongoing studies comparing single-fraction and hypofractionated stereotactic radiosurgery. The scope of this work is to evaluate the potential benefits of spatiotemporal fractionation for the treatment of multiple brain metastases. Materials and Methods Spatiotemporal fractionation (STF) schemes aim at partial hypofractionation in the tumor along with near- uniform fractionation in normal tissues. This is achieved by delivering distinct dose distributions in different fractions, which are designed such that each fraction delivers a high single dose to complementary parts of the tumor while creating a similar dose bath in surrounding normal tissues. In the context of multiple metastatic lesions, a simplified approach to spatiotemporal fractionation ( s STF) is proposed, which aims at