2930 - Early Insights into Potential Clinical Benefits of Biology-Guided Radiotherapy for Breast Cancer

Purpose/Objective(s): Biology-guided radiotherapy (BgRT) performed with a PET/CT Linac utilizes radiotracer emissions from tumors to direct radiotherapy beamlets in real-time. First introduced into clinical practice in 2023 for lung and bone tumors using fluorodeoxyglucose (FDG), studies are ongoing to investigate expanding BgRT indications to other anatomies. Additionally, specific breast radiotracers like fluoroestradiol (FES) could hold therapeutic promise in BgRT. This study evaluates hypothetical BgRT plans for breast cancer lesions to gather preliminary insights into the potential clinical benefits of BgRT (using FDG or FES) in this patient population.

Materials/Methods: FES PET/CT scans from three patients with four ER+ breast lesions and a FDG PET/CT scan from one patient with two lesions were retrospectively selected from a single institution. One of the patients with ER+ lesions had bilateral breast lesions. On the FDG scan, the patient with two lesions had one in the breast and one in the axilla. The PET/CT images were processed in Mirada RTx and were delineated using 2x the mean background AC threshold. PET/CT data and contours were imported into a BgRT-treatment planning system (TPS). A simulation tool within the TPS converted the diagnostic PET images into simulated BgRT planning PET images. Planning target volumes (PTVs) and Biology-Tracking Zones (BTZs) were defined by a 5 and 10 mm isotropic GTV expansion for BgRT planning, respectively. PTV max dose, coverage and volume, CI, CI80, CI50 and treatment time were recorded in FES and in FDG BgRT plans.

Results: The FES BgRT plans created for the four lesions resulted in an average PTV volume of 29.0 cc. Average PTV max dose and coverage were 128.2% and 90.0%, respectively. Average CI, CI80 and CI50 were 1.28, 2.79, 7.40, respectively, with an average treatment time of 40 min. FDG BgRT plans created for the two lesions resulted in an average PTV volume of 59.1 cc. Average PTV max dose and coverage were 128.1% and 92.5%, respectively. Average CI, CI80, CI50 were 1.20, 2.43, 5.14 respectively, with an average treatment time of 39 min and 54 sec.

Conclusion: Based on these PET/CT images, the six lesions met the BgRT eligibility criteria enabling plan creation for both radiotracers. BgRT plans with both tracers offered good target coverage. Further research is needed to validate these findings, which provide an early indication of potential clinical benefits. Specifically, investigations into treatment deliverability should be conducted.