FES and FDG PET imaging performance comparison for potential use in biology-guided radiotherapy for breast cancer.

e12575 Background: SCINTIX biology-guided radiotherapy (BgRT) uses F18-FDG emissions from lung and bone tumors to direct radiotherapy beamlets almost in real-time. Expanding BgRT indications with more specific radiotracers like F18-FES holds therapeutic promise. This study investigates F18-FDG and F18-FES PET imaging metrics in estrogen receptor positive (ER+) breast cancer lesions for potential BgRT use. Methods: Ten FES PET/CT scans from patients with 16 ER+ lesions in total were retrospectively selected from one institution. Eight of these patients also underwent FDG PET/CT (9 lesions in total) which were compared to their corresponding FES scans. PET/CT images were processed in RTx (Mirada Medical, Oxford, UK) to delineate target volumes and activity concentrations (AC) using a 2× mean 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. The gross tumor volumes (GTVs) for the lesions were imported. Planning tumor volumes (PTVs) and Biology-Tracking Zones (BTZs) were defined by a 5 and 10 mm isotropic GTV expansion, respectively. In addition, a background structure (BTZ shell) was created via a 5 mm expanded shell from the BTZ. The mean AC (ACmean) was calculated for each lesion’s GTV and PTV, BTZ and BTZ shell, and the maximum AC (ACmax) was measured for GTV and BTZ ring on the diagnostic PET images in RTx. AC, NTS and values were also measured on the simulated planning PET images in the BgRT-capable TPS. The GTVs, & PTVs targets’ volumes, GTV superior-inferior length (SIL), and the BTZ distance to nearest organs at risks (OARS) were recorded. Results: Twenty-five lesions were analyzed (breast: 14, axilla: 7, lung: 2, thoracic spine: 2). BgRT eligibility (AC > 5, NTS > 2.7) was met by 4/10 FES patients (5/16 lesions) and 2/8 FDG patients (3/9 lesions).Table 1 summarizes the FES and FDG metrics. Notably, one lesion was visible with FES but undetected with FDG. One patient with two lesions qualified for BgRT with FDG only, another with one lesion qualified with FES only, and one lung lesion met eligibility with both tracers. Conclusions: FES and FDG PET/CT provided complementary insights for BgRT planning, and both radiotracers could serve as potential bioguides for breast cancer BgRT. FDG demonstrated higher AC values, while FES showed greater specificity. Larger studies are needed to validate these findings and explore staging comparisons. Descriptive statistics of FES and FDG. GTV PTV BTZ 3mm BTZ ring GTV 3mm BTZ ring Mean AC (kBq/ml) Max AC (kBq/ml) FES 7.6 ± 1.9 4.7 ± 1.1 3.2. ± 0.7 1.6 ± 0.5 1.6 ± 0.5 1.6 ± 0.5 FDG 12.6 ± 0.8 7.8 ± 0.7 5.6 ± 0.8 2.8 ± 0.6 2.8 ± 0.6 2.8 ± 0.6 GTV GTV Nearest PET-avid OARs AC NTS TBR Volumes (cc) SIL (cm) (cm) (kBq/ml) FES 9.0 ± 9.3 2.4 ± 1.1 5.8 ± 3.4 6.6 ± 2.2 9.0 ± 2.3 4.7 ± 1.6 FDG 25.4 ± 32.7 2.9 ± 1.5 3.4 ± 4.3 10.9 ± 4.3 7.3 ± 1.7 4.4 ± 1.4