Feasibility of Biology-Guided Radiotherapy for Pancreatic Tumors: An Assessment of Normalized Target SUV

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Biology-guided radiotherapy (BgRT) is a novel method of delivering radiation using emissions from injected radiotracers to provide tracked radiotherapy. Pancreatic tumors in particular may benefit from BgRT given the potential for reducing irradiated volumes and thereby reducing gastrointestinal toxicity. For BgRT to be feasible, there must be sufficient contrast between tumor targets and the surrounding background. We assessed pancreatic lesion contrast using normalized target standardized uptake values (NSUVs) calculated from FDG-PET scans. Patients were identified with IRB approval from our institution (2019-1073). We measured NSUVs on scans from patients with pancreatic cancer before and after induction chemotherapy, which in most cases was FOLFIRINOX or gemcitabine with nab-paclitaxel. Tumors were contoured on PET/CT scans. We calculated the average SUV for two margins around the gross tumor volume (GTV) that did not include the GTV: one for a 5-mm margin and the other for a 10-mm margin. NSUVs were calculated as the SUVmax of the GTV divided by the average SUV of the margin, and then averaged across axial, coronal, and sagittal views. SUVs were measured retrospectively on a GE AW Server 3.2. Since not all PET/CT scans were done with intravenous contrast, the major limitation of this study was identifying the GTV in those patients. Non-parametric Mann-Whitney U and Kruskal-Wallis tests were used to compare NSUV before and after chemotherapy and by tumor location. We scanned tumors in 53 patients with pancreatic adenocarcinoma, 29 at baseline and 28 after induction chemotherapy; 4 patients had scans available at both times. No patient had received pancreatic surgery or radiotherapy. Most tumors were located in the head of the pancreas (33) followed by the body (9), tail (6), neck (3), and uncinate process (2). Before therapy, the SUVmax range was 8.64‒72.25; the mean NSUV was 3.84 (range 1.77‒7.33) for 5-mm margins and 4.05 (1.77‒7.91) for 10-mm margins. After therapy, the SUVmax range was 5.13‒57.43; the mean NSUV was 3.02 (range 1.21‒9.70) for 5-mm margins and 3.28 (1.21‒8.67) for 10-mm margins. Decreases in NSUV after chemotherapy were significant for both the 5-mm (p = 0.001) and 10-mm (p = 0.01) margins. For the 4 patients with evaluable scans before and after induction chemotherapy, the NSUV after induction chemotherapy changed by ‒1.15 for 5-mm margins and by ‒1.12 for 10-mm margins. No differences were observed in terms of tumor location (p>0.05). Although exact NSUV threshold criteria are yet to be defined, using thresholds of 2.5, 3, or 3.5 and a 10mm margin, 86%, 72%, and 59% of pre-therapy patients and 68%, 50%, and 29% of post-induction chemotherapy patients, respectively, would be considered suitable for BgRT. BgRT may be feasible in patients with pancreatic cancer.