G. Gibbard1, K. A. Al Feghali1, D. de Jong1, A. Maniyedath1, A. Amini2, and A. Liu2; 1RefleXion Medical, Inc., Hayward, CA, 2Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA
Purpose/Objective(s): Biology-guided radiotherapy (BgRT) with fluorodeoxyglucose (FDG) signal collected via an on-board positron emission tomography (PET) system integrated in an O-ring gantry linac was cleared by the Food and Drug Administration (FDA) for lung and bone lesions. This study aims to determine if BgRT is feasible in the liver, an organ which has a FDG signal background, and if plans are clinically acceptable for FDG-avid metastatic liver lesions. Materials/
Methods: Ten patients with metastatic liver lesions were retrospectively selected from one academic institution. Diagnostic PET/CT images of these treatment sites were collected and resampled to match the CT used for planning. Diagnostic PET images were converted to mimic PET images that are acquired on a PET-linac and would be used to guide the delivery. For BgRT planning, the planned tumor volume (PTV) was generated with 5 mm margin from gross tumor volume (GTV) and a Biology Tracking Zone was generated including the anticipated full range of target motion. BgRT plans were created to 36Gy in 4 fractions for liver lesions. BgRT plan creation was first tested to determine if the Activity Concentration (AC) and Normalized Target Signals (NTS) on converted PET images would meet BgRT AC and NTS hard constraints required for plan creation. GTV and PTV were collected. BgRT plan quality was then evaluated for PTV coverage, including V100%, D98% and mean Dose (Dmean), PTV point dose (Dmax), conformity index (CI) and Heterogeneity index (HI), and organ at risk (OAR) mean doses. Results: Six lesions met the constraints for NTS and AC for plan creation in four patient datasets. The average GTV size was 14.9 +/- 10.4 cc, with a PTV of 35.8 +/- 21.0 cc. The diagnostic PET PTV mean AC, liver mean AC, and derived AC and NTS on converted PET images average and standard deviation were 16.6 +/- 3.2 kBq/ml, 9.6 +/-1.3 kBq/ml, 13.97 +/- 2.89 kBq/ml and 6.85 +/- 3.16, respectively. All plans met OAR dose constraints such as max dose on duodenum, ribs, skin, small bowel, and stomach. For the six plans, lesion PTV coverage, V100%, D98%, and Dmean were 91.02 +/- 5.59%, 34.8 +/- 1.5 Gy and 39.5+/- 1.3 Gy. The PTV Dmax, CI and HI were 124.8 +/-10.2%, 1.13+/- 0.04 and 1.25+/-0.10. The Dmean for Liver minus PTV, right rib, right lung, stomach, duodenum and small bowel were 7.2 +/- 2.0, 2.2 +/- 0.5, 0.5 +/-0.8, 2.4 +/-1.4, 1.4 +/- 1.4 and 0.4 +/- 0.2 Gy, respectively. Conclusion: This initial study demonstrates the feasibility of BgRT planning for liver lesions, indicating successful adherence to AC and NTS constraints, and satisfactory plan quality meeting clinical requirements.
