2423 - Effect of Biodistribution Changes via Increase in Target Size in Biology-Guided Radiotherapy (BgRT)

M. Surucu¹, G. Bal², A. Maniyedath², N. Kovalchuk¹, and B. Han¹; ¹Department of Radiation Oncology, Stanford University, Stanford, CA, ²RefleXion Medical, Inc., Hayward, CA

Purpose/Objective(s): Biology guided radiotherapy (BgRT) uses real-time PET data from the tumor to guide the planned dose to the tumor. The aim of this study is to test the effect of tumor growth on the accuracy of BgRT delivery. Materials/

Methods: A custom-built fillable patient-specific quality assurance (PSQA) insert containing a ball target and a C-shape OAR was used. The ball shaped target is divided into 4 quadrants. Three of the 4 quadrants were filled with FDG using a target-to-background ratio of 8:1. PET avid area was contoured as GTV and a 5 mm margin was added to generate the PTV. An additional 5 mm margin was added to create Biology Tracking Zone (BTZ), where the PET emissions will be used to plan and treat using BgRT. The same PET biodistribution of three compartments filled (3/4) is used during the first set of BgRT deliveries to test the accuracy of this non-uniform biodistribution. In the second set of BgRT deliveries, all 4 quadrants of the ball target were filled with FDG to demonstrate the growth of tumor in the 4^th compartment. Both BgRT deliveries were repeated multiple times as PET activity decayed down to 5 kBq/ml activity concentration (AC) or Normalized Target Signal (NTS) down to 2. Resulting BgRT delivery doses were captured with the patient-specific quality assurance (PSQA) and evaluated with 3%/2mm gamma criteria.
Results: For the 3/4^th filled target BgRT experiment, the AC during PET modeling was 12.3, and during PreScan PET was AC was 19.7, 13.6, 10.9, 7.2, 6.1 and 5.1 kBq/ml. The Normalized Target Signal (NTS) during modeling was 12.3 while after PreScan PET NTS was 10.7, 8.6, 9.6, 6.2, 6.5 and 5.9. In tumor growth case where all quadrants were filled, the AC was 13.7, 12.1, 9.8, 9.5, and 5.5 while the NTS was 8.3, 7.2, 7.2, 7.9 and 7.0. The patient-specific quality assurance (PSQA) gamma pass rates were 93.2%, 97.4%, 96%, 100% and 92.9% for the 3/4 deliveries and 96.3%, 93.5%, 99.1%, 96.9% and 95.7% for the 4/4 delivery.
Conclusion: The plan designed for the 3/4 biodistribution was successfully treated for both 3/4 filled case and 4/4 filled case. The accuracy of delivery was maintained as the FDG decayed as well as the shape of the tumor changed, suggesting that the BgRT delivery is robust even though the AC decreases to the half of the values that BgRT planned was based on.