A. Groll, S. Sharma, J. Schmall, A. Da Silva, M. Narayanan, and G. Kuduvalli Ph.D; RefleXion Medical, Inc., Hayward, CA
Purpose/Objective(s): This work discusses delivery accuracy metrics for target point-of-view films used to evaluate BgRT. BgRT is a tracked delivery methodology that relies on the use of a positron emission tomography (PET) radiopharmaceutical to track and deliver the prescription dose. Intended for one or multiple PET avid targets with asynchronous non-bulk motion, delivery metrics must quantify target coverage and plan accuracy. For BgRT, we evaluated 3%/3mm gamma index inside the PTV againsta novel iso-dose line to PTV “margin” metric for spatial analysis. Two studies were performed as a proof of principle demonstration of margin importance. Materials/
Methods: In our first study, a large torso phantom was prepared with an FDG fillable 22 mm ball target and a c-shaped avoidance structure. A 10 Gy/fraction plan was created on the medical technology company system using a planning PET image with target-to-background ratio (TBR) of 8.34:1. Contouring for the plan used a 22 mm GTV and a 32 mm PTV. The treatment used a TBR of 8.12:1 with target and background concentration of 70.07 kBq/ml and 8.63 kBq/ml, respectively. In our second study using the same phantom, a 10 Gy/fraction plan using a 26 mm target was prepared with a planning PET image with a TBR of 12.4:1. Contours for the plan used a 26 mm GTV and a 36 mm PTV. The treatment used a TBR of 12:47:1 with target and background concentration of 75.82 kBq/ml and 6.08 kBq/ml, respectively. However, the delivery was performed to a 22 mm target to model a biodistribution change. The PTV in the plan was not changed. Variable 3D sinusoidal target motion was used for all planning and treatment studies; study 1 and 2 used max amplitudes of 16 mm and 13 mm, respectively. To use the “margin” metric, both plans had ~95% PTV coverage with a conformity index (CI) of 1.03. All studies used Ashland EBT-XD film cut to approximately 10 x 10 cm2 to measure delivered dose. Analysis of the films included the calculation of the 3%/3mm gamma criterion within the PTV. Margin loss was calculated as the maximum spatial contraction from the PTV to the film recorded 97% of prescription dose iso-dose line. Acceptable margin loss was limited to 3 mm, a criterion created by the PET signal spatial uncertainty from motion. Results: The irradiated film from study 1 demonstrated 0 mm of margin loss with respect to the PTV. Film 3%/3mm gamma within the PTV was 97.33%. Film from study 2 demonstrated 0 mm of margin loss but a PTV 3%/3mm gamma of 75.33%. In study 1, both the margin metric and PTV gamma produced favorable results. In study 2, PTV gamma failed, but the margin metric passed. The margin metric accounted for the biodistribution change from the 26 mm to 22 mm target, indicating coverage was maintained during treatment. Conclusion: As no dosimetry metric is currently established for tracked delivery, a margin metric may be a more appropriate measure of target coverage than the conventional 3%/3mm measurement.
