2226 - Hybrid Single Target: A New Strategy of Jointly Optimized Planning and Delivery for Biology-Guided Radiotherapy and Image-Guided Radiotherapy

Purpose/Objective(s):

The PET/CT linac is a dual-treatment modality device that delivers both biology-guided radiotherapy (BgRT) and image-guided radiotherapy (IGRT). We hypothesize that these modalities could be combined to generate a hybrid single target (HST) treatment plan (a larger volume for IGRT and a smaller one for BgRT – similar to an integrated boost volume). Using an ¹⁸F-fluorodeoxyglucose (FDG)-fillable phantom, we studied the ability of the device to deliver jointly optimized dose to overlapping fields via BgRT and IGRT using a PET-avid uniform target.

Materials/Methods: Both in-house software and ArcCHECK were used to obtain the delivery metrics on two different plans, during two distinct experiments. A static ArcCheck phantom with an insert containing a 22-mm spherical target and a C-shape organ-at-risk were used for hybrid single target planning and delivery. PTV was created by a 5-mm uniform expansion from the 22-mm GTV. In one plan, the biology-tracking zone (BTZ) was equal to the PTV, and for the second plan, the BTZ was a 5-mm uniform expansion from the PTV. The PTV was prescribed 80Gy in 5 fractions. The GTV and the PTV were the targets for the BgRT and IGRT plans, respectively. Delivered doses were recorded using the ArcCheck and an in-house software for margin loss/gain, coverage, CI/CI80/CI50 and gamma passing rates were evaluated.

Results:

ArcCHECK 3%/3mm gamma relative dose averages for each plan PTV were 99.2% and 98.2%, respectively. The in-house software showed the hybrid single target deliveries achieved a margin loss and gain average of 0.45/0.6 mm and 0.2/0.5mm, respectively. The calculated average percent deviation from the plan to deliveries for coverage, CI, CI80 and CI50 for the two plans were –1.4/-0.3%, -0.5/0.5%, -0.8/-1.7% and –0.2/-7.3%.

Conclusion:

This work serves as a proof of concept that jointly optimized BgRT and IGRT plans with overlapping fields can be both planned and delivered on a PET/CT linac. This HST approach presents new therapeutic opportunities for targets with partially PET-avid volumes, enabling the use of multiple isodose level prescriptions.This approach holds the potential to improve normal tissue sparing and could provide enhanced treatment optimization for static targets. Additional motion phantom experiments will need to be performed to evaluate this approach in moving targets.