Author: Sarah Dumont, Trevor Ketcherside, An Liu, William T. Watkins, Qiuyun Xu 👨🔬
Affiliation: RefleXion Medical, Department of Radiation Oncology, City of Hope National Medical Center 🌍
Purpose:
The RefleXion X1 SCINTIX algorithm convolves beam fluence with real-time PET distributions for tracking and plan adaptation, but radiotherapy Planning Target Volumes (PTVs) are often significantly larger than PET-active gross tumor volumes (GTVs). This work evaluates PET-guided RT and adaptation to large PTVs with relatively small PET-active sub-volumes.
Methods:
Treatment planning, delivery, and adaptation for large PTVs was evaluated utilizing a Na-22 point source in the ArcCheck phantom. A 5.3 cm3 GTV was expanded to PTV sizes ranging from 40-864 cm3 in spherical and cylindrical expansion up to 8-cm Superior/Inferior (S/I). SCINTIX plans were developed to deliver 35 Gy in 7 fractions to GTV and PTV, with constraints V100>95% and Heterogeneity Index (HI)<125%. Plans which met the constraints were delivered with and without intra-fraction shifts to evaluate the system’s ability to perform real-time adaptive treatment while tracking on small PET-active volumes. Plans were evaluated including V100, HI, CI100, and CI50. Delivered dose accuracy was estimated using 3%/3mm Gamma (G3/3).
Results:
Plans with S/I PTV dimension > 6 cm could not simultaneously meet V100 and HI constraints. All plans with S/I dimension <5.0 cm met V100 (96.7%±1.3%) and HI (1.23±0.02) with excellent CI100 (1.12±0.12) and CI50 (4.40±0.86). Delivery of plans was successful with G3/3=96.7%±1.85% including PTV volumes >200cm3. PTVs>450cm3 were undeliverable due to memory overloading. Real time tracking on intra-fraction shifts was observed in all deliverable plans, but real-time plan adaptation did not occur when shifting mid-treatment. The S/I limitation is due to the PET-detector size (5.2 cm); when intra-fraction motion was applied beyond this region the SCINTIX system interrupted treatment requiring re-imaging and continuation of treatment.
Conclusion: This study has validated PET-guided treatment for large PTVs with small PET-active sub-volumes. The results show potential to track and adapt treatment if any sub-volume of a PTV is PET active.