Robustness of Bgrt Delivery to Sudden Patient Shifts Evaluated In the X2 Platform
Abstract
Purpose
To evaluate the ability of a RefleXion X2 system with extended PET-FOV with a developmental Anchor Point Tracking algorithm to autonomously follow PET-avid target and deliver dose, accounting for large sudden target displacements.
Methods
A custom 18F-FDG–fillable 22 mm spherical target insert, and a C-shaped organ-at-risk (OAR) insert were placed in a cylindrical phantom containing an FDG–water background. The phantom was placed in ArcCHECK detector array to evaluate the delivered BgRT doses. Target and OAR-to-background activity ratios were maintained close to 8:1, while maintaining ~5 kBq/mL for the background. BgRT treatment plans prescribing 50 Gy in 5 fractions to PTV (GTV+5mm) were generated on the RefleXion X2 platform using BTZ (Biology-Tracking Zone) sized to accommodate the investigated step shifts. The prescan evaluation was performed in the planned configuration and prior to delivery, the phantom was physically displaced by 1.0 cm, 1.5 cm, and 2.0 cm in the inferior direction to simulate sudden patient shifts. The spherical target insert included an EBT-XD film placed in the coronal plane for dose verification. Dosimetric performance was evaluated using 3%/2 mm gamma analysis using ArcCheck, with additional assessment of radial margin loss criteria defined as the inward change in the delivered 97% isodose line towards the PTV using film analysis.
Results
For 1.0 cm, 1.5 cm, and 2.0 cm step shifts, 3%/2mm gamma pass rates of 99.2%, 97.0%, and 98.5% were observed, respectively. Film measurements showed good agreement with planned dose, with PTV margin loss values of 2.3 mm, 0.6 mm, and 0.9 mm for the corresponding shifts, within added 5 mm GTV margin.
Conclusion
The BgRT X2 system maintained accurate dose delivery and target tracking for sudden step shifts up to 2.0 cm, supporting the robustness of BgRT using wider PET field of view and the developmental anchor point tracking algorithm.