Dosimetric Performance for Large PET-Avid Tumors Enabled By Anchor Point Tracking and Extended PET Field-of-View
Abstract
Purpose
To evaluate the delivery accuracy and treatment plan quality of the RefleXion X2 system with a developmental anchor point tracking delivery algorithm for large PET-avid targets, with specific emphasis on the treatment planning and performance improvements.
Methods
Three 18F-FDG fillable spherical targets with diameters of 4, 5, and 6 cm were fabricated. Each target accommodated an EBT-XD film placed in the coronal plane between the two hemispheres, along with an ionization chamber socket enabling absolute point-dose measurements. A C-shaped structure was used to represent the PET-avid organ at risk (OAR), such as the heart. Both the spherical targets and the C-shaped OAR were filled with 18F-FDG and were placed within a large torso-shaped phantom, which was subsequently filled with a uniform background solution of water and FDG at approximately 5 kBq/ml background, resulting in a target-to-background ratio (TBR) of approximately 8:1. Dosimetric performance was evaluated using two-dimensional dose profiles derived from film measurements, complemented by an A14SL Exradin ionization chamber for point-dose evaluations. Treatment plan quality was further assessed using target coverage, conformity index (CI), and CI50.
Results
Treatment plans for all 3 targets achieved ~95% PTV coverage with CI values of 1.02, 1.01, and 1.01 and CI₅₀ values of 3.55, 3.34, and 3.26, respectively, for the 4, 5, and 6 cm targets. Ion chamber measurement errors for static targets were 1.3% (4 cm), 0.03% (5 cm), and −1.0% (6 cm). Further, the films demonstrated greater than 90% gamma within PTV and good agreement between planned and measured dose in the coronal plane for all targets.
Conclusion
This study demonstrates the ability of the X2 system, leveraging extended PET-FOV detectors with a developmental tracking algorithm, to plan and accurately deliver BGRT treatments to large targets between 4–6 cm in diameter while maintaining target coverage and plan conformity.