In vivo Prompt Gamma-Ray Spectroscopy for Personalized, Adaptive Particle Radiation Therapy of Bone Cancer
Abstract
Purpose
A novel in vivo method to monitor bone cancer during particle therapy that uses prompt gamma-ray spectroscopy (PGS) to quantify bone density and calcium (Ca) content. Upon bone irradiation with a particle beam, Ca-specific gamma emissions are induced by inelastic nuclear reactions. Tracking these Ca-related signatures inter-fraction can provide personalized, quantitative metrics of treatment response. The resulting information can then support adaptive therapy decisions and trigger follow-up re-planning as needed, without any additional radiation exposure.
Methods
In silico simulations using Geant4, and supported by experimental measurements, were performed in which particle beams irradiated bone-like scaffold phantoms with varying porosity, representing different Ca contents and densities. Tissue-equivalent inserts from the GammexTM 467 phantom were used as references. Prompt gamma-ray spectra were acquired with a detector system based on CeBr3 scintillator surrounded by BGO for background suppression. A detector study varying CeBr3 and BGO geometries and anticoincidence coverage was performed and summarized using photopeak efficiency and total background suppression. Finally, using the selected configuration, ion-Ca induce de-excitation lines were identified and quantified as a function of Ca content and scaffold density.
Results
The optimized detector geometry improved the baseline photopeak efficiency and total background suppression by approximately 70% and 25%, respectively. The most prominent ion-Ca related de-excitation lines identified were from the 38K* (0.13 and 0.33 MeV), 39Ca* (2.47 MeV), and 40Ca* (3.74, 3.90 MeV). Ca mass density in the bone scaffolds and tissue-equivalent composition was predicted with sub-1 wt% accuracy.
Conclusion
The optimized CeBr3 and BGO detector design improves PGS signal-to-noise ratio, supporting practical in vivo deployment and enabling sub-percent mineral quantification in realistic bone-equivalent phantoms using Ca-specific prompt gamma lines. This approach can provide inter-fraction, quantitative treatment-response metrics to support adaptive particle therapy and follow-up re-planning.