SU‐C‐204‐03: In‐Vivo Range Verification and Tissue Response to Proton Radiotherapy Using Prompt Gamma Volume Histograms

Purpose: To study if prompt gamma (PG) volume histograms created from PG images acquired during treatment delivery can be used to identify changes to beam range and tissue composition in response to proton radiotherapy. Methods: Monte Carlo simulations of a single field from a single treatment fraction (100 cGy) for prostate cancer were performed for the cases of: 1) an ideal patient setup, 1) a setup shift in the superior direction and 2) a reduction in oxygen concentration in the tumor. For each case, the 3 ‐Dimsional dose delivery and elemental PG emission in the patient for the treatment fraction were recorded and imported into a commercial treatment planning system. Changes in the dose volume histograms (DVH), as well as the PG volume histograms (PG‐VH) for the PG emission from oxygen and for the total PG emission (from all elements) were analyzed. Results: For the 1 cm superior shift, the prostate DVH and total PG‐VH both shifted toward lower doses with the shape of the curves remaining nearly unchanged, resulting in the DVH V95 dropping from 100 cGy to 90 cGy. For the total PG‐VH, the V95 fell from 50 to 38. For the reduced oxygen case, both the DVH and PG‐VH had a much different shape than for the ideal case, with a significant downward slope in the curves as a function of dose. Conclusions: The shift in the prostate PG‐VH for a superior shift in the patient setup correlated well with the DVH indicating it is possible to detect setup errors by analyzing the PG‐VH from prompt gamma images obtained during daily proton treatment delivery. Additionally, it may be possible to detect changes in the elemental concentrations of irradiated tissues by analyzing the shape of the PG‐VH.