SU‐FF‐T‐405: Dose to Water Versus Dose to Medium in Proton Beam Therapy

Abstract

Purpose: Dose in radiation therapy is traditionally reported as dose‐to‐water, Dw. Monte Carlo (MC) dose calculations report dose‐to‐medium, Dm. A methodology is needed to convert Dm into Dw for comparing MC and planning system results. This work addressed the following questions for proton therapy: What formalism allows the conversion from Dm to Dw?1. Can we convert MC calculated Dm into Dw retroactively or do we have to convert during particle tracking?2. What is the difference between Dm and Dw when analyzing patient data in terms of dose distributions, dose volume histograms and absolute doses?3. Is there a difference in the predicted beam range between Dm and Dw?Method: We did develop a formalism to convert Dm into Dw for proton beam dose calculations. Three different methods are introduced, an approximate method based on relative stopping power which allows retroactive conversion, a method considering energy dependent relative stopping powers, and a method incorporating nuclear interaction events as well. A total of 33 patient fields were analyzed. MC calculated dose distributions, dose volume histograms and absolute doses to assess the clinical significance of differences between Dm and Dw were compared. Results: We found that the difference between the three conversion methods was within 1% in most cases when analyzing mean doses to contoured structures. Further, we found that the difference between Dm and Dw can be up to 10% for high CT numbers. The difference is clinically insignificant for soft tissues. For proton beams stopping in bony anatomy, the predicted beam range can differ by 2–3 mm when comparing Dm and Dw. Conclusion: For comparison between MC and analytical dose distributions in proton therapy, a dose conversion is required. However, retroactive conversion seems to be sufficiently accurate in most cases, except for the end of range.