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SU‐FF‐T‐405: Dose to Water Versus Dose to Medium in Proton Beam Therapy
Author(s) -
Paganetti H
Publication year - 2009
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3181887
Subject(s) - nuclear medicine , monte carlo method , proton therapy , dosimetry , proton , absorbed dose , formalism (music) , stopping power , physics , radiation treatment planning , histogram , mathematics , radiation therapy , nuclear physics , detector , statistics , medicine , optics , computer science , art , musical , visual arts , image (mathematics) , artificial intelligence
Purpose: Dose in radiation therapy is traditionally reported as dose‐to‐water, D w . Monte Carlo (MC) dose calculations report dose‐to‐medium, D m . A methodology is needed to convert D m into D w for comparing MC and planning system results. This work addressed the following questions for proton therapy: What formalism allows the conversion from D m to D w ? 1. Can we convert MC calculated D m into D w retroactively or do we have to convert during particle tracking? 2. What is the difference between D m and D w 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 D m and D w ? Method: We did develop a formalism to convert D m into D w 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 D m and D w 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 D m and D w 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 D m and D w . 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.