Clinical evaluation for the difference of absorbed doses calculated to medium and calculated to water by Monte Carlo method

BACKGROUND: To evaluate the difference of absorbed doses calculated to medium and to water by a Monte Carlo (MC) algorithm based treatment planning system (TPS), and to assess the potential clinical impact to dose prescription. METHODS: Thirty patients, 10 nasopharyngeal cancer (NPC), 10 lung cancer and 10 bone metastases cases, were selected for this study. For each case, the treatment plan was generated using a commercial MC based TPS and dose was calculated to medium (D(m)). The plan was recalculated for dose to water (D(w)) using the same Monitor Units (MU) and control points. The differences between D(m) and D(w) were qualitatively evaluated by dose-volume parameters and by the plan subtraction method. All plans were measured using the MapCheck2, and gamma passing rates were calculated. RESULTS: For NPC and Lung cases, the mean differences between D(w) and D(m) for the targets were less than 2% and the maximum difference was 3.9%. The maximum difference of D(2%) for the organs at risk (OARs) was 6.7%. The maximum differences between D(w) and D(m) were as high as 10% in certain high density regions. For bone metastases cases, the mean differences between D(w) and D(m) for the targets were more than 2.2% and the maximum difference was 7.1%. The differences between D(w) and D(m) for the OARs were basically negligible. At 3%&3 mm criterion, the gamma passing rate of D(w) plan and D(m) plan were close (> 94%). CONCLUSION: The differences between D(w) and D(m) has little clinical impact for most clinical cases. In bony structures the differences may become clinically significant if the target/OAR is receiving doses close to its tolerance limit which can potentially influence the selection or rejection of a particular plan.