Purpose: Conventional photon dose calculation algorithms typically report the absorbed dose‐to‐water(Dwater). Monte Carlo(MC) dose calculation algorithms, however, by default reports the absorbed dose‐to‐medium(Dmedium). It has been suggested that for clinical comparisons, MC‐Dmedium results should be converted into Dwater to ensure valid comparisons. The goal of this study is to assess if the difference between Dwater and Dmedium is clinically significant for MC calculated IMRT plans. Method and Materials: Ten patients with H&N and ten patients with prostate cancer were selected for this study. Existing IMRT plans were re‐calculated using an EGS4‐based MC dose calculation system. Dmedium was converted to Dwater by multiplying Dmedium results by average water‐to‐medium stopping power ratio. Dwater and Dmedium results for target and critical structures were evaluated using the DVH‐based indices: D2(dose to 2% of the structure volume), D50(dose to 50% of the structure volume), D98(dose to 98% of the target volume), and Dmean(mean dose). Results: For H&N, although the changes in average dose‐volume indices were less than 1.5%, up to 6.2% differences in PTVCTV D2 were observed for individual patients. The cord and brainstem D2 indices changed up to 2.5% and 2.7% respectively. For prostate, the differences in the indices for targets were less than 1%. The changes in critical structure indices were less than 1%, except for two patients in which changes up to 2.7% in rectum D50 index were observed. The increases in the range of 4.5– 11.5% in the femur dose‐volume indices were observed in converting from Dmedium to Dwater due to the high calcium content of the hard bones. Conclusion: This study showed that converting dose‐to‐medium to dose‐to‐water in MC‐based IMRT plans may significantly change the structure doses for some cases, especially when hard bone containing structures such as the femurs are present.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging