Author(s)

Muhammad Nauman Usmani, Norihisa Masai, Ryoong-Jin Oh, Hiroya Shiomi, Daisaku Tatsumi, Hideharu Miura, Toshihiko Inoue, Masahiko Koizumi

Department of Medical Physics & Engineering, Osaka University Graduate School of Medicine, Osaka, Japan.
Miyakojima IGRT Clinic, Osaka, Japan.

Dose in radiation therapy has been reported as the water-equivalent dose using conventional dose calculation algorithms. The Monte Carlo (MC) algorithm employs characterization of human tissues by elemental composition and mass density. It enables more accurate dose calculation for radiation therapy treatment planning and typically reports absorbed dose to medium. Whether one should use dose to medium or tissue (D_m) in place of dose to water (D_w) for MC treatment planning remains the subject of debate. The aim of the current study is to evaluate the differences between dose-volume indices for D_m and D_w MC-calculated IMRT plans. Thirty-seven spine patients were selected for this study. The IMRT optimization and MC calculations were performed using the iPlan RT Dose^TM ver 4.1.2 (Brainlab, Munich, Germany) treatment planning system (TPS) with an X-ray Voxel Monte Carlo (XVMC) dose calculation engine. D_w and D_m results for target and critical structures were evaluated using the dose-volume-based indices. Systematic differences between dose-volume indices computed with D_w and D_m were up to 5.2%, 4.2%, and 4.5% for D2, D50 and D98 indices of the clinical target volume (CTV), respectively and up to 1% for the critical structure dose indices. Our study demonstrates that employing D_m in place of D_w in MC-calculated IMRT treatment plans introduces a significant systematic difference in target DVHs. We recommend that for diffused target structures (such as spine tumors), dose to water is a better quantity for dose prescription in photon beam treatment planning using existing MC TPS. While for critical structures, it would be reasonable to report D_m always. However in future with the availability of finer spatial resolution, D_m will be the most suitable variable for both target and critical structures’ dose prescription and reporting in MC treatment planning.

Dose to Medium; Dose to Water; Monte Carlo; IMRT

M. Usmani, N. Masai, R. Oh, H. Shiomi, D. Tatsumi, H. Miura, T. Inoue and M. Koizumi, "Comparison of Absorbed Dose to Medium and Absorbed Dose to Water for Spine IMRT Plans Using a Commercial Monte Carlo Treatment Planning System," International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, Vol. 3 No. 1, 2014, pp. 60-66. doi: 10.4236/ijmpcero.2014.31010.