TITLE:
Comparison of Absorbed Dose to Medium and Absorbed Dose to Water for Spine IMRT Plans Using a Commercial Monte Carlo Treatment Planning System
AUTHORS:
Muhammad Nauman Usmani, Norihisa Masai, Ryoong-Jin Oh, Hiroya Shiomi, Daisaku Tatsumi, Hideharu Miura, Toshihiko Inoue, Masahiko Koizumi
KEYWORDS:
Dose to Medium; Dose to Water; Monte Carlo; IMRT
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.3 No.1,
February
20,
2014
ABSTRACT: 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 (Dm) in place
of dose to water (Dw) 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 Dm and Dw 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 DoseTM ver 4.1.2 (Brainlab, Munich, Germany) treatment
planning system (TPS) with an X-ray Voxel Monte Carlo (XVMC) dose calculation
engine. Dw and Dm results for target and critical structures were evaluated using the dose-volume-based indices. Systematic
differences between dose-volume indices computed with Dw and Dm 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 Dm in place of Dw 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 Dm always. However in future with the availability of finer spatial resolution, Dm will be the most suitable variable for both target and critical structures’
dose prescription and reporting in MC treatment planning.