The use of non-standard CT conversion ramps for Monte Carlo verification of 6 MV prostate IMRT plans

Abstract

Monte Carlo (MC) dose calculation algorithms have been widely used to verify the accuracy of intensity-modulated radiotherapy (IMRT) dose distributions computed by conventional algorithms due to the ability to precisely account for the effects of tissue inhomogeneities and multileaf collimator characteristics. Both algorithms present, however, a particular difference in terms of dose calculation and report. Whereas dose from conventional methods is traditionally computed and reported as the water-equivalent dose (D w ), MC dose algorithms calculate and report dose to medium (D m ). In order to compare consistently both methods, the conversion of MC D m into D w is therefore necessary.This study aims to assess the effect of applying the conversion of MC-based D m distributions to D w for prostate IMRT plans generated for 6 MV photon beams. MC phantoms were created from the patient CT images using three different ramps to convert CT numbers into material and mass density: a conventional four material ramp (CTCREATE) and two simplified CT conversion ramps: (1) air and water with variable densities and (2) air and water with unit density. MC simulations were performed using the BEAMnrc code for the treatment head simulation and the DOSXYZnrc code for the patient dose calculation. The conversion of D m to D w by scaling with the stopping power ratios of water to medium was also performed in a post-MC calculation process.The comparison of MC dose distributions calculated in conventional and simplified (water with variable densities) phantoms showed that the effect of material composition on dose-volume histograms (DVH) was less than 1% for soft tissue and about 2.5% near and inside bone structures. The effect of material density on DVH was less than 1% for all tissues through the comparison of MC distributions performed in the two simplified phantoms considering water. Additionally, MC dose distributions were compared with the predictions from an Eclipse treatment planning system (TPS), which employed a pencil beam convolution (PBC) algorithm with Modified Batho Power Law heterogeneity correction. Eclipse PBC and MC calculations (conventional and simplified phantoms) agreed well (<1%) for soft tissues. For femoral heads, differences up to 3% were observed between the DVH for Eclipse PBC and MC calculated in conventional phantoms. The use of the CT conversion ramp of water with variable densities for MC simulations showed no dose discrepancies (0.5%) with the PBC algorithm. Moreover, converting D m to D w using mass stopping power ratios resulted in a significant shift (up to 6%) in the DVH for the femoral heads compared to the Eclipse PBC one. Our results show that, for prostate IMRT plans delivered with 6 MV photon beams, no conversion of MC dose from medium to water using stopping power ratio is needed. In contrast, MC dose calculations using water with variable density may be a simple way to solve the problem found using the dose conversion method based on the stopping power ratio.

Influence of internal fixation systems on radiation therapy for spinal tumor

“…Therefore, the TPS must be modified to take such scenarios into account. Currently, the common correction methods for TPS are the EPL, Batho, and ETAR methods 23 , 31 , 32 . The EPL method is not used widely due to its excessive simplification of the model (31) .…”

“…The specific method used in clinical research is as follows: (32) if there are no important organs and fast‐reacting tissues near the metal implants (2–3 cm) and the implant is made up of a low‐density metal, modified TPS is used to evaluate the influence of implant on dose distribution. In addition, the influence of X‐ray scattered by the metal in a CT image is also considered in this system.…”