A Study to Evaluate Influence of Bladder Contrast Agent in Monte Carlo Dose Calculation Algorithm for Advanced Radiotherapy Techniques

Purpose of this study is to evaluate the Monte Carlo (MC) dose calculation accuracy between contrast and non-contrast-base Computed tomography (CT) images used for advanced radiotherapy techniques in a treatment planning system (TPS). In this study, a total of ten prostate cancer patients were chosen from the clinical database. All the plans were generated for with contrast CT (WC-CT) and WOC without CT contrast (WO-CT) using a treatment planning system. Dose prescribed to planning target volume (PTV) was 46Gy in 23 fractions followed by boost dose. During treatment planning, the reference WC-CT images were recalculated by forcing electron density equal bladder density and generated WOC-CT plan for comparison. Advanced techniques such as step & shoot IMRT (SS-IMRT), Dynamic IMRT and Volumetric modulated arc therapy (VMAT) were analyzed for WC-CT and WOC-CT. Dosimetric indices such as dose coverage to PTV, OAR, Conformity index (CI), Homogeneity index (HI), Integral dose (ID) and total Monitor units (MUs) were compared using dose volume histogram (DVH). From DVH results, it was observed that WC-CT and WOC-CT plans did not show any significant difference in CI, HI, ID and PTV dose (D95%, Mean dose, Dmax) (P>0.05). Similarly, insignificant dose difference was found in OAR volumes such as bladder, rectum, left femur, right femur and bowel for all the three techniques (SS-IMRT, DY-IMRT, & VMAT) between WC-CT and WOC-CT (P>0.05). However, a significant variation was seen in total MUs for all the techniques (P<0.05). A significant variation of total MU was observed between WC-CT and WOC-CT for SS-IMRT, DY-IMRT & VMAT were 874.3±104.2, 844.5±122.1; 987.00±132.57, 961.8±97.6; and 1221.3±168.6, 1206.5±210.5. The study results concluded that bladder electron density in WC-CT images up to 1.4 did not make any significant dose difference in CI, HI, ID, PTV and OAR volumes except total MUs in all three techniques. For advanced radiotherapy techniques, WC-CT images up to 1.4 can be used without compromising plan quality.