Effects of contrast materials in IMRT and VMAT of prostate using a commercial Monte Carlo algorithm

Contrast materials help in contouring in radiotherapy. The primary aim of this study is to investigate the effects of contrast materials in bladder on the dosimetry during prostate intensity modulated radiation therapy and volumetric modulated arc therapy. The study also investigates the difference of the two dose calculation options namely ‘dose to medium (Dm)’ and ‘dose to water (Dw)’ in a commercial Monte Carlo based treatment planning system. Eight IMRT treatment plans were retrospectively studied which were used to treat high risk prostate cancer patients. The treatment plans generated in Monaco treatment planning system use seven coplanar beams and calculated ‘Dm’ as the clinical option. These plans were recalculated, keeping the segments, beam angle and monitor units the same, with different relative electron densities assigned to the structure ‘bladder’ to mimic the presence of contrast material. The same plans were recalculated using the ‘Dw’ option. Further, keeping the IMRT constraints and plan calculation properties the same, these plans were re-optimised with the delivery method changed to volumetric modulated arc therapy and calculated using both ‘Dm’ and ‘Dw’ options. For all the four scenarios, it was found that for the target volumes CTV and PTV, ‘minimum dose’ is the only endpoint studied having a significant difference with the presence of contrast material. For bladder, the endpoint V40 Gy is affected. Any significant dosimetric effect is found only when the relative electron density of the contrast material is 1.2 or more. Also, the dosimetric difference is greater when ‘Dm’ option is used for calculation. For rectum, the dosimetry remains unaffected. Hence, contrast materials should be contoured and assigned appropriate relative electron densities during IMRT and VMAT treatment planning of prostate. Also, the difference in dose reported with the two dose calculation options (Dm and Dw) in the presence of contrast materials is significant.