Enhancement of prostate tumor volume definition with intravesical contrast: a three-dimensional dosimetric evaluation.

PURPOSE: To assess the impact of intravesical contrast during computed tomography (CT) simulation on prostate tumor volume definition and dose distribution. METHODS AND MATERIALS: Sixteen patients with localized adenocarcinoma of the prostate underwent CT-based virtual simulation in preparation for definitive radiotherapy. Patients were immobilized with a foam cradle and an initial CT was performed after oral but without intravesical contrast (noncontrast scan). A second scan was performed following administration of intravesical contrast (contrast scan). Beam apertures were designed on the noncontrast scans and digitized into the contrast scan file. Beam apertures were also designed on the contrast scans. Isodose plans were generated for several beam apertures and arrangements. RESULTS: There was enhanced visualization of the prostate at the cephalad portion of the field for 15 of the 16 cases. The mean differences between the noncontrast and contrast volumes was significant (p = 0.0001). The mean percent underdosage to the prostate ranged from 3.9% to 18.6%, depending upon the target volume and beam arrangement. CONCLUSION: This study demonstrates the necessity of using intravesical contrast for defining the location of the prostate during CT simulation. The underestimation of the extent of the prostate when omitting intravesical contrast leads to significant underdosage. The value of intravesical contrast is most evident when small (prostate only) conformal fields are used.