Addition of magnetic resonance imaging to real time trans-rectal ultrasound-based treatment planning for prostate implants

Purpose
The greater soft tissue contrast of magnetic resonance imaging (MRI) allows improved accuracy in prostate contouring compared to transrectal ultrasound (TRUS) and helps in identifying specific regions within the prostate. This study attempts to evaluate the potential benefit of MRI-TRUS fusion in treatment planning for more accurate prostate contouring and tumor dose escalation.

Material and methods
14 patients with previous MRI-guided prostate biopsy and an low-dose-rate (LDR) permanent prostate seed implant have been selected. The prostate and tumor (5 patients) were contoured on the MRI images by a radiologist. The prostate was also contoured on TRUS images during LDR procedure together by a urologist and radiation oncologist. MRI and TRUS images were rigidly fused to compare prostate contours in MRI and TRUS. Prostate was then re-contoured by the radiation oncologist using this fusion. Moreover, V₁₀₀, V₁₅₀, and D₉₀ differences were evaluated for localized tumor compared to prostate with negative values indicating cold tumor regions. These cases were re-planned to simulate dose escalation.

Results
The prostate volume was contoured 8 ±10% smaller in TRUS images, compared to MRI images. The mean percent difference in tumor (compared to prostate) V₁₀₀ was 0.3 ±–0.4%, V₁₅₀ was –0.7 ±–24.8%, and D₉₀ was 0.2 ±–12.1%. For the posteriorly located tumors (2 cases), V₁₀₀ was 0.0 ±–0.3%, D₉₀ was 9.5 ±–3.0%, and V₁₅₀ was 26.1 ±–5.4%. For anteriorly located tumors (3 cases), V₁₀₀ was 0.4 ±–0.4%, D₉₀ was –6.0 ±–11.9%, and V₁₅₀ was –18.5 ±–14.4% (became 15.6 ±14.6% after re-plan).

Conclusions
The MRI-TRUS image fusion is a feasible tool for the visualization of the prostate gland, particularly at the apex and base of the gland. Tumor identification presents the potential for dose escalation using fusion, especially for anteriorly located tumors.