The purpose of this study was to evaluate the correlation between real-time intra-operative ultrasound-based dosimetry (USD) and day 0 post-implant CT dosimetry (CTD) ¹³¹Cs permanent prostate brachytherapy. Fifty-two consecutive patients who underwent prostate brachytherapy with ¹³¹Cs were evaluated. Real time operating room planning was performed using VariSeed 7.1 software. Post-needle placement prostate volume was used for real-time planning. Targets for dosimetry were D₉₀ >110%, V₁₀₀ >90%, V₁₅₀ <50%, and V₂₀₀ <20%. The CT scan for post-operative dosimetry was obtained on day 0. The mean values for USD, CTD, and the linear correlation, respectively, were, for D₉₀: 114.0%, 105.61%, and 0.15; for V₁₀₀: 95.1%, 91.6%, and 0.22; for V₁₅₀: 51.5%, 46.4%, and 0.40; and for V₂₀₀: 15.8%, 17.9%, and 0.42. The differences between the mean values for USD and CTD for D₉₀ (p<0.01), V₁₀₀ (p<0.01), and V₁₅₀ (p<0.05) were statistically significant. For D₉₀, 30.8% of patients had a >15% difference between USD and CTD and 51.9% of patients had a >10% difference between these values. In contrast, the USD and CTD for V₁₀₀ were within 5% in 55.8% of patients and within 10% in 86.5% of patients. This study demonstrates a correlation between the mean intra-operative USD and post-implant day 0 CTD values only for V₂₀₀. Significant variation in D₉₀, V₁₅₀, and V₂₀₀ values existed for individual patients between USD and CTD. These results suggest that real-time intra-operative USD does not serve as a surrogate for post-operative CTD, and that post-operative CTD is still necessary.

Key words: Cesium-131; Prostate cancer; Brachytherapy; Dosimetry; Intra-operative planning.