Urethral and periurethral dosimetry in prostate brachytherapy: is there a convenient surrogate?

Abstract Purpose: To assess and compare two models for a surrogate urethra to be used for postimplant dosimetry in prostate brachytherapy. Methods and Materials: Twenty men with a urinary catheter present at the time of postimplant computed tomographic imaging were studied. Urethral and periurethral volumes were defined as 5-mm and 10-mm diameter volumes, respectively. Three contours of each were used: one contour of the true urethra (and periurethra), and two surrogate models. The true volumes were centered on the catheter center. One surrogate model used volumes centered on the geometrical center of each prostate contour (centered surrogate). The other surrogate model was based on the average deviation of the true urethra from a reference line through the geometrical center of the axial midplane of the prostate (deviated surrogate). Maximum point doses and the D 10 , D 25 , D 50 , D 90 , V 100 , V 120 , and V 150 of the true and surrogate volumes were measured and compared (D n is the minimum dose [Gy] received by n% of the structure, and V m is the volume [%] of the structure that received m% of the prescribed dose) as well as the distances between the surrogate urethras and the true urethra. Results: Doses determined from both surrogate urethral and periurethral volumes were in good agreement with the true urethral and periurethral doses except in the superior third of the gland. The deviated surrogate provided a physically superior likeness to the true urethra. Certain dose-volume histogram (DVH)-based parameters could also be predicted reasonably well on the basis of the surrogates. Correlation coefficients ≥0.85 were seen for D 25 , D 50 , V 100 , V 120 , and V 150 for both models. All the other parameters had correlation coefficients in the range of 0.73 – 0.85. Conclusions: Both surrogate models predicted true urethral dosimetry reasonably well. It is recommended that the simpler deviated surrogate would be a more suitable surrogate for routine clinical practice.