Critical discussion of different dose–volume parameters for rectum and urethra in prostate brachytherapy

Abstract

Purpose To compare different dose–volume histogram (DVH) parameter concepts used for rectum and urethra. Methods and Materials Thirty-eight postplan CT scans were used to contour the rectum with only one outer contour and as a wall structure. DVH analysis included dose to absolute and relative volumes of both contour types, from RD 0.1cc to RD 10cc and from RDmax to RD30, respectively. Volume parameters are reported (RV50–RV300) in cubic centimeters and percentages. The analysis of urethral dose parameters was based on 55 CTs with a urethral catheter. Relative (UD100 to UDmax) and absolute volume parameters (UD 0.5cc to UD 0.1cc, UV100, UV150) were evaluated, and also correlated to prostate parameters. The analysis was repeated for 10 MRI-based interstitial high-dose rate cases. Results The correlation between organ and wall results was high for RD1, RD 2cc, and RD 0.1cc, with differences of <5%. DVH parameters reporting dose to a relative volume (e.g., RD10) or a relative volume related to a certain dose (e.g., RV100 [%]) are sensitive to the number of contoured slices. Dmax has a high uncertainty due to the sampling algorithm. RV100 (145 Gy) of 1.5 cc is similar to an RD 2cc of 130 Gy. The urethral UD10 and UD 0.1cc correlate with a mean difference of 1%. The ratios of UD5/UD30, UD10/UD30, and UD5/UD10 were 1.12, 1.09, and 1.03, respectively. The correlation between D90 and D10 for prostate to urethra UD10 was poor. Conclusions Only absolute volume parameters are stable in relation to different contouring concepts. When delineating the outer rectum contour, only RD 2cc and RD 0.1cc can be used. RV 100 in cc correlates to RD 2cc. Reporting UD5, UD10, and UD30 together is redundant. Additional information is given when reporting UV100 or UV150.