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Critical discussion of different dose-volume parameters for rectum and urethra in prostate brachytherapy
Christian Kirisits1, Gregor Goldner, Daniel Berger
1Department of Radiotherapy, Medical University of Vienna, Austria. Christian.Kirisits@meduniwien.ac.at
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 (145Gy) of 1.5cc is similar to an RD(2cc) of 130Gy. 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.
