Background and purpose: To compare treatment planning methods in high-dose-rate (HDR) brachytherapy of prostate cancer. In particular, to assess quantitatively the dosimetric superiority, if any, of the anatomy-based dwell position (ABDP) and inverse optimization (IO) over the conventional cylindrical dwell position (CDP), geometric optimization (GO), and dose-point optimization (DO) in terms of the determination of dwell positions and dwell times.
Patients and methods: Between September 2002 and April 2003, 10 cases of treatment-planning CT images were taken for external radiotherapy for prostate cancer. Treatment planning computer software and the CT data were used to create hypothetical HDR brachytherapy applicator needles, which were properly implanted in the prostate. Six different plans including IO with ABDP (IO(ABDP)), IO with CDP (IO(CDP)), GO with ABDP (GO(ABDP)), GO with CDP (GO(CDP)), DO with ABDP (DO(ABDP)), and DO with CDP (DO(CDP)) were made for each case, that is, 60 plans in total. All plans were normalized so that the D(95) should be equal to 100% of the prescribed dose. Dose-volume histograms from all 60 plans were analyzed, and multiple implant quality indices, including CI, EI, DNR, %V(R 75), %V(B 75), and %V(U 150) for each plan, were extracted and compared. Then, the best settings for IO(ABDP) regarding dwell position and dose limit were sought for.
Results: ABDP showed a statistically significantly lower EI (P<0.001), %V(R 75) (P=0.002), and %V(B 75) (P=0.015) than CDP. IO showed a statistically significantly lower %V(U 150) than GO (P=0.009), or than DO (P<0.001). Given a definition that a figure exceeding three-fold of the minimum figure of the index is clinically unacceptable, only in IO(ABDP) all index figures were clinically acceptable, while in the other five plans at least one index figure was unacceptable.
Conclusions: In the CT-based treatment planning for prostate HDR brachytherapy, ABDP is useful to achieve a high conformity, which leads to a reduction of the doses to the bladder, rectum, and surrounding normal tissue. IO is useful to lower the urethral dose without sacrificing conformity. IO(ABDP) is recommended on the basis of the current study. However, this conclusion has been drawn from the idealized hypothetical settings, and some possibility remains that this conclusion is not always applicable to the real implants.