Dual Source Strength Planning for Focal Low-Dose-Rate Brachytherapy of Prostate Cancer: Robustness to Source Displacement

Abstract

Seed displacement during and following low-dose-rate prostate brachytherapy (LDR-PB) is quite common. In extreme cases, seeds can be completely lost through ejaculation, transurethral routes or through the venous plexus to the lung. More common smaller displacements are caused by implantation error due to factors such as needle deflection, gland motion, and prostate edema. Regardless of the cause, unpredictable changes in the seed locations from the original plan may significantly affect dose coverage, leading to overdosing adjacent critical structures and/or underdosing the target region. We have analyzed the robustness of dosimetric parameters, to local source displacements, in inter-needle dual source strength (DSS) treatment planning for focal LDR-PB. Using a Monte Carlo approach, we compared the robustness of DSS plans with four alternative planning strategies: high source strength (HSS), intermediate source strength (ISS), low source strength (LSS), and intra-needle dual source strength (D2SS). Ultrasound images of 50 patients who ultimately received standard whole prostate implants were selected from the BC Cancer Agency database. A range of prostate volumes were deliberately included, but other than this criterion, the selection process was random. The rectum, and a urethral avoidance structure (urethral surrogate) were delineated, and a simulated focal PTV was created from the right half of the prostate gland (CTV) minus a portion of the gland midline and the urethral avoidance structure. A 1-2 mm margin inside the urethral avoidance structure was included and the full length of the prostate was covered. Seed strengths used were: 0.941 U for HSS, 0.417 U for LSS, 0.679 U (the average of 0.941 and 0.417) for ISS and a combination of 0.941 U and 0.417 U for DSS and D2SS plans. In DSS plans, which have been used at our institution for patients receiving focal LDR-PB [1], same strength sources were required within each needle, while in D2SS plans sources of different strength were allowed within the needles. Plans were created by manual optimization with the following constraints: PTV V100% ≥ 95% and PTV V150% between 50% and 70% of PTV volume, PTV D90% between 95% and 125% of the prescription dose, urethral avoidance structure V100% ≤ 30% of the structure’s volume and rectal V50% ≤5% of rectal volume. Changes in the planned dosimetric parameters due to source displacements were studied using a Monte Carlo approach (500 iterations). Random source displacement statistics applied were 0±2.5 mm lateral and posterior motions, and 4±3mm inferior motion. The average number of needles and sources used in LSS plans is approximately 2 times that of HSS (13.7±2.3 vs. 7.4±1.1 needles and 45.9±12.2 vs. 23.2±6.0 sources) and 1.5 times the number used in the other 3 plan types. The plots in figure 1 illustrate less robustness of the LSS plans to source displacement in terms of PTV V100%, V150% and D90. A negative value indicates a decrease in the dose parameter after displacing the sources. CI100 robustness in LSS plans is also less than that of DSS and HSS plans. There is no significant difference in PTV dose metrics in the other four plan types. DSS plans show significantly less increase in the urethral V125% compared to the HSS, ISS, and D2SS plans. Our results show an overall similarity in robustness of PTV dose measures between DSS and HSS plans, however, in terms of urethral dose robustness, DSS planning is superior to HSS, and in general significantly superior, for focal planning, to the LSS strategy. Dual source strength planning is a feasible approach for focal LDR-PB, and appears to result in increased robustness to source displacements. This strategy could also be extended to whole gland treatment with potentially similar benefits.