Prostate brachytherapy treatment planning using a standard plan

In conventional treatment planning for prostate brachytherapy, a radiation dosimetrist chooses source positions required to cover prostate plus treatment margin to the prescribed dose. When constructing a plan, the placement of the initial seeds is constrained only by factors such as staying within the treatment margin boundary and urethral and rectal sparing, while the final seeds are placed with the additional constraint of filling in low dose regions. To investigate how this process can be streamlined, the authors devised a plan, consisting of a particular configuration of radioactive sources, to cover with a 5 mm margin a majority of prostate shapes having volumes between 30 and 40 cc. They then analyzed how well this plan covered a random sampling of prostates with volumes in the ranges 20 to 30 cc (5 patients), 30 to 40 cc (17 patients), and 40 to 50 cc (5 patients). V100, the percentage of target volume receiving 100% or more of the prescription dose, was 98% or greater for 15 or 17 patients (88%) with prostate volumes between 30 and 40 cc (average margin was 5.3 mm). For larger prostates (40-50 cc), the average V100 dropped from 98% to 95%. For smaller prostates (20-30 cc), the average margin increased from 5.3 mm to 7.4 mm. This shows that a standard plan can be used for most prostates within a 10 cc volume range. The use of a standard plan would reduce treatment planning time and would facilitate user familiarity with the loading patterns.