Standardization of prostate brachytherapy treatment plans

Purpose: Whereas custom-designed plans are the norm for prostate brachyther apy, the relationship between linear prostate dimensions and volume calls i nto question the routine need for customized treatment planning. With the g oal of streamlining the treatment-planning process, we have compared the tr eatment margins (TMs) achieved with one standard plan applied to patients w ith a wide range of prostate volumes. Methods and Materials: Preimplant transrectal ultrasound (TRUS) images of 5 0 unselected University of Washington patients with T1-T2 cancer and a pros tate volume between 20 cc and 50 cc were studied. Patients were arbitrarily grouped into categories of 20-30 cc, 30-40 cc, and 40-50 cc. A standard 19 -needle plan was devised for patients in the 30- to 40-cc range, using an a rbitrary minimum margin of 5 mm around the gross tumor volume (GTV), making use of inverse planning technology to achieve 100% coverage of the target volume with accentuation of dose at the periphery and sparing of the centra l region. The idealized plan was applied to each patient's TRUS study. The distances (TMs) between the prostatic edge (GTV) and treated volume (TV) we re determined perpendicular to the prostatic margin. Results: Averaged over the entire patient group, the ratio of thickness to width was 1.4, whereas the ratio of length to width was 1.3, These values w ere fairly constant over the range of volumes, emphasizing that the prostat e retains its general shape as volume increases. The idealized standard pla n was overlaid on the ultrasound images of the 17 patients in the 30- to 40 -cc group and the V100, the percentage of target volume receiving 100% or m ore of the prescription dose, was 98% or greater for 15 of the 17 patients. The lateral and posterior TMs fell within a narrow range, most being withi n 2 mm of the idealized 5-mm TM, To estimate whether a 10-cc volume-interva l stratification was reasonable, the standard plan generated from the 30- t o 40-cc prostate model was applied to 5 patients each from the 20- to 30-cc group and the 40- to 50-cc group. Using the standard plan designed for the 30- to 40-cc group, the TMs were closer to 10 mm than to 5 mm for the smal ler volume glands and too small for the larger volume ones, assuming an ide al margin of 5 mm. Conclusion: The application of standardized plans to prostate brachytherapy is feasible. Stratifying the volume in 10-cc intervals appears to be adequ ate, suggesting that the majority of cases appropriate for treatment with b rachytherapy might be treated with three standard plans. While the authors believe that the use of a limited number of standard treatment plans is Fea sible, practical, and medically acceptable, it should be emphasized that th e use of a standard plan should always be previewed by computer-aided appli cation to the particular patient's planning images. (C) 2001 Elsevier Scien ce Inc.