A detailed examination of the difference between planned and treated margins in 125I permanent prostate brachytherapy

To determine whether potential extraprostatic extension (EPE) of prostate adenocarcinoma is covered by the prescribed dose when permanent 125I implants are planned with a 5-mm treatment margin.

The postimplant dosimetry of 60 consecutive 125I prostate implants was analyzed to determine whether there was a residual 3-mm margin, adequate for treatment of potential EPE. The implants were peripherally loaded and planned with a nominal 5-mm symmetric dose margin. Extraprostatic seeds were not used at midgland, although extraprostatic seeds were placed at the base and apex. The radial distance between the edge of the prostate and the prescription isodose line (145 Gy) was measured at the left lateral, left posterolateral, posterior, right posterolateral, and right lateral positions at the base, midgland, and apex in both the preplan and postimplant dosimetry.

The mean postimplant margin at the base (4 +/- 2 mm) was significantly less (p < 0.01) than planned (6 +/- 2 mm) by 2 mm. The planned and postimplant margins at the midgland (5 +/- 1 mm and 5 +/- 2 mm) and apex (7 +/- 2 mm and 7 +/- 3 mm) were indistinguishable (p = 0.31 and p = 0.69, respectively). At the base, 69% of the measurements were > or = 3 mm compared with 89% and 91% at the midgland and apex, respectively. Overall, 83% of the margin measurements were > or = 3 mm. The prostate postimplant V100 and D90 were 96 +/- 4% and 193 +/- 27 Gy, respectively.

A 5-mm planning dose margin can potentially treat most EPE. However, the postimplant margin, like other dosimetric parameters, is sensitive to source placement errors and the percentage of EPE treated depends upon how well the plan is executed. Because implant quality is operator dependent, we would not recommend brachytherapy alone for patients with a high risk of EPE.