Limitations of the minimum peripheral dose as a parameter for dose specification in permanent 125I prostate implants.

PURPOSE The objective of this work is to investigate whether the minimum peripheral dose is a practical parameter for dose specification in permanent 125I implants of the prostate. METHODS AND MATERIALS The investigation was carried out by use of a computer model of ellipsoidal 125I implants in which the average dimension and elongation factor were varied to provide a wide range of geometries. Both ideal and nonideal implants were investigated. The 125I seeds were confined to the target volume except for a portion of the study in which the effect of placing seeds outside the target volume was investigated. RESULTS The minimum peripheral dose was found to be very sensitive to the seed placement. The irregularities in the seed spacing that inevitably occur in actual implants tend to lower the minimum peripheral dose. As a result, the minimum peripheral dose is generally significantly less than planned by an amount that is unpredictable, and often exceeds 25%. However, the percentage of the target volume that receives a dose less that the prescribed minimum peripheral dose is generally less than 10%. Implanting seeds outside the target volume improves the dose uniformity, but does not appear to offer any advantage in dose coverage, and increases the volume of normal tissue irradiated. CONCLUSION If a minimum peripheral dose is prescribed for a permanent 125I prostate implant, and the implant is planned using an idealized implant having precisely spaced seeds, the prescribed dose will rarely, if ever, be achieved. Reasonable agreement with the prescribed dose can be achieved only if the requirement for coverage is relaxed from 100 to 90%, or if the total source strength is increased by 20% to compensate for the anticipated imperfections in seed placement.