Modification of prostate implants based on postimplant treatment margin assessment.

Purpose: To quantify the extent of additional source placement needed to perfect an implant after execution by standard techniques, assuming that uniform 5 mm treatment margins (TMs) is the criteria for perfection.

Materials And Methods: Ten consecutive, unselected patients treated with 1-125 brachytherapy were studied. Source placement is planned just inside or outside of the prostatic margin, to achieve a minimum 5 mm TM and a central dose of 150%-200% of the prescription dose. The preimplant prostate volumes ranged from 24 to 85 cc (median: 35 cc). The number of sources implanted ranged from 48 to 102 (median: 63). Axial CT images were acquired within 2 h postoperatively for postimplant dosimetry. After completion of standard dosimetric calculations, the TMs were measured and tabulated at 45 degrees intervals around the prostate periphery at 0.0, 1.0, 2.0, and 3.0 cm planes. Sources were then added to the periphery to bring the TMs to a minimum of 5 mm at each measured TM, resulting in a modified implant. All margin modifications were done manually, without the aid of automated software.

Results: Patients' original (unmodified) D90s ranged from 111% to 154%, with a median of 116%. The original V100s ranged from 94% to 99%, with a median of 96%. No patient required placement of additional sources to meet a minimum D90 of 90% or a minimum V100 of 80%. In contrast, patients required from 7 to 17 additional sources (median: 11) to achieve minimum 5 mm TMs around the entire prostatic periphery. Additional sources equaled from 12% to 24% of the initial number of sources placed (median: 17%). By adding sufficient peripheral sources to bring the TMs to a minimum 5 mm, patients' average V100 increased from 96% to 100%, and the average D90 increased from 124% to 160% of prescription dose. In the course of achieving a minimum 5 mm TM, the average treatment margin for all patients combined increased from 5.5 to 9.9 mm. The number of sources needed to bring the TMs to a minimum 5 mm was loosely correlated with the preimplant prostate volume and the change in prostate volume from implant-related swelling. Adding sufficient sources to achieve minimum 5 mm TMs increased the prostate volume receiving greater than 200% of the prescription dose (V200) from 39% to 58%, and increased the average urethral point dose (2.00 cm inferior to the bladder) from 154% to 171% of the 144 Gy prescription isodose.

Conclusions: Minimum 5 mm TMs are not uniformly achieved with current implant techniques. It seems that doing so, even in experienced hands, will require a reappraisal of our implant techniques, or the addition of intraoperative dosimetric analysis with the capacity to substantially modify the implant with extra sources.