Marker seed migration in prostate localization.

PURPOSE Marker seed location was analyzed to test the hypothesis that there is no intraseed migration within the prostate, a premise fundamental to the technique of marker seed localization of this organ. Despite increasing interest in the use of implanted seeds as fiducial markers for gland location, there are few data available with which to evaluate the validity of this technique, particularly over the entire course of external beam radiation therapy. METHODS AND MATERIALS Between May 2001 and December 2001, after obtaining fully informed written consent, 9 patients with early stage prostate cancer were enrolled on an institutionally reviewed protocol. Patients had four to five marker seeds implanted into the prostate under transrectal ultrasound guidance before definitive radiotherapy. The porous gold seeds were each 1.2 x 2.0 mm in dimension. Seed locations from orthogonal radiographs based on the initial simulation and weekly orthogonal films were digitized using a CMS Focus planning system, thereby facilitating the determination of intraseed spacing. The digitization of the isocenter from each orthogonal pair of radiographs was used to determine digitizing error for seed localization. Pubic symphysis, bilateral femoral heads, and isocenter were also digitized and will be analyzed at a later date. RESULTS Overall, the average migration of all the seeds in the patients was 1.2 +/- 0.2 (SD) mm. The greatest average movement of any seed in any patient was 1.9 mm over the entire 7-week course of radiotherapy. The smallest average movement was 0.6 mm. The greatest change in intraseed spacing in any of the patients during the full course of therapy was 6.6 mm. One seed in 1 patient was lost at the start of the third week of therapy and censored from analysis. Digitizing error in seed localization was calculated to be 0.20 +/- 0.03 (SD) mm. CONCLUSIONS As an aggregate, there is negligible seed migration within the prostate over the entire course of definitive radiotherapy. However, there are small, detectable movements in individual seed locations, perhaps resulting from topographic changes in the gland secondary to seed placement, anatomic changes in bladder and rectum, or treatment itself. With respect to seed migration, prostate marker seeds represent an accurate and reliable surrogate of gland location during a full course of radiotherapy.