Anisotropy of Edema Formation Evidenced by Implanted Gold Fiducials in Permanent Prostate Brachytherapy

Abstract

Purpose: The dosimetric quality of seed implants most likely will deteriorate after implantation mainly due to edema. One of such scenarios is a significant increase of UD10. Edema has long been assumed to be isotropic. However, the isotropic formation of edema cannot explain the increase of UD10, which leads us to think that the edema formation may not be isotropic. In this study we provide for the first time direct evidence about the anisotropy of edema formation by measuring the change of inter-fiducial distance before and after seed implantation with the help of implanted gold fiducials. Materials and Methods: Sixteen patients with T1N0M0 to T2aN0M0 prostate carcinoma underwent 103Pd (Prospera, NAS Medical, Chatsworth, CA) boost brachytherapy after 45 Gy of external beam irradiation are utilized for this study. The volume ranges from 21.0 to 67.6 cc (with a median of 34.6 cc). Patients are implanted with 3 standard gold fiducials (Civco, Inc., Kalona, IA) that is 3 mm long and 0.8 mm in diameter. They are placed at base just right of urethra (Superior), midgland at extreme left (Center) and apex slightly right (Inferior). After 2 weeks, patients are scanned with a GE Lightspeed CT unit for external planning with 2.5 mm slice thickness. Those fiducials form an acute triangle viewed from AP scout view. CT data set are sent to CMS XiO treatment planning system (Elekta A.B., Stockhom, Sweden). The coordinates of fiducials are obtained. Using those coordinates inter-fiducial distance is calculated. During 5 weeks external treatment, no fiducial migration is observed. After external beam treatment, patients undergo seed implants. Post-implant CT is done 24 hours later. The coordinates of fiducials are obtained again. Inter-fiducial distance is calculated with new coordinates. Inter-fiducial distance differences are obtained. Results: The maximum superior-center distance change is 4.5 mm, the minimum is –1.8 mm (median 1.9 mm). Four out of 16 (25.0%) show inter-fiducial distance decrease. For center-inferior, the maximum distance change is 4.3 mm, the minimum is –1.5 mm (median 0.9 mm). Six out of 16 (37.5%) show interfiducial distance decrease. While for inferior-superior, the maximum distance change is 4.7 mm, the minimum is –0.5 mm (median 1.6 mm). Five out of 16 (31.3%) show interfiducial distance decrease. Conclusions: Using interfiducial distances among implanted gold fiducials before seed implants as references, the change of these distances after seed implants is directly caused by edema formation. The existence of decreases in inter-fiducial distances is strong indication that the edema formation is not isotropic. With further study, the anisotropy of edema formation may help to explain the increase UD10. This study is done for 103Pd, similar result is expected to hold also for 125I and 131Cs.