The effect of seed orientation deviations on the quality of¹²⁵I prostate implants

Abstract

We quantified the effect of seed orientation deviations on five prostate seed implant cases at our institution. While keeping their positions fixed, the iodine-125 seeds were assigned orientations sampled from a realistic probability distribution derived from the post-implant radiographs of ten patients. Dose distributions were calculated with both a model that explicitly includes anisotropy (TG43 anisotropy function) and a point source model (TG43 anisotropy factor). Orientation deviations had only a small influence on prostate dose-volume histograms: the 95% confidence intervals on the volumes receiving 100%, 150% and 200% dose were at most +/-0.8%, +/-1.1% and +/-0.6% of the prostate volume, respectively. The dose-volume histograms of anisotropic seed distributions were marginally better than those with isotropic point-source seeds. Anisotropy caused a displacement of cold spots (regions receiving <100% of the prescribed dose) in <1% of the prostate volume. Our results indicate no net benefit to prostate dosimetry in using more isotropic seeds. Furthermore, we propose a new 'weighted anisotropy function' to better account for the effects of anisotropy when seed orientation is unknown. Conceptually, the TG43 anisotropy factor described in AAPM TG43 averages the effect of anisotropy over all solid angles, with the implicit assumption that all seed orientations are equally probable. In prostate implants, however, seeds are preferentially oriented parallel to the needle axis. The proposed weighted anisotropy function incorporates this non-uniform probability.

Quantifying the effect of seed orientation in postplanning dosimetry of low‐dose‐rate prostate brachytherapy

“…Nath et al 18 have studied needle divergence of varying degrees on spherical target volumes and actual patient implants and found that they degrade the dosimetric quality of an implant between pretreatment and implantation dosimetry as well as the tumor biological effective dose when compared to the reference implant. Corbett et al 19 have investigated the problem by first creating a probability distribution of seeds orientation as identified from postimplant radiography of ten patients. They subsequently assigned the seeds orientation for five prostates seed implant case by sampling orientations from this probability distribution.…”

Clinical Investigations on the Spinal Osteoblastic Metastasis Treated by Combination of Percutaneous Vertebroplasty and125I Seeds Implantation Versus Radiotherapy

“…As a matter of fact, beyond the treatment target volumes, the dose drops rapidly with distance, leading to limited normal tissue damages. 5,6 Percutaneous vertebroplasty (PVP) is a recently developed treatment for spinal metastases. It has been widely used in the treatment of osteolytic metastases and achieved satisfactory efficacy.…”

Photoacoustic imaging of prostate brachytherapy seeds

“…For the 4-mm acquisition, theoretical contrast calculations suggest that PA imaging would have still outperformed US imaging had the seed been imaged in the short-axis orientation, with a slightly reduced contrast improvement (i.e., PA imaging over US imaging) of 24.4 dB (compared to a 27.9-dB improvement) being realized in this case. A study by Corbett et al found, however, that the majority of seeds are placed approximately parallel (i.e., ±16°) to the transducer face during a clinical procedure [20]. Thus, although the case of short-axis imaging is an important one to consider, seeds tend to be oriented for optimal PA imaging.…”