A unified strategy to focal brachytherapy incorporating transperineal biopsy, image fusion, and real-time implantation with and without rectal spacer simulated in prostate phantoms

Purpose:
To develop an approach to the diagnosis and treatment of prostate cancer using one platform for fusion biopsy, followed by focal gland ablation utilizing permanent prostate brachytherapy with and without a rectal spacer.

Material and methods:
Prostate phantoms containing multiparametric magnetic resonance imaging (mpMRI) regions of interest (ROI) underwent fusion biopsy, followed by image co-registration of positive sites to a treatment planning brachytherapy program. A partial hemi-ablation and both posterior lobes using a Mick applicator and linked stranded seeds were simulated. Dummy sources were modeled as iodine-125 (¹²⁵I) with a prescribed dose of at least 210 Gy to gross tumor (GTV) and clinical target volume (CTV), as defined by mpMRI visible ROI and surrounding negative biopsy sites. Computer tomograms (CT) were performed post-implant prior to and after rectal spacer insertion. Different prostate and rectal constraints were compared with and without the spacer.

Results:
The intra-operative focal volumes of CTV ranged from 6.2 to 14.9 cc (mean, 11.3 cc), and the ratio of focal volume/whole prostate volume ranged between 0.19 and 0.42 (mean, 0.31). The intra- and post-operative mean focal D₉₀ of GTV, CTV, and for the entire prostate gland was 265 Gy and 235 Gy, 214 Gy and 213 Gy, and 66.1 Gy and 57 Gy, respectively. On average, 13 mm separation was achieved between the prostate and the rectum (range, 12-14 mm) on post-operative CT. The mean doses in Gy to 2 cc of the rectum (D_2cc) without spacer vs. with spacer were 39.8 Gy vs. 32.6 Gy, respectively.

Conclusions:
Doses above 200 Gy and the implantation of seeds in clinically significant region for focal therapy in phantoms are feasible. All rectal dosimetric parameters improved for the spacer implants, as compared with the non-spacer implants. Further validation of this concept is warranted in clinical trials.