Session Item

To determine different approaches of focal brachytherapy with and without rectum spacer derived in an in-vitro prostate cancer simulation.

Two different focal strategies are examined: a lateralized approach (left-right) and the both posterior lobes strategy, which is the predominant position of a clinically significant prostate cancer. Two sets of implants were used in an in-vitro study using the Simulated Inanimate Models of prostate with a dominant lesion (Gross tumor volume, GTV)(SIM^TM): the loose Mick seeds in comparison with the linked stranded I¹²⁵ seeds. The prescribed dose to the clinical target Volume (CTV), and the GTV were at least 200 Gy. Furthermore a rectum spacer (barrigel®) was injected to decrease the dose to the rectum. Post-implant Computed Tomographies of the phantoms were performed with a post-implant dosimetry.  The rectum D1cc and D2cc were compared with and without spacer implant.

The focal volumes of the CTV varied from 5 to 15 cc (mean 8 cc, the percentages of ‘‘focal volume/whole prostate volume’’ varied between 17% and 38% with a mean of 26%). The focal volumes of GTV varied from 0.7 to 1 cc. The  total number of needles necessary to implant the focal volume varied between 7 and 10 (mean, 9). The total number of seeds (activity 0.500 to 0.600 mCi) varied from 27 to 41 (mean, 33). The mean focal D90 of GTV and CTV were 273 Gy (range, 195 Gy – 434Gy) and 227 Gy (range, 168-277 Gy). The D90 for the entire prostate gland was 60.8 Gy (range, 32.6 - 111 Gy). No differences were observed between loose or stranded technique.

The prescription of doses above 200 Gy and the implantation of seeds in the clinical 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 clinical validation of this concept is warranted.