A Pilot Study of Robotic Template Guidance for Needle Placement in Transperineal Prostate Brachytherapy

Section 1- Background:

Adenocarcinoma of the prostate will affect over 220,900 U.S. males this year, making it the most prevalent cancer in the nation.1 Prostate specific antigen (PSA) is a very useful tumor marker in early detection of this malignancy, as well as a reliable marker for control after definitive therapy with either surgery or irradiation. Widespread use of prostate-specific antigen (PSA) screening has resulted in approximately 45% of patients being detected with early stage disease. For this group of patients, brachytherapy or implantation of radioactive sources into the prostate is a treatment option that has risen dramatically in use over the last several years due to its effectiveness and convenience.

The use of brachytherapy or implantation of radioactive sources into the prostate for adenocarcinoma has advantages over external beam radiation in that a very high dose can be delivered to the tumor while limiting the doses to the surrounding normal tissue (i.e., bowel and bladder). It is well established that outcomes after treatment with brachytherapy are critically related to the technical quality of source placement within the gland. A spatially desirable placement of the radioactive sources with achievement of optimum dose distributions within the prostate is key to the success of brachytherapy with regard to both killing tumor as well as minimizing toxicity.

The development of transrectal ultrasound (TRUS) of the prostate, with the ability to map the prostate in several planes, as well as the associated development of trans-perineal implantation of the prostate, has allowed the development of the modern prostate implantation method. Ultrasound images are taken before or during the implant procedure, and a source distribution plan is developed that optimizes dose to the prostatic tissues while sparing the urethra and rectum. Needles are placed into the prostate via a transperineal template, and the sources are placed into the prostate according to the source distribution plan.

Currently, the template utilized for needle guidance is limited to 0.5 centimeter x-axis and y-axis grid spacing, limiting the ability of the needle positions to conform to the shape of the prostate, which is spherical (Fig 1). The most commonly utilized method of dosimetric planning involves placing needles around the periphery of the prostate capsule, in order to best cover potential microscopic extracapsular extension as well as to avoid the dose sensitive urethra which runs through the center. The grid arrangement of the current template limits the ability of these needle positions to best approximate the external contour of the prostate (Fig 2). Also, placement of sources in the most posterior row adjacent to rectum is critical. Sources too close to rectum may cause increased risk of rectal bleeding, whereas source too far from the capsule may result in risk of underdosing disease in that area. Greater freedom in needle positioning would allow for more optimum placement of sources.