LDR (low-dose rate) brachytherapy uses permanent seed implants to treat early stage prostate cancer. In this technique, radioactive 'seeds' about the size of a grain of rice are positioned in the prostate using needles that are guided into position by ultrasound and a template or grid, through which the needles are inserted to control their spacing. From 50 to more than 100 individual seeds may be placed in a single session using 20 to 50 needles. Radiation is then delivered to the prostate slowly over the following month or so as the sources decay. As the seeds emit radiation, they reduce in strength and in a few weeks no longer emit any measurable amounts of radiation. The most commonly used LDR treatment site is the prostate, which involves permanent placement of tiny radioactive seeds in the prostate tissue.
Treating with brachytherapy requires the seeds be placed so the radiation dose adequately covers the entire prostate, but minimizes dose to the surrounding healthy tissue. To accomplish this task, Varian has developed the VariSeed™ LDR prostate treatment planning system which uses trans-rectal ultrasound (TRUS) images to help design the intended placement of sources. It then helps provide 'dynamic dosimetry' to update the dose map in real time as the individual seeds are deposited in the prostate. This allows the physician to compensate in real time for any gaps that may arise as a result of prostate deformation by the needles, or other needle placement issues that may occur. VariSeed can also automatically detect seeds from a post implant computed tomography (CT) image set to confirm the received dose distribution.