Corey Watts with Prof. Bruce Thomadsen and Timothy Burns

UW-Madison Department of Medical Physics

The field of medical physics consists of the use of cancer treatments involving radiation from internal and external sources. One of its principal procedures, called brachytherapy, is the implantation of radioactive iodine-125 seeds into the area of a prostate tumor. Physicians and physicists create detailed treatment plans based on the size and location of the tumor and the strength of the radioactive seeds in order to maximize the radiation dose absorbed by the tumor and minimize that absorbed by surrounding tissue. It is critical for the accurate implementation of the treatment plan that the radioactivity of each implanted seed is precisely known.

Trends in the manufacture and sale of brachytherapy seeds have led many vendors to deliver seeds for a treatment plan within an already-sealed, sterile package. To develop a technique for properly assaying the strength of each seed without sacrificing sterility, we performed a feasibility study involving radiographic dosimetry of the seeds using X-ray diagnostic films. This entailed exposing a sensitive film to the packaged seeds for a set period of time, developing the film, and employing imaging and mathematical analysis on the optical density resulting from the seeds to determine each one’s relative strength. At the conclusion of the study, refinements in technique had reduced error in radiographic strength assessments from our earliest procedures, but discrepancies between measured and actual relative strengths were as yet too great to fully affirm the feasibility of the procedure.