Radiation oncologists map prostate for accurate IMRT

As radiation therapy develops more sophisticated targeting, researchers are turning to new imaging methods to direct it. Fused ProstaScint and CT images and daily ultrasound scanning both show promise as tools to concentrate intensity-modulated radiation therapy (IMRT) and spare surrounding healthy tissue.

Researchers at the Mayo Clinic in Scottsdale, AZ, use ProstaScint scans fused with axial CT images of the pelvic region to target hot spots within the prostate. Because the imaging agent is taken up by bone, blood vessels, the bladder, and other anatomic landmarks, coregistration can be closely approximated, according to lead author Dr. Steven Schild, vice chair of radiation oncology at the clinic.

For 43 patients, Schild and colleagues delivered treatment doses of 75.6 Gy over 42 fractions to the entire prostate, defined using CT-derived data. They set strict limits for how much peripheral radiation exposure would be acceptable for the bladder and rectum. The ProstaScint-enhancing region within the prostate was believed to correlate with the region of greatest tumor burden and was simultaneously boosted to 82 Gy.

Among 38 patients followed for three months, only one showed a grade 3 genitourinary toxicity. This resolved within one month following therapy. Sixty percent had genitourinary symptoms that were treated with Flomax, and 48% had grade 2 gastrointestinal toxicity that was treated with Imodium. Prostate-specific antigen levels dropped from an average of 6.9 before treatment to 1.9 one month following therapy. Long-term follow-up has been limited, as this technology is new and integrating the systems has required considerable work. ProstaScint scans have utility in improving the staging of prostate cancer and in localizing the regions of greatest tumor burden within the prostate. Both of these benefits can help the radiation oncologist improve treatment.

Researchers in Dijon, France, are using daily ultrasound studies to account for minor shifts in the prostate that could skew IMRT targeting. A software protocol that overlays 3D ultrasound on a simulation-planning CT scan has been able to track movement of 4 to 6 mm on the x, y, and z axes.

The five-minute scan has been incorporated into routine practice since the initial study of 37 patients.

Both techniques underscore an evolution in therapy and targeting, Schild said.