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Radiation Oncologists Map the Prostate to Increase the Accuracy of Intensity-Modulated Radiation Therapy

Radiation Oncologists Map the Prostate to Increase the Accuracy of Intensity-Modulated Radiation Therapy

SCOTTSDALE, Arizona-As radiation therapy develops more sophisticated targeting, researchers are turning to new imaging methods to direct it. Fused ProstaScint (Cytogen, Princeton, New Jersey) and computed tomography (CT) images, as well as daily ultrasound scanning, show promise as tools to concentrate intensity- modulated radiation therapy (IMRT) and spare surrounding healthy tissue, a report presented at the 90th Scientific Assembly and Annual Meeting of the Radiological Society of North America indicates. Researchers at the Mayo Clinic in Scottsdale, Arizona, use Prosta- Scint 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 investigator Steven E. Schild, MD, vice chair of radiation oncology at the Mayo Clinic. In a 43-patient study sponsored by Cytogen, Dr. Schild and his colleagues delivered treatment doses of 75.6 Gy over 42 fractions to the entire prostate, defined using CT-derived data (abstract SCC19-07). They set strict limits on the degree of peripheral radiation exposure that 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 3 months, only 1 patient had a grade 3 genitourinary toxicity. This resolved within 1 month following therapy. Sixty percent had genitourinary symptoms that were treated with tamsulosin (Flomax), and 48% had grade 2 gastrointestinal toxicity that was treated with loperamide (Imodium). Prostate- specific antigen levels dropped from an average of 6.9 before treatment to 1.9 at 1 month following therapy. Long-term follow-up has been limited, as this technology is new and integrating the systems has required considerable effort. Prosta- Scint 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. French Ultrasound Study Researchers in Dijon, France, Dr. Schild said, 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 5-minute scan has been incorporated into routine practice since the initial study of 37 patients, he reported, adding that both ProstaScint and ultrasound techniques underscore an evolution in therapy and targeting.

 
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