MIAMI BEACH, Fla.—Men with localized prostate cancer (PCa) treated with intensity modulated radiation therapy (IMRT) have 26% fewer late bowel and rectal adverse effects compared with those who undergo three-dimensional conformal radiation therapy (3D-CRT), according to study findings presented at the 53rd annual meeting of the American Society for Radiation Oncology.

“This study supports the continued use of IMRT in the management of prostate cancer. It is a safe and very well-tolerated therapy with fewer complications than 3D-CRT,” said investigator Jeff Michalski, MD, Professor of Radiation Oncology at Washington University Medical Center in St. Louis, Mo.

Investigators also observed a significant 15% higher rate of rectal side effects in white men compared with other races, regardless of the radiation treatment modality. “The racial differences were definitely surprising and we are still unsure as to why this exists,” Dr. Michalski said. “While it could be a real difference in the tolerance to treatment, it could also represent cultural differences in reporting side effects and physician interpretation of patient descriptions.”

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IMRT is a newer, more specialized form of 3D-CRT that allows radiation to be more exactly shaped to fit the tumor and further limits the amount of radiation received by healthy tissue near the tumor. IMRT may also safely allow a higher dose of radiation to be delivered to the tumor, potentially increasing the chance of a cure.

These latest data are from a preliminary analysis of acute and late toxicity in men receiving high-dose radiation therapy on a phase 3 Radiation Therapy Oncology Group (RTOG) dose-escalation trial. Researchers wanted to compare the toxicity rates of high-dose radiation therapy to standard dose radiation treatment, using IMRT and 3D-CRT. The toxicities were scored from the grade of zero (no toxicity) to four (severe toxicity). The study also examined what patient characteristics might be associated with toxicity.

The study involved 748 men, of whom 491 were treated with 3D-CRT and 257 with IMRT. Each patient had a total of 44 treatments. The median follow-up was 4.6 years for the 3D-CRT group and 3.5 years for the IMRT group.

The researchers found a statistically significant reduction in acute Common Toxicity Criteria for Adverse Events (CTCAE) genitourinary and gastrointestinal (GI) adverse effects occurring within 90 days from treatment start with IMRT-delivered therapy. Dr. Michalski’s group also observed a trend toward a clinically meaningful reduction in late GI adverse effects occurring more than 90 days from treatment start. IMRT was associated with a statistically significant decrease in acute Grade 2+ rectal/bowel and urinary toxicity.

High radiation doses to large volumes of the rectum and acute GI adverse effects were significantly associated with late rectal adverse effects.