An injectable tissue spacer can reduce rectal dose and the risk of radiation burns to the rectum of patients undergoing intensity-modulated radiation therapy (IMRT) for prostate cancer, according to a study presented at the Cancer Imaging and Radiation Therapy Symposium held April 29 and 30 in Atlanta.
More than 90% of men with prostate cancer who undergo treatment are cured, but many suffer debilitating side effects from the treatment. One of the most common toxicities in patients who receive radiation therapy is damage to the rectum. The use of injectable tissue spacers to reduce radiation dose to the rectum has been effective for patients treated at the Cancer Center of Irvine in California, according to Dr. Kenneth Tokita, the center's medical director.
Tokita told attendees at the symposium, which was sponsored by the American Society for Radiation Oncology (ASTRO) and RSNA, that he and his colleagues wanted to determine if transperineal injection of a tissue-spacer compound into the prostate-rectal interspace could reduce the risk of radiation burns to the rectum.
Between December 2009 and August 2010, 34 patients had the tissue-spacer injection. They then received either IMRT in 33 to 45 fractions of 180 cGy or high-dose-rate brachytherapy plus IMRT.
The patients had MRI exams preinjection, postinjection, and every two weeks until the conclusion of treatment to monitor any changes in the spacer compound. Adaptive radiation therapy was applied in cases where the tissue spacer degraded significantly, and treatment plans were revised accordingly.
The spacer generated an additional 1-cm mean separation between the prostate and the rectum, resulting in a significant reduction in rectal dose, Tokita explained. The additional prostate-rectal spacing decreased the maximum rectal dose by 11.5% and the mean rectal dose by 30%. The rectal wall V60 (percentage of volume receiving at least 60 Gy) and V70 (percentage receiving at least 70 Gy) decreased by 19.12% and 19.87%, respectively.
"Biweekly MRI scans revealed that the tissue-spacer separation dissipated over time," Tokita said. "The mean separation over time revealed an initial decay of 20% during the first four weeks, followed by a more rapid decay of 60% over the following two weeks. This dissipation enabled the patient's anatomy to return to its baseline state."
The spacing provides significant dosimetric advantages for treatment planning and delivery, Tokita noted. Severe rectal radiation burns, the most serious risk of injury from the IMRT treatments, were essentially eliminated. This enables dose to the posterior prostate to be increased without concerns of damaging the rectum, according to Tokita.
"Removing rectal injury from the treatment essentially makes radiation therapy the treatment of choice for prostate cancer," he concluded. "The ability to reach almost perfect cure rates and minimal injury is the dream of all cancer specialists. We are now wondering where else this may benefit cancer patients in radiation therapy treatments."
