Tracking breathing-related prostate movement enables more precise delivery of radiation, study finds.

BOSTON—Real-time positional tracking of the prostate could improve external beam radiation treatment (EBRT) of prostate cancer, data suggest.

Respiratory-induced motion may be greater than previously reported in the literature, where maximum motion of less than 1 mm has been described. In the new study, researchers found that individual respiratory-related prostate motion may be as much as 2 mm and should be considered when using very narrow planning target volume (PTV) safety margins for prostate EBRT.

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“Many people think that the prostate is a static organ, meaning that it doesn’t move in relation to the bony pelvis, but that is not the case,” said lead study investigator Tasha McDonald, MD, a senior resident physician at Oregon Health & Science University in Portland.

“The prostate is a moving organ, and we know it moves because of many factors, including how full other organs are, such as the bladder or rectum.” He presented findings here at the American Society for Therapeutic Radiology and Oncology annual meeting.

Although previous studies have demonstrated that the prostate moves during the breathing cycle, Dr. McDonald and her colleagues were able to track movement in real time by using the new image-guided Calypso Medical System It works by using tiny transponders that are inserted into the prostate and report the exact location and motion patterns inside the body over time.

“Our research demonstrates that the prostate moves during the breathing cycle, mostly up and down, as much as 2 mm. We were able to determine this motion by evaluating the Calypso tracings of patients.” Dr. McDonald said.

Prostate positional variability can alter the effectiveness of radiation therapy when the precise target location is unknown. PTV margins traditionally have been designed to compensate for inter-fraction prostate setup variability. PTV margins, however, may be reduced when using daily image-guidance, according to Dr. McDonald. 

Few data are available regarding the magnitude and clinical relevance of high-frequency respiratory-induced prostate motion. “This is important information,” she said, “because in low-risk prostate patients, we treat the prostate and seminal vesicles with a small margin—5-7 mm—to account for prostate motion and setup error.

By knowing all the factors that contribute to prostate motion, we will be able to determine appropriate margins.” If the margins are too large, she noted, there can be more toxicity to normal tissue, and if the margins are too small, we could miss the prostate.

Greater accuracy allows the delivery of higher-dose radiation while leaving healthy cells alone. This process simultaneously reduces adverse effects, specifically rectal and bladder toxicities and erectile dysfunction (ED), as well as offering men a better chance for a cure.

In this current study, prostate motion during radiation delivery was measured using the Calypso System in 20 patients. Prior to radiotherapy planning, three transponders were implanted in the prostate and used for daily localization and continuous real-time positional tracking. Respiratory motion was cross-verified using the clinically tested Varian Real Time Position Management (RPM) system. A total of 450 motion traces were analyzed.

The investigators found that 95% of analyzed real-time motion-tracking traces demonstrated identifiable respiratory-induced prostate motion. The observed frequency of breathing motion correlated with the respiratory frequency derived from the RPM system.

In addition, the researchers found that the frequency of breathing motion was patient-specific and relatively consistent over a course of EBRT. Dr. McDonald said craniocaudal prostate respiration motion was the largest, followed by anteroposterior movement. Respiratory-induced lateral motion never exceeded 0.5 mm. 

Overall, in 14 patients, respiratory-induced prostate motion exceeded 1 mm more than 75% of the time (mean 1.3 mm), and the largest measured amplitude of respiratory motion was 2.5 mm.

“This doesn’t change the techniques for salvage or adjuvant radiation, but when treating prostate cancer definitively, it may help define the treatment volume and minimize toxicity,” Dr. McDonald told Renal & Urology News.

“When urologists evaluate the side effects of radiation versus surgery, these findings are important. As we get better at understanding the motion of the prostate during radiation, we may be able to actually reduce the toxicity of radiation. Just as urologists are getting better using robotics to minimize toxicity and decrease side effects, such as incontinence and ED, this may be a way to reduce toxicity from radiation. It will allow us to better the track the prostate and minimize the margins that we treat, so we [affect] less healthy tissue.”