MRI Detects Prostate Cancer Recurrence at Very Low PSA

Megan Brooks

May 06, 2011

May 6, 2011 (Atlanta, Georgia) — In patients with a rising or persistently elevated prostate-specific antigen (PSA) level after prostatectomy, dynamic contrast-enhanced (DCE) endorectal magnetic resonance imaging (MRI) is effective in identifying local prostate cancer recurrence, even in patients with extremely low PSA levels, according to a new study.

"I see DCE endorectal MRI becoming more standard in the evaluation of patients with a rising PSA or persistently elevated PSA after prostatectomy," lead investigator Seungtaek Choi, MD, assistant professor of radiation oncology at the University of Texas M.D. Anderson Cancer Center in Houston, told Medscape Medical News.

"Currently," he explained, "most physicians get a CT [computed tomography] scan of the abdomen and pelvis and a bone scan to evaluate these patients. I think that DCE endorectal MRI of the pelvis could provide more information than a CT, as the MRI scan is much better at differentiating prostate tissue/cancer from rectum/bladder/muscles."

Dr. Choi presented the study here at the Cancer Imaging and Radiation Therapy Symposium, which was cosponsored by the American Society for Radiation Oncology and the Radiological Society of North America.

Positive Predictive Value Tops 88%

Dr. Choi and colleagues reviewed the charts of 389 patients treated between January 2004 and October 2010 for a rising or persistently elevated PSA level after prostatectomy; 143 underwent a DCE MRI of the pelvis with an endorectal coil before starting salvage treatment.

Of this group, 113 had an undetectable PSA level (<0.1 ng/mL) after surgery and 30 had a detectable PSA level after surgery (median PSA, 0.2 ng/mL; range, 0.1 to 1.7 ng/mL). At MRI, median PSA was 0.3 ng/mL (range, 0.1 to 8.0 ng/mL).

On MRI, a focal area of rapid enhancement with contrast in the prostate/seminal vesicle fossa was defined as being suspicious for local recurrence; 35 of the 143 patients had this finding on MRI. Twenty-six of these patients underwent ultrasound-guided biopsy of the prostate/seminal vesicle fossa, with 23 showing cancer. The positive predictive value of DCE endorectal MRI for cancer was 88.5%.

Nine patients were treated with salvage radiation therapy to the prostate/seminal vesicle fossa without pathologic confirmation of their suspicious MRI; 8 of these patients remained disease-free after a median of 22 months (7 patients with a PSA level below 1.0 ng/mL and 1 with a PSA level of 1.0 ng/mL).

The median PSA level for patients with a biopsy-proven local recurrence was 1.35 ng/mL. Eight patients with a biopsy-proven recurrence had a PSA below 1.0 ng/mL at the time of the biopsy.

Among 9 patients who had prostate/seminal vesicle fossa biopsies performed without any suspicious MRI findings, only 1 had a pathologically proven local recurrence, yielding a negative predictive value for DCE endorectal MRI of 88.9%.

"I think that people have classically thought that patients with low PSA levels do not have clinically evident cancers," Dr. Choi noted. "Our study showed that we are now able to find cancers even at low PSAs" (the lowest level at which a cancer was seen was 0.3 ng/mL).

He said being able to locate the cancer in the prostate fossa is "very important" for several reasons. "It should better select for patients who will benefit from salvage radiation therapy to the prostate bed. Currently, we often radiate patients without knowing where the cancer is exactly," he pointed out.

It should also better allow the radiation oncologist to plan the treatment, he added. "At our center, we treat more aggressively when the patient has a clinically evident recurrence on MRI. We usually add androgen-deprivation therapy and try to give a higher dose to the suspicious area on MRI," Dr. Choi explained. "Our hope is that doing so will increase the chance of cure for these patients."

"We feel that such treatment is warranted, as the patients who we treat for salvage are usually younger and have proven that they have a more aggressive variant of prostate cancer," Dr. Choi said.

Ready for "Prime Time"

Dr. Choi thinks DCE endorectal MRI in this patient population is ready for "prime time," although there is some variability in the skill of the radiologist reading the MRI, he said, adding that, "as this scan gets done more frequently, radiologists will get more experience at reading them."

"Currently, all of our patients with a rising or persistently elevated PSA get a DCE endorectal MRI of the pelvis," Dr. Choi told Medscape Medical News. "If there is a suspicious finding, the patient undergoes a biopsy for pathologic confirmation. At some point, we may stop doing that, but I think it's always helpful to confirm the existence of cancer before giving radiation therapy."

Dr. Choi said that although no further formal studies are planned, he suspects that they will continue to update the data as more patients get scanned. "We should be able to have a larger pool of patients to better judge the accuracy of the MRI," he told Medscape Medical News.

Dr. Choi has disclosed no relevant financial relationships.

Cancer Imaging and Radiation Therapy Symposium. Presented April 29, 2011.

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