Applied Radiology

CASE SUMMARY

Patient 1 is a 62-year-old man with known chronic tophaceous gout who was seen in 1997 for an annual check-up that revealed an enlarged prostate and noninflamed multiple gouty tophi, the largest of which involved the left pre-patellar bursa. He also had an elevated prostate specific antigen (PSA) for which he underwent a prostatectomy. Three years later, he presented with an elevated PSA and an indium-111-labeled capromab pendetide (ProstaScint, Cytogen Corp., Princeton, NJ) scan was ordered. The ProstaScint scan showed increased activity involving the para-aortic and right iliac lymph nodes, indicating metastatic prostate cancer. It also demonstrated an area of increased activity involving the left prepatellar region (Figure 1).

Patient 2 is a 61-year-old man with known advanced right-knee osteoarthritis and prostate cancer that had been treated with cryoablation in May 2000. On physical examination, he was noted to have an enlarged prostate and was suspected to have extracapsular extension of disease. The right knee showed no signs of active inflammation. A ProstaScint scan performed as part of his work-up revealed activity limited to the prostatic bed with no evidence of extra prostatic nodal activity. Increased activity in the region of the patient's right knee was noted incidentally (Figure 2).

DIAGNOSIS

Patient 1: Chronic tophaceous gout, causing increased ProstaScint accumulation in the prepatellar bursa.

Patient 2: Advanced osteoarthritis, causing increased activity on ProstaScint scan.

IMAGINING FINDINGS

These cases demonstrated positive ProstaScint scans, which did not prove to be carcinoma.

DISCUSSION

It is important to recognize other causes of increased activity with this radionuclide. ProstaScint scan is a technically demanding procedure with several potential pitfalls: suboptimal quality control of the camera, suboptimal patient preparation, inadequate imaging time, and inaccurate computer processing. Therefore, it is best performed and interpreted at sites with experience and expertise. ¹¹ The gamut for false-positive activity with ProstaScint scan includes such common causes: injection site and its regional draining lymph nodes, gastrointestinal (GI) tract activity, tortuous vessels, and misinterpreting expected areas of radiopharmaceutical biodistribution (such as the liver, spleen, bone marrow, salivary gland, male genitalia, blood pool activity, kidney, and bladder). False-positive activity may also have some uncommon causes: renal cell carcinoma, ⁷ pelvic kidney, ⁸ neurofibromas, ⁹ hepatic hemangioma, ¹⁰ gout, and osteoarthritis.

These are the first reports of false-positive ProstaScint scans from joint disease. There have been a few reported cases of abnormal uptake of ProstaScint from the causes listed above. Indium-111-labeled capromab pendetide (ProstaScint) is a new radiopharmaceutical that is FDA-approved for the imaging of prostate cancer patients at high risk for metastatic disease and for patients who have had a prostatectomy and present with a rising serum PSA level.

Capromab pendetide is a whole murine monoclonal antibody directed against prostate membrane specific antigen (PMSA), a transmembrane glycoprotein expressed by prostate epithelial cells. Prostate membrane specific antigen is higher in prostate adenocarcinoma cells than in nonmalignant cells, and higher in metastatic lesions than in the primary lesion. Typically, ProstaScint scans has a biodistribution of activity involving the liver, spleen, kidneys, bone marrow, male genitalia, and blood pool. It is excreted in the GI tract. It is used to stage newly diagnosed prostate cancer and to identify residual disease, local recurrence, and metastasis in post-prostatectomy patient. ^1-5 It enables more accurate disease staging and monitoring than is afforded by other imaging modalities, such as CT and MRI. ⁶

Gout encompasses a group of disorders that occur alone or in combination and include hyperuricemia, arthritis, tophaceous deposition of urate crystals in and around joints, interstitial deposition of urate crystals in renal parenchyma, and urolithiasis. Approximately 10 years after the first acute attack, tophi become apparent on physical examination, as in the first case. Osteoarthritis also exhibits a verity of clinical expressions that typically lack clinical signs of inflammation. We postulate that the increased uptake was from low-grade clinically occult inflammation at the sites of these patient's joint disease.

CONCLUSION

False-positive ProstaScint scans occur from numerous causes. These are the first reported cases of joint disease that caused false-positive studies. Radiologists interpreting these scans need to be aware of the causes of false-positive scans.