CASE
A 65-year-old man presented for consultation because his prostate-specific antigen (PSA) level was elevated. The patient's medical history is significant for a radical retropubic prostatectomy 11 years ago. Before the surgery, the patient's serum PSA level was 3.0 ng/mL. Adenocarcinoma of the prostate was diagnosed by biopsy. The pelvic lymph nodes were negative at the time of the surgery; therefore, the disease was determined to be confined to the prostate. Pathology confirmed stage II adenocarcinoma with a Gleason score of 3+3. The patient's serum PSA was subsequently undetectable; he was lost to follow-up for several years. When the patient returned in 2002, his PSA level was 0.2 ng/mL. The following year, it had increased to 0.3 ng/mL, and in 2005, it was 1.9 ng/mL. One month before this consultation, the patient's PSA level was 9.4 ng/mL. At this visit, the PSA level was 10.56 ng/mL. The patient had no symptoms other than erectile dysfunction, a common side effect of prostatectomy. He had no new areas of bone pain and no new respiratory, GI, genitourinary, or neurologic complaints.
CT with contrast of the pelvis and abdomen showed no obvious evidence of metastatic disease. A whole-body bone scan displayed some osteoarthritic changes but no definitive evidence of meta
stasis. Immunoscintigraphy with capromab pendetide (ProstaScint), a murine monoclonal antibody that reacts with prostate-specific membrane antigen (PSMA), was ordered. A low level of reactivity was seen in the prostatic fossa, but it was less intense than would be seen in recurrent disease. No reactive adenopathy or evidence of bony metastatic disease was apparent. However, focal radiotracer reactivity appeared in the upper lobe of the left lung (Figure 1). Chest CT revealed a spiculated lesion measuring 2.8 cm in diameter. The nodular density resembled a primary lung carcinoma; however, the lesion's location correlated with the area of reactivity seen on the scintigram (Figure 2). Biopsy confirmed that the tumor was a prostate cancer metastasis, which was surgically removed in October 2006. The margins of the excision were negative, but vascular channel invasion by carcinoma existed. Treatment with leuprolide (Lupron) injections for systemic metastasis of prostate cancer was initiated.
One month after resection of the lung tumor, the patient's PSA level was 2.06 ng/mL, and it continued to decrease. Follow-up serum PSA levels were 0.07 ng/mL in March 2007 and 0.16 ng/mL in June 2007. At this time, the patient chose to proceed with pulse hormonal therapy. In September 2007, the patient's PSA level had increased to 2.19 ng/mL. He continued to experience erectile dysfunction and noted having hot flashes, both of which were attributed to the hormone treatment. He had no other symptoms of metastatic disease.
DISCUSSION
Prostate cancer is the most prevalent cancer in men, with an estimated 186,320 new cases and an estimated 28,660 deaths in 2008.1 Cancer deaths as a result of prostate cancer are second only to deaths resulting from lung malignancies. The lifetime risk of developing prostate cancer is more than 16%.1 Radical prostatectomy, a common procedure for cases of localized prostate cancer, includes removal of the nearby lymph nodes as well as the prostate. The procedure is intended to be curative; in theory, PSA levels should decrease after surgery and remain undetectable. Unfortunately, a significant number of men have measurable serum PSA levels after primary treatment, a situation known as biochemical recurrence. Furthermore, clinical evidence of distant metastases develops within 10 years after radical prostatectomy in 15% of patients.2
An elevated PSA level after treatment is caused by local disease recurrence in the prostatic fossa or a distant metastasis. Locating the cancerous tissue is essential to determine whether treatment should be localized or systemic; however, tracking the source of an elevated PSA level can be a difficult task. Prostate cancer most often metastasizes to bone and pelvic lymph nodes. As such, CT of the pelvis/abdomen and radionuclide bone scan are commonly used modalities to find metastases, but these scans have limited usefulness.3 Metastatic disease can occur in any tissue; therefore, it may not be found with these modalities. An autopsy study found evidence of metastasis to the lung in 46% of men with prostate cancer.4 However, lung metastasis with no known bone or lymph node involvement is extremely rare and has been described in only a handful of case reports. Therefore, imaging studies of the lungs are usually not warranted when the findings on pelvic/abdominal CT and bone scan are negative and the patient has no respiratory symptoms.
PSMA is a protein expressed by both normal and malignant prostate epithelial cells. Imaging with capromab pendetide is indicated for biopsy-proven prostate cancer tumors that are thought to be clinically localized after standard radiographic evaluation (chest radiography, bone scan, CT, or MRI) and are at high risk of metastasizing to the pelvic lymph nodes. The imaging study is also indicated when PSA levels are elevated postprostatectomy, standard metastatic evaluation results are negative or equivocal, and occult metastatic disease is strongly suspected.5