Micrometastasis of Prostate Cancer to Lymph Nodes: Detection by Means of Reverse Transcription--Polymerase Chain Reaction (1997)

In the clinical management of patients with prostate cancer, recognition of metastatic spread is critical to determining therapeutic options. Curative surgical procedures directed at the primary tumor are used in patients with clinically localized disease. Among patients with cancer histologically confined to the organ and with no evidence of disease in regional lymph nodes, some will have distant metastases or local recurrences postoperatively (1,2). In these patients, the number of metastatic or residual cancer cells is so small that occult metastases or local minimal spread of the disease cannot be detected by conventional radiologic imaging or histopathologic examination. Methods for detecting such minimal disease spread need to be more sensitive and specific than those now commonly used. Immunohistochemical techniques, using antibodies directed against tumorspecific antigens or tissue-specific antigens, are potential tools for recognizing the presence of small foci of metastatic disease (3,4). Molecular probes are also used to detect small numbers of neoplastic cells within large areas of primarily non-neoplastic cells (5). Use of the polymerase chain reaction (PCR) has dramatically increased the sensitivity of this approach. PCR-based methods analyzing tumor-specific or tissue-specific messenger RNA (mRNA) expression (i.e., reverse transcription‐PCR [RT‐ PCR]) or detecting tumor-specific mutations in oncogenes and tumor suppressor genes have been applied for the detection of micrometastases or minimal residual diseases, both of which are difficult to discover by traditional morphologic and immunohistochemical analyses (6‐9). In 1992, Moreno et al. (8) initially reported the clinical application of RT‐PCR for detecting micrometastatic prostate cancer cells in the peripheral circulation of patients with prostate cancer. Since this report, there has been a rapid expansion of the use of RT‐PCR for the detection of micrometastatic prostate cancer cells in peripheral blood, regional lymph nodes, bone marrow, and surgical field blood (10). Some reviews (10‐13)on the clinical application of RT‐PCR in patients with prostate cancer have been published. In this editorial, we focus on the detection of prostate cancer micrometastasis to lymph nodes using RT‐PCR. In 1993, we reported (14) use of an RT‐PCR assay targeting prostate-specific antigen (PSA) mRNA to detect micrometastatic prostate cancer cells in lymph nodes. This assay could detect PSA mRNA from 0.1 LNCaP prostate cancer cell or a single LNCaP cell that had been mixed wit h1◊1 0 6 peripheral blood mononuclear cells before RNA extraction. Forty-four lymph nodes obtained from 22 patients with prostate cancer were analyzed by means of RT‐PCR. The assays detected PSA mRNA in five lymph nodes with histologic and/or immunohistochemical evidence of metastatic cells and in four of 39 lymph nodes with no histologic or immunohistochemical evidence of metastasis. In our study, two (11%) of 18 patients with no histologic evidence of lymph node involvement were positive for metastases to the lymph nodes when tested by RT‐ PCR. This initial observation has been confirmed in recent studies (15,16). Takahashi et al. (15), analyzing fine-needle aspirates of pelvic lymph nodes by means of RT‐PCR, detected PSA mRNA not only in all six cytologically positive and two cytologically class III cases but also in two (17%) of 12 cytologically negative lymph nodes. When testing formalin-fixed, paraffin-embedded pelvic lymph node specimens obtained from patients who had undergone staging pelvic lymphadenectomy at radical prostatectomy, Edelstein et al. (16) detected PSA mRNA in 16 (44%) of 36 lymph node specimens that appeared to be tumor free by conventional pathologic detection methods. In this issue of the Journal, Ferrari et al. (17), examining patients with localized prostate cancer exhibiting high-risk features, further report that RT‐PCR assays targeting PSA and prostate-specific membrane antigen (PSM) mRNA detect PSA- and/or PSM-expressing cells in lymph nodes removed by laparoscopic dissection in 23 (79%) of 29 patients with no pathologic evidence of lymph node involvement. The wide variety of RT‐PCR sensitivities (11%‐ 79%) in the various studies may be attributed to differences in the subjects enrolled and to methodologic differences in the RT‐PCR assays, including the collection of lymph node specimens, the preparation of mRNA, and the PCR conditions (14‐ 17). However, all of these studies demonstrate that PSA- and/or PSM-expressing cells are present in a portion of pelvic lymph nodes in patients with prostate cancer exhibiting no pathologic evidence of lymph node metastasis, and the researchers conclude