Sentinel lymph node mapping in colorectal cancer--a review.

Colon and rectal cancers are the leading causes of morbidity and mortality from all gastrointestinal malignancy, worldwide. In the United States, colorectal cancer is the third most common cause of cancer-related mortality. It is estimated that 133,200 new cases will be diagnosed in the year 2000, and approximately 56,500 people will die of the disease. 10 As in most solid neoplasms, the presence of regional lymph node metastasis in apparently localized colorectal cancer is the most significant risk factor for the development of local-regional and distant metastatic recurrences. Adjuvant systemic therapy has now been demonstrated to reduce the risk of premature death by approximately 30% in node positive individuals. 4 It is apparent that the accurate pathologic evaluation of the regional lymphatics is crucial not only for proper prognostication but for therapeutic planning as well. Over the last two decades, a variety of pathologic techniques have been described to improve the accuracy of the staging of the regional lymph nodes in colorectal cancer, including immediate nodal harvesting, fat clearing technique, 22 pinning, and stretching methods. 6 These techniques are designed to reduce the incidence of missed regional lymph nodes at the time of gross examination, to increase the total number of lymph nodes examined per case, and potentially to stage more accurately the individual. Wong and coworkers 23 suggested that a minimum of 14 nodes needs to be examined to accurately stage the regional lymph nodes in the mesentery. It is well known that only a fraction of the nodes resected usually is submitted for histopathologic evaluation. A more detailed analysis of the regional nodes by multilevel serial sectioning 17 can increase the identification of metastatic disease. This has resulted in a number of newer methods to detect micrometastasis, including immunohistochemistry 7,9 and molecular analysis by reverse transcriptase and polymerase chain reaction (RT-PCR). 15 These techniques have increased the number of patients with nodal micrometastasis. Despite the effectiveness of these approaches in identifying patients with nodal metastasis, these methods remain labor intensive, and the costs are prohibitive. The techniques of sentinel lymph node (SLN) mapping as described by Morton et al 16 in melanoma, and Giuliano et al 8 in breast cancers have been used to identify the first lymph node(s) that receive lymphatic drainage from the primary tumor site. These lymph node(s) have been termed sentinel lymph node(s). The sentinel lymph node(s) are the node(s) at greatest risk for harboring metastatic disease when present. The status of SLNs also reflects the status of the remainder of the nodal basin, with >95% accuracy in most published series, for melanoma and breast cancers. 5,18 % Similar techniques have been adopted for other solid tumors in many institutions. 2 This article reviews the technique of SLN mapping as it applies to colorectal cancer and summarizes the authors' personal experience with this technique, including its indications, limitations, benefits, implications, and future directions.