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Occult Primary      (see NCCN on occult)

Occult primary tumors are defined as histologically proven metastatic malignant tumors whose primary site cannot be identified during pretreatment evaluation. Even after postmortem examination, the primary tumor is not identified in 20%-50% patients. Occult primary tumors, or cancers of unknown primary site, account for 5% to 10% of all diagnosed cancers. These tumors are manifested by a wide variety of clinical presentations. For most patients with occult primary tumors, the disease is incurable. Patients with cancer of unknown primary site demonstrate common characteristics and present with general complaints such as anorexia, weight loss, etc.

Multiple sites of involvement are shown in more than 50% of patients with occult primary tumors. The common sites of involvement are the liver, lungs, bones, and lymph nodes. Clinical absence of primary tumors, early dissemination, aggressiveness, and unpredictability of metastatic pattern are characteristic of these tumors. Life expectancy is very short with a median survival of about 6-9 months. Patients with occult primary tumors often have chromosomal abnormalities in the short arm of chromosome 1. Abnormalities in the p53 gene have been identified in more than 70% of patients with unknown primary tumors.

Patients with occult primary tumors may present with favorable or unfavorable sets of prognostic signs. The following features are proposed as the favorable prognostic factors: poorly differentiated carcinoma with midline distribution; women with papillary adenocarcinoma of peritoneal cavity; women with adenocarcinoma involving only axillary lymph nodes; squamous cell carcinoma involving cervical lymph nodes; isolated inguinal adenopathy (squamous carcinoma); etc.

Unfavorable features include: adenocarcinoma metastatic to the liver or other organs; non-papillary malignant ascites (adenocarcinoma); multiple cerebral metastases (adeno- or squamous carcinoma); multiple lung/pleural metastases (adenocarcinoma); multiple metastatic bone disease (adenocarcinoma).

There are two widely held misconceptions regarding occult primaries. First, both patients and physicians think that if a primary cancer is found, an effective chemotherapy can be identified, which will significantly improve survival. In fact, for most solid tumors that have metastasized, chemotherapy is only palliative and does not significantly improve long-term survival. Secondly, the statement “common things are common,” suggests that knowing the patterns of metastases will identify the primary sites. While it is true that certain patterns of metastases suggest possible primaries, occult primaries can metastasize to any site. Therefore, one should not rely on patterns of metastases to determine the primary site.

Occult primary tumors occur equally in men and women, usually in the sixth decade of life. A primary tumor site is found in fewer than 30% of patients who present initially with an occult primary tumor. At presentation, half of patients with an occult primary tumor have multiple sites of involvement such as lymph nodes, lung, bone, liver, pleura, and the brain. Poor prognostic factors associated with an occult primary tumor include male gender, a pathologic diagnosis of adenocarcinoma, the presence of hepatic involvement, and an increasing number of involved organ sites.

Most of these patients are treated with chemotherapy... radiation is useful for areas of symptomatic metastases (see here) and is commonly used where the source is thought to be the breast (see here) and on lymph node metastases to the neck (cervical) nodes (see here) or inguinal (groin) nodes (see here.) see review of squamous cancers here.