Cancers of unknown primary site (CUPs) is a generic term for a group of metastatic tumors in which the location of the primary cancer cannot be determined by conventional diagnostic methods. Cancers of unknown primary site account for approximately 3-5% of all malignant tumors. No biological features common to this type of tumor have been identified. However, recent evidence has shown that even in the case of a resting primary tumor, multiple factors such as metastatic qualities of the tumor cells themselves, location-specific transformation of circulating tumor cells, and oncogenic induction at the site of metastasis may contribute to metastatic spread of the tumor. Diagnosis The diagnosis of cancer with an unknown primary site requires the aid of pathological evaluation. Based on the pathological features, cancers of unknown origin can be classified as: (1) highly and moderately differentiated adenocarcinoma, (2) poorly differentiated carcinoma (including poorly differentiated adenocarcinoma), (3) squamous cell carcinoma, (4) undifferentiated tumor, and (5) neuroendocrine differentiated carcinoma. Immunohistochemistry is primarily used to identify tissues of origin for cancers of unknown primary focus and to exclude chemosensitive and curable tumors (e.g., lymphomas and germ cell tumors) (Figure 1). If the findings confirm a diagnosis of cancer or adenocarcinoma, immunologic staining for prostate-specific antigen (PSA) is recommended for male patients or for female patients with axillary lymph node metastases for estrogen and progesterone receptor immunologic staining to rule out hormone-sensitive tumors and for specific treatment. Keratin (keratins) CK7 and CK20 staining may provide information on the location of primary cancer foci, and chromogranin A (chromogranin A) and synaptophysin staining are primarily used to determine neuroendocrine differentiation (Table 1). Gene expression analysis assays have now entered the clinical medical market and are mainly used to determine the tissue of origin of cancers of unknown primary focus. These experiments help to determine the potential site of primary tumorigenesis in patients. However, it is not known whether primary tumor-specific therapeutic modalities are effective against metastases and further validation remains to be done. Table 1 Routine immunohistology of biopsies from patients with cancer of unknown origin Figure 1 Routine immunohistology of biopsies from patients with cancer of unknown origin Staging and risk assessment Cancer of unknown origin is usually classified as a metastatic tumor, and patients with this type of cancer generally present with a poor prognosis. However, appropriate diagnostic studies can screen out some patients with cancer of unknown primary focus, thus benefiting patients in direct treatment. The following recommendations outline routinely feasible evaluation protocols. (1) Complete physical examination (including head and neck, rectal, pelvic, and breast examinations), routine blood and biochemical tests, and CT scans of the chest, abdomen, and pelvis [IV, B]. (2) Endoscopy should follow the principle of signs, symptoms or laboratory abnormalities. Serologic testing for alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), plasma chromogranin A test, and prostate-specific antigen (PSA) is recommended for male patients to determine the presence or absence of extragonistic germ cell tumors, neuroendocrine tumors, and prostate cancer, and for endocrine therapy. Table 2 Diagnostic and staging guidelines for cancer of unknown primary site Patients with cancer of unknown primary site can be classified into different subtypes based on clinical and pathological features (Table 2). A small number of patients (15-20%) belong to the clinicopathologic subtype and are associated with a good prognosis. These patients generally have chemosensitive and curable tumors, and disease progression can be well controlled with the help of appropriate multidisciplinary therapy. However, the majority of patients with cancer of unknown origin (80-85%) do not belong to this particular subtype. These patients have a moderate sensitivity to treatment and median overall survival is generally shorter than 1 year (6 to 10 months). Among patients with cancer of unknown origin, there are two prognostic groups: (1) those with good status (0-1) and normal lactate dehydrogenase (LDH) levels, with a median survival of 1 year; and (2) those with good status (0-1) or normal lactate dehydrogenase (LDH) levels, with a median survival of only 4 months. Figure 2 Clinical control of patients with cancer of unknown primary focus Recommendations for clinical control of patients with cancer of unknown primary focus generally include identification of specific subtypes, exclusion of cancers of unknown non-primary focus, and use of prognostic parameters in clinical practice (Figure 2). Guidelines for the diagnosis and staging of patients with cancer of unknown primary focus (Table 2) CT/FDG-PET can be used for clinical control of patients with cancer of unknown primary focus, particularly in patients with cervical adenopathy and single metastases. Treatment Treatment regimen design should be based on the clinicopathologic subtype and prognostic characteristics of the patient. 10-15% of patients with cancer of unknown primary site should be selected for the same treatment option as patients with cancer of known primary site, i.e., metastasectomy [III, B]. 30-60% of patients can achieve long-term disease control, with clinical control being the most critical for long-term patient survival (Table 3). Retrospective analysis showed that the clinical behavior, biological characteristics, and treatment response and outcomes of patients with cancer of unknown favorable risk primary did not differ significantly from those of patients with cancer of known primary. Table 3 Treatment options for patients with cancer of unknown favorable risk primary site Clinical studies have found that the prognosis of patients with cancer of unknown low-risk primary site is not promising despite receiving multiple combinations of chemotherapy. A recent meta-analysis study showed no significant advantage of platinum salts, paclitaxel or novel cytotoxic compounds (gemcitabine, perillyl alkaloids or irinotecan) in the treatment of patients with cancer of unknown primary site. A recent randomized prospective phase III study comparing gemcitabine/irinotecan with paclitaxel/carboplatin/oral etoposide in patients with low-risk cancers of unknown origin showed low toxicity and no significant difference in survival between the two treatment regimens. In addition, another randomized phase II study found that the efficacy/toxicity ratio of the cisplatin- gemcitabine regimen was superior to that of the cisplatin- irinotecan regimen. Currently, the only major therapeutic goals for patients with cancer of unknown primary origin are: modest prolongation of survival with symptomatic relief and improvement of quality of life. Therefore, low-toxicity chemotherapy regimens should be the first choice for this type of low-risk patients. The suitability of targeted agents for patients with cancer of unknown primary focus is not known. Retrospective data show that tumors with “colorectal cancer” confirmed by immunohistochemistry or molecular characterization experiments are therapeutically responsive to colorectal site-specific agents (oxaliplatin or irinotecan) [IV, B]. The sample size of these experimental data is very low and still needs to be validated. Table 4 Commonly used low-toxicity palliative chemotherapy regimens for patients with low-risk cancers of unknown primary foci In patients with suspected primary tumor loci identified by immunohistochemistry or microarray studies in clinical trials, a combination of chemotherapy and targeted agents or site-specific therapy is recommended. Common chemotherapy regimens for patients with low-risk cancers of unknown primary site are shown in Table 4. Treatment response assessment A separate test for treatment response assessment is recommended after two or three rounds of chemotherapy. In particular, patients with low-risk but over-treated cancers of unknown primary focus should be given special attention to their quality of survival. There is no evidence yet that follow-up of asymptomatic patients is necessary. However, patients are advised to undergo specific screening based on clinical symptoms.