CHAPTER 47: CARCINOMA OF UNKNOWN PRIMARY

Carcinoma of unknown primary (CUP) is a biopsy-proven (mainly epithelial) malignancy for which the anatomic site of origin remains unidentified after an intensive search. CUP is one of the 10 most frequently diagnosed cancers worldwide, accounting for approximately 3–5% of all cancers. Most investigators do not include lymphomas, metastatic melanomas, and metastatic sarcomas that present without a known primary tumor as CUP because these cancers have specific stage- and histology-based treatments that guide management.

With the increasing availability of additional sophisticated imaging, invasive diagnostic techniques, and the emergence of effective targeted therapies in several cancers, an individualized management algorithm with an impact on quality of life and survival is critical. The reasons cancers present as CUP remain unclear. One hypothesis is that the primary tumor either regresses after seeding the metastasis or remains so small that it is not detected. It is possible that CUP falls on the continuum of cancer presentation in which the primary has been contained or eliminated by the natural body defenses. Alternatively, CUP may represent a specific malignant event that results in an increase in metastatic spread or survival relative to the primary. Whether the CUP metastases truly define a clone that is genetically and phenotypically unique to this diagnosis remains to be determined.

No characteristics that are unique to CUP relative to metastases from known primaries have been identified. Abnormalities in chromosomes 1 and 12 and other complex cytogenetic abnormalities have been reported. Aneuploidy has been described in 70% of CUP patients with metastatic adenocarcinoma or undifferentiated carcinoma. The overexpression of various genes, including Ras, bcl-2 (40%), her-2 (11%), and p53 (26–53%), has been studied in CUP samples, but they have no effect on response to therapy or survival. The extent of angiogenesis in CUP relative to that in metastases from known primaries has also been evaluated, but no consistent findings have emerged.

CLINICAL EVALUATION

Obtaining a thorough medical history from CUP patients is essential, paying particular attention to previous surgeries, removed lesions, and family medical history to assess potential hereditary cancers. Physical examination, including a digital rectal examination in men and breast and pelvic examinations in women, should be performed. Determining the patient's performance status, nutritional status, comorbid illnesses, and cancer-induced complications is essential since these may affect treatment planning.

Role of serum tumor markers and cytogenetics

Most tumor markers, including CEA, CA-125, CA 19-9, and CA 15-3, when elevated, are nonspecific and not helpful in determining the primary tumor site. Men who present with adenocarcinoma and osteoblastic metastasis should undergo a prostate-specific antigen (PSA) test. In patients with undifferentiated or poorly differentiated carcinoma (especially with a midline tumor), elevated β-human chorionic gonadotropin (βhCG) and α fetoprotein (AFP) levels suggest the possibility of an extragonadal germ cell (testicular) tumor. Cytogenetic studies had a larger role in the past, although interpretation of these older studies can be challenging. In our opinion, with the availability of immunohistochemical stains, cytogenetic analyses are indicated only occasionally. We reserve them for undifferentiated neoplasms with inconclusive immunohistochemical stains and those for which a high suspicion of lymphoma exists.

Role of imaging studies

Chest radiographs are always obtained in CUP workups but findings are often negative, especially with low-volume disease. A computed tomography (CT) scan of the chest, abdomen, and pelvis is indicated in the search for the primary, evaluate the extent of disease, and select the most favorable biopsy site. Older studies suggested that the primary tumor site is detected in 20–35% of patients who undergo a CT scan of the abdomen and pelvis, although by current definition these patients would not be considered as having CUP. Older studies also suggest a latent primary tumor prevalence of 20%; with more sophisticated imaging, this prevalence is <5% today.

Mammography should be performed in all women who present with metastatic adenocarcinoma, especially in those with adenocarcinoma and isolated axillary lymphadenopathy. Magnetic resonance imaging (MRI) of the breast is a recognized follow-up modality in patients with suspected occult primary breast carcinoma following negative mammography and sonography results. The results of these imaging modalities can influence surgical management; a negative breast MRI result predicts a low tumor yield at mastectomy.

A conventional workup for a squamous cell carcinoma and cervical CUP (neck lymphadenopathy with no known primary tumor) includes a CT scan or MRI and invasive studies, including indirect and direct laryngoscopy, bronchoscopy, and upper endoscopy. Ipsilateral (or bilateral) tonsillectomy (with histopathology) has been recommended for these patients. Fluorodeoxyglucose positron emission tomography (FDG-PET) scans are useful in this patient population and may help guide the biopsy; determine the extent of disease; facilitate the appropriate treatment, including planning radiation fields; and help with disease surveillance. A smaller radiation field encompassing the primary (when found) and metastatic adenopathy decreases the risk of chronic xerostomia. Several studies have evaluated the utility of positron emission tomography (PET) in patients with cervical CUP. These trials have included a small number of patients; primary tumors were identified in ~21–30%.

The diagnostic contribution of PET to the evaluation of other CUP (outside of the neck adenopathy indication) is controversial. PET-CT can be helpful for patients who are candidates for surgical intervention for solitary metastatic disease because the presence of disease outside the primary site may affect surgical planning.

Invasive studies, including upper endoscopy, colonoscopy, and bronchoscopy, should be limited to symptomatic patients or those with laboratory, imaging or pathologic abnormalities that suggest that these techniques will result in a high yield in search for a primary cancer.

PATHOLOGIC DIAGNOSIS OF CARCINOMA OF UNKNOWN PRIMARY

A detailed pathologic examination of the most accessible biopsied tissue specimen is mandatory in CUP patients. Pathologic evaluation typically consists of hematoxylin-and-eosin stains and immunohistochemical tests. Electron microscopy and cytogenetics are rarely useful.

Light microscopy evaluation

Adequate tissue obtained by fine-needle aspiration or core-needle biopsy should first be stained with hematoxylin and eosin and subjected to light microscopic examination. On light microscopy, 60–65% of CUPs are adenocarcinoma, and 5% are squamous cell carcinoma. The remaining 30–35% are poorly differentiated adenocarcinoma, poorly differentiated carcinoma, poorly differentiated neoplasm. A small percentage of lesions are diagnosed as neuroendocrine cancers (2%), mixed tumors (adenosquamous, or sarcomatoid carcinomas), or undifferentiated neoplasm (Table 47-1).

Role of immunohistochemical analysis

Immunohistochemical stains are peroxidase-labeled antibodies against specific tumor antigens that are used to define tumor lineage. The number of available immunohistochemical stains is ever-increasing. However, in CUP cases, more is not necessarily better, and immunohistochemical stains should be used in conjunction with the patient's clinical presentation and imaging studies to select the best therapy. Communication between the clinician and pathologist is essential. No stain is 100% specific, and overinterpretation should be avoided. PSA and thyroglobulin tissue markers, which are positive in prostate and thyroid cancer, respectively, are the most specific of the current marker panel. However, these cancers rarely present as CUP, so the yield of these tests may be low. Figure 47-1 delineates a simple algorithm for immunohistochemical staining in CUP cases. Table 47-2 lists additional tests that may be useful to further define the tumor lineage. A more comprehensive algorithm may improve the diagnostic accuracy but can make the process complex. With the use of immunohistochemical markers, electron microscopic analysis, which is time-consuming and expensive, is rarely needed.

There are >20 subtypes of cytokeratin (CK) intermediate filaments with different molecular weights and differential expression in various cell types and cancers. Monoclonal antibodies to specific CK subtypes have been used to help classify tumors according to their sites of origin; commonly used CK stains in CUP are CK7 and CK20. CK7 is found in tumors of the lung, ovary, endometrium, and breast and not in those of the lower gastrointestinal tract, whereas CK20 is normally expressed in the gastrointestinal epithelium, urothelium, and Merkel cells. CK20+/CK7– strongly suggests a primary tumor of the colon; 75–95% of colon tumors show this pattern of staining. CK20–/CK7+ suggests cancer of the lung, breast, ovary, endometrium, and pancreaticobiliary tract; some of these can also be CK20+. The nuclear CDX-2 transcription factor, which is the product of a homeobox gene necessary for intestinal organogenesis, is often used to aid in the diagnosis of gastrointestinal adenocarcinomas.

Thyroid transcription factor 1 (TTF-1) is a 38-kDa homeodomain-containing nuclear protein that plays a role in transcriptional activation during embryogenesis in the thyroid, diencephalon, and respiratory epithelium. TTF-1 nuclear staining results are typically positive in lung and thyroid cancers. Approximately 68% of adenocarcinomas and 25% of squamous cell lung cancers stain positive for TTF-1, which helps differentiate a lung primary tumor from metastatic adenocarcinoma in a pleural effusion, the mediastinum, or the lung parenchyma.

Distinguishing pleural mesothelioma from lung adenocarcinoma can be challenging. Calretinin, Wilms' tumor gene-1 (WT-1), and mesothelin have been suggested as useful markers for mesothelioma.

Gross cystic disease fibrous protein-15, a 15-kDa monomer protein, is a marker of apocrine differentiation that is detected in 62–72% of breast carcinomas. UROIII, high-molecular-weight cytokeratin, thrombomodulin, and CK20 are the markers used to diagnose lesions of urothelial origin.

ROLE OF DNA MICROARRAY AND REVERSE TRANSCRIPTASE POLYMERASE CHAIN REACTION IN CARCINOMA OF UNKNOWN PRIMARY

In the absence of a known primary, developing therapeutic strategies for CUP is challenging. The current diagnostic yield with imaging and immunochemistry is ~20–30% for CUP patients. The use of gene expression studies holds the promise of substantially increasing this yield. Gene expression profiles are most commonly generated using quantitative reverse transcriptase polymerase chain reaction (RT-PCR) or DNA microarray.

Neural network programs have been used to develop predictive algorithms from the gene expression profiles. Typically, a training set of gene profiles from known cancers (preferably from metastatic sites) are used to train the software. The program can then be used to predict the putative origin of a test tumor and presumably of true CUP. Comprehensive gene expression databases that have become available for common malignancies may also be useful in CUP. Investigators have used expression data from normal differentiated tissues to identify conserved expression profiles found in malignant tissue as a basis for predicting the tissue of origin (ToO). These approaches have been effective in blind testing against known primary cancers and their metastasis. However, because, by definition, the primary tumor site is not identifiable in CUP, validation of site prediction in this setting can be challenging, and any predictions currently must be supported by clinical and pathologic correlation. Prospective validation trials are currently evaluating the role of molecular studies identifying ToO in CUP and its impact on management. Early trials suggest that the profiling approach is feasible from archived formalin-fixed paraffin-embedded (FFPE) core-needle biopsies. Quantitative RT-PCR on fine-needle aspiration samples is very useful in clinical practice. Data from some studies suggest that a putative primary profile can be applied to 80–85% of cases. At present, the best confirmation of molecular profiling studies in CUP is an indirect validation based on the patient's presentation and clinical course. Current studies are geared to understanding profiling tools' accuracy, clinical effectiveness, and how these assays complement immunohistochemistry and help guide therapy.

Patients with CUP should undergo a directed diagnostic search for the primary tumor on the basis of clinical and pathologic data. Subsets of patients have prognostically favorable disease, as defined by clinical or histologic criteria, and may substantially benefit from aggressive treatment and expect prolonged survival. However, for most patients who present with advanced CUP, the prognosis remains poor, with early resistance to available cytotoxic therapy. The current focus has shifted away from empirical chemotherapeutic trials to understanding the metastatic phenotype, ToO profiling, and evaluating molecular targets in CUP patients. Our understanding of this challenging disease is growing steadily in the era of sophisticated diagnostics and therapeutics. A strategy that integrates information from a patient's clinical presentation including imaging and pathology with directed immunohistochemistry and molecular profiling in selected cases will help us select the best treatment for our patients.