Metastatic cancer of unknown primary: diagnostic challenges

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Fig. 3. K-nearest-neighbor (KNN) from Rosetta Cancer Origin Test (courtesy of Rosetta Genomics). The full KNN analysis compares the expression across all 64 microRNAs used for the test on the 1,282 training samples. The majority diagnosis among the nearest five samples is determined.
This figure shows a simplified version of KNN results in three-dimensional space. Only the expression of three microRNAs (instead of 64 microRNAs) and only the most likely differential diagnoses (instead of all 1,282 tumors in the training set) are shown. Our patient sample is depicted as a blue dot. The five nearest neighbors in the complete training set are marked with a black dot on top of the symbol representing the reference diagnosis. Four of the nearest neighbors were urothelial carcinomas, and one of the nearest neighbors was a biliary tract cancer. (TCC: urothelial carcinoma, SqCC: squamous cell carcinoma, hsa-miR-205, hsa-miR-375, hsa-miR-934: three microRNAs selected for this image.)

Molecular expression profiles based on RT-PCR or microarray analysis of tumor mRNA or microRNA have been developed in an attempt to streamline diagnosis and create a more objective diagnostic test. These tests are validated to generate a molecular profile of the metastasis or unknown cancer and identify a tissue of origin by comparing the molecular profile of the “unknown” with the profiles of previously established known tumors.⁴ Several such tests are commercially available.

Our case illustrates the potential of molecular testing to identify the origin of a CUP when other testing such as imaging and traditional histopathology fails to give a clear diagnosis. Based on the patient’s history, metastatic urothelial carcinoma was in the differential diagnosis, but other primary tumors that are more likely to cause lytic bone metastases could not be ruled out. Another challenge was that no biopsy of the metastases could be obtained during the lifetime of this patient. Even the autopsy results (gross tumor morphology, light microscopy, and immunohistochemical staining) were inconclusive for a cancer of origin, and urothelial cancer was not suspected based on histomorphology. The microRNA-based tumor profile, however, unambiguously identified urothelial carcinoma as the primary origin, demonstrating the utility of such an assay not only for identifying the tumor of origin in typical CUP but also for revealing a primary tumor that is not considered during the initial immunohistochemical workup of a tumor.

Fig. 4. Based on the Cancer Origin Test results, additional stains for GATA3 (A) and uroplakin (B) were performed and were positive, confirming the molecular identification of this tumor as urothelial carcinoma.

In retrospect, a small biopsy of sufficient size for molecular tumor profiling could possibly have been obtained, since molecular CUP tests require less tissue compared with what may be needed for a comprehensive IHC workup. The impact of identifying the primary in a patient with CUP was demonstrated in a recent prospective study that used a real-time RT-PCR assay to determine the tumor of origin in 247 patients with cancer of unknown primary. In the study, 194 patients received assay-directed, site-specific treatment. Median survival time for these patients was found to be 12.5 months,⁸ which compares favorably to nine months for CUP patients treated empirically.⁷ While this study suggests the potential of novel molecular diagnostic tests to direct treatment of cancers of unknown primary, there are relatively few studies of its kind, and no one molecular assay for the identification of tumor origin has been adopted as standard of care in clinical practice. In addition, two prospective blinded studies suggest superior accuracy of molecular CUP testing compared with extended IHC panels,^9,10 and turnaround times of five to 10 days for the molecular profiles should fulfill the clinical need in conjunction with traditional pathology testing.

Conclusions. A microRNA-based molecular assay for cancer of unknown primary correctly identified urothelial carcinoma as the primary cancer in this patient with widely metastatic disease. Such tests can be performed on small biopsies and help guide patient management in conjunction with traditional pathology/IHC and imaging studies. In particular, patients who are found to have a tissue of origin that is known to be responsive to site-specific treatment will be expected to achieve longer survival times compared with empiric treatment for CUP.

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Erica Schramm is a fourth-year medical student at Cooper Medical School at Rowan University, where Dr. Kocher is chair and associate professor of biomedical sciences and Dr. Edmonston is associate professor of pathology. Dr. Edmonston is director of molecular pathology, Department of Pathology and Laboratory Medicine, Cooper University Hospital, Camden, NJ.