Occult primary: Clinical practice guidelines in oncology

Occult primary tumors, or cancers of unknown primary, account for 5% to 10% of all diagnosed cancers, and are manifested by a wide variety of clinical presentations, while conferring a poor prognosis for most patients. Even after postmortem examination, the primary tumor is not identified in 20% to 50% of patients. Multiple sites of involvement are observed in more than 50% of patients. Although certain patterns of metastases suggest possible primaries, occult primaries can metastasize to any site. In most patients, occult primary tumors are refractory to systemic treatments, and chemotherapy is only palliative and does not significantly improve long-term survival. However, special pathologic studies can identify subsets of patients with tumor types that are more responsive. Treatment options should be individualized for this selected group to achieve improved response and survival rates. Important updates for the NCCN guidelines include the additions of tables on tumor-specific markers and their staining pattern as well as analysis of undifferentiated carcinoma. For the most recent version of the guidelines, please visit NCCN.org