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Signature ✍️
- “Cancer of unknown primary site” refers to a histologically confirmed metastatic cancer with the primary site of origin eluding detection despite a standard diagnostic work-up
- While this diagnostic dilemma is a problem for only ~2-4% of all cancer patients, survival outcomes are generally worse than in metastatic cancers with known primaries
- The reason for undetectable primary is still unknown but hypothesized to be due to rapid, early dissemination of a primary tumor that is either very small or that has regressed below the threshold of radiographic detection
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DDx 🏳️🌈
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Principles❗️
The Diagnostic Arc ⛰️
- As a diagnosis of exclusion, “cancer of unknown primary site” bears resemblance to the “fever of unknown origin” DDx (i.e. both require a systematic, first-pass work-up for cryptic cancers prior to landing in that diagnostic space)
- The implication of this is patients with newly discovered metastatic disease require a standard cancer work-up (CT torso with IV contrast) as well as additional more sensitive testing based on the patient’s clinical presentation (see “cryptic cancers” framework)
Final Steps 👣
- Once due diligence has been taken, a best effort can be made to classify the cancer through serial immunostaining (see DDx) & molecular profiling
Epidemiology 🌎
- Adenocarcinoma is most common histologic subtype, accounting for ~70-80% of causes, which is followed by poorly-differentiated carcinomas
- Autopsy series have identified the lung, pancreas, GI tract, & urogenital tract as the most common hidden primary sites
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