XXXX of unknown origin.

@entity1 @entity706 @entity2971 @entity5 @entity900 @entity2 @entity189 @entity3 @entity420 @entity4304
@entity3 of unknown origin (MUO) constitutes between 5% and 10% of all noncutaneous @entity5 . An MUO is defined as a @entity3 for which the site of origin is not suggested by thorough history, physical examination, chest x-ray studies, routine blood and urine studies, and histologic evaluation. Two major groups of MUO can be defined: MUO to lymph nodes only (N1-3), and MUO to visceral sites. The prognosis may be quite good for @entity1 with MUO limited to lymph nodes in the mid to high cervical, axillary, and groin areas. However, MUO in other lymph node areas is far more serious, with the possible exception of that in @entity1 with a new syndrome, i.e., "advanced poorly differentiated @entity189 ." There is some suggestion that these @entity1 may respond to @entity900 -based combination chemotherapy. @entity1 with MUO to visceral sites have a poor prognosis. However, @entity3 from some @entity5 are sensitive to chemotherapy and a limited search for these @entity5 should be undertaken. These @entity5 include @entity706 , @entity2971 , small cell @entity2 , @entity420 of the breast, ovary, endometrium, thyroid, or prostate, and possibly @entity4304 . We start by reviewing the biochemical events of @entity3 that may be targets for therapy. The importance of a correct tissue diagnosis is then considered, including the role of standard histochemistry, electron microscopy, enzyme histochemistry, and immunohistochemistry. The relatively limited value of radiologic tests in localizing the primary site of origin of the @entity5 is emphasized, as well as the limited role of currently available biomarkers. We conclude by discussing the treatment of each of the subtypes of MUO.
