A reappraisal of limited-stage undifferentiated carcinoma of the lung. Does Stage I small cell undifferentiated carcinoma exist?

Patients with small cell undifferentiated carcinoma of the lung (SCUC) have a poor prognosis. Surgical excision is avoided if the diagnosis can be made with small biopsy specimens or cytologic preparations. We reviewed 323 consecutive patients with pulmonary neoplasms diagnosed as SCUC, oat cell carcinoma, and undifferentiated or poorly differentiated carcinoma. At the time of diagnosis, only 18 patients had neoplasms classified as clinical Stage I, and only 1 of these had SCUC after histologic review. Fifteen patients had atypical carcinoid, a tumor with features intermediate between ordinary bronchial carcinoid and SCUC. In 2 instances, there was insufficient tissue for definitive diagnosis. Cumulative survival of the 15 patients with Stage I atypical carcinoid tumor was 80% at 1 year and 60% at most recent follow-up (mean follow-up 20 months). Mean survival for the 305 remaining patients was 7.9 months. Atypical carcinoid may be misdiagnosed as SCUC or poorly differentiated carcinoma, particularly with limited tissue samples or cytologic preparations. Stage I ASCUC exists but is exceedingly rare. Many examples of purported Stage I SCUC probably represent atypical carcinoid. Because atypical carcinoid has a far better prognosis than SCUC, precise diagnosis is important and surgical resection should be considered.