Reasons for Separate Classification

Doubling Time:
Small cell carcinoma has a doubling time which is several fold higher than non-small carcinoma. This results in an increased growth fraction of the tumor which causes a significantly increased metastatic potential. Thus, patients with small cell carcinoma are more likely to present with systemic disease.
Paraneoplastic Syndromes:
Small cell carcinomas are much more likely to produce paraneoplastic syndromes than other types of lung carcinoma. This is felt to be due to the biochemical properties of the cells.
The therapy for small cell carcinoma is different. Small cell has shown an increased sensitivity to chemotherapy over other types of bronchogenic carcinoma. In non-small cell carcinoma, surgical intervention is the primary form of treatment.