MSH: An anaplastic, highly malignant, and usually bronchogenic carcinoma composed of small ovoid cells with scanty neoplasm. It is characterized by a dominant, deeply basophilic nucleus, and absent or indistinct nucleoli. (From Stedman, 25th ed; Holland et al., Cancer Medicine, 3d ed, p1286-7),NCI: A neuroendocrine carcinoma composed of small malignant cells which are often said to resemble "oat cells" under the microscope. Small cell carcinoma most often affects the lungs. Clinically, this is often a rapidly growing cancer that spreads to distant sites early.