CALCA and medullary thyroid gland carcinoma: The spindle cell variant of a MTC must be recognized and distinguished from other entities with spindle cell morphology that may affect the thyroid gland, such as spindle cell atypical thyroid adenoma, spindle epithelial tumor with thymus-like differentiation, poorly differentiated carcinoma, and anaplastic carcinoma, which are negative for neuroendocrine markers and calcitonin on immunohistochemistry.