Image Quiz: Cytology-A subcutaneous mass in a senior dog

Mast cell tumor is correct!

The key characteristic of these cells is cytoplasmic granules that are purple (metachromatic) and similar to the color of a properly stained nucleus. The presence of granules will sometimes interfere with nuclear staining, leaving the nuclei pale-blue instead of purple. The fried egg appearance of these cells, with a moderate amount of cytoplasm encircling the nucleus, can also be a valuable clue, particularly when the mast cells are poorly granular. The apparent poor granularity of many of these mast cells could be an artifact caused by the use of a rapid-acting dip-type stain, or it could reflect the fact that the neoplastic cells are poorly differentiated (i.e. perhaps a high-grade mast cell tumor). Excision of the mass, with histologic evaluation, is required for proper tumor grading.