Doctors classify cancer cells by examining them under a microscope. Different cancers have different appearances. Classifying a cancer based on its microscopic appearance helps your doctor determine:

• The course or outcome (prognosis) of the cancer
• The most effective treatment for a specific cancer

Common terms used to describe the appearance of cancer cells include:

• Clear cell (the inside of the cells appears clear)
• Spindle cell (spindle-shaped cells)
• Large cell (the cells are larger than normal cells)
• Small cell (the cells are smaller than normal cells)
• Squamous (the cells are flat in appearance)
• Adenocarcinoma (the cells are gland-like in appearance)

Other factors that help classify a cancer include:

• Area of the body in which the cancer originated, such as the liver or breast. Cancers from certain organs may have a similar appearance. For example, the most common type of kidney cancer is classified as clear cell. On the other hand, breast cancer rarely has a clear cell appearance. So clear cells on a breast biopsy may indicate that the cancer didn't originate from the breast but perhaps spread (metastasized) from another area of the body, such as the kidneys.

• Type of tissue from which the cancer evolved, including carcinomas and sarcomas. Carcinoma is a cancer that begins in the skin or in tissues that line or cover internal organs. Sarcoma is a cancer of the bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue.