If you haven’t seen the movie Fletch, you must stop reading this right now and go rent it. Chevy Chase plays a newspaper reporter who “changes his identity more often than he changes his underwear.” One of his characters is Dr. Rosenpenis (all the doctors in the hospital are named Rosen-something: Rosenbaum, Rosenkrantz, etc.). Looking for health information on a man he is investigating, Dr. Rosenpenis finds his way to the medical records department, where he informs the nurse that he needs to see the chart on the guy because some other doctor told him the patient had “melanoma…carcinoma…some kind of -noma.”

In a way, Chevy Chase’s flippant comment is a good distillation of tumor nomenclature. Most tumors do end in “-oma.” But, as you might expect, it’s a little more complicated than that.

Benign tumors, in general, are designated by placing the -oma designation at the end of the cell type. So: an adenoma is a benign tumor arising from glandular cells (adeno- means glandular), a leiomyoma is a tumor arising from smooth muscle cells (leio- means smooth, myo- means muscle), and a chondroma is a benign tumor arising from chondrocytes. There are certain benign tumor names that don’t follow this rule. A papilloma is a benign tumor that has finger-like projections, a polyp is a benign tumor that projects upward, forming a lump, and a cystadenoma is a benign tumor that has hollow spaces (cysts) inside.

Malignant tumors also usually have the -oma designation at the end. However, there are a couple additions depending on the type of tissue in which the malignancy arises. Malignancies that arise in epithelial tissue (skin, glands) are designated carcinomas, whereas malignancies that arise in mesenchymal tissue (bone, cartilage, muscle, blood vessels, etc.) are designated sarcomas. So: an adenocarcinoma is a malignant tumor of glandular cells, a squamous cell carcinoma is a malignant tumor arising in squamous cells, a chondrosarcoma is a malignant tumor arising in chondrocytes, an angiosarcoma is a malignant tumor arising in blood vessels, and a leiomyosarcoma is a malignant tumor arising in smooth muscle cells.

Of course, there have to be some malignancies that don’t follow the rules. Lymphoma is a malignant tumor arising in lymphocytes, mesothelioma is a malignant tumor arising in mesothelial cells, melanoma is a malignant tumor arising in melanocytes, and seminoma is a malignant tumor arising in sperm cell precursors. All of these names sound benign, but they are definitely not.

Finally, there are some exceptions that are just plain weird. There are non-tumors that sound like tumors. For example, a hamartoma is not a tumor, but a  mass of disorganized indigenous tissue. A choristoma is not a tumor, but heterotopic rest of cells. Some tumor names seem to come out of nowhere: a nevus is a benign tumor of melanocytes, leukemia is a malignant tumor of white blood cells, and a hydatidiform mole is a benign tumor of chorionic villi.