mast cell tumor
Meaning of mast cell tumor: Overwhelmingly a diagnosis in dogs; the human counterpart is usually a harmless skin lesion of infancy rather than a cancer.
Definition of mast cell tumor
A mast cell tumor is a growth made up of mast cells, the tissue-dwelling immune cells packed with granules of histamine and other irritant chemicals. In dogs it is the commonest skin tumor and behaves as a true cancer. In people the equivalent lesion is a mastocytoma, usually a single soft brown patch or lump on an infant, which typically settles by itself.
Are mast cells really white blood cells?
Only in the loosest sense, and the shorthand hides the point. A mast cell starts in the bone marrow like every other white cell, but it leaves the bloodstream while still immature and finishes growing up in tissue. Mature ones are essentially absent from normal blood, so unlike a neutrophil or a lymphocyte they cannot be counted on a routine blood test. They settle where the body meets the outside world: skin, the lining of the gut and airways, and the tissue wrapped around small vessels and nerves.
What happens when the granules empty?
Each cell is loaded with sealed packets of histamine, heparin and digestive enzymes, held ready to be dumped in seconds. That is useful against parasites and is the engine of allergic reaction, but a lump densely packed with such cells can release its contents from ordinary handling. Firm stroking of one of these lesions makes it swell, redden and itch within minutes, a response distinctive enough to have its own name. When large numbers of cells release together, flushing, low blood pressure, cramping and wheezing can follow.
How does the human form compare with the veterinary one?
| Feature | Mastocytoma in a child | Mast cell tumor in a dog |
|---|---|---|
| Behavior | Almost always benign, often fades over years | Malignant, graded, able to spread |
| Number | Usually one, sometimes a scattering | Single or multiple |
| Response to rubbing | Swells and reddens | Swells and reddens |
| Related condition | Cutaneous mastocytosis | Spread to nodes, spleen and marrow |
Because the veterinary tumor is so common, most writing under this heading concerns animals. The human skin lesion is better filed as a mastocytoma and is treated as a benign tumor. A genuinely malignant mast cell tumor in a person, sometimes called mast cell sarcoma, is exceedingly rare, and the far commoner human problem involving these cells is systemic mastocytosis, an overgrowth spread through marrow and other organs rather than a single lump.
Also known as
Used in a sentence
A mast cell tumor was included in the differential for the raised brown lesion.
Good to know
Mature mast cells live in tissue rather than circulating, so these tumors form local collections that a blood count cannot detect, and firm handling of one can release histamine and produce swelling, redness and itching around it.
Word origin
Where “mast cell tumor” comes from
Mastocytoma builds on the Greek kytos, meaning a hollow vessel and so a cell, with -oma, a swelling or tumor; mast translates a German word for fattening, since the granules made the cells look stuffed with food.