A skin mass or lump can be within the skin, in the tissues under the skin or attached to the skin and underlying tissues. Thus, any proliferation of cells, pocket of fluid or debris, or swelling of the cat's tissues can cause a mass. The health impact depends on the severity of the cause of the growth and the success in treatment of the problem.

The most common causes of skin growth are tumors, malignant and benign, bruises, infections and other causes.

Malignant Tumors

Malignant tumors are proliferations of abnormal cancerous cells that endanger the cat by either invading nearby tissues or spreading to other areas of the body (metastasis). There are too many types of skin tumors to list here, but some of the more common ones are:

Mast cell tumors. These are made up of great numbers of mast cells, a type of cell that is normally found within the skin. These cells normally release substances that are involved in allergic and immune responses. The massive amounts of these substances that can be released from tumors can be very dangerous to the animal. Tumors of this type can invade locally and metastasize.

Melanomas. These are tumors of the cell type that provides pigment to skin. They may be malignant or benign. Melanomas on the feet or in the mouth are likely to be very aggressive and dangerous. These tumors are usually black in color.

Fibrosarcomas. These tumors affect the cell type that helps to make up the connective tissue found under the skin. They rarely metastasize but are very destructive locally. On rare occasions, this tumor type has been shown to be induced by vaccines at vaccination sites in cats.

Squamous cell carcinomas are malignant tumors of skin cells that often occur at areas of unpigmented or lighter colored skin due to damage from the sun. This is especially true of white cats. They rarely metastasize but are destructive locally. They may also be non-solar induced.