A skin lipoma is a fatty tissue growth which is benign. Therefore, by definition, they are always non-harmful and there is never reason to worry. Nevertheless, it is important to confirm that a suspected lipoma really is a benign growth, rather than malignant skin cancer or a symptom of another serious disease. There are several guidelines to determine when to worry about a lipoma in the skin.
According to the Brighton Laser Clinic, skin lipomas occur in about 1% of the overall population, and statistically occur more commonly among middle-aged people and among women. Provided that the growths are confined to the skin and do not affect internal organs, they are not considered serious and do not need to be removed, unless they are large enough to cause pain or interfere with freedom of movement of an affected limb. The removal of a lipoma is only minor surgery, but it may grow back afterward in the same location. According to the Merck Manual, a benign lipoma may grow up to three inches in diameter.
There is reason to be concerned, however, if you have an unknown growth which has just appeared, and especially if it continues to grow rapidly. Such growths may, in fact, not be lipomas, but a form of cancer, such as liposarcoma, which grows in fat cells. Liposarcoma is rare and occurs most commonly in the thigh, whereas benign lipomas are more common in the arms. The Mayo Clinic recommends that suspicious lipoma-like growths be evaluated to rule out liposarcoma.
In addition, a lipoma is usually painless on its own, though it may cause discomfort by interfering with movement or pressing against surrounding tissues. This is not a good guide to determining whether a growth is cancerous or not, since cancerous growths are also usually painless. However, a growth which hurts may be caused by some other condition, such as an infection, and should therefore still be checked out by a doctor. Painful lipomas may be caused by Dercum's disease, which predominantly affects older women and can cause weight gain as well as more severe symptoms. According to the National Human Genome Research Institute, this disease cannot be cured, but there are severa options for relieving pain, including the drug lidocaine, surgery (also discussed above), and liposuction in the case of abdominal growths.
In all cases, if you are concerned about a suspect growth under the skin you should have it assessed by a qualified medical professional, such as your family doctor, rather than attempting to self-diagnose the problem. If your doctor is also concerned, he or she may take a biopsy (make a small incision to remove some of the growth for testing), or order an ultrasound or even an MRI, in order to make sure there is nothing to worry about.