There are several different possibilities here, although if the area has been present for a while and is not growing, then there is almost certainly nothing to worry
about, as the vast majority of these types of growths are benign. However, rarely, a soft growth like this may be a sign of a serious problem, such as a cancerous tumor. For this reason, it is a good idea to have your primary care doctor
look at the area the next time you see them. If the area is growing rapidly or is painful or changing in any other way, then you need to get in right away for an evaluation.
By far the two most common causes of smooth growths under the skin would be simple types of cysts, or lipomas. Both of these are almost always benign and do not require any specific treatment. Occasionally, a cyst or a lipoma may become large enough that it is visible under the skin or is painful. In this case, it may be an option to have the cyst or lipoma surgically removed. Surgical removal of these growths is simple and can be done in the office most of the time by a surgeon