The sudden appearance of a mass or lump in the body can be fear inducing, no matter where it appears. Concerns for cancer or an unusual injury run rampant, and the appearance of a new lump in the foot is no different. While the new mass is certainly not a sign that is normal, most people can rest assured that cancer is unlikely, and fairly uncommon.
A lump or mass under the skin is a sign that tissue of some form has expanded beyond its typical size. Some causes of a mass may be fast growing, and some may be slow growing that is simply not noticed until the area is either visible externally or is painful and prompts a personal exam. In the foot, the most common cause of a quickly appearing mass is a ganglion cyst. This generally benign fluid-filled mass is what most people have when they see a new lump in their foot or ankle. An in-depth explanation of ganglion cysts and ganglion cyst explanation and treatment can be found by following the highlighted link. Cancer within a cyst like this is possible, but incredibly rare. Other common masses in the foot and ankle that can seem like they suddenly appear include fat tissue masses, masses of fibrous tissue called fibromas, tendon thickening from underlying bone spurs (especially behind the heel) as well as simple cases of soft tissue swelling/fluid retention (such as often seen in front of the outer ankle). These masses are rarely ever cancerous, and while they can cause pain do not require urgent surgical intervention.
On the other hand, serious cancer can form in the foot, and less commonly may be a metastatic mass from a cancer elsewhere in the body. These cancers can include rare tumors of skeletal muscle, bone, fat tissue, nerve tissue, blood vessels, and cells that form the tissue in and around tendon. Some of these rare cancers are potentially fatal, and others create serious but not lethal problems for the health of the foot or ankle. Treatment for these cancers may need to involve the entire body, and surgery or amputation may be needed in the most serious of cases. Once again, though, these cancers are RARE.
If you notice a mass develop in your foot, the most sensible decision is to start with a foot and ankle specialist who deals with foot anatomy on a daily basis. A thorough exam by a podiatrist can lead to an accurate early diagnosis of the condition. If the mass is suspicious, proper testing can be arraigned, as well as coordination of appropriate treatment. However, over 99% of the time this is not necessary, and the benign lump can be easily treated in the office.