Squamous cell carcinoma is a malignant skin cancer that involves a mutation and abnormality in the cells located in the top layer if the skin, called the keratinocytes.  It is the second most common skin cancer next to basal cell carcinoma, and often can occur anywhere in the body that is exposed to ultraviolet radiation, such as from the sun.  People with fair skin are obviously at greater risk to develop this cancer, but it can also be seen in those with darker skin tones.  People with immune deficiencies, such as organ transplant patients, those on chemotherapy, or those with HIV, are also at greater risk for developing this skin cancer  This cancer can also arise from skin injuries, such as burns, scars, and longstanding wounds.  Less than 3% of cases occur in the foot, but squamous cell carcinoma does appear in many non-pigmented foot masses.  This cancer is particularly dangerous, as it can metastasize to other organs and cause death.
 
Squamous cells can also be found in other tissue in the body, such the digestive tract, reproductive tract, and lungs, and cancer of these tissues shares the same name.  For the purposes of this article, the skin form and its relationship to the foot will be the topic of discussion.
 
When found in the foot, the squamous cell carcinoma can have several different appearances.  It can appear as a slowly growing red scaly patch, a hard thick papule, a open wound with raised edges, or even a thick hard mass like a wart.  One of the most common presentations in the foot is of a red crusty lesion that may be mistaken for peeling irritated skin or athletes foot infection.  This presentation often indicates squamous cell carcinoma in-situ, often referred to as Bowen's disease.  This is an early form of squamous cell carcinoma in which the cancer cells have not yet invaded past the top epidermal layer of the skin, and are self-contained.  There is potential for this form to convert to invasive squamous cell carcinoma, however this is infrequent, especially is early intervention takes place.
 
An even earlier form of this disease, before it becomes cancerous, comes from a skin condition called actinic keratosis.  Similar to squamous cell carcinoma in-situ, these benign lesions appear as small (less than 1 inch across) raised scaly growths with color that can range from tan to brown to red.  Up to 10% of actinic keratosis can eventually turn into squamous cell carcinoma, and 40-60% of squamous cell carcinoma's start off as untreated actinic keratosis.
 
Treatment for squamous cell carcinoma depends on the severity of the cancer stage.  Early stages, particularly the in-situ form, can be treated with either surgical excision or a topical chemotherapy drug.  More advanced cases may require aggressive surgical excision, possible amputation, or IV chemotherapy/radiation therapy.  Like all skin cancers, early identification is the key to effective treatment.  Because of the variability in how these tumors appear, any skin lesion that raises suspicion should be biopsied to determine the underlying tissue type, and if identified squamous cell carcinoma should be treated as soon as possible.