Squamous Cell Carcinoma
Squamous Cell Carcinoma is the second most common type of skin cancer that is caused by ultraviolet rays (sunshine), arsenic ingestion, some wart viruses (human papilloma virus), and radiation.
Invasive Squamous Cell Carcinoma
Although uncommon, this tumor does have the potential to metastasize (spread to the lymph nodes and internal organs) and can be fatal. In general, this risk is very low and estimated to be approximately 3%. Tumors that occur within a burn scar and those that are on the lip or ear are more likely to metastasize than those elsewhere and carry a risk of up to 15%.
In most cases, surgery (Mohs' micrographic surgery, excision, or electrodessication and curettage) or radiation is used to treat these tumors. There are pros and cons to each treatment method with differences in cure rate, healing time, post-treatment restrictions, and cosmetic outcome.
Squamous Cell Carcinoma in situ (non-invasive)
Early, non-invasive tumors are called squamous cell carcinoma in-situ (or intraepithelial squamous cell carcinoma or Bowen’s disease), which does not pose a metastasis risk at this early stage. Tumor cells are present only in the epidermis, the top layer of skin, where there are no blood vessels and therefore no risk of spreading. This early variant is not life-threatening unless untreated, in which case it can become invasive with time (months to years).
Keratoacanthoma or Squamous Cell Carcinoma, Keratoacanthoma Type
There is much confusion over this tumor in recent years. This tumor was considered to be a "benign" or "pre-malignant" tumor for decades. These tumors typically grow very rapidly for several months, the stop growing, and then spontaneously involute (go away leaving a scar). More recently, reports arose of "metastatic keratoacanthomas"--tumors that did in fact metastasize (spread). It is still questionable whether or not these "metastatic lesions" were in fact keratoacanthomas or a squamous cell carcinoma. Unfortunately, it is sometimes difficult to know for sure. For this reason, nearly all textbooks are advocating removal of these lesions despite the incredibly rare risk of this occurring. This has resulting in numerous procedures that, in all likelihood, are unnecessary. Hopefully, in the upcoming years this will become clearer and a true distinction can be made between keratoacanthoma and squamous cell carcinoma.
While surgical removal is currently recommended for most, the choice to clinically observe the lesions is viable and is a great option for many. In my experience, over 95% of these lesions resolve after biopsy within 6-8 weeks. When lesions exhibit aggressive behavior, fail to involute, or increase in size, consideration should be given for complete surgical removal.