Keratoacanthoma (KA), also known as molluscum sebaceum , is a hyperkeratotic dome-shaped nodule that can grow up to one to three centimeters within a few months. It is a low-grade skin tumor found on sun-exposed areas such as the head, hands, arms and trunk. It is most important for the ophthalmologists because it can occur on the eyelids. Although KA is characterized by a benign growth, its differentiation from squamous cell carcinoma (SCC) is of upmost importance. For some, KA represents a spontaneous regressive form of SCC ,  while other researchers favor a strict differentiation between these two entities based on gene expression  and/or expression of cutaneous markers.
KA is primarily associated with sun exposure in elderly people. Chemical carcinogens also play a role in its pathogenesis. No genetic basis has been yet identified.
Gross Pathology 
Keratoacantoma appears as a solitary flesh-colored dome shaped lesion with central keratin-filled plug.
There is no single histological finding that can characterize keratoacanthoma. Rather, histologic diagnosis requires a constellation of findings:
- Exoendophytic lesion composed of layers of well-differentiated squamous epithelium resulting from acanthosis
- Central keratin-filled crater that may enlarge as the lesion evolves and matures
- Larger mature keratinocytes are found towards the center of tumor cell nests
- Cytoplasm of large keratinocytes give eosinophilic hue on H & E stain.
- Buttressing (lipping) of the lesion edges around the central crater, creating a symmetrical appearance
- Adjacent dermis may contain small micro abscesses composed of mixed inflammatory infiltrate (eosinophils and neutrophils)
Note: Histologic features that favor keratoacanthoma over squamous cell carcinoma include hemispheric flask like shape with overhanging edges and central keratin plug
Variants (from a dermatologic point of view)
- Giant: 
- Multinodular keratoacanthoma) :
- Subungual keratoacanthomas :
- Multiple Keratoacanthoma : These may be seen in Torre’s syndrome (Muir-Torre syndrome) where multiple keratoacanthomas may be associated with underlying primary visceral cancer, most often of the GI tract.
KA commonly presents in old men with fair skin on sun-exposed area. It is a self-limited benign growth characterized by a rapidly enlarging, solitary flesh-colored nodule, followed by a period of stability, which later involutes, leaving behind a scarring lesion.  
KA lesions classically present as a quickly growing, firm, flesh-colored papule. The proliferative phase is characterized by a symmetrical, smooth surfaced lesion that progressively ulcerates, forming a hyperkeratotic dome-shaped nodule; often described as a horn like projection. It can also demonstrate a cup-shaped appearance with the central ulceration. Within 1-2 months, it can rapidly grow to 1-2 cm. If on the nose or eyelids, extensive local tissue destruction may occur. Although the duration of the lesion may be variable (few months to a year), it often spontaneously regresses leaving behind a hypopigemented scar.
Excisional or deep incisional biopsy of the lesion is the preferred tool for establishing the diagnosis of KA. 
The diagnosis can be done by Histology.
Differential diagnosis 
- Actinic Keratosis
- Squamous cell carcinoma
- Cutaneous Horn
- Molluscum contagiosa
General therapy usually consists of cryosurgery, electrodessication and curettage, excision, Mohs micrographic surgery and Radiation treatment (X-ray). Medication therapies such as antineoplastic agents are usually prescribed for patients who have usually more aggressive lesions. Some examples include topical and intralesional 5-fluorouracil, methotrexate and bleomycin.
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