General: second most common skin cancer, malignancy of keratinocytes in the epidermis


Pathogenesis: radiation, chemicals (arsenic), occupation exposures (soot, coal, tar), HPV alter cellular genetic material 


HPI: chronic skin lesion, not healing, occasionally bleeds or ulcerates 


Risk factors: sun/radiation exposure (geographic, occupational), carcinogen exposure, > 60 y/o male, lighter skin pigmentation, family/past medical history of skin cancer, 


Physical exam: hard, scaling, indurated, crusted, erythematous to flesh-colored patch/plaque/nodule, may bleed or ulcerate, often on sun-exposed areas (head, neck, arms), near scars or lesions, can involve mucous membranes and lower lip 


Differential: keratoacanthoma (rapid growth, central crater of keratin), basal cell carcinoma, seborrheic keratosis, wart, hypertrophic actinic keratosis, SCC in situ on the glans penis = erythroplasia of Queyrat


Histology: hyperkeratosis, abnormal keratinocytes, can invade dermis, can classify via grades 1-4


Complications: can metastasize (increased risk if > 2 cm, high grade, dermal invasion, location on ear/mucous membranes) through lymphatic system, SCC from AK has low risk of metastasis


Treatment: (1) reduce sunlight exposure (hats, long-sleeves, SPF 30 sunscreen, avoid mid-day sun), (2) excision, (3) electrodessication and curettage, (4) Mohs, could also try radiation


References: AAD Basic Dermatology Curriculum, Dermatology by Bolognia et. al., Lookingbill and Mark's Principles of Dermatology, First Aid USMLE Step 1 2020, First Aid USMLE Step 2 CK