Nejm 2001;344:975; 1992;327:1649
Cause:Neoplasia from sun exposure directly (eg, Canadian fishermen—Nejm 1975;293:411) or medical PUVA, eg, for psoriasis; or from actinic keratoses, or other chronic irritation, eg, sites of chronic osteomyelitis drainage, which have an especially bad prognosis (Nejm 1980;303:367); or in renal and other transplant pts on chronic immunosupression, possibly from diminished surveillance (Nejm 2003;348:1681; 1995;332:1052)
>100 000/yr in US; 100-150/100 000/yr, over age 75 yr incid is 1000-1500/100 000/yr increasing rapidly even in younger population (Jama 2005;294:681)
Associated with UVB sun exposure in whites; arsenical rx with arsenical keratoses of palm; XERODERMA PIGMENTOSA (autosomal recessive inability to repair UV-damaged DNA) (Nejm 1986;314:1423), rx with po isotretinoin (Nejm 1988;318:1633) and strict photo-avoidance
Sx:Skin sore that won't heal
Si:Skin ulceration with varying degrees of subcutaneous and intradermal invasion; scale and erythema; cutaneous horn sometimes, r/o seborrheic and actinic keratosis
Mortality <1/500, 1500 deaths/yr in US
30% incr risk of nondermal cancers like multiple myeloma, lymphoma, leukemias as well as incr risk of dermal basal cell carcinomas (Am J Epidem 1995;141:916) as well as worse prognosis when get other cancers (Ann IM 1999;131:655)
r/o: Wart recently treated with podophyllin. “Liver spots,” actinic or solar lentigines
ACTINIC KERATOSIS (senile or solar), may be multiple, may progress to squamous cell, may progress to squamous cell carcinoma; prevent w sunscreen (Nejm 1993;329:1147); rx w (Med Let 2004;49:42; 2002;44:57): excision, freezing, topical 5-FU (Effudex) 2% or 5% bid × 2-4 wks <$75 generic, diclofenac gel bid × 3 mo 50% cure $105, aminolevulinic acid (Levulan) + blue light $110, imiquinod (Aldara) 5% crm q hs × 8-16 wk $520, tretinoin (retinoic acid) 0.1% cream qd (Nejm 1992;326:368), gentle cryotherapy, or laser (Med Let 1997;39:10)
KERATOACANTHOMA, looks clinically and pathologically very similar, rx like squamous cell cancer
Lab: Path:Biopsy, excisional or wedge if dx uncertain; in situ lesions called Bowen's disease
Rx:
Prevent w PABA sunscreens; 5-FU 5% cream bid × 4 wk to actinic keratoses, produces inflammation, vesiculation, and resolution. 
-carotene does not prevent new tumors (Lancet 1999;354:723; Nejm 1990;323:789) but low-fat diet does dramatically (Nejm 1994;330:1272)
Therapeutic (J Am Acad Derm 1993;28:628): Surgical excision or referral for Mohs micrographic surgery if high risk (>2 cm; into sc tissue, histopathology > well-differentiated grade I, or involvement of scalp, nose, ears, eyelids, or lips); radiation; isotretinoin 0.5 mg/kg bid po × months if resistant (to surgery + radiation) or recurrent extensive disease (Ann IM 1987;107:499)