A 34-year-old, ostensibly healthy Caucasian male presented with a 3 cm × 3 cm solitary skin lesion located on the distal portion of the dorsal aspect of the right arm. The lesion first appeared during the patient’s last year in high school, approximately 17 years earlier. Although the lesion had always been asymptomatic, the patient was now concerned because of a recent expansion in size. The patient had extensive ultraviolet light exposure as a teenager while working as a lifeguard; excessive ultraviolet exposure continued as an adult due to outdoor employment as a geologic surveyor. The patient denied the use of sunscreen. Family history was negative for similar lesions. The patient was not taking any medication.
B. Squamous cell carcinoma
“Porokeratosis” refers to a collection of papular skin diseases which are characterized by gradual centrifugal spread, central atrophy and dyschromia, and a peripheral, palpable ridge-like border. Solitary, larger lesions are known as porokeratosis of Mibelli (PM). The lesion in this case is a classic clinical example of PM. Other common forms of porokeratosis include: disseminated superficial actinic porokeratosis, linear porokeratosis, porokeratosis palmaris et plantaris disseminata, and punctate porokeratosis. The peripheral ridge, or cornoid lamella, is due to a clonal proliferation of atypical keratinocytes showing abnormal terminal keratinocyte differentiation. While the atypical keratinocytes show faulty differentiation, they do not show an increased rate of proliferation. Most PM lesions involve the limbs and eventually reach several centimeters in diameter; “giant porokeratosis” lesions can grow to 10 cm or 20 cm in diameter.
PM typically appears during childhood in fair-skinned individuals, although many lesions begin in adolescent years. Occasionally there is a family history of PM or another type of porokeratosis, suggesting a genetic predisposition. Excessive ultraviolet light exposure has been considered a significant risk factor for the development of PM. Adult onset PM may follow immunosuppression, either due to medication or illness. Immunomodulating drugs used to treat autoimmune diseases or to prevent organ transplant rejection may trigger porokeratosis,[2,3] and reported disease associations include HIV infection, diabetes mellitus, liver disease, and hematologic or solid organ malignancy. Localized cutaneous immunosuppression due to long-term application of a potent topical steroid has been reported to induce PM. Regardless of the age of onset or underlying predisposing factors, PM affects men twice as often as women.
As was true in this case, PM may remain unchanged for many years or rapidly grow between long periods of inactivity. Mutations in p53 may be an important etiologic force. The diagnosis of porokeratosis is usually clinical, but may be verified by characteristic histologic features. Nonmelanoma skin cancer can develop within PM; squamous cell carcinoma occurs much more frequently than basal cell carcinoma in this setting. Very rarely, porokeratosis-associated squamous cell carcinomas may metastasize and cause death.
There is no known reliable cure for PM short of complete surgical excision; this modality is certainly most appropriate when malignant degeneration develops. Medical treatments are generally disappointing. However, lesional appearance of stable PM may improve following: application of fluorouracil (5-FU) or calcipotriol creams, cryotherapy, diamond fraise dermabrasion, or laser ablation. CO2, Q-switched ruby, Nd:YAG, and 585 nm flashlamp-pumped pulsed dye lasers have been used with varying degrees of success. Of all nonsurgical treatment options, prolonged (about 6 months) daily application of topical 5% imiquimod cream has been shown to be the most effective.[7,8] Photodynamic therapy has also shown some promise in management of PM, but a standardized methodology has not yet been determined.
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