The world's most common cancer rarely spreads, and a third appear where sun never reaches
Basal-cell carcinoma makes up at least 32% of all cancers worldwide, yet it grows slowly and seldom travels beyond its starting point. Sunlight is behind about two-thirds of cases, but the rest turn up on skin the sun barely touches, and whether sunscreen actually prevents it is still unproven.
The cancer takes its name from the basal cells in the deepest layer of the epidermis, which its cells resemble under the microscope, though some tumours probably arise from hair-follicle germ cells called trichoblasts instead. It usually shows up as a painless, pearly or shiny bump crossed by tiny blood vessels, sometimes with an ulcer; the old name rodent ulcer reflects that. Superficial types can pass for a red eczema-like patch, and infiltrative forms can look like scar tissue, which makes them hard to spot by eye. In the United States, about 35% of white men and 25% of white women develop one during their lives.
Ultraviolet light is the main driver. It creates thymine dimers, kinks in DNA that repair systems mostly but not entirely fix, so damage accumulates into mutations, and sunburn also dampens the skin's local immune surveillance. Childhood exposure is especially harmful, tanning beds add to the dose, and fair skin, radiation therapy, long-term arsenic exposure and weak immunity all raise risk. A small share of cases come from Gorlin syndrome, caused by a mutation in the PTCH1 gene on chromosome 9 that disrupts the hedgehog signalling pathway and also brings jaw cysts and pits on the palms and soles. A Cochrane review found too little evidence to say whether sunscreen alone prevents the disease, while noting its certainty was low.
Nodular tumours, the classic form, account for half of cases and cluster on the head and neck. Diagnosis rests on a biopsy, often a simple shave under local anaesthetic, with special staining such as BerEP4 for unclear samples. Removal is the usual treatment, and small tumours can simply be cut out.
For many facial and recurrent cancers, doctors turn to Mohs surgery, devised by Frederic Mohs in the 1930s. Tissue is removed and checked under the microscope while the patient waits, with the entire margin inspected before deciding whether to cut more. Standard laboratory slicing, by contrast, may examine less than 5% of the margin. Freezing, scraping, creams and light-based treatments are alternatives, and rare cases that spread can be treated with chemotherapy or targeted drugs.
Source: Basal-cell carcinoma
