Sunshine on healing skin — one step forward, two steps back?

The most common pre-cancerous lesion dermatologists treat is the actinic keratosis or “AK.” These rough growths are often frozen with liquid nitrogen and gone in a zap. Sometimes, however, a patch of skin is covered with precancerous lesions. One almost doesn’t know where to start. For such a patient, the correct treatment is often not to spray twenty or thirty AKs with liquid nitrogen. If the whole patch of skin is affected with precancerous lesions, then spot treatments like liquid nitrogen zaps will just result in a game of whack-a-mole. One AK is beaten down only to have one or two more pop up by the next visit.

Field damage demands field treatment. Rather than spot-treating individual AK’s, an entire patch of skin can be treated with certain creams and therapies to reduce the number of precancers and the amount of sun damage in an entire area. Field treatments like fluorouracil cream, imiquimod cream, and photodynamic therapy (“blue light therapy”) can reduce the AK burden across an entire zone such as the entire scalp or face. These treatments reduce the number of future cancers and get the number of AK’s back down to a reasonable number.

I often recommend field treatments to my patients but with one big caveat. I tell them, “Don’t undergo a field treatment unless you can stay out of the sun during the treatment and for a week or so afterwards.” I advise patients that I am worried that sun damage on skin that is being treated or is recovering may not only cancel out the benefits but might leave them worse off.

Why the worry? Am I overcautious? I believe not. I’m concerned about more than just a little sunburn. I’m concerned about the sun hitting cells when they are at their most vulnerable: when they are replicating – such as when cells fill in tiny wounds and even micro-gaps created from the killing of precancers like AKs.

Every system has its vulnerabilities, and our DNA repair processes are no different. Our cells’ repair processes work well during the resting phases of cell life when they are happily performing their usual functions. But when a cell decides to duplicate its DNA and divide into daughter cells in a process called mitosis, some of the repair processes are temporarily shut down.

Remember in the movie Jurassic Park when getting the park back online requires temporarily turning everything off? The electricity to the fences had to be shut off, too, and some of the carnivorous dinosaurs escaped and wreaked havoc. Well, the same thing happens in our cells when they duplicate their DNA and divide from one cell into two cells. When cells synthesize new DNA, many of the DNA repair processes are temporarily turned off to allow DNA duplication and cell division to take place. If a mutation, such as from sunlight, occurs when the repair systems are off, then the mutation is often not corrected. The resulting daughter cells are permanently affected with the mutation. Such mutated cells can develop into cancer.

Field treatments like fluorouracil, imiquimod and photodynamic therapy kill bad cells. The healthy neighbor cells must then start dividing to take the place of the bad cells. More cells dividing means more cells with DNA repair processes in the “off” position. Treatments aimed at eliminating cancerous lesions temporarily create a vulnerable state.

Advice to avoid the sun during AK treatments is not just a case of preventing your usual sunburn or of healing skin being more sensitive. Healing skin is more susceptible to mutations and DNA damage from sunburns than resting skin is. Biology argues against taking a “cheat day” and getting sun exposure during or after any treatment that requires skin to heal. If you find yourself in need of field treatment such as PDT, fluoruracil or imiquimod, remember it is only a good idea if you can stay out of the sun during the entire treatment and for the time it takes afterwards for your skin to repair itself.