One of the frustrations I often hear from patients is that their sunspots never seem to go away.
They have a few rough spots frozen during a skin check. The skin heals, everything settles, and then, six months later, there are more. Sometimes in exactly the same area.
For someone who has already had several skin cancers removed, this can be understandably worrying. Why does it keep happening? Is the treatment not working? And does it mean another skin cancer is developing?
The answer often lies in something called field cancerisation.
It’s a term most patients haven’t heard of, but it helps explain why treating individual sunspots is sometimes not enough, particularly in people with extensive sun damage or a history of skin cancer.
The problem isn’t always the spot you can see
Actinic keratoses, sometimes called solar keratoses, are those rough, scaly patches that commonly appear on the face, scalp, ears, hands and forearms after years of sun exposure.
Some are pink or red. Others are barely visible, but you can feel them when you run your fingers over the skin. Patients often describe them as little patches of sandpaper that never quite disappear.
They develop because ultraviolet radiation has damaged the DNA of skin cells over many years.
We regard actinic keratoses as precancerous changes because some can eventually develop into squamous cell carcinoma (SCC), one of the common forms of skin cancer in Australia.
Not every actinic keratosis will become cancer. In fact, many never do. The difficulty is that we can’t reliably tell which ones will progress.
What concerns me more than one or two isolated lesions is a patient who has large numbers of them, particularly when new lesions keep appearing despite regular treatment.
Take someone with a bald scalp who has spent decades working outdoors. They may have twenty or thirty visible actinic keratoses across the scalp and forehead. Freezing each one might clear those individual lesions, but it doesn’t change what has happened to the skin around them.
And that is where field cancerisation becomes important.
What does field cancerisation actually mean?
Think about an area of skin that has been exposed to the sun for forty or fifty years.
The damage isn’t confined to the rough patches you can see. The surrounding skin has been exposed to the same sunlight, often for just as long.
Some of those skin cells will already have developed DNA changes, even though the skin still looks relatively normal.
That wider area of damaged skin is what we call a cancerised field.
It’s particularly common on the scalp, forehead, temples and forearms, where sun exposure tends to be both prolonged and repetitive.
This explains why a patient can have several actinic keratoses successfully treated with liquid nitrogen, only to develop new ones nearby.
It doesn’t necessarily mean that the original treatment failed. The next lesion may have developed from another group of sun-damaged cells that were already present.
Once we recognise this pattern, it makes sense to reconsider our approach. Rather than freezing new lesions every few months indefinitely, we can look at treating the broader area of damaged skin.
Why some patients need a different approach
There is quite a difference between someone who develops their first actinic keratosis at 65 and someone of the same age who has already had five or six squamous cell carcinomas removed.
Both may have sun-damaged skin, but their risk is not the same.
Patients with multiple previous skin cancers, extensive actinic keratoses or lesions that recur rapidly deserve closer attention.
I am particularly cautious with patients whose immune systems are suppressed.
This includes organ transplant recipients, people with certain blood cancers and patients taking medications that suppress immune function.
In these patients, squamous cell carcinomas can develop more frequently and sometimes behave more aggressively.
It is also important not to assume that every thick, crusted spot is simply another actinic keratosis.
A lesion that becomes painful, develops a firm raised base, bleeds, grows quickly or refuses to heal needs proper assessment. Occasionally, what appears to be a stubborn sunspot turns out to be an invasive squamous cell carcinoma.
In that situation, a biopsy may be more appropriate than another round of freezing or a course of cream.
This is why assessing the skin properly matters as much as choosing the treatment.
So, how do we treat an entire area of sun damage?
For isolated actinic keratoses, liquid nitrogen is still a very useful treatment. It’s quick, practical and generally effective for suitable individual lesions.
But if someone has widespread actinic keratoses across the scalp or forehead, treating one spot at a time becomes less useful.
This is when I would consider a field treatment.
One of the most established options is 5-fluorouracil cream, often referred to as 5-FU.
Rather than treating individual lesions, the cream is applied to a defined area of sun-damaged skin. It works by interfering with the growth of abnormal skin cells.
What surprises many patients is how much the skin can react.
An area that initially looked quite normal may become red, inflamed and crusted during treatment. This happens because microscopic abnormalities can be present beyond the obvious sunspots.
I always think it is important that patients understand this before starting. Otherwise, the reaction can be alarming, particularly when it involves the whole forehead or scalp.
The redness and inflammation are expected to a degree, but there is no benefit in trying to produce the most severe reaction possible. Excessive pain, significant ulceration or signs of infection are reasons to seek medical advice.
The length and frequency of treatment depend on the formulation, location and individual circumstances. Some patients find the treatment quite manageable. Others need more support, particularly if they are treating a large area.
It can be inconvenient, and the skin may look worse before it looks better. But for the right patient, it can be a very worthwhile treatment.
There is also evidence that 5-FU may temporarily reduce the risk of developing squamous cell carcinoma. In one clinical trial involving patients at high risk of skin cancer, treatment of the face and ears reduced the number of SCCs requiring surgery during the following year.
That is encouraging, although the protective effect was not maintained throughout the longer follow-up period. Field treatment is not a permanent solution to a lifetime of sun damage.
What about other treatments?
Fluorouracil is not the only option.
Imiquimod cream works differently, stimulating a local immune response against abnormal skin cells. It can be effective, although it also produces inflammation and is not appropriate for every patient.
Photodynamic therapy, or PDT, is another option, particularly for areas such as the face and scalp. It involves applying a photosensitising medication and then activating it using an appropriate light source.
Some patients prefer PDT because treatment is carried out in a clinical setting rather than applying cream at home over several weeks. However, conventional PDT can be uncomfortable, and availability and cost may influence the decision.
Daylight PDT is generally less painful and can be a good option for appropriately selected lesions.
There are also newer and combination topical treatments, although their suitability depends on the type of lesions, the treatment area and the patient’s medical history.
Sometimes the best approach is a combination. We might treat a few thicker lesions individually and then use a field treatment for the surrounding skin.
There isn’t one treatment that suits everyone, and there is certainly no reason every patient with a few sunspots needs to undergo extensive field therapy.
What happens when the sun damage is severe?
Some patients develop what I would describe as a persistent cycle of actinic keratoses and skin cancers.
They have regular skin checks, undergo treatment, and return a few months later with another collection of lesions.
This is particularly challenging in people with significant immune suppression or a long history of squamous cell carcinoma.
For these patients, the aim is to get ahead of the disease where possible, rather than simply responding to each new lesion.
That might mean scheduling field treatments periodically, examining the skin more frequently and investigating suspicious changes promptly.
Occasionally, patients with an exceptionally high burden of skin cancers may benefit from additional preventive medications under specialist supervision.
One medication that often comes up in discussion is nicotinamide, a form of vitamin B3.
An Australian study found that nicotinamide 500 mg twice daily reduced new non-melanoma skin cancers by around 23% over twelve months in high-risk patients who were not immunosuppressed.
However, it is important to put that finding into context.
The benefit was observed during treatment, and a separate study in organ transplant recipients did not show the same reduction in skin cancers.
Nicotinamide can be worth discussing in selected patients, but it is not a substitute for treating suspicious lesions, protecting the skin from the sun or attending regular checks.
For some patients, particularly transplant recipients with repeated squamous cell carcinomas, management may also involve their dermatologist and transplant team working together.
These decisions need to take into account the person’s overall health, medications and the behaviour of their previous skin cancers.
Why I don’t consider field treatment a once-off cure
One thing I try to make clear to patients is that successful treatment doesn’t mean the skin has been returned to its original state.
We can remove actinic keratoses. We can treat a cancerised field and significantly improve the condition of the skin. We may also reduce the development of new lesions for a period of time.
But we cannot erase every effect of decades of UV exposure.
New lesions can still develop, and someone who has already experienced multiple skin cancers remains at increased risk.
For that reason, a patient with extensive field cancerisation needs an ongoing plan.
Some people may only require treatment every few years, while others need more regular intervention. Patients with multiple previous skin cancers often benefit from skin examinations every three to six months, depending on their history and level of risk.
I would also encourage patients not to wait for their next appointment if something changes.
A new spot that grows quickly, becomes sore or bleeds repeatedly is worth having examined sooner.
And although it can seem a little late to start being careful with the sun after forty years of outdoor work, sun protection still matters.
A broad-brimmed hat, protective clothing, shade and regular SPF 50 or 50+ sunscreen help reduce further UV damage. They are particularly important for patients who have already developed significant actinic keratoses or skin cancers.
The conversation worth having at your next skin check
If you have just one or two actinic keratoses, straightforward treatment may be all that is needed.
But if you find yourself having the same areas frozen repeatedly, or you have a history of multiple squamous cell carcinomas, it may be worth asking whether you have field cancerisation.
That question can change the conversation from treating individual lesions to managing your skin cancer risk over the longer term.
Sometimes cryotherapy remains the most appropriate treatment. Sometimes a course of field therapy will be more useful. And sometimes a lesion needs a biopsy before anything else is done.
What matters is understanding the extent of the sun damage, recognising which lesions are concerning and choosing a treatment plan that makes sense for your skin and your medical history.
The goal isn’t to promise that you’ll never develop another sunspot or skin cancer. It’s to manage the damage we know is there, reduce future problems where we can, and identify any developing skin cancer as early as possible.
For patients with extensive sun damage or a history of multiple skin cancers, that longer-term approach is often much more valuable than simply freezing the next rough spot.
References
American Academy of Dermatology. Guidelines of Care for the Management of Actinic Keratosis. Journal of the American Academy of Dermatology, 2021.
Kandolf L, et al. European Consensus-Based Interdisciplinary Guideline for Diagnosis, Treatment and Prevention of Actinic Keratoses, Epithelial UV-Induced Dysplasia and Field Cancerization. Journal of the European Academy of Dermatology and Venereology, 2024.
Weinstock MA, et al. Chemoprevention of Basal and Squamous Cell Carcinoma With a Single Course of Fluorouracil, 5%, Cream. JAMA Dermatology, 2018.
Chen AC, et al. A Phase 3 Randomized Trial of Nicotinamide for Skin-Cancer Chemoprevention. New England Journal of Medicine, 2015.
Allen NC, et al. Nicotinamide for Skin-Cancer Chemoprevention in Transplant Recipients. New England Journal of Medicine, 2023.
This article provides general patient education and is not a substitute for individual medical assessment. Suspicious, changing or non-healing lesions should be examined by a qualified clinician.