Prognostic sub-classifications of T1 cutaneous melanomas based on ulceration, tumour thickness and Clark level of invasion. Results of a population-based study from the Swedish Melanoma Register



Melanoma incidence is rising in most fair-skinned populations,[1-4] and although this cancer is very difficult to treat when advanced, fortunately, the majority of cases diagnosed today are at an early stage.[2, 4] Staging criteria for melanoma have been developed and refined over time to guide clinical management,[5] but as the distribution shifts towards thinner and thinner tumours, it is necessary to derive accurate long-term survival figures for early-stage lesions to guide management and counsel patients.

This issue of the journal carries a report from the Swedish Melanoma Register[6] describing survival in patients diagnosed with thin melanomas (< 1 mm). This adds to a limited literature of population-based estimates of survival for thin melanomas, joining reports from high-incidence populations in Queensland[7] and New South Wales,[8] and the U.S. Surveillance, Epidemiology and End Results (SEER) register.[9] The fact that these new Swedish data reflect the mortality experience of more than 13 000 patients with thin melanomas across an entire population is relevant to the interpretation of the study findings, as the staging criteria being assessed were developed largely from referral centres. Such centres typically experience higher rates of mortality than the general population (attributable to referral of higher-risk patients); it is essential that population-based risks be reported to ensure that decisions are made with the most valid information possible. Strengths of this report are the large sample size, high quality of data capture relating to histological characteristics coupled with excellent follow-up of patients through the Swedish health system, all but eliminating the likelihood that subtle biases might have distorted the mortality estimates.

Overall, the findings are reassuring, and confirm the very low overall mortality for patients diagnosed with thin melanomas (3·4% and 4·4% at 10 and 15 years, respectively). As for previous reports, the authors observed that the risks of death were not uniform for all patients with thin melanomas, and they identified a number of independent predictors of poor survival. Both melanoma thickness and Clark level adversely affected prognosis (even when these factors were adjusted one for the other in multivariate models), underscoring the vital significance of depth of invasion for melanoma outcomes. In addition, the authors observed significantly poorer survival associated with anatomical site (melanomas arising on the head/neck faring worse than trunk or limbs), and with ulceration. Data relating to mitotic rate were not available except for the most recently diagnosed melanoma cases, and so the authors were unable to contribute population-based data to this issue.

In a novel analysis, the authors cross-classified each patient into one of 18 mutually exclusive categories according to their tumour thickness, Clark level and ulceration. In so doing, they identified three distinct subgroups based on their risks of dying from thin melanoma. Almost 70% had ‘low risk’ thin melanomas (< 0·75 mm, level II or III, nonulcerated) with 10-year mortality of only 1·5%. The patients at ‘intermediate risk’ had either thin (0·75 mm) but ulcerated melanomas, or thicker (0·76–1·00 mm) but nonulcerated melanomas, and experienced 10-year mortality of 6·1%. A much smaller group (approximately 3%) of patients had thicker and ulcerated melanomas, which conferred a 10-year mortality of 15·6%.

These data confirm again the notion that thickness matters for melanomas, and underscore the imperative to diagnose and treat these cancers as early as possible. For patients and their doctors, they also provide the reassurance that the overwhelming majority of thin melanomas will never lead to harm. For the small proportion of thin melanomas with ulceration, these data give cause for more aggressive follow-up.

Conflicts of interest

None declared.

Funding sources

No external funding.