RALEIGH, N.C. – The ratio of melanoma in situ to invasive melanoma jumped 12-fold among American health professionals during a recent 3-decade period.
Just how much of this steep rise in melanoma in situ diagnoses represents a true increase in the development of in situ lesions as opposed to the product of improved screening techniques or overdiagnosis due to medicolegal pressure remains a matter for conjecture. The new analysis of melanoma trends in the Nurses' Health Study and the Health Professionals Follow-Up Study provides evidence suggesting that all three factors may be involved, according to Erin X. Wei of Massachusetts General Hospital, Boston.
Her analysis included roughly 121,000 female nurses followed prospectively in the Nurses’ Health Study since 1976 and 51,000 men followed in the Health Professionals Follow-Up Study since 1986. During the follow-up period, new cases of invasive melanoma increased dramatically, but they were far outpaced by the rise in cases of melanoma in situ, she said at the annual meeting of the Society for Investigative Dermatology.
For example, while the ratio of in situ to invasive melanoma among women was 0.09:1 in 1977, by 2002 it had reached 1:1.
Support for the notion that the rise in melanoma in situ is due in part to overdiagnosis in response to medicolegal concerns comes from the finding that the bulk of the increase involves lesions reported in this study as having Not-Otherwise-Specified pathology. Lentigo maligna cases have remained relatively flat over time, she noted.
Also, in this large patient population, as well as in studies from elsewhere around the world, the increase in melanoma in situ has not been accompanied by a shift in the pattern of invasive melanoma thickness or by a reduction in melanoma mortality, as would be expected in response to a major increase in detection of invasive melanoma precursor lesions.
But perhaps in situ melanomas are often not precursors to invasive melanoma. If they were precursors, one would expect them to be diagnosed at a younger average age than invasive melanoma. Such was not the case in this analysis. Indeed, while roughly 40% of invasive melanomas were diagnosed prior to age 60, that was true for only 20% of melanoma in situ. Nurses diagnosed with melanoma in situ were an average of 6 years older than those diagnosed with invasive melanoma; among men, the difference was 2 years.
Melanoma in situ and invasive melanomas tended to differ in their anatomic distribution. Invasive melanomas were far less likely than in situ lesions to occur on the head and neck, and much more likely to occur on the lower extremities. This suggests some in situ melanomas are indolent and do not become malignant over time, Ms. Wei continued.
In support of the existence of a true biologic increase in the incidence of melanoma in situ, Ms. Wei and her coworkers found that older individuals were two- to threefold more likely to develop their lesions on chronically sun-exposed areas than on intermittently sun-exposed areas.
An intriguing gender difference in the anatomic distribution of melanoma in situ was evident. Among men, more than 98% of all melanoma in situ were found on the upper half of the body. In women, the lesions were more evenly distributed anatomically, with 27% of in situ lesions being found on the lower extremities.
The Nurses’ Health Study and Health Professionals Follow-up Study are funded by the National Institutes of Health. Ms. Wei reported having no financial conflicts.