Malignant melanoma: early diagnosis and screening
Sneyd, Mary Jane
The incidence rate of melanoma has shown large increases in New Zealand over the past 30 years, with the result that New Zealand has one of the highest incidence rates of melanoma in the world. Primary prevention and early diagnosis are the main methods of melanoma control. Of these, the aspect of melanoma control likely to have the biggest impact in the short term is early diagnosis. In New Zealand, general practitioners are usually the first health professionals required to make diagnostic and management decisions about skin lesions but little information exists about the diagnosis and management of skin cancer by general practitioners. Therefore a survey was performed to assess current levels of knowledge and management practices of general practitioners and dermatologists concerning the early detection and treatment of skin cancer. From the results of this survey it appears that a visual examination of the skin by a medical practitioner has potential for use as a screening test for melanoma, with diagnostic accuracy of 78%, sensitivity of 81% and specificity of 98%, estimated from assessing photographs of skin lesions. Furthermore, this survey suggested a high degree of expertise among New Zealand doctors, particularly with regard to decisions about biopsy. Depth distribution is one of the better indicators of the effectiveness of melanoma control programmes and the best predictor of future mortality, but this information has not been routinely collected. Several studies of the incidence and depth distribution of melanoma in New Zealand have been published but these were often limited by incomplete case ascertainment. Therefore a survey was carried out, with complete ascertainment of melanoma patients in three areas of New Zealand, in order to investigate a wide range of clinical and histological characteristics of melanoma. The results from this survey support a trend of increasing incidence in New Zealand. It has been suggested that the increase in incidence could be due to rapid recognition of earlier lesions on exposed body sites, particularly in women, but this hypothesis was not supported by the data in this study. Interestingly, unlike the female excess of melanomas estimated from Cancer Registry incidence data before 1992, the age-standardized incidence rates for each sex in this study were approximately the same. This may reflect bias and incompleteness of routine reporting of melanoma. In this study the proportion of invasive melanomas diagnosed at less than 0.76mm thick was 49%, well below the target set by the Department of Health in 1990, and there was a significant association between depth of invasive melanoma and age. The results also showed that the highest density of melanomas occurred on body sites chronically exposed to sunlight, such as the face and ears. It has been shown that there is a seasonal variation in the incidence of melanoma and this survey supported some cyclical trends in melanoma incidence, with peaks in incidence over the summer months. The case-control approach has been useful in the evaluation of screening for breast, cervical and colon cancers and was used here in the evaluation of screening for melanoma. However, as there have been no organized screening programmes for melanoma in New Zealand, information about screening experience was only obtainable by interviewing patients. A small case-control study was carried out to evaluate the effect of screening by skin examination on the incidence of total, superficial and advanced melanoma and to describe the main risk factors for melanoma in New Zealand. This case-control study showed a significantly decreased risk of melanoma in people who had performed any type of screening skin examination, but whether this is consistent with a direct effect of screening on melanoma incidence remains unclear. In addition this study showed that skin examinations are frequently performed in New Zealand and that most melanomas are discovered by the patients themselves. As in many similar studies, there was a strong association of melanoma risk with host phenotypic factors, particularly with red hair, fair skin, freckles and numbers of naevi. However, there was little evidence supporting an association between sun exposure and melanoma risk.
Advisor: Skeggs, David; Sharples, Katrina
Degree Name: Doctor of Philosophy
Degree Discipline: Preventive and Social Medicine
Research Type: Thesis