Importance Merkel cell carcinoma (MCC) is an uncommon but highly invasive form of skin cancer. The mechanisms that cause MCC are yet to be fully determined.
Objectives To compare the incidence and survival rates of MCC in Queensland, Australia, known to be a high-risk area, with MCC incidence and survival elsewhere in the world. We also analyzed incidence trends and differences in survival by key demographic and clinical characteristics.
Design, Setting, and Participants Retrospective cohort study of population-based administrative data for MCC collected by the Queensland Cancer Registry and supplemented with detailed histopathologic data. Deidentified records were obtained of all Queensland residents diagnosed as having MCC during the period from 1993 to 2010. A subsample of histopathologic records were reviewed by a senior dermatopathologist to determine the potential for misclassification. A total of 879 eligible cases of MCC were included in the study.
Main Outcomes and Measures Incidence rates were directly age standardized to the 2000 United States Standard Population. Trends were examined using Joinpoint software with results expressed in terms of the annual percentage change. The period method was used to calculate 5-year relative survival, and adjusted hazard ratios were obtained from multivariate Poisson models.
Results There were 340 cases of MCC diagnosed in Queensland between 2006 and 2010, corresponding to an incidence rate of 1.6 per 100 000 population. Men (2.5 per 100 000) had higher incidence than women (0.9 per 100 000), and rates peaked at 20.7 per 100 000 for persons 80 years or older. The overall incidence of MCC increased by an average of 2.6% per year from 1993 onwards. Relative survival was 41% after 5 years, with significantly better survival found for those younger than 70 years at diagnosis (56%-60%), those with tumors on the face or ears (51%), and those with stage I lesions (49%).
Conclusions and Relevance Incidence rates for MCC in Queensland are at least double those of any that have been previously published elsewhere in the world. It is likely that Queensland’s combination of a predominantly white population, outdoor lifestyle, and exposure to sunlight has played a role in this unwanted result. Interventions are required to increase awareness of MCC among clinicians and the public.
Merkel cell carcinoma (MCC) is a rare and highly invasive form of neuroendocrine skin cancer that was first described by Toker1 as trabecular carcinoma in 1972. The neoplasms are composed of small, round, blue cells sharing histomorphologic and immunohistochemical features with various neuroectodermally derived cells including cutaneous Merkel cells.2,3 It is currently unclear whether MCCs actually derive from cutaneous Merkel cells or share a common precursor.2,4,5
Merkel cell carcinomas can be difficult for clinicians to identify because of their nondescript features.6 However, just recently, Jalilian and colleagues7 reported that the 4 most frequent clinical features were cherry-red color, shiny surface, sharp circumscription, and nodular structure. They also outlined significant dermoscopic features including linear, irregular, and polymorphous vessels; poorly focused vessels; and milky pink areas.7 Despite the rather characteristic histopathologic features, immunohistochemical staining is required for a definitive diagnosis to differentiate MCC from a broad spectrum of small, round, blue-cell neoplasms (eg, metastasis of an oat cell carcinoma of the lung).2,5,8 The tumors typically show rapid growth, meaning that many patients develop metastatic disease, and recurrence is common despite surgical removal of the primary lesion.2,8
Merkel cell carcinoma is most often diagnosed among elderly white patients on sun-exposed body sites such as the head and neck.2,5,8 Immunosuppression, which occurs when the body’s normal immune responses are inhibited due to medical intervention or disease, is another recognized risk factor,2,5 which is suggestive of a viral cause.8 The discovery of a polyomavirus (MCPyV)9 in most MCC tumors has offered new insights into its pathogenesis,8,10,11 although much remains unknown. For example, while there is an established association between MCC and UV irradiation,8 it is not clear whether UV irradiation contributes directly to the development of MCC or if it acts in combination with immunosuppression.10
The state of Queensland in northeastern Australia has a subtropical climate and high levels of UV radiation year-round combined with a predominantly fair-skinned population and an outdoor lifestyle. Queensland consequently has the highest reported incidence rate of skin cancers worldwide, particularly invasive melanoma.12- 14 The main aim of the present study was to compare the incidence of MCC in Queensland with that found elsewhere in the world. We also examined incidence trends and investigated differences in survival by key demographic and clinical characteristics.