Epidemiology
Though MF is the most common subtype of CTCL, it is still a relatively
uncommon malignancy. United States data is obtained from
cancer registries that take part in the Surveillance, Epidemiology
and End Results (SEER) program. The most recent data reported
that the annual incidence is 4.5 cases/million or 0.36 per 100,000
person-years.9 Additional data indicates that the incidence of MF
may be increasing, however this rise may be due to enhanced diagnosis
and/or reporting instead of a true increase in incidence.2,9
MF more commonly affects older adults, with a median age
at diagnosis of 55-61 years; it is rarely seen before the age of
30.10-12 There has been some recognition that MF may arise in
children and adolescents, though it remains less common.13,14
The pediatric incidence may be under-reported because physicians
are more hesitant to biopsy and because data on younger
patients may not be analyzed separately.2
The incidence of MF is higher among men, with the sex difference
increasing with age.15,16 The male-female ratio of MF
across several large series was found to range from 1.7-2.2.10,11,17
The incidence of MF is higher among black patients than
among white patients in all age groups, and is lower for Asians
as compared to whites; however, racial differences diminish
with age.9,15 A 1999 US study indicates that there is a lack of
population data about Asian and American Indian sub-groups;
no data is reported for Hispanic groups.9 Older data for Los Angeles
County indicates that Hispanics had a lower incidence of
MF as compared to whites.18
There is considerable geographic variation in incidence, correlating
with factors such as high family income and high
percentage of population with advanced degrees.15 In addition,
there is an association between incidence and high physician
density. This suggests that a detection bias may be partially responsible
for the geographic differences seen.2,17
Finally, as definitions of ICD-O codes have changed, shifts in
the distribution of CTCLs within subcategories have made it
more difficult to assess incidence tendencies for sub-types of
CTCL, such as MF.15 It has been suggested that these redistributions
may have contributed to the apparent stabilization in
the incidence of MF since the 1980’s.9
Risk Factors
Though
the exact pathophysiologic mechanism of MF remains
unclear, several risk factors have been identified. Occupational
exposure has been suggested as a potential risk factor
for the development of MF.2 A study by Cohen et al reported a
relative risk of 4.3 for MF in patients in industrial employment.
Additional studies in textile, machinery, chemical, and service
industries were incongruous about the correlation between exposure
and MF risk.19-21
Genetics may play a partial role in the etiology of MF.2,16,19 In a
US study, skin disease in a first-degree relative was noted in
up to 26% of MF patients, and 5% of patients noted an affected
parent.16 Though a small regional study found an association
between MF and a history of malignancy, a subsequent more
extensive study demonstrated no association, likely negating
the previous findings. Morales et al suggest this discrepancy
may be due to Berkson’s bias.2,22,23
Several other factors have been examined, including history of
atopic disease and antecedent skin disease, viral and fungal infection,
smoking and alcohol history, drug use, and ultraviolet
exposure.21,22 The data is limited, but thus far an association has
not been proven for these factors.
