History
Seven month old boy with a tumorous lesion at the wrist between
the median nerve and main radial artery.
Pathology
Gross inspection of the resected lesion revealed a subcutanous
multilobulated whitish mass, measuring 3cm in diameter, with
poorly demarcated borders blending into the surrounding fatty
tissue.
Histologically it was composed of three different types of tissue:
eosinophilic areas of spindle cells growing in short interweaving
bundles, between accumulations of more polymorphous or stellate-shaped
cells forming rounded nests in a basophilic myxoid background.
Intermingled were moderate amounts of mature fat tissue, seemingly
encircled by the other two components.
Discussion
The histology of this „tumour“ is characteristic
and unique. Immunohistochemistry is not
necessary to identify the lesion. Age, location and the striking
triphasic organoid pattern are the diagnostic hallmarks.
FHI was given its name by Franz M. Enzinger in 1965 after evaluation
of 30 cases from the
files of the Armed Forces Institute of Pathology. Formerly it
was designated “ by R. D. Reye as „subdermal fibromatous
tumour of infancy“, reporting six cases in 1956.
It is a lesion of young children, usually under two years of
age, but a minority of cases can also occur in older children
or be congenital. The majority presents as a painless at times
rapidly growing nodule, usually solitary, although multiple lesions
have occurred. The location is prevalently in the upper region
of the body, with a high percentage in the shoulder region, upper
arm, axilla and neck. Other frequently found locations are the
external genital area and the inguinal region. Gender does not
affect the distribution, but there is a predilection for boys.
An association with syndromes or a high familial incidence have
not been reported. The size ranges from 0.5 cm to very large
masses over 20 cm. Spontaneous regression does not occur. There
is no evidence of malignant transformation and long-term clinical
follow-up indicates a benign biological behavior.
Histologically it is principally composed of three distinct
kinds of tissue, which can vary in
their relative proportions :
1) dense fibrous collagen forming tissue growing in partly branching
and interweaving bundles, with fingerlike projections into the
surrounding fat tissue,
2) primitive mesenchymal tissue arranged in nests, concentric
whorls or bands and
3) mature adipose tissue intimately admixed with the other components.
There is usually no clear demarcation, although there are some
reports of older lesions showing some encapsulation. Extension
into underlying tissue, like muscle or fascia, is a known feature,
but visceral involvement has not been reported. The fibrous tissue
shows immunohistochemical evidence of myofibroblastic differentiation
as indicated by actin and desmin reactivity. The second component
consists of concentric immature mesenchymal ball-like structures,
often squeezed between or adjacent to the fibrous septa, or at
times isolated in the adipose tissue. The cells within these
areas are stellate, oval, round or spindle-like and the matrix
is fairly mucoid and Alcian blue positive. There is a rich capillary
network around the myxoid and the collagenized regions, with
a high proportion of infiltrating lymphocytes. The amount of
the third component, the fat tissue, varies from case to case.
It has been questioned if it really represents an essential integral
part of the tumour. The amount of the fatty tissue, always much
greater than normally present in the subcutis, and its striking
admixture with the other two elements have been viewed as evidence
in favour of its tumourous origin. The presence in some cases
of preadipocytes, suggestive of a maturation process, also supported
this assumption.
Although there is rarely a problem in recognition of this tumour,
a few other entities that may
occasionally simulate the picture of FHI, especially when dealing
with a small biopsy:
- Nodular fasciitis: rare in children,
circumscribed, abundant proliferation of plump fibroblasts
with frequent mitoses, extravasated erythrocytes,
multinucleated giant cells, feathery appearance,
myxoid background, no structured pattern, but rather
fibroblast-culture-like.
- Calcifying aponeurotic fibroma, especially
in small children: localisation(!), no centrally
located fat tissue, dense collagenous stroma, multinucleated
giant cells, calcifications in the later stages, high local recurrence
rate.
- Neural fibrolipoma: typical localisation,
infiltration of major nerves with neural
degeneration and atrophy, association with macrodactyly in 30%
of cases.
- Dermatofibrosarcoma protuberans (plaque
form): early to middle adult life, uniform
slender fibroblasts in a storiform pattern, areas of higher mitotic
activity with nuclear atypia
indistinguishable from fibrosarcoma (WHO: intermediate malignancy),
metastasis possible, IHC: CD 34+.
- Fibrous histiocytoma: rare in
children under two years of age, usually no extension
into the subcutis, multiplicity in one third of cases,
inflammatory cells and histiocytes (xanthoma cells)
sometimes to a remarkable degree (in deep situated
lesions), multinucleated giant cells, hemangiopericytoma-like
areas, overlying epidermal changes.
- Diffuse infantile fibromatosis: localisation(!),
the diffuse form mainly found in small
children has small ovoid cells in a myxoid background intermingled
with residual atrophic
muscle fibres and lipocytes without an organoid pattern, cellular
areas with high mitotic rates (DD: fibrosarcoma!),high recurrence
rate.
The treatment of choice for FHI is local excision. Even with
incomplete excision the recurrence rate is low and reported to
be between 10 to 15%. The prognosis is excellent.
