“A Five Month Old Girl with a Right Cheek
Mass”
California Tumor Tissue Registry’s
Case of the Month
CTTR COTM Vol. 15:2
November, 2012
www.cttr.org
A healthy Caucasian girl was delivered vaginally at full term, with no complications. At
two weeks, a soft, non-discrete, non-tender right cheek mass was noted. The mass was
non-pulsatile, did not bleed or interfere with feeding or respiration, nor did the size of the
mass fluctuate with crying or movement. There were no abnormalities of the overlying
skin or mucosa. The mass was closely observed for the next few months. Ultrasound
demonstrated a 4 x 3 x 1.4 cm solid whorled mass anterior to the parotid; MRI revealed a
well-circumscribed mass involving the right masseter muscle, adjacent to the mandible
with increased T2-signal within the bone marrow of the mandible. At five months of age,
she underwent intraoral excision of the mass under anesthesia. The patient, now 16
months old, recovered from surgery without complications. Post-operative MRI
demonstrated a residual and/or recurrent 3 cm right masseter mass with no other nodules
or masses have been observed. No family history of similar lesions or syndromic
conditions was elicited.
Resection of the lesion yielded an aggregate of approximately 2.0 x 1.5 x 0.4 cm
homogenous brown soft tissue. Microscopically, the lesion appeared to be wellcircumscribed but extended to the inked margins (Fig. 1). Skeletal muscle differentiation
and maturation were observed with cross striations and haphazard arrangement of
irregular fascicular bundles of muscle cells with scant to moderate eosinophilic
cytoplasm (Figs 2-4). No definite immature cells were identified, and no myxoid stroma
was noted. No areas of hypercellularity, increased mitotic activity, pleomorphism or
necrosis were observed.
Diagnosis: “Fetal rhabdomyoma, intermediate (cellular) type, cheek”
Evelyn Choo MD1, Rachel Conrad MD1, Anwar Raza, MD1, Donald Chase MD1,2
1. Department of Pathology and Human Anatomy, Loma Linda University and
Loma Linda Medical Center, Loma Linda, California
2. California Tumor Tissue Registry, Loma Linda, California
Fetal rhabdomyoma is a rare but benign skeletal muscle neoplasm, first described by
Dehner in 1972. It typically occurs in the head and neck region of young children and
demonstrates varying degrees of skeletal muscle differentiation.
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The term rhabdomyoma encompasses a complex classification system of several distinct
benign skeletal muscle neoplasms. It is organized by location (cardiac versus
extracardiac), and the extracardiac type is divided into adult, genital and fetal categories
based on tissue differentiation and clinical presentation. The adult form tends to occur in
head and neck region of 40-60 year old males and displays prominent eosinophilic
polygonal cells with vacuolated cytoplasm. The genital form typically presents as a
polypoid lesion in vulva and vagina of middle-aged women; microscopically, long straplike muscle fibers with prominent cross-striations are seen in a collagenous and myxoid
matrix. The fetal form shows immature skeletal muscle differentiation and is more
common in the head and neck region of children below age four.
The fetal form is further subdivided into a myxoid type and an intermediate type. The
myxoid subtype (also known as “classic”) is comprised almost entirely of immature
primitive spindle cells in a myxoid stroma and tends to appear in postauricular soft tissue.
In contrast, a wider spectrum of myocyte maturation is seen in the intermediate subtype
(also known as “cellular” or “juvenile”), initially described by Di Sant’Agnese and
Knowles in 1980. This subtype of rhabdomyoma appears more often in soft tissue of face
or mucosal tissue and represents a more differentiated lesion than classic fetal subtype.
Clinically, fetal rhabdomyoma presents in children less than four years old, may often be
congenital and is more common in males than females (2.4:1). The lesion consists of a
solitary, well-defined, non-tender mass involving soft tissue or mucosa. It grows very
slowly and seldom ulcerates the overlying skin or mucosa. Multiple extracardiac
rhabdomyomata have been associated with nevoid basal cell carcinoma syndrome (also
known as Gorlin-Goltz syndrome) and may show a mutation in PTCH on chromosome
9q22. However, no correlation is seen with tuberous sclerosis as in cardiac
rhabdomyoma.
The gross appearance of fetal rhabdomyoma consists of a well-circumscribed, smooth,
polypoid mucosal lesion, typically two to six centimeters in diameter. Lesions are
typically solitary. Cross-sectioning reveals a grey to pink surface, often with a myxoid
appearance.
Microscopically, these lesions display either a myxoid or an intermediate pattern. The
myxoid version is comprised of a myxoid matrix surrounding scattered or short bundles
of immature spindle-shaped or oval cells with immature skeletal muscle fibers. The small
uniform nuclei contain delicate chromatin and are enveloped by tapered eosinophilic
cytoplasmic processes. Rare cross-striations are present. The intermediate type closely
resembles classic adult head and neck rhabdomyoma, and contains a wider spectrum of
myocyte differentiation with few immature spindle-shaped muscle precursors and
numerous prominent strap-shaped muscle cells. The myocytes have central vesicular
nuclei, scant to moderate eosinophilic cytoplasm, and frequent cross-striations with
occasional vacuolation and glycogen. They are haphazardly arranged in irregular
fascicular bundles. Ganglion-like rhabdomyoblasts with prominent nucleoli may be
present. In both myxoid and intermediate types, the absence of significant atypia,
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necrosis, or mitotic activity is noted. Invasion and destruction of surrounding tissues
should not be seen.
Immunohistochemical staining shows positivity for muscle-specific actin (MSA),
myoglobin, and desmin; focal positivity may be seen for smooth muscle actin (SMA),
S100, glial fibrillary acidic protein (GFAP), and vimentin.
Electron microscopy will show thick and thin myofilaments in the more mature
myocytes; z-bands and glycogen may also be demonstrated in the cytoplasm. The
immature myocytes may lack specific ultrastructural features of myocyte differentiation.
The recommended therapy for fetal rhabdomyoma is complete excision. Prognosis is
excellent. Local recurrence is rare and is typically associated with incomplete resection.
Metastases have never been described in the literature. Only two controversial cases of
possible malignant transformation have been reported but may have been misdiagnosed
as benign initially, and recurrent cases should be carefully evaluated for a misdiagnosis of
rhabdomyosarcoma.
The differential diagnosis of fetal rhabdomyoma includes rhabdomyosarcoma (both
embryonal and spindle cell types), adult rhabdomyoma, genital rhabdomyoma, Triton
tumor (neuromuscular hamartoma), rhabdomyomatous mesenchymal hamartoma of the
skin, and infantile fibromatosis.
1. Rhabdomyosarcoma consists of pronounced diffuse cellular immaturity, necrosis,
mitotic figures, and infiltration of adjacent tissues with frank destruction. The
presence of nuclear atypia in rhabdomyosarcoma is the most important criteria to
distinguish it from fetal rhabdomyoma.
2. Adult rhabdomyoma has distinctive polygonal “spider” cells with vesicular nuclei,
central nucleoli, and eosinophilic cytoplasm. It tends to occur in the head and
neck region of adult males.
3. Genital rhabdomyoma demonstrates predominately mature long, strap-like
striated muscle cells and occasional immature myocytes in a background of
myxoid material and collagen. It typically presents as an asymptomatic, slowgrowing, small polypoid lesion in the vulvo-vaginal region of middle-aged
women. Rare occurrences in the male urogenital tract have also been described.
4. Triton tumor is composed of nerve fascicles mixed with mature striated skeletal
muscle. It is associated with neurofibromatosis in younger patients and typically
affects the axial skeleton.
5. Rhabdomyomatous mesenchymal hamartoma consists of normal dermal elements
with mature striated skeletal muscle.
6. Infantile fibromatosis is not as well-circumscribed as fetal rhabdomyoma. It tends
to involve regions deeper than the subcutis, has a fasciculated spindle cell pattern,
lacks cytoplasmic striations, and contains interspersed fat cells.
Fetal rhabdomyoma is a rare neoplasm of immature skeletal muscle. Distinguishing it
from the more common, more aggressive rhabdomyosarcoma is important. Clinical
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characteristics (location, behavior) and microscopic traits (mitoses, necrosis, and
especially nuclear atypia) may be helpful. Prognosis is excellent after surgical excision.
Recognition of this benign neoplasm is important in communicating accurate prognostic
information and in preventing over-aggressive treatment.
Suggested Reading:
1. Dehner LP, Enzinger FM, Font RL. Fetal rhabdomyoma: an analysis of nine
cases. Cancer. 1972;30:160-166.
2. Di Sant’Agnese PA, Knowles DN. Extracardiac rhabdomyoma: a
clinicopathologic study and review of the literature. Cancer. 1980;56:780-789.
3. Kapadia SB, Barr FG. Rhabdomyoma. In: Fletcher CDM, Unni K, Mertens F,
eds. World Health Organization Classification of Tumours. Pathology and
Genetics of Tumors of Soft Tissue and Bone. Lyon, France: IARC Press,
2002:142-145.
4. Premalata CS, Kumar RV, Saleem KM, Fathima LJ, Das K. Fetal rhabdomyoma
of the lower extremity. Pediatr Blood Cancer. 2009;52(7):881-883.
5. Walsh SN, Hurt MA. Cutaneous fetal rhabdomyoma: a case report and historical
review of the literature. Am J Surg Pathol. 2008;32(3):485-491.
6. Weiss SW, Goldblum JR. Enzinger & Weiss’s Soft Tissue Tumors. 5th ed.
Philadelphia: Mosby-Elsevier, 2008:583-592.
7. Yang S, Zhao C, Zhang Y, Liao S. Mediastinal fetal rhabdomyoma in nevoid
basal cell carcinoma syndrome: a case report and review of the literature.
Virchows Arch. 2011; 459:235-238.
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