Diagnostic Cytology
Fig. 18.12 Embryonal RMS. Cluster of "small round blue cells" of
embryonal RMS. The differential diagnosis includes ES/PNET (H&E x HP).
common locations. It also occurs in the pelvis and is the most
common bladder tumor in young children, although the alveo-
lar subtype is less frequent in the pelvis. According to NCI cri-
teria embryonal RMS is considered a favorable subtype over
alveolar rhabdomyosarcoma.72 Pleomorphic RMS is the rarest
type and may be difficult to distinguish from other high-grade
sarcomas such as MFH.
The aspiration biopsy sample of RMS is usually very cellular,
with loose tissue fragments and single cells. Myxoid and colla-
genous stroma is often abundant.
Embryonal RMS shows a combination of primitive round
cells with other forms of rhabdomyoblasts such as strap cells.
The cells of embryonal rhabdomyosarcoma are "small round
blue cells" and must be distinguished from the other small round
blue tumor cells occurrent in childhood such as Ewing sarcoma
(ES)/primitive neuroectodermal tumor (PNET) (Fig. 18.12).
When cytoplasmic cross-striations are found they serve as a clue
to the correct classification of this tumor. Myxoid stroma is seen
in the background. A tigroid background has been reported but
not often encountered.73
Alveolar RMS shows a relatively uniform population of round
cells with eccentrically located nuclei and cytoplasmic vacuoles
(Fig. 18.13).
Pleomorphic RMS are rich in mitotic figures, and have bizarre
shapes and atypical nuclei. Cytoplasmic cross-striations may be
found in some cells.
RMSs are positive for MyoD1 and myogenin; myoglobulin
is considered less useful due to background staining and lower
specificity.74 Rhabdomyoblasts are usually glycogen positive.
Molecular studies can be useful in alveolar RMS with a con-
sistent t (2; 13) (q35; q14) and a variant translocation t (1; 13)
(p36; q14). Embryonal and pleomorphic RMS do not show a
consistent cytogenetic abnormality.
Cytologically, the demonstration of rhabdomyoblasts is most
supportive of the diagnosis of RMS. Degenerate and regenerat-
ing striated muscle fibers can be mistaken for rhabdomyoblasts.
The botryoid subtype of embryonal RMS, found in bladder
and vagina of children, is characterized by abundant myxoid
stroma and a denser, more cellular area. There are few cytologic
examples of this tumor.
Fig. 18.13 Alveolar RMS. FNAB of alveolar RMS. Note the eccentric nuclei
and soft cytoplasm (Papanicolaou x HP).
Key features of embryonal RMS
• Primitive "small round blue cells" mixed with strap
• Cross-striations possibly present;
• Myxoid stroma; and
• Subtype more favorable than other RMS.
Key features of alveolar RMS
• Uniform population of round cells;
• Eccentrically located nuclei;
• Cytoplasmic vacuoles; and
• t(2;13)(q35;q14).
Key features of pleomorphic RMS
• Atypical nuclei;
• Bizarre shapes;
• Cross striations; and
• Mitotic figures.
Vascular Tumors
Hemangiomas are common benign vascular neoplasms which
can present in almost any body location and at any age. They
are easily recognized in superficial locations and enter the dif-
ferential diagnosis of deep-seated vascularized lesions. They are
characterized histologically by a proliferation of thin-walled
vascular channels lined by bland endothelial cells (Fig. 18.14A).
Cytologically, three-dimensional arcades or compact dense coils
composed of bland elongated spindle cells associated with scat-
tered single spindle cells can be seen in a background containing
abundant blood (Fig. 18.14B). The cells have scant to moderate
amount of cytoplasm, bland nuclei, nuclear grooves, and incon-
spicuous nucleoli. Because of the potential misdiagnosis of the
specimen as unsatisfactory for evaluation, recognition of the cel-
lular pattern of hemangiomas combined with the radiological
suspicion of a vascularized lesion is an important finding for a
correct diagnosis.75,76
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