Page 165 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

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Rhabdomyoma and rhabdomyosarcoma
VetBooks.ir Tumours of the skeletal muscle deriving from myofibroblasts or primitive mesenchymal cells
capable of differentiation into skeletal muscle cells.

Clinical features
• Very rare skin neoplasms in dogs and cats. The benign form is more rare than the
malignant counterpart.
• No age, breed or sex predilection reported. However, several cases of rhabdomyosar-
coma described in the literature have been observed in young dogs (< 2 years old).
• Rhabdomyoma has been primarily reported in the larynx and ear pinnae in dogs and
cats, respectively. Rhabdomyosarcoma may be observed in several locations in both
species. In cats, it may also develop at the site of previous injection or vaccination.
• Most tumours present as firm, poorly demarcated, often multilobulated masses,
which arise in the underlying muscle. Alopecia and ulceration are common.
• Rhabdomyoma is a slow-growing tumour.
• Rhabdomyosarcoma is characterized by a more aggressive behaviour and metastatic
potential.

Cytological features
• Variable cytological features, often non-specific.
• The cellularity is variable, from low to high.
• Background: clear and variably haemodiluted.
• Cell morphology greatly depends on the histological variant.
• They are often individualized, round to polygonal and with a low N:C ratio. Additional
populations of elongated cells and smaller cells with a high N:C ratio can also be seen (un-
differentiated rhabdomyoblasts).
• Nuclei are round, central to paracentral. They have granular chromatin. Prominent round
nucleoli are often visible in the malignant forms.
• Cytoplasm is abundant, basophilic, often granular, with poorly defined borders. Cytoplas-
mic cross-striation and paranuclear clear halo may be seen occasionally.
• Cellularity and cellular atypia are higher in malignant forms and may include marked an-
isocytosis/anisokaryosis, multinucleation and bizarre mitotic figures. Multinucleated cells
may have nuclei arranged in a linear fashion within the cytoplasm (strap cells).
Variants
Four different histological variants of rhabdomyosarcoma have been reported in the literature
and include botryoid, embryonal, alveolar and pleomorphic forms. The botryoid types mainly
affect the urinary bladder, whereas the other forms have been described originating from striated
muscle of the outer body surface.
• Embryonal rhabdomyosarcoma:
• This is the most common variant and has been reported in dogs and only rarely in cats.
• Head and neck are preferred anatomical locations.
• Cells may exhibit different stages of development from round to polygonal and often
show marked signs of atypia.
• A spindle-cell sub-variant has also been described in literature.