Page 137 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

VetBooks.ir 9 Mesenchymal Tumours
and Other Neoplasms

Mesenchymal tumours originate in the dermis and in the subcutaneous tissue. They can be
classified based on the cell type they arise from:
• Connective tissue (fibrocytes/fibroblasts):
• Fibroma and fibrosarcoma.
• Myxoma and myxosarcoma.
• Keloidal fibroma and fibrosarcoma.
• Vessels (endothelial cells and perivascular cells):
• Haemangioma and haemangiosarcoma.
• Perivascular wall tumours (PWTs).
• Nerves:
• Peripheral nerve sheath tumours (PNSTs) including schwannoma.
• Smooth muscle cells of vessels and hair follicles:
• Leiomyoma and leiomyosarcoma.
• Subcutaneous adipose tissue (adipocytes):
• Lipoma and liposarcoma.
By extension, other neoplasms included in this category are:

• Melanocytes:
• Melanocytoma and melanoma.
• Langerhans cells and interstitial dendritic cells:
• Histiocytic disorders.

In this chapter, rhabdomyoma and rhabdomyosarcoma are discussed briefly, as clinically they
may appear as subcutaneous masses, even if they arise from the beneath muscle layer.

Cytological diagnosis of cutaneous mesenchymal lesions
The cell morphology depends on the histotype. Most mesenchymal tumours exfoliate spindle-
shaped cells with oval nuclei and elongated cytoplasm forming characteristic tails; for this rea-
son, the term spindle cell tumour is often used. In certain cases, specific cytological features are
present, allowing the identification of the lineage of origin and a more detailed diagnosis. This is
particularly true for neoplasms such as lipoma, liposarcoma and perivascular wall tumours. In
other cases, cytological features are not considered typical for a specific mesenchymal neoplasm
and a more generic term of soft tissue sarcoma or simply sarcoma is used.
It should also be emphasized that the identification of mesenchymal neoplasms is not free
from interpretative doubts. Well-differentiated sarcomas may, in some instances, lack cyto-
logical criteria of atypia; on the contrary, in reactive fibroplasia and scar tissue, aspiration may
exfoliate dysplastic fibroblasts, which could be misdiagnosed as neoplastic. In these cases, the

124 © Francesco Cian and Paola Monti 2019. Differential Diagnosis in Small Animal Cytology
(F. Cian and P. Monti)