Myxoid sarcoma or myxosarcoma in rabbits
Esther van Praag, Ph.D.
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The word “sarcoma” comes from the Greek and means “fleshy growth”. Sarcoma is nowadays used to describe relatively rare a group of malignant tumors that involve the connective tissue. Although sarcomas are well-recognized tumors, their characteristics lead to confusion. Indeed, some type of sarcoma may present a combination of features of various different types of sarcoma. This lead to the widely accepted conclusion that the neoplastic development of a primitive mesenchymal cell can lead into different direction, thus different types of sarcoma.
The tumors are found is all parts of the body: forelimbs, hind limbs, head, neck, shoulder, chest, abdomen or hip; as well as in all types of tissues: muscle tissue, fat tissue, in the blood vessels, in the tissue surrounding joints, and in tendons.
Four principles apply for sarcoma tumors:
• Location: a superficially located tumor is less likely to be malignant than a deeper located tumor.
• Size: bigger tumors are more likely to become malignant than small tumors.
• Growth: rapid growth tumors are more likely to be malignant than slow growing ones.
• Vascularization: malignant tumors are rich in blood vessels, whereas benign tumors are avascular or poorly vascularized.
Sarcoma tumors are locally invasive into the surrounding tissues. Although their metastatic rate is low, they can metastase through the bloodstream to other organs.
Myxosarcoma is defined has a fibrosarcoma rich in connective tissue proteoglycans (mucins). This kind of tumor is therefore also sometimes called myxoid fibrosarcoma. In veterinary medicine, the term “myxosarcoma” remains, however, a general term for otherwise specific sarcoma tumors like myxofibrosarcoma, liposarcomas or malignant fibrous histiocytomas.
A virus called "malignant rabbit fibroma virus" has been isolated in rabbits, which can lead to fibrosarcoma, and perchance to fibrosarcoma derived myxosarcoma.
Myxosarcoma differentiate between low-grade, intermediate and high-grade lesions. The tumors can be small and (multi)nodular, or large. Their degree of malignancy is generally low, but increases with successive regrowth (recurrence). All have invasive properties into surrounding tissues.
Typically, a tumor has a myxoid matrix containing spindle- to polyhedral cells. The neoplastic cells can have various shapes: round, ovoid or elongated; their nuclei is hyperchromatic. Sheets of interdigitated cells, rich in myxoid matrix can be present. Pseudolipoblasts are commonly observed. Vascularization of the tumor is typical, with curvilinear capillaries.
The complete excision of the myxosarcoma lesions is necessary. Indeed, a characteristic of myxosarcoma and myxofibrosarcoma tumors is their high rate of recurrence (according to species, up 70% within year of surgical removal). At recurrence, these lesions gain a higher grade of aggressiveness and an increased potential to become malignant. Prognosis is guarded.
All my gratitude to Susan L. (USA), to Christine Harvey, DVM (USA), to E. Kufuor-Mensah, DVM (USA) and to Dr. Taylor, DVM (USA) for kindly allowing the use of illustrative material for this text. Thanks are also due to Timothy, for his patience during picture-sessions.
Schaff Z, Grimley PM, Michelitch J, Banfield WG. Spontaneous myxosarcoma in a cottontail rabbit (Sylvilagus floridanus): observation of tubular structures in the endoplasmic reticulum of tumor cells. J Natl Cancer Inst. 1973; 51(1):293-7.
Strayer DS, Sell S, Skaletsky E, Leibowitz JL. Immunologic dysfunction during viral oncogenesis. I. Nonspecific immunosuppression caused by malignant rabbit fibroma virus. J Immunol. 1983; 131(5):2595-600..