Congenital myofibroblastic fibrosarcoma of the temporal bone in a newborn Arabian foal.

Overview

• This research article describes a rare case of a congenital malignant soft tissue tumor called myofibroblastic fibrosarcoma (MFS) found at birth in an Arabian foal, specifically originating from the temporal bone region.
• The study provides detailed clinical, histological, and immunohistochemical findings that led to the diagnosis of MFS, highlighting its uniqueness as the first reported congenital case in animals.

Introduction to Myofibroblastic Fibrosarcoma (MFS)

• MFS is a type of malignant tumor affecting soft tissues, characterized by the proliferation of myofibroblasts, which are cells that have features of both fibroblasts and smooth muscle cells.
• This tumor is generally rare in domestic animals and even more uncommon as a congenital (present at birth) condition.
• The significance of this study is heightened by the fact that congenital MFS has not been previously reported in animals.

Case Description

• The subject was a newborn Arabian foal presenting a large solitary subcutaneous mass located in the left temporal area of the head.
• Upon gross examination, the mass was hemispheric, unencapsulated, elastic, fibrous, and had a white-yellowish coloration.
• The mass was attached firmly to the temporal bone, indicating close association with the skeletal structure.

Histological Findings

• The tumor consisted predominantly of spindle-shaped fibroblasts arranged in fascicles or interwoven bundles.
• Neoplastic cells showed low nuclear atypia, meaning there was minimal deviation from normal cell nucleus structure, which sometimes correlates with less aggressive tumors.
• Mitotic rate (a measure of cell division activity) was low, indicating slow proliferation of the tumor cells.
• There was evidence of local invasion into surrounding soft tissues and the temporal bone, but no metastasis (spread to distant sites) was detected.
• The stroma (supportive tissue) of the tumor was abundant in collagen, which is typical for fibrosarcomas.

Immunohistochemical Analysis

• Immunohistochemistry involves staining tumor cells with antibodies to detect specific proteins to assist in diagnosis:
• The tumor cells showed strong and diffuse labeling with:
• Vimentin – a marker for mesenchymal origin cells
• Calponin – associated with smooth muscle differentiation
• Desmin – a protein found in muscle cells
• Alpha-smooth muscle actin (α-SMA) – marker of smooth muscle and myofibroblast cells
• S100 protein was mildly expressed, which is a marker sometimes seen in nerve-associated or melanocytic cells but can appear in some fibrous tumors.
• H-caldesmon was weakly positive; this protein is specific to smooth muscle cells and helps distinguish myofibroblastic tumors from smooth muscle tumors.
• The tumor cells were negative for:
• Pancytokeratin AE1/AE3 – ruling out epithelial origin tumors
• P63 – often used to detect squamous and myoepithelial cells

Diagnostic Conclusion

• The combination of histopathological features and immunohistochemical profile supported the diagnosis of congenital myofibroblastic fibrosarcoma in the foal.
• Diagnosis was reinforced by the low-grade cellular atypia, local invasive behavior without metastasis, and the specific immunohistochemical markers consistent with myofibroblastic origin.
• This case is notable as the first documented incidence of congenital MFS in an animal, expanding the understanding of tumor pathology in veterinary medicine.

Significance and Implications

• Provides insight into differential diagnosis for soft tissue masses in newborn animals.
• Highlights the importance of combining histological examination with immunohistochemistry to accurately characterize and diagnose rare tumors.
• Offers a reference case for veterinary pathologists encountering similar tumors, especially in horses.
• May prompt further research into congenital soft tissue tumors in animals and their biological behavior.