• Loose sheets of ovoid to plump spindle cells associated with some myxoid to fibrillary stroma
• Singly-occurring plump to stellate cells with tapering cytoplasmic processes and some plasmacytoid cells
• Scattered lymphocytes and occasional osteoclast-like multinucleated giant cells also noted
• No necrosis present

Further investigations: Ultrasound scan showed a well-defined heterogeneous hypoechoic nodule within the subcutaneous layer measuring 1.0 x 0.9 x 0.8 cm. No fatty hilum suggestive of a normal lymph node was seen. No invasion of underlying structures was present.

Differential diagnoses:

• Mesenchymal lesions/neoplasms
  • Nodular fasciitis
    • Spindle to stellate cells occurring in clusters and as singly dispersed cells; can be associated with mxyoid stroma
    • Cells have abundant cytoplasm and tapering cytoplasmic processes
    • Inflammatory cells often seen 
    • Can have relatively frequent mitoses
    • If cell block is available – lesional cells are positive for SMA
  • Schwannoma
    • Clusters of slender spindle cells associated with fibrillary stroma
    • Nuclei may appear wavy 
    • Nuclear palisading may be seen
    • Generally lack single cells in the background
    • Occasional larger, atypical nuclei (so-called ancient change) may be present
    • If cell block is available – lesional cells are positive for S100
• Epithelial neoplasms
  • Pleomorphic adenoma 
    
    • Potential differential diagnosis if suitable location (e.g. angle of jaw, submandibular region etc)
    • Clusters of epithelioid to spindle epithelial cells blending into metachromatic fibrillary stroma
    • Scattered singly dispersed plasmacytoid to plump cells in the background

The lump was excised a few months later. 

• Circumscribed subcutaneous nodule composed of a cellular proliferation of spindle cells in a fascicular to storiform pattern with areas of microcystic change; myxoid and oedematous areas
• Tissue culture-like arrangement of cells seen at the periphery
• Admixed lymphocytes, osteoclast-like multinucleated giant cells and extravasated red blood cells
• Scattered mitoses seen, but no atypical forms identified
• No necrosis seen
• Spindle cells are positive for SMA, and negative for S100, CD34, desmin and beta-catenin

Nodular fasciitis

Comments:

• The supportive clinical features in this case include – rapid growth over a relatively short period of time and subcutaneous location of the lesion. Sometimes, there can be a preceding history of trauma to the area.
• Nodular fasciitis is known mimic of malignancy on both cytology and histology – awareness of its typical features and correlation with clinical information are crucial.