a, b Histopathological findings. The pathological specimen revealed that spindle cells with different-sized nuclei were mixed throughout the tissue. Some regions showed nuclear polymorphism or a storiform pattern, and locally, there were approximately 7 mitoses/10 HPFs. The margin was relatively clear, but spindle-shaped tumour cells infiltrated the surrounding interstitium and basal myoepithelium, and the patient was diagnosed with malignant peripheral nerve sheath tumour (MPNST). c Immunostaining showed MIB-1-positive staining. The MIB-1 LI was approximately 25%. d Immunostaining showed S-100-positive staining.