After an aggressive recurrence 11 months later, pathology was consistent with high-grade MPNST including heightened mitotic activity and loss of S100 expression.
We present 3 cases of cutaneous S100-positive spindle cell tumors in elderly patients, in which first findings led to initial misdiagnoses as cutaneous MPNST and benign peripheral sheath nerve tumor (neurofibroma).
The tumor was resected; histologic appearance and immunophenotype, including patchy but strong positivity for S100 protein, indicated a diagnosis of malignant peripheral nerve sheath tumor.
They also support the fact that about one third of MFSS and one fourth of PDSS are positive for S-100 protein, a finding of diagnostic relevance when considering their distinction from other spindle to round cell sarcomas, especially malignant peripheral nerve sheath tumors.