Malignant Transformation of Schwannoma to S-100 Negative Epithelioid Malignant Peripheral Nerve Sheath Tumor: A Case Report

The malignant transformation of schwannomas is exceptionally rare and typically results in a malignant peripheral nerve sheath tumor (MPNST). We report a unique patient with malignant transformation of a schwannoma into an S-100 negative epithelioid MPNST. A 72-year-old male patient presented with an abdominal mass, and imaging revealed a 5 cm tumor in the rectus abdominis muscle, which was diagnosed as schwannoma through biopsy. The patient opted for observation instead of surgery. Eight years later, the tumor had enlarged to 10 cm, accompanied by worsening pain. Imaging showed intra-abdominal dissemination and metastases to mediastinal and para-aortic lymph nodes. Comprehensive whole-body positron emission tomography and computed tomography imaging and meticulous re-examination of the original schwannoma biopsy definitively excluded alternative primary malignancies and established histopathological continuity between the current lesion and the pre-existing schwannoma. Immunohistochemical staining indicated the tumor was S-100 negative, SOX10 negative, H3K27me3 positive, SMARCB1/INI1 positive and EMA negative, leading to the diagnosis of S-100 negative epithelioid MPNST. Due to the rapid tumor growth, palliative radiation therapy was administered, and the patient received best supportive care. This patient was identified as an S-100 negative epithelioid MPNST.