Case representation
A 3-year-old male child was referred with a right-sided neck swelling. Anatomical imaging with an MRI scan at the referring center demonstrated a mass in the right masticator space measuring 3.3.x2.7x3.2 cm involving the medial and lateral pterygoid muscles. Review of histological material received from the referring center, obtained from an incisional biopsy of this mass, showed primitive mesenchymal cells with loose, myxoid stroma, and variable degrees of skeletal muscle differentiation. Immunohistochemistry showed strong and diffuse Desmin expression, nuclear expression of Myogenin in fewer than 50% of tumor cells, and nuclear expression of MyoD1 in approximately 50% of tumour cells (Figure 1). The histological pattern was consistent with embryonal RMS. There was no evidence of FOXO1 rearrangement by fluorescentin-situ hybridization (FISH). Stage evaluation with PET/CT and bone marrow examination was negative for metastatic disease. The patient was started on chemotherapy according to the ’intermediate risk’ guidelines of the institutional protocol. After the initial 9 weeks of chemotherapy, response evaluation demonstrated tumor progression on MR and PET/CT imaging. At this point, his chemotherapy was intensified according to the ’high-risk’ guidelines. Since the tumor was surgically not amenable to resection, he received focal radiotherapy (59.4Gy) for local control, followed by consolidative adjuvant chemotherapy according to the high-risk protocol. He was then treated with maintenance therapy for 12 months. At the end of his chemotherapy, the tumor mass was stable on MRI and demonstrated a complete metabolic response on PET/CT. However, the risk of progression/relapse is extremely high.