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Synovial sarcoma is a malignant soft-tissue sarcoma (STS) that occurs in children and young adults but very rarely in neonates. We herein report a neonatal case of synovial sarcoma. Informed consent for the publication of this case report and any accompanying images was obtained from the patient’s parents. A 2-month-old female infant was referred to the present hospital due to swelling of the left forearm. She was born to non-consanguineous parents uneventfully at 41 weeks of gestation and weighed 3,510 g. She had no family history of early-onset malignancy. Her mother noticed a swelling at 2 weeks after birth. On admission, she had no clinical symptoms except for the swollen left forearm (Fig. 1a). Magnetic resonance imaging showed a well-defined lobulated mass (45 9 30 9 30 mm) between the brachialis and brachioradial muscles (Fig. 1b,c). Computed tomography (CT) of the chest and positron emission tomography-CT of the whole body showed no evidence of metastatic lesions. She underwent incisional biopsy of the tumor. On microscopy, spindle cells were increased with a storiform pattern. Immunostaining of the tumor cells was positive for AE1/AE3, CAM5.2, epithelial membrane antigen, Bcl2, and a-smooth muscle actin, and negative for S100, CD34, and desmin. Polymerase chain reaction of the tumor specimen was positive for SYT-SSX1/2 translocation (Fig. 1d). The SYT rearrangement was identified on fluorescence in situ hybridization. All of the aforementioned findings were compatible with a diagnosis of synovial sarcoma. Her post-surgical grade, according to the Intergroup Rhabdomyosarcoma Study grouping system, was group III. The patient received three courses of neoadjuvant chemotherapy with ifosfamide (IFO) and doxorubicin (DXR). IFO 33 mg/kg (the first course) or 50 mg/kg (subsequent courses) was given for 3 days and DXR 0.625 mg/kg for 2 days per course. Although the chemotherapy modestly reduced the size of the tumor, its effect was transient; therefore, the patient underwent amputation of the left arm. Almost all of the tumor cells were viable without significant necrosis. The patient received another course of chemotherapy with the same regimen for relapse prevention. At the time of writing, the patient had been disease-free for 8 months since the operation. Synovial sarcoma is a rare malignant STS driven by a translocation between SYT and SSX. Although it is the most common non-rhabdomyosarcoma STS (NRSTS) in children, neonatal onset is extremely rare. Rhabdomyosarcoma represents approximately one-third of all STS in infants, followed by fibrosarcoma and malignant rhabdoid tumor. Synovial sarcoma was observed in 239 of 3,316 patients 1–18 years of age but in none of 302 infants with STS. As such, determining the appropriate therapeutic strategies for early infantile synovial sarcoma can be difficult. We could identify only five infants with synovial sarcoma in the literature (Table S1). Given that synovial sarcoma in children is chemosensitive compared with other NRSTS, the standard approach is wide surgical excision combined with radiotherapy and chemotherapy, according to risk stratification. High survival rates were reported for intermediateand high-risk groups in the European Pediatric Soft Tissue Sarcoma Study Group (EpSSG) NRSTS 2005 with multimodal therapy including IFO-DXR chemotherapy and radiotherapy, but the omission of adjuvant chemotherapy has been suggested for the low-risk group, defined as “initial complete resection, arising from non-axial Correspondence: Yasutomo Funakoshi, MD, Nagasaki University Hospital, 1 Chome-7-1 Sakamoto, Nagasaki City, Nagasaki 8528501, Japan. Email: [email protected] Received 25 December 2017; revised 22 February 2018; accepted 3 April 2018. doi: 10.1111/ped.13581