Embryonal Rhabdomyosarcoma of the Diaphragm in a Two-Year-Old Boy.

Although primary tumors of the diaphragm are rare in children, rhabdomyosarcoma is the most common malignant tumor arising in the diaphragm. Rhabdomyosarcoma is a malignant tumor that arises from primary mesenchymal cells that differentiate into skeletal muscle. Here, we report on a two-year-old boy in whom embryonal rhabdomyosarcoma of the diaphragm developed and who presented with abdominal distension due to significant ascites. Contrast-enhanced computed tomography revealed two mass lesions: a lesion that had originated from the right diaphragmatic crus, part of the diaphragm, extending to the omental bursa, and another lesion in the left inguinal canal. Furthermore, massive ascites and diffuse peritoneal thickening suggested peritoneal dissemination. On 2-deoxy-2-(fluorine-18)-fluoro-D-glucose positron emission tomography combined with computed tomography, these lesions exhibited high uptake, suggesting malignant tumors. Pathological examination of biopsy specimens of the left inguinal tumor revealed embryonal rhabdomyosarcoma. According to the Intergroup Rhabdomyosarcoma Study IV risk classification system, the patient was categorized as high risk, meeting the criteria for embryonal-type rhabdomyosarcoma, stage 4, and group IV. The ARST0431 therapy was promptly started. Because tumor tissues disappeared on imaging after chemotherapy, surgical resection was omitted. No recurrence has been observed 16 months after the completion of treatment, including chemotherapy and radiotherapy. Even for a group IV rhabdomyosarcoma of the diaphragm, an embryonal type might improve the prognosis with multidisciplinary treatment. Long-term follow-up should carefully monitor not only recurrence but also side effects from the toxicity of total abdominal irradiation.