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Extraskeletal osteosarcoma is a rare soft tissue sarcoma and typically appears as a solid mass with variable mineralization. A few cases of extraskeletal osteosarcoma have shown extensive hemorrhagic changes or bleeding due to its high-grade malignancy; however, to the best of our knowledge, no previous reports have described the pathological characteristics of tumors with non-hemorrhagic cystic change. The present report discusses three cases involving cystic lesions with a solid area at the periphery that arose in soft tissues. The large cystic spaces contained only yellowish-brown fluid and little or no blood, with no clear pathological necrotic tissue. The solid component comprised a focal area of osteoid production by highly anaplastic sarcomatous cells. All of the cases showed high-grade malignancy histologically, and the average maximum diameter was approximately 175 mm. The tumors were located at the adductor muscles in two cases and at the intermuscular region of the hamstring muscles in one case. All of the patients consulted a doctor after more than half a year had passed since the recognition of swelling or a mass on their affected extremities. Surgical treatment was performed for all patients. One patient had lymph node metastasis, and another had lymphoedema after surgery, but no histological invasion to the lymphatic ducts was observed in the excised specimen. Degenerative changes in the fluid content over a long time-course as a result of tumor bleeding or necrosis, rather than lymphorrhea by lymphatic channel invasion of the tumor, might have contributed to the formation of the cystic lesion. Only 1 patient who underwent chemotherapy remained clinically disease-free over 10 years after surgery. Chemotherapy regimens for osteosarcoma rather than those for soft tissue sarcoma are mostly effective for extraskeletal osteosarcoma, so the diagnosis by a biopsy is essential. It is important to consider extraskeletal osteosarcoma in the differential diagnosis of soft tissue tumor with cystic form and calcification and a long clinical course before consulting a doctor. [ABSTRACT FROM AUTHOR]