Alveolar soft part sarcoma (ASPS) is a rare malignant soft tissue tumor that predominantly affects young adult women. ASPS constitutes less than 1% of all malignant soft tissue tumors. ASPS typically presents as a gradually enlarging mass in the extremities, head, and neck. However, it can also occur in other parts of the body. Herein, we report a case of ASPS occurring in the pectoralis major muscle of a 69-year-old man. He was referred to our institute with a soft tissue mass in the left thoracic region. CT, MRI, and fine-needle biopsy raised the suspicion of a soft tissue malignancy in the pectoralis major muscle. FDG-PET/CT confirmed the absence of additional lesions beyond the left thoracic region. The patient underwent radical resection of the tumor. Histopathological examination of the resected tumor confirmed the diagnosis of ASPS. Follow-up one year after surgery revealed no signs of local recurrence or distant metastasis. This case is particularly noteworthy as ASPS in the pectoralis major muscle is a rare occurrence. A literature search identified the present case as only the second reported case, and the first in Japan.
