BackgroundRadiation therapy (RT) is considered a risk factor for the development of sarcoma in patients with breast cancer. However, there are few reports regarding post-irradiation sarcoma (PIS).Case presentationThe patient was a 59-year-old woman who presented with a chief complaint of induration in the lower outer quadrant of the left breast. She underwent breast-conserving surgery (BCS) for breast cancer located in the left upper inner region and received endocrine therapy following RT (50 Gy/25 fractions/5 weeks) for breast conservation 6 years previously. Core needle biopsy revealed leiomyosarcoma. There was no distant metastasis. Repeat BCS including part of the pectoralis major muscle was performed. Chest wall resection was not performed because of the lack of invasion. Based on the morphological and immunohistochemical features, a diagnosis of leiomyosarcoma was made. All of the resection margins in the specimen were tumor-free. She has been disease-free for over 20 months.ConclusionsPost-irradiation leiomyosarcoma is an extremely rare tumor with high malignant potential, and thus, multidisciplinary therapy and close follow-up are advised.