We reported a case of post-irradiation leiomyosarcoma that developed following BCS. The incidence of PIS may continue to increase because of the combined use of BCS and RT as a standard therapy for breast cancer. There are some prior reports regarding PIS. However, most of these reports described post-irradiation angiosarcoma instead of leiomyosarcoma. Yap et al.  reported that among sarcomas occurring within the RT field, angiosarcoma accounted for 56.8 % of the lesions, with malignant fibrous histiocytoma (MFH) accounting for 15.9 %, fibrosarcoma and chondrosarcoma accounting for 6.8 %, and leiomyosarcoma accounting for 4.5 % of the lesions.
The latency period between RT and the appearance of PIS ranges from 4 to 36 years with a mean of 12 years, and the average radiation dose was 53 Gy . There was no difference between patients treated with megavoltage radiation and those treated with orthovoltage radiation concerning the type of sarcoma, its location, or survival rates . This means that the occurrence of PIS is a stochastic effect instead of a deterministic effect.
PIS is often difficult to diagnose in patients with fibrosis legions after RT, and thus, the diagnosis may occur late . In our case, we could diagnose post-irradiation leiomyoma after re-biopsy. Therefore, it may be necessary to perform excisional biopsy to exclude the possibility of PIS.
Appropriate morphological features should distinguish leiomyosarcoma of the breast. In addition, immunohistochemical analysis is a useful diagnostic tool. The differential diagnosis of leiomyosarcoma includes various types of sarcoma, such as fibrosarcoma, MFH, and phyllodes tumor. In leiomyosarcoma, tumor cells are positive for α-SMA, desmin, and vimentin and negative for cytokeratin, epithelial membrane antigen, and S-100 . In our case, immunohistochemistry revealed that the tumor cells were positive for α-SMA, desmin, and vimentin.
Among the treatment options for primary leiomyosarcoma, surgery is generally considered the primary treatment . In contrast, des Guetz et al. reported that in a neoadjuvant therapy group of 19 patients who received chemotherapy, nine patients achieved a clinical partial response . In addition, two post-irradiated histologic subtypes, leiomyosarcoma and undifferentiated sarcoma, were associated with greater clinical responses than the remaining subtypes . These data suggest that chemotherapy should be considered in the treatment of PIS, in contrast to primary sarcomas. We have to include both repeat surgery and chemotherapy in the treatment plan in cases of recurrence of distant metastasis.
The prognosis for patients with PIS is reported to be dismal, with a median survival time ranging from 2.3  to 2.6 years , and the 2-year survival rate has been reported as 32 % . However, survival was much better in patients who responded to neoadjuvant chemotherapy and those who underwent surgery.
We reported the first case of post-irradiation leiomyosarcoma of the breast in Japan. In addition, this malignancy is rare globally. However, the lesion in our patient may have been primary leiomyosarcoma, and leiomyosarcoma of the breast is extremely rare. Further research is required. Both surgery and chemotherapy are important treatment options for post-irradiation leiomyosarcoma, and close follow-up are advised.