Background: Buccal mucosa metastasis is rare in breast cancer patients. Case Presentation: A 50-year-old female breast cancer patient with invasive lobular carcinoma (ILC) in his eight years follow-up period presented with a swelling in the right eye. Head and neck MRI was showed soft tissue formation with dimensions of 59x31 mm was detected in the axial plane, extending from the level of the right infratemporal fossa to the right buccinator muscle and right maxillary sinus in front of the alveolar process. In the right periorbital region, there was a thickness increase of up to 1 cm in the thickest part of the globe, which was evident in the anterolateral. In addition, there was no bone destruction in the right infratemporal region in maxillofacial CT, and no additional organ metastasis was detected in PET-CT performed for systemic scanning. In the histopathological examination of the tissue, carcinoma infiltration was detected. Radiotherapy was applied to the right buccal and orbital region of the patient as 30 Gy in 12 fractions (2.5 Gy per fraction) with volumetric modulated arc therapy technique and then 15 Gy in 6 fractions (2.5 Gy per fraction) boost radiotherapy with the adaptive technique were applied. At the follow-up 2 months later, clinical response was observed in the patient, and significant regression of the lesion was observed in facial and orbital MRI. At the 15th month follow-up of the patient, clinical and radiological remission continues. All clinical complaints regressed. Conclusion: ILC is a subtype of breast cancer that is characterized by unusual metastasis. Therefore, during the follow-up of these patients, it is necessary to monitor both laboratory values and clinical complaints very carefully.

Keywords: Radiotherapy, lobular carcinoma, metastasis, breast cancer, buccal mucosa.