The patient was a 65-year-old woman who was diagnosed with right-sided breast cancer 8 years ago and was treated by mastectomy followed by chemotherapy and 50Gy of radiation therapy. Then 4 years later, her chest CT scan showed ground-glass opacification in contact with the irradiated area. Initially, the condition was thought to be radiation pneumonitis. However, gradually the mass became solid and extended in a belt-like manner. So, the patient was referred to our department and tissue biopsy was performed using endobronchial ultrasonography with a guide-sheath (EBUS-GS) and video-assisted thoracic surgery (VATS). The findings led to the final diagnosis of pulmonary mucosa-associated lymphoid tissue (MALT) lymphoma. Our search for references did not reveal any previous reports of pulmonary MALT lymphoma associated with radiation therapy, suggesting that it might be a rare condition. Our findings suggest that histological diagnosis is of critical importance in the clinical course of radiation pneumonitis, particularly in cases of exacerbation.