Successful Management of Occult Breast Cancer with a Background of Anti-Melanoma Differentiation-Associated Gene 5 Antibody-Positive Interstitial Pneumonia: A Case Report.

Background: Anti-MDA5 (melanoma differentiation-associated gene 5) antibody-positive dermatomyositis is a severe subtype of dermatomyositis associated with rapidly progressive interstitial lung disease, which carries an extremely high mortality rate. Prompt diagnosis and therapeutic intervention are crucial for survival. Here, we report a rare case of occult breast cancer in a patient with anti-MDA5 antibody-positive associated interstitial pneumonia. Following the control of the lung disease with immunosuppressive therapy, the patient successfully underwent neoadjuvant chemotherapy (NAC) and curative surgery.

Methods: A 63-year-old woman presented with progressive dyspnea. Imaging tests revealed diffuse ground-glass opacities in both lungs and enlarged left axillary lymph nodes. Blood tests showed elevated KL-6 levels and anti-MDA5 antibodies. Although no skin lesions or myositis were observed, she was diagnosed with anti-MDA5 antibody-positive associated interstitial pneumonia. Immunosuppressive therapy, including steroid pulse therapy, tacrolimus, cyclophosphamide pulse therapy, and plasma exchange, was initiated, leading to an improvement in her lung condition. She was then initially referred to the department of plastic surgery for further evaluation of the enlarged left axillary lymph node. Excisional biopsy of the enlarged left axillary lymph node revealed triple-negative occult breast cancer (cTXN1M0, Stage IIA). After the patient was referred to our department, NAC was initiated, achieving a clinical partial response while avoiding exacerbation of the interstitial pneumonia. After completing NAC, a left axillary lymph node dissection was performed, and the final pathological diagnosis was ypTXN2aM0 (Stage IIIA). Postoperative radiotherapy was omitted due to the risk of worsening the interstitial lung disease, and capecitabine was administered for 6 months. The patient has remained recurrence-free for 3 years following treatment.

Conclusions: This case highlights the successful management of triple-negative breast cancer under the constraints of anti-MDA5 antibody-positive associated interstitial pneumonia. To ensure the smooth implementation of breast cancer treatment while controlling interstitial pneumonia, close collaboration with respiratory physicians was essential for a successful outcome.