Incidental pulmonary leiomyoma in a patient with invasive mucinous breast carcinoma: A diagnostic challenge.

Pulmonary leiomyomas are rare benign smooth muscle tumors that are often discovered incidentally, particularly during oncologic staging, and can closely mimic metastatic lesions. We present the case of a 73-year-old postmenopausal woman newly diagnosed with low-grade ductal carcinoma in situ of the right breast with a focal invasive mucinous carcinoma component. As part of staging, contrast-enhanced chest CT revealed 2 well-circumscribed solid nodules in the left lower lobe, initially raising suspicion for metastatic spread. ^18F-FDG PET/CT demonstrated low metabolic activity, and subsequent CT-guided biopsy confirmed a well-differentiated smooth muscle neoplasm consistent with pulmonary leiomyoma. In view of the benign histology, low FDG avidity, and radiologic stability, surgical excision was deferred in favor of surveillance. The patient underwent breast-conserving surgery with radiotherapy and adjuvant endocrine therapy, later switched from letrozole to exemestane due to side effects. During multi-year follow-up, the pulmonary nodules remained stable without evidence of recurrence or metastasis. This case underscores the diagnostic dilemma of incidental pulmonary nodules in oncology patients and highlights the importance of integrating multimodality imaging with histopathology to avoid unnecessary interventions.