Two cases of primary small cell carcinoma of the breast showing non-mass-like pattern on diagnostic imaging and histopathology.

Primary small cell carcinoma of the breast is a rare breast cancer. We report two cases of this entity showing a non-mass-like pattern on multimodality images and histopathology. Both women presented with a breast mass, and one had axillary lymphadenopathy. Both cases revealed fine calcification on mammography (MMG) and an irregularly shaped, hypoechoic lesion on ultrasonography. Computed tomography (CT) and magnetic resonance imaging (MRI) showed non-mass-like enhancement in both cases. Dynamic MRI showed medium initial enhancement followed by persistent delayed enhancement in one patient, whereas rapid initial enhancement with plateau delayed enhancement was observed in the other. The breast lesions showed very high signal intensity on diffusion-weighted MRI. Positron emission tomography showed moderate accumulation of 2-fluoro-2-deoxyglucose in the breast tumor and lymph node metastasis. The non-mass-like enhancement on CT and MRI and the segmental fine calcification on MMG indicate the abundance of components of ductal carcinoma in situ and the breast origin of the small cell carcinoma.