Navigating the diagnostic pitfalls of cystic and solid breast masses: a comparative case report of metaplastic carcinoma and benign lesions.

Background: Cystic and solid masses (CSMs) present a significant diagnostic challenge due to their heterogeneous internal architecture and a relatively high malignancy rate (23%-31%). Metaplastic breast carcinoma (MBC), an aggressive and rare subtype of triple-negative breast cancer, often presents as a CSM, frequently leading to clinical misinterpretation and delayed treatment. Case description: We report two middle-aged women presenting with rapidly enlarging, tender CSMs. Both cases exhibited similar initial clinical and sonographic features. In Case 1, the mass was initially misdiagnosed as "plasma cell mastitis" (PCM) at an external institution following a fine-needle aspiration. The definitive diagnosis of MBC was ultimately established through a vacuum-assisted breast biopsy, which was pursued after a supplementary breast magnetic resonance imaging (MRI) was obtained. In contrast, Case 2 followed a benign clinical course and was pathologically confirmed as a benign breast cyst with fibrocystic changes. Conclusion: Diagnosing CSMs requires integration of clinical history, physical examination, imaging findings, and histopathological evaluation. Pathological analysis remains the gold standard for definitive diagnosis. These cases highlight key diagnostic considerations, including the utility of supplemental breast MRI, the diagnostic value of cystic fluid characteristics, and the importance of obtaining an adequate tissue sample during biopsy.