Teaching Case

Twenty-seven-year-old patient referred to imaging because of a rapidly growing palpable mass of the right breast. No family history of breast cancer.

On US (Fig. 1) we found a complex cystic mass with thickened, irregular walls and mural nodules (a) showing vascularisation on colour US (b). After fine needle aspiration (c) there was incomplete resolution of the mass. Although the result of cytological analysis was benign, we subsequently performed core-needle biopsy of the residual solid component because of discordant imaging appearance, thus obtaining final diagnosis.
On MRI (Fig. 2), the lesion appeared as a well-defined hypointense mass on unenhanced axial T1-weighted image (a), showing hyperintense cystic aspect associated with internal septa and mural nodules on axial T2-weighted imaging (b). Irregular walls and internal nodules showed intense contrast enhancement (second subtracted post-contrast image in c).

Background: MC is a rare but aggressive type of breast cancer, characterised by the differentiation of neoplastic epithelium into squamous cells and/or mesenchymal-like elements. Imaging appearance can overlap with both invasive ductal carcinoma and benign lesions, which further complicates the diagnosis [1]. MC typically presents as a rapidly growing mass, with most reports documenting large size at the time of clinical onset (> 2 cm) [2].

Clinical Perspective: Regardless of tumour size, surgical intervention should be the first step in managing patients with operable tumors, since neoadjuvant chemotherapy is minimally effective in reducing tumour burden and preventing disease progression. The response to systemic chemotherapy is usually poor [2].

Imaging Perspective:
MC can mimic benign conditions, presenting as a round or oval mass on mammography, and hypoechoic solid or complex cystic mass on US. Complex cyst appearance correlates with cystic degeneration and necrosis found on pathologic evaluation [3]. On MRI, MC more often presents as a mass with spiculated margins, with intermediate to high signal intensity on T2-weighted images, and iso-intensity or hypo-intensity on T1-weighted imaging. Although hyperintensity on T2-weighted imaging is often associated with benign lesions, it can be secondary to necrosis or mucoid production in malignancies [4].
MC can also present with the cystic pattern we showed. In general, the problem raised by cystic pattern is the differentiation with complicated breast cysts. Complicated cysts contain low-level internal echoes or intracystic debris, while complex cysts show thick walls, thick septa, and a variable extent of intracystic solid components. The risk of malignancy in complicated cysts is less than 2%, thus prompting conservative management with short-interval imaging follow-up or aspiration. Complex cystic breast masses have a substantial chance of being malignant (among 23% and 31%), needing percutaneous biopsy for the diagnosis [5].

Take-Home Message, Teaching Points:
1. A rapidly growing palpable mass should raise the suspicion of malignancy, regardless of patients’ age and cystic appearance on imaging.
2. It is important to differentiate a complicated cyst from a complex cyst because the management significantly differs.
3. Reliance solely on benign cytologic findings in the fine-needle aspirate is not advisable, because the residual solid component of a complex cyst may harbor malignancy. When a complex cyst contains true solid components, this is indication for biopsy.