Which breast cancer type may not form a distinct lump and is best imaged with MRI?

• A. Ductal carcinoma in situ
• B. Lobular carcinoma
• C. Inflammatory breast cancer
• D. Medullary carcinoma

Answer: (B)

Invasive lobular carcinoma often grows infiltratively in the breast without forming a discrete lump. The cancer cells spread in single-file patterns through the tissue, which makes masses subtle or even undetectable on mammography and sometimes on ultrasound. Because MRI offers superior soft-tissue contrast and can reveal non–mass-like enhancement, it is particularly good at showing the extent of lobular cancer when a lump is not obvious. MRI can demonstrate diffuse, regional, or multifocal enhancement that corresponds to the infiltrative spread and can also detect contralateral or occult disease, guiding surgical planning and treatment decisions.

Ductal carcinoma in situ typically presents with microcalcifications on mammo and is best assessed with mammography, not MRI alone. Inflammatory breast cancer presents with skin changes and diffuse involvement; imaging supports staging but the clinical presentation is a key driver of diagnosis and management. Medullary carcinoma often forms a more obvious mass and is not the classic scenario where MRI uniquely outperforms other modalities.