Imaging patterns and diagnostic challenges in metastatic invasive lobular carcinoma.

Invasive lobular carcinoma (ILC) is the second most common subtype of breast cancer after invasive carcinoma of no special type (NST), previously referred to as invasive ductal carcinoma. Loss of E-cadherin-mediated adhesion drives its characteristic diffuse infiltration, producing subtle or ill-defined lesions on imaging and contributing to underdiagnosis. ILC is heterogeneous, comprising a classical form and several variants-some of which, such as pleomorphic ILC, are considered more aggressive and carry distinct prognostic implications. ILC demonstrates a distinctive metastatic tropism, with a higher prevalence of peritoneal, gastrointestinal, ovarian, and leptomeningeal involvement than NST, whereas pulmonary metastases are reported less often. ILC Metastases are frequently low-volume or infiltrative and may mimic benign or inflammatory conditions, complicating staging and follow-up. This didactic review provides a head-to-toe survey of metastatic ILC, combining radiology and nuclear medicine perspectives. It highlights diagnostic pitfalls and key imaging hallmarks, with emphasis on whole-body diffusion-weighted MRI (WB-DWI) and PET/CT strategies beyond^18F-fluorodeoxyglucose ({\,}^18F-FDG), given the often low or heterogeneous FDG avidity of ILC, including^18F-fluoroestradiol (FES) and fibroblast activation protein inhibitor (FAPI) imaging. By integrating biological insights with state-of-the-art imaging, this review emphasises how the distinctive metastatic behaviour of ILC can lead to delayed detection. Increased awareness of these patterns and need for dedicated imaging approach among radiologists and nuclear medicine physicians is necessary to reduce missed or late diagnoses.