The term "classic invasive lobular carcinoma" of the breast defines breast malignancies of vastly different nature.

PURPOSE

To describe in detail the special features of a previously unappreciated "classic invasive lobular carcinoma" which is confined to the terminal ductal lobular units (TDLUs) and differs considerably from the extensive classic invasive lobular carcinoma, and to suggest specific terminology.

METHOD

All invasive breast cancer cases without associated microcalcifications diagnosed in our Institution with the histopathologic diagnosis of classic invasive lobular carcinoma during the years 1996-2019 (n = 560) formed the basis of this study. The cases were prospectively classified according to their imaging biomarkers (mammographic features) and followed up to Dec 31, 2021, to determine long-term patient outcome. An additional 2600 invasive breast cancer cases (diagnosed other than invasive lobular carcinoma) without associated microcalcifications served as a reference group. Detailed histopathologic analysis used large format (10x8 cm) thin section technique and staining methods including hematoxylin-eosin (H&E), E-cadherin, cytokeratin CK 5/6, a transmembrane glycoprotein (CD44) and anti-actin or anti-smooth muscle myosin heavy chain.

RESULTS

The imaging biomarkers differentiated two separate disease subgroups, having the same histopathologic diagnosis, classic invasive lobular carcinoma. One of these has the imaging biomarker of extensive architectural distortion with no central tumour mass, occupies the extralobular mesenchyme and has a long-term survival of 56%. The other subgroup forms stellate or circular non-calcified tumour masses usually smaller than 20 mm, which appear to arise in the intralobular mesenchyme, and has a significantly better long-term survival of 84%.

CONCLUSIONS

There is a striking difference between the subgross histopathology and the mammographic appearance (imaging biomarkers) of two breast malignancies having the same histopathologic diagnosis, "classic invasive lobular carcinoma". The large difference in the long-term outcome of these two tumour types is even more striking. Using the same specific term, "classic invasive lobular carcinoma", to describe these two separate entities can adversely affect management decisions.