How Pathology Affects Appearance, Features of Invasive Lobular Carcinoma That Every Imaging Physician Should Know

2035 Objectives: (1) Discuss characteristic pathologic features of Invasive Lobular Cancer (2) Discuss multimodality imaging features of ILC with particular focus on CT, bone scintigraphy, and PET/CT (3) Examine case examples of typical imaging features that an imaging physician will encounter.\n Introduction: Invasive lobular carcinoma (ILC) is the second most common histology of breast malignancy, accounting for approximately 10-15% of cases (1). The imaging features are variable and in general imaging is less sensitive for ILC as compared with invasive ductal carcinoma (IDC), owing in part to a higher false negative rate of diagnosis (2). The classic histologic appearance is of uniform tumor cells in a single-file line infiltrating the breast parenchyma, usually in part due to the lack of E-cadherin, without causing significant desmoplastic response (7). These features decrease the degree of architectural distortion and mass-forming tendencies of this tumor histology, thus causing challenge in both a clinical and radiologic diagnosis. Despite this, multimodality imaging plays a critical role in diagnosis, surgical planning, and long-term treatment of ILC. The purpose of this exhibit is to succinctly elucidate radiologic features of ILC on multimodality imaging focusing on the most common modalities that a nuclear medicine reader will encounter.\nFigure 1.\nFigure 2.\nFigure 3.\nMethods/Results: FDG PET/CT: On FDG PET/CT primary lobular breast cancers typically show lower metabolic uptake when compared with IDC. Additionally, sclerotic osseous lesions of ILC were less likely to demonstrate focal FDG uptake as compared to IDC (3).\nMDP bone scintigraphy: In one study, patterns of ILC osseous metastases were described as, multiple well defined uniform small sclerotic lesions (USSL), large sclerotic lesions (solitary or multiple), mixed lytic/sclerotic, and purely lytic. 60.8% of patients in the study had the USSL pattern, and all scintigraphy exams were considered negative. Thus, we need to recognize a negative bone scan in the setting of this distinct USSL pattern as a false negative exam (4).\nFigure 4.\nMammography: Cited false-negative rates of mammography are in the range of 19-43%, significantly higher than typical IDC (1). This is most likely from a combination of decreased mass-forming tendency and low relative density of this subtype as well as a very low percentage/degree of calcification, less than 2%.\nUltrasonography: Most commonly described as a heterogeneous hypoechoic mass with irregular contour as well as posterior acoustic shadowing. Reported sensitivities for the detection of ILC range from 68% to 98% (5)\nMRI: Breast MRI has a reported sensitivity for ILC of 95%. Due to the multicentric nature of ILD, MRI is superior to mammography in the detection of tumor extent and multicentricity. MRI is often performed to determine extent for treatment/surgical planning of ILC because studies have shown that the additional characterization by MRI leads to clinical and surgical management change in 50% and 28% respectively (5).\nMRI imaging findings of ILC include and irregular mass with spiculated or ill-defined margins, multiple enhancing foci with interconnecting enhancing strands, and enhancing septae. On dynamic contrast enhanced MRI ILC can demonstrate delayed enhancement in contrast to the classic rapid enhancement/washout pattern of IDC (5).\n Conclusions: It is important for the imaging physician to recognize the pathologic and histologic basis that leads to challenging in detecting ILC on imaging, especially on typical modalities such as bone scan and FDG PET/CT where uptake and metabolic uptake is often low level, potentially leading to higher numbers of false negative examinations.