Invasive lobular carcinoma: clinicopathological features and subtypes

Our results showed that intermediate tumor grade, positive hormone receptor status, and larger tumor size were associated with ILC. A lower proliferation rate (Ki67) was found more frequently with ILC (48.8%) compared with IDC (35.8%) (p=0.03), and the subtype distributions differed significantly between both histological groups: luminal A was significantly more common with ILC (47.6%) compared with IDC (34.5%) (p=0.004). We found no significant difference regarding ALN involvement between ILC and IDC. However, in the patients with the luminal A subtype, we found an association between positive nodal status and ILC.

Our analysis showed that age at diagnosis did not differ significantly between patients with ILC or IDC. This has been reported previously in several studies;17,18,21,39,40 however, ILC was associated with older age at diagnosis.7,9,13,14,34,41–43 According to our findings, lower tumor grade in ILC was observed in different studies comparing these tumors with IDC.8,9,13,14,34,40,41 Our results showed that ILC was associated with a larger tumor size, as seen in several previous trials.1,7–10,13–15,17,34,40,41 In contrast, some studies reported no difference in tumor size between ILC and IDC.18,21,39,42

As in our study, positive ER status was found more frequently with ILC in several trials.10,13,21,39 Additionally, ILC was associated with a higher rate of hormone receptor (HR)-positivity.7,9,40,41,44 In contrast, more ER-negative tumors were observed with ILC than with grade-matched IDC in a British trial comparing both histologic groups. 1 Our analysis showed that HER2 expression did not differ significantly between ILC and IDC; however, reports of more HER2-negative tumors with ILC exist,7,40,41,44 and several studies revealed that ILC was more often slowly proliferative;7,40,44 our results were similar.

In this study, we demonstrated a significantly different distribution in BC subtypes between both histological groups. In ILC tumors, the luminal A, luminal B, HER2-positive, and triple-negative subtypes accounted for 47.6%, 50.0%, 1.2%, and 0%, respectively. In 1.2% of ILCs, the subtype was unknown. Iorfida et al. evaluated ILC regarding its biological features. 45 According to IHC reactivity, approximately half of all ILCs were classified as luminal B (48.5%). Luminal A, HER2-positive, and triple-negative subtypes accounted for 34.9%, 0.4%, and 1.5%, respectively; the status of 14.7% of the ILCs could not be determined. 45

In a Belgian study, ER, PR, and HER2 status were used to define different subtypes. 14 ILCs were more frequently ER+/PR+/HER2− (85.3%) compared with non-ILCs (67.0%). The HER2-positive and triple-negative subtypes were very rare among ILCs (0.7% and 1.3%, respectively) relative to non-ILCs (4.9% and 11.2%, respectively) (p