Diffuse invasive breast carcinoma of no special type.

Diffuse invasive breast carcinomas are rare tumors associated with unfavorable prognostic parameters. This growth pattern is often related to invasive lobular cancer (ILC). Diffuse ductal breast carcinoma of no special type (NST) is largely under-recognized in the literature. We identified 70 diffuse invasive breast carcinomas in a consecutive series of 1249 invasive tumors. Based on morphology and E-cadherin expression, 15/70 were NST and 55/70 were ILC. Subgroups differed in mammographic appearance, as more NST tumors than ILCs formed stellate masses (53 vs. 18 %, p = 0.000436) while ILCs displayed more architectural distortion. NST tumors were significantly more often radiologically extensive than ILCs (80 vs. 38 %, p = 0.0042987). Subgroups did not differ significantly in disease extent on histology, lymph node status, progesterone receptor status, and molecular phenotype, with a difference of borderline statistical significance in estrogen receptor status (87 vs. 100 %, p = 0.0434783). Significantly more NST tumors were HER2 positive (27 vs. 4 %, p = 0.0050463) and showed high Ki67 proliferation index (60 vs. 25 %, p = 0.0121808). The most striking differences occurred in the histology grade of the in situ (high grade in 53 vs. 4 %) and invasive (high grade in 27 vs. 2 %) tumor components and in the distribution of the in situ component (diffuse in 73 vs. 11 %). We conclude that diffuse invasive breast carcinomas of NST comprise a small subgroup of breast carcinomas. Most of these cancers are non-high grade and of luminal phenotype, but extensive and lymph node positive with worse prognostic parameters than diffuse ILC.