Three cases of invasive solid papillary carcinoma of the breast displaying a wide variety of histological patterns with diagnostic challenges

We report three cases of invasive solid papillary carcinoma (SPC) of the breast that exhibited a broad spectrum of histological patterns, posing significant diagnostic challenges in the biopsy and mastectomy specimens. Although initially diagnosed as invasive breast carcinoma of no special type (IBC-NST) on biopsy, the resected specimens revealed the characteristic features of SPC, including solid growth patterns with delicate fibrovascular cores, in addition to tubular and glandular structures. The tumors consisted predominantly of intricately confluent nodules with well-circumscribed, rather than ragged, contours immunohistochemically lacking myoepithelial lining. Within these nodules, we observed rosettes, tubules, cribriform-like glands, and IBC-NST-like nests at the periphery of the confluent areas. These structures led to misinterpretation on biopsy. Both the confluent nodules and peripheral nests were diffusely positive for synaptophysin and INSM1. Additionally, compact, round, non-confluent nodules with sparse or invisible fibrovascular cores were also present. They immunohistochemically demonstrated diffuse neuroendocrine differentiation and preservation of myoepithelial linings. We propose that the confluent nodules with well-circumscribed contours and IBC-NST-like nests represent invasive SPC, while the non-confluent nodules correspond to SPC in situ (SPCIS) or neuroendocrine ductal carcinoma in situ. Differentiating invasive SPC from IBC-NST is crucial in breast pathology. The key lies in recognizing the distinctive morphology of invasive SPC and its frequent coexistence with SPCIS.