Sneaky DCIS-looking Invasive Ductal Carcinoma of the Breast in the Extensive DCIS Background

Abstract Introduction/Objective In the most cases, invasive ductal carcinoma (IDC) of the breast is identifiable when they present with classic infiltrative growth pattern. However, subset of IDC can occur in a very sneaky way, significantly mimicking the appearance of ductal carcinoma in situ (DCIS). In this condition, it’s much more easier to miss the invasive component without pulling ancillary staining when morphologic findings are extremely compatible with DCIS, especially the diagnosis of DCIS was made on the previous biopsy. Methods/Case Report Retrospective analysis of the histologic and immunohistochemical findings of pure DCIS and DCIS-looking invasive ductal carcinoma. Results (if a Case Study enter NA) Here, we reported a 55 year-old female who was noted to have microcalcification at the 11:00 o’clock of the right posterior breast on routine mammographic examination in 09/2023. Biopsy of the calcification area in 10/2023 reported high grade DCIS (ER+ PR-). Histologic examination of subsequent mastectomy specimen showed two separate DCIS-looking areas. Immunohistochemical (IHC) staining showed that myoepithelial markers, p63 and smooth muscle myosin heavy chain (SMMHC), were retained at the periphery of all the expanded acini in one area. Unexpectedly and surprisingly, p63 and SMMHC were completely lost at the periphery of part of the DCIS-looking acini in another area, immunohistochemically compatible with the diagnosis of invasive ductal carcinoma admixed with DCIS. Conclusion Knowing that invasive ductal carcinoma of the breast can present as DCIS-looking morphology, especially given the condition that the diagnosis of DCIS was rendered on the previous biopsy, will enhance awareness of pathologists to recognize sneaky DCIS-looking invasive ductal carcinoma in the extensive DCIS background. In turn, this will prevent misdiagnosis and under-treatment of patients with invasive ductal carcinoma of the breast.