Metastasis of in situ solid papillary carcinoma of the breast to the lungs:a rare case report

Solid papillary carcinoma (SPC) of the breast is considered a special subtype of ductal carcinoma <i>in situ</i> (DCIS), characterized by distinctive histological features and growth patterns. Although SPC itself is not very common, it is particularly noteworthy that distant metastasis from its non-invasive form is highly uncommon. This contrasts sharply with infiltrative solid papillary carcinoma (ISPC), which lacks the typical myoepithelial cell layer seen in SPC and displays a map-like infiltrative growth pattern, often involving adipose and fibrous stromal infiltration. This study reports a case of a patient who was diagnosed with <i>in situ</i> SPC following a total mastectomy, and subsequently developed pulmonary metastatic ISPC five years later. The case supports the concept of a morphological continuum from <i>in situ</i> SPC to invasive carcinoma and highlights that even non-invasive SPC may possess the distant metastasis, albeit rarely. These findings provide new insights into the biological behavior of this rare breast cancer subtype.