Isolated splenic metastasis presenting at diagnosis of HER2-positive de novo metastatic breast cancer: a case report.

Splenic metastasis is uncommon, and truly isolated splenic metastasis (ISM) from breast cancer is a clinical rarity. Contemporary autopsy series reports splenic involvement in <1% of breast cancer-related deaths, and fewer than 15 well-documented ISM cases have been published. We report a 71-year-old woman presenting with a right-breast mass, regional lymphadenopathy, and a solitary splenic lesion detected on staging computed tomography (CT) and positron emission tomography-CT. Biopsy confirmed hormone receptor-negative, HER2-positive invasive ductal carcinoma. Six cycles of docetaxel, trastuzumab, and pertuzumab led to a 59% reduction in the primary tumour and complete radiological resolution of the splenic lesion. The patient remains progression-free 22 months after initiating therapy, maintained on trastuzumab and pertuzumab. Although extremely rare, ISM can present as the initial manifestation of de novo metastatic breast cancer. Awareness of this possibility may facilitate early systemic therapy and obviate the need for diagnostic splenectomy.