RESUMO
Breast cancer is the most common malignancy diagnosed in women. Invasive lobular breast cancer (ILC) is the second most common histologic subtype after invasive ductal carcinoma. Metastatic occult primary breast cancer, although rare, is a well-known clinical entity that usually presents with axillary lymphadenopathy without a detectable breast tumour. A perimenopausal woman in her 50s presented with abdominal pain, fatigue, and weight loss. Imaging showed peritoneal carcinomatosis with ascites, ovarian masses, and a lesion in the ascending colon. Gastric and colon biopsies showed infiltration from lobular breast cancer. Diagnostic workup, including mammography, breast ultrasound, and breast MRI, showed no evidence of breast pathology or axillary lymphadenopathy. First-line treatment with goserelin, letrozole, and palbociclib commenced with clinical improvement and radiological response. This case illustrates the challenges faced by clinicians in the diagnosis and treatment of lobular breast cancer without an identifiable primary lesion or axillary lymphadenopathy.