Abstract PO5-20-09: LOCOREGIONAL MANAGEMENT OF DE NOVO STAGE IV HORMONE-POSITIVE BREAST CARCINOMA AFTER CDK 4/6 INHIBITOR TREATMENT

Abstract Systemic therapy remains the backbone for treatment of metastatic breast carcinoma. Locoregional treatment of the primary tumor remains controversial. While it has an established role in the palliative setting, its survival benefit remains unclear. The rationale behind locoregional approach is the reduction of tumor burden and the removal of cancer stem cells which may propagate the disease. In previous literature, hormone-positive breast cancer seems to be the best candidate for locoregional treatment in the presence of synchronous metastases. We present a case of a 43 year-old, female, with no known comorbidities, who sought consult due to note of a hard lump on her right breast. Core needle biopsy of the right breast mass was performed which showed invasive breast carcinoma, no special type, Nottingham histologic grade 2. On immunohistochemistry, tumor was ER +8, PR +8, Ki 67 90%, Her-2 neu negative. Baseline positron emission tomography/CT (PET/CT) scan showed hypermetabolic, heterogeneously-enhancing lobulated right breast mass, 3 × 6.7 × 7.7 cm, with smaller hypermetabolic adjacent nodules. There was also note of multiple enhancing, hypermetabolic lung and pleural nodules, and lymphadenopathies in the mediastinal, right axillary, right cardiophrenic and perigastric areas. Well-defined, hypermetabolic heterogeneously hypoenhancing masses were also seen in segments II and VII of the liver. There was also note of right-sided pleural effusion. Ultrasound-guided thoracentesis was done which turned positive for malignant cells on pleural fluid analysis. She underwent 4 cycles of chemotherapy with Doxorubicin and Paclitaxel. Reevaluation PET-CT scan revealed partial response. Patient was then started on Ribociclib plus Letrozole with ovarian function suppression for 18 months. On latest reevaluation PET/CT scan, there was no evidence of hypermetabolic metastatic disease, with demonstration of hypermetabolic activity only in the right breast. The patient then underwent right total mastectomy, with histopathologic findings of residual invasive carcinoma, Nottingham histologic grade 3, residual tumor size of 3.5 cm, with 95% tumor cellularity and lymphovascular invasion identified, ypT2. All surgical margins were negative for tumor. Currently, the patient has no evidence of disease, with ribociclib and letrozole continued until disease progression or unacceptable toxicity. In this case, surgery of the primary tumor was performed in the presence of a responsive metastatic disease, with the primary tumor being the only evidence of disease. With the advent of newer therapy options like cyclin-dependent kinase 4/6 inhibitors offering outstanding survival benefit for metastatic disease, a more aggressive and synergistic approach with locoregional therapy may be done to eradicate the tumor burden, translating to better long-term survival. Locoregional management of the primary tumor should be discussed in the context of a multidisciplinary team approach and should be individualized based on the individual patient’s need. Citation Format: Raye Angeli Abella, Rubi Li. LOCOREGIONAL MANAGEMENT OF DE NOVO STAGE IV HORMONE-POSITIVE BREAST CARCINOMA AFTER CDK 4/6 INHIBITOR TREATMENT abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-20-09.