Abstract PO2-20-09: Hiding in plain sight: A heterogeneous response in a patient with metastatic HR+/HER2- breast cancer

Abstract INTRODUCTION: Metastatic breast cancer is an infamously heterogeneous disease—characterized by a range of clinical presentations and treatment responses. In light of this heterogeneity, ASCO guidelines recommend patients with new, accessible metastatic tumors undergo a biopsy to inform treatment decisions. Here, we present a patient with HR+/HER2- disease with a long history of unusually heterogeneous responses—and a repeat metastatic biopsy that provided an answer. CASE: A 55-year-old woman presented for consultation of treatment-refractory metastatic breast cancer. She was originally diagnosed with a right-sided Stage III HR+/HER2- invasive ductal carcinoma (IDC) at the age of 33. Upon staging, she was incidentally found to have a right lower-lobe lung carcinoid tumor that was removed by lobectomy. She underwent a partial mastectomy, axillary lymph node dissection, followed by adjuvant chemo- and radiation therapy. She tolerated one week of tamoxifen which was discontinued due to adverse effects. Fifteen years later, screening mammography revealed a suspicious lesion in the right breast—biopsy consistent with a new IDC. She underwent bilateral mastectomies which showed a pT1cNxM0 G2 IDC with associated DCIS (ER+/PR+/HER2 IHC 2+, FISH-negative, Oncotype RS 12). Genetic testing was negative. The patient trialed adjuvant tamoxifen, which was discontinued due to repeat adverse effects. Two years later, rising tumor markers prompted restaging scans that demonstrated an enlarging liver lesion. Liver biopsy was consistent with metastatic adenocarcinoma of the breast (ER+/PR+/HER2 IHC 0), GATA3 positive. Patient was started on letrozole with lupron and palbociclib with disease control for two years until progression in the bone, prompting a switch of letrozole to fulvestrant with continued palbociclib. Her disease was stable for two additional years until progression of multiple liver lesions. She was then started on single-agent paclitaxel. At this point, she was seen in our office for consultation and next-line therapy options. Repeat staging scans showed heterogeneous response in the liver with overall progression. ctDNA testing revealed a mutation in MEN1 (p.Q354*, 2.8% VAF) without other actionable alterations. Patient opted to enroll into a clinical trial of an experimental ER-antagonist but experienced progression of her liver lesions on first staging scans. A second liver biopsy was performed on a progressive lesion which showed ER-/PR-/HER2- disease. She opted to enroll into a Phase 1b/2 trial with sacituzumab govitecan-hziy in combination with a PARPi. Staging scans again showed a mixed response in the liver. Given a history of mixed responses and discordant receptor staining, the differential diagnosis included loss of hormone receptor expression in the breast carcinoma versus metastases from an occult neoplasm. Additional immunohistochemical studies were performed on a subsequent liver biopsy showing positivity for chromogranin, synaptophysin, and INSM1, and negative results for ER, PR, GATA3, and TRPS1. Next-generation sequencing of the progressive lesion revealed a MEN1 SNV (p.Q354*, VAF 82%) as well as loss of 11q (MEN1 locus) suggestive of biallelic loss of MEN1—providing molecular evidence of a metastatic carcinoid tumor. A history of mixed response in the liver was concluded to be two coexisting neoplasms—a metastatic HR+/HER2- breast cancer as well as a late recurrence of a right-lower lobe carcinoid tumor from 22 years prior. DISCUSSION: The unanticipated discovery of an occult, coexisting neoplasm in the setting of a metastatic HR+/HER2- breast cancer illustrates the value of meticulous pathological review, repeat molecular testing and the use of next-generation diagnostic tools such as tumor sequencing in the advanced setting—particularly when a clinical story defies conventional expectations—to personalize care and optimize oncologic management in the face of complex biology. Citation Format: Nolan Priedigkeit, Jane Brock, Olivia Cunningham, Molly Skeffington, Melissa Hughes, Nancy Lin, Susan Lester, Heather Parsons. Hiding in plain sight: A heterogeneous response in a patient with metastatic HR+/HER2- breast cancer 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 PO2-20-09.