PS5-06-14: Unexpected B-hCG elevation in metastatic inflammatory breast cancer: A diagnostic dilemma

Abstract Introduction: The beta subunit of human chorionic gonadotropin (B-hCG) is often used as a marker of pregnancy, but can be produced by several different sources, including the placenta, trophoblastic tumors, nontrophoblastic tumors, and the pituitary gland. While not used as a routine tumor marker for breast malignancies, studies have shown that hCG from a placental source appears to be protective against breast cancer, while ectopic production of B-hCG appears to be associated with poor prognosis. As part of routine testing during chemotherapy at our institution, all patients undergo a pregnancy test due to the risks of chemotherapy on the developing fetus. Here, we present a case of incidentally found false positive pregnancy test in a post-menopausal patient with metastatic inflammatory breast cancer. Case Description: We present a case of a 49 year old female who was initially diagnosed in May 2024 with stage IV HER2+ inflammatory breast cancer, with metastases to multiple lymph nodes and pulmonary nodules. She progressed through several lines of therapy, including docetaxel, trastuzumab, and pertuzumab, dose dense doxorubicin and cyclophosphamide, and fam-trastuzumab deruxtecan. She is currently on therapy with tucatinib, capecitabine, and trastuzumab. In May 2025, she had routine prechemotherapy labs done including a urine pregnancy test which resulted positive. She had two prior urine pregnancy tests which were negative in July and September 2024. We subsequently performed a serum B-hCG quantitative test in May 2025 showed a B-hCG of 82.4 mIU/mL. The patient had reported abstinence and amenorrhea for the past year. It was determined to be a false pregnancy test and her therapy was continued. The clinical case was discussed with our OB/GYN colleagues with consensus to verify menopausal status by hormone levels. Labwork showed an estradiol of <11 pg/mL and FSH of 94.8 IU/L, consistent with a post-menopausal state. She had a repeat set of labs in June 2025 which showed a serum B-hCG of 87.4 mIU/mL, estradiol of 16 pg/mL, and FSH of 97.5 IU/L, again consistent with a post-menopausal state. Discussion: Patients undergoing chemotherapy are often screened for pregnancy, but quantitative measures of B-hCG as a tumor marker are not typically done for breast malignancies. Some case reports have described breast malignancies presenting with elevated B-hCG which was described as having choriocarcinomatous features. There have been case studies evaluating the prognostic implication of elevated B-hCG in breast malignancies however levels in these studies were only very mildly elevated (e.g. <5 mIU/mL). For this patient, it was thought that the elevated B-hCG could be due to a pituitary source, however typically in these cases the B-hCG is only mildly elevated (e.g. in one population of women the mean level of hCG was 9.5 mIU/mL with a range of 2.1-32 mIU/mL). Our patient in this case had a B-hCG value of 82.4-87.4 mIU/mL. However, her low levels of estradiol and elevated FSH confirmed a post-menopausal state. Patients with trophoblastic tumors typically have significantly elevated B-hCG, greatly above the level seen in this patient. Conclusion: In summary, this case highlights an instance of moderately elevated B-hCG in a patient with metastatic inflammatory breast cancer, which has not been well described in the literature to our knowledge. We sought to describe this case to alert providers to the possibility of false pregnancy tests in patients with breast cancer with no other known gynecological cancer. As with this patient, next steps include measurement of estradiol and FSH to confirm a post-menopausal state. Citation Format: K. Klein, D. Ramirez. Unexpected B-hCG elevation in metastatic inflammatory breast cancer: A diagnostic dilemma [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS5-06-14.