Incidental discovery of second primary malignancies in patients diagnosed with breast cancer: An institutional case series.

e12552 Background: National guidelines recommend tissue biopsy of suspected recurrent breast cancer, to confirm diagnosis and to characterize disease phenotype, receptor status, and molecular markers. Despite this, clinicians frequently rely on imaging alone to diagnose recurrent breast cancer; indeed, our group recently published data revealing that only half of patients in Washington state diagnosed with metastatic breast cancer between 2008 and 2017 underwent confirmatory biopsies for suspected metastatic disease on imaging. Here we present evidence of how this failure to fully evaluate suspected breast cancer recurrence could potentially lead to misdiagnosis. Methods: We surveyed our breast medical oncology faculty to identify patients treated at Fred Hutchinson Cancer Center (FHCC) from 2018 to 2022 in which a second primary carcinoma (not including non-melanoma skin cancers) was discovered as part of breast cancer workup and treatment. Results: We identified 10 cases of second primary diagnoses. Of these, the majority (7/10) were lung cancer; other diagnoses included neuroendocrine tumors (1), chronic lymphocytic leukemia (1), and thyroid cancer (1). In the case of lung cancers, patients were typically diagnosed during staging or CT evaluation for radiation or reconstruction, in which a pulmonary nodule raised suspicion for malignancy. In five cases, patients were treated with curative intent for both of their early-stage primary malignancies, and remain disease-free on endocrine therapy alone. In two cases, patients with biopsy-proven metastatic breast cancer were found to have new suspicious lung lesions, which on biopsy were found to represent lung adenocarcinoma; these patients were treated surgically for their lung cancer and continued long-term therapy for metastatic breast cancer. Conclusions: While imaging can raise strong suspicion for metastatic recurrence of breast cancer, the assumption that an apparently metastatic lesion 1) indeed represents metastasis and 2) represents metastatic spread of the known primary rather than another malignancy, risks treating both the primary tumor and the newly detected condition inappropriately. Biopsy is critical for confirmation of metastatic disease vs second primary or other etiology. As patients with metastatic breast cancer are surviving longer as our cancer-directed therapies improve, appropriate diagnostic measures for a suspicious non-breast malignancy are recommended for optimal care. Novel imaging modalities such as 18F-fluoroestradiol (FES) PET and others could be explored in discriminating between metastatic lesions and unrelated conditions.