MON-325 Co-Occurrence of Breast Cancer and Neuroendocrine Tumors: More Than a Coincidence?

Background: In Ontario, age-standardized incidence of female breast cancer was 145.1 per 100000/year in 2012. Neuroendocrine tumors (NET) are less common, with an incidence of 5.86 per 100000/year in 2009 in the same population. Evidence is scarce about the possible genetic mutations that may predispose patients with NET to develop breast cancer. Clinical cases: We report a series of 9 cases of co-occurrence of breast cancer and NET. All patients (n=9) were female, with a mean age of 61.4 years (35-85) at breast cancer diagnosis and 63.4 years (51-89) at NET diagnosis. Six patients had a lumpectomy, 2 had a mastectomy and one was not a surgical candidate. Five tumors were invasive ductal carcinomas, one was a ductal carcinoma in situ and two were lobular carcinomas. Seven tumors were ER+, 5 were PR+ and only one was HER-2 neu +. Mean tumor size was 1.1 cm (0.4-2.0). Three tumors had a histologic grade 2 and one had a grade 3. All tumors were Stage 1a. Three patients had locoregional recurrence. Two patients developed breast cancer metastases, one in cervical lymph nodes and bone metastases and one had liver metastases. In terms of adjuvant therapy, two patients received chemotherapy, four received radiotherapy, 6 received an aromatase inhibitor and one received trastuzumab. Mean follow-up time for the breast cancer was 7.44 years (2-26). Of the 9 patients, 4 had a pancreatic NET, 3 had a small bowel NET and two had a lung NET. Five patients had surgical resection of the primary tumor, three were not surgical candidates and one was followed by active surveillance. Mean tumor size was 5.2 cm (1.9-8.1); one patient had Stage 1, two had Stage 2 and one had Stage 3 disease. One pancreatic NET was an insulinoma. Mean Ki-67 index was 12.6 % (1.0-33.0). One NET had a Grade 1, 6 had a Grade 2 and 1 had a Grade 3. Five tumors developed metastases during follow-up, 5 to the liver, two to mesenteric lymph nodes, two to bones. 6 were treated with a somatostatin analogue and 4 with everolimus. Two received capecitabine-temozolomide chemotherapy, which targets both malignancies, one received peptide receptor radionuclide therapy (PRRT with 177-Lutetium) and three are currently being considered for PRRT. Two patients had hepatic metastasectomy. Mean NET follow-up time was 5.67 years (1-10). Two patients were known cases of Multiple Endocrine Neoplasia type 1 harboring pathogenic MEN1 variants. At time of submission, we are awaiting germline candidate gene mutational analyses including MEN1, MEN4/CDKN1B, BRCA1/2, MUTYH, CHEK2 and Lynch syndrome. Conclusion: This case series serves as a reminder that patients with one malignancy are more often predisposed to develop a second neoplasm, prompting clinicians to consider this possibility when new lesions appear during follow-up. In particular, NET patients maybe at increased risk of breast cancer, the underlying basis of which remains to be determined .