Synchronous Triple Malignancy Presentation of Multiple Myeloma and Prefibrotic Primary Myelofibrosis in a Patient With Mucinous Breast Carcinoma: A Case Report From Rural South Africa.

BACKGROUND: Multiple primary malignant neoplasms (MPMNs) diagnosed synchronously are rare, with an incidence of less than 1%. The occurrence of triple synchronous malignancies is exceedingly uncommon, and to our knowledge, no prior reports describe the concurrent presentation of a myeloproliferative neoplasm, plasma cell dyscrasia and breast cancer. Such cases pose considerable diagnostic and therapeutic challenges, particularly in settings with limited access to advanced molecular testing and oncologic therapies. CASE PRESENTATION: A 67-year-old woman presented with a pathological femoral fracture, marked leucocytosis and thrombocytosis. Histological examination of the fracture site revealed a kappa-restricted plasmacytoma meeting diagnostic criteria for multiple myeloma (R-ISS Stage 3). Bone marrow biopsy demonstrated hypercellularity with atypical clustered megakaryocytes, consistent with prefibrotic primary myelofibrosis, and molecular analysis confirmed a JAK2 V617F mutation. During evaluation, a right breast mass was confirmed on core biopsy as an invasive carcinoma with focal mucinous differentiation (ER/PR-positive, HER2-negative and Ki-67 25%), consistent with a luminal B-like profile. The patient commenced therapy with melphalan and prednisone for myeloma and anastrozole for breast carcinoma, with multidisciplinary input guiding treatment sequencing. DISCUSSION: This case represents an exceptionally rare triad of synchronous malignancies spanning myeloid, plasma-cell and epithelial lineages. While myeloproliferative neoplasms are associated with an elevated risk of secondary neoplasia, the simultaneous occurrence of three independent malignancies raises the possibility of shared pathogenic mechanisms such as clonal haematopoiesis or an underlying germline predisposition. The presentation highlights the diagnostic complexity of distinguishing independent primaries from metastatic disease and illustrates the importance of maintaining diagnostic vigilance. Limited access to next-generation sequencing and novel therapies further complicates management in resource-constrained healthcare environments. CONCLUSION: Triple synchronous primary malignancies are exceedingly rare and diagnostically challenging. This case emphasises the necessity of a systematic, multidisciplinary approach to evaluation and management, even in low-resource settings, to ensure accurate diagnosis and optimal coordination of care across multiple oncologic pathologies.