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BackgroundOur study aimed to assess the prognostic value of poorly differentiated clusters (PDCs) in invasive breast cancer.MethodsA total of 146 cases of operable invasive ductal carcinoma that was not otherwise specified (IDC-NOS), from 2002 to 2009, were pathologically reviewed. Cancer clusters with five or more cancer cells and lacking gland-like structures were counted from a field containing maximum clusters in H & E slides under a × 20 objective lens (0.950 mm2 field of vision).ResultsTumors with <5, 5 to 9, and ≥10 clusters were graded as G1, G2, and G3, respectively (n =41, 60, and 45 tumors, respectively). An interobserver test showed good reproducibility, with a Cohen’s kappa coefficient of 0.739. The PDC grade was significantly associated with N stage (P <0.001), lymphovascular invasion (P =0.007), tumor budding grade (P <0.001), relapse rate (P <0.001), and death rate (P <0.001). Survival analyses revealed that the PDC grade was a significant prognostic factor for disease-free survival (hazard ratio 3.811; P <0.001) and overall survival (hazard ratio 3.730; P =0.001), independent of T stage, N stage, or tumor budding grade.ConclusionsThe PDC grade is an independent prognostic factor of IDC-NOS. Considering the simplicity and availability of this method relative to conventional clinical pathology, PDCs may serve as a novel prognostic histological characteristic in IDC-NOS.