Differentiation-based histologic grading of colorectal carcinoma (CRC) is widely used, but its clinical impact is limited by insufficient prognostic value, interobserver disagreement, and the difficulty of its application to CRC with specific histologic types such as mucinous and medullary carcinoma. A recently proposed novel grading system based on quantifying poorly differentiated clusters (PDCs) claims to have the advantages of reproducibility and improved prognostic value, and might apply to heterogeneous CRC. We aimed to validate the clinicopathologic significance of the PDCs-based grading system and to determine the relationship between this grading system and microsatellite instability (MSI). Two hundred and one patients who had undergone radical surgery were reviewed. Based on the number of PDCs, 85, 58, and 58 tumors were classified as grade (G) 1 (42.3%), G2 (28.9%), and G3 (28.9%), respectively. PDCs-based grade was significantly associated with T, N, and M stages; lymphovascular invasion; conventional histologic grade; and frequent tumor budding (all P <0.001). In multivariate analysis, PDCs-based grade was found to be an independent prognostic factor for disease-free survival (P = 0.022; hazard ratio, 3.709 [G2], 7.461 [G3]). G3 CRC significantly correlated with high MSI (MSI-H) compared to G1 and G2 (P = 0.002; odds ratio, 5.750). In conclusion, this novel grading would provide valuable prognostic information to a greater number of patients and would require continued verification. PDCs-based grading is feasible for CRCs with heterogeneous morphology, and we propose that the association between G3 and MSI-H be further evaluated in different histological subtypes of CRC.
Keywords: Carcinoma; Colorectal Neoplasms; Microsatellite Instability; Neoplasm Grading; Prognosis.