KI-67 heterogeneity in well differentiated gastro-entero-pancreatic neuroendocrine tumors: when is biopsy reliable for grade assessment?

Purpose: Ki-67 heterogeneity can impact on gastroenteropancreatic neuroendocrine tumor grade assignment, especially when tissue is scarce. This work is aimed at devising adequacy criteria for grade assessment in biopsy specimens. Method: To analyze the impact of biopsy size on reliability, 360 virtual biopsies of different thickness and lengths were constructed. Furthermore, to estimate the mean amount of non-neoplastic tissue component present in biopsies, 28 real biopsies were collected, the non-neoplastic components (fibrosis and inflammation) quantified and the effective area of neoplastic tissue calculated for each biopsy. Results: Heterogeneity of Ki-67 distribution, G2 tumors and biopsy size all play an important role in reducing the reliability of biopsy samples in Ki-67-based grade assignment. In particular in G2 cases, 59.9% of virtual biopsies downgraded the tumor and the smaller the biopsy, the more frequent downgrading occurs. In real biopsies the presence of non-neoplastic tissue reduced the available total area by a mean of 20%. Conclusions: By coupling the results from these two different approaches we show that both biopsy size and non-neoplastic component must be taken into account for biopsy adequacy. In particular, we can speculate that if the minimum biopsy area, necessary to confidently (80% concordance) grade gastro-entero-pancreatic neuroendocrine tumors on virtual biopsies ranges between 15 and 30 mm², and if real biopsies are on average composed of only 80% of neoplastic tissue, then biopsies with a surface area not <12 mm² should be performed; using 18G needles, this corresponds to a minimum total length of 15 mm.