Histological Classification of Neuroendocrine Tumors

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Differences exist in the clinical behavior of neuroendocrine tumors (NETs) based on the site of origin and histologic characteristics, Matthew H. Kulke, MD, points out. To compensate for this, Pamela L. Kunz, MD, believes that several standard variables should be part of the pathology report for NETs, including Ki67, mitotic index, grade, and tumor features/size. Using primarily the mitotic index and Ki67, tumors can be differentiated by low, intermediate, and high grades.

Histological features can be used both predictively and prognostically, believes James C. Yao, MD. In terms of prognosis, tumor size is used frequently to determine the aggressiveness of the tumor. In general, Yao believes, the smaller the tumor the lower the grade and generally the more benign. Identifying an intermediate to high-grade tumor has predictive potential, since more aggressive malignant tumors are treated with chemotherapy.

The primary site of the tumor plays a role in its aggressiveness, Rodney F. Pommier, MD, points out. Tumors that are found in the GI tract, particularly the small bowel, are generally thought of as malignant, even when small.

Uncommon forms for NETs, such as goblet cell carcinoids and mixed adenocarcinomas, tend to behave more aggressively, the panel agrees. For tumors with mixed histology, the best approach is to treat the most aggressive component. However, Yao notes, responses to treatment in poorly differentiated NETs are not equivalent to those seen with well-differentiated tumors.