Tumors of the neuroendocrine system comprise a morphologically and biologically heterogeneous group of neoplasms that can arise in virtually all tissues and organ systems. These tumors are currently defined on the basis of a common phenotype that is characterized by the expression of multiple genes encoding a wide spectrum of neural and neuroendocrine traits. Although the tumors express a common neuroendocrine program, their clinical and pathologic features are highly variable and range from cases with an organoid growth pattern and essentially benign behavior to highly aggressive neoplasms with small or large cell features. Additionally, a subset of tumors may have evidence of both neuroendocrine and nonneuroendocrine differentiation. For many cases, prediction of biologic behavior for neuroendocrine neoplasms has proven to be elusive and has resulted in a series of often confusing systems of nomenclature and classification.
In this issue of Pathology Case Reviews, the editors provide a brief overview of neuroendocrine tumor classification and nomenclature, together with a review of immunohistochemical approaches to their diagnosis. In his paper, Dr William Travis highlights the spectrum of pulmonary neuroendocrine tumors, with an emphasis on diagnostic criteria for each of the tumor categories utilizing the 2004 World Health Organization (WHO) classification. Dr Ronald DeLellis provides an overview of neuroendocrine neoplasms of the thyroid, with an emphasis on the pathology and molecular characteristics of medullary thyroid carcinoma. Dr Yogeshwar Dayal's paper reviews the current classification and pathologic features of gastrointestinal neuroendocrine tumors, while Drs Günter Köppel and Martin Anlauf contribute an overview of pancreatic endocrine tumors using the 2004 WHO classification. Dr Robert Osamura and his colleagues provide a series of interesting cases that underscore the fact that neuroendocrine tumors may arise in a variety of diverse and often unexpected sites, while Dr Ann Marie McNicol's contribution highlights the pathology and molecular characteristics of pheochromocytomas and paragangliomas. Finally, Drs Lee and Meier's paper provides an interesting example of carcinoid heart disease.
While some aspects of recent systems of nomenclature of this enigmatic group of tumors remain problematic, we hope that the articles in this issue of the journal will provide a series of useful guidelines for their diagnosis and classification.