NEUROENDOCRINE TUMORS OF LUNG
Bruce Mackay 
University of Texas MD Anderson Cancer Center
Houston, Texas

A current issue of considerable interest in pulmonary pathology is the presence of neuroendocrine differentiation in lung carcinomas, and attention has been focussed on its range of occurrence, morphologic features, and clinical significance. Neuroendocrine properties are exemplified in the carcinoid tumors, and small cell carcinomas are accepted as forming part of the spectrum, but it is recognized that some non-small cell lung tumors also possess certain neuroendocrine characteristics even though they may not display light microscopic evidence for this. Electron microscopy has a limited but significant role in identifying and defining the properties of neuroendocrine lung tumors.

Carcinoid Tumors
The ultrastructural features of lung carcinoids are well known to electron microscopists. The dominant property is the presence of dense-core granules and they are probably present in every tumor though they vary in number and to some degree in appearance. Most carcinoids have round granules, but in fewer than 10%, the granules are large and angular. Morphometric analysis of the granules in lung carcinoids has confirmed that they range in caliber from an upper limit of approximately 400 nm down to about 100 nm. In any one tumor, the granules fall within a narrow range of diameters. As a rule, the granules are numerous and present in most of the cells, but they vary in frequency and some cells have few or no visible granules within a plane of section. There does not appear to be a relationship between the size or appearance of the granules and the immunostaining properties.

Typical light and electron microscopy of carcinoid tumor. Note the numerous, fairly uniformly sized neurosecretory granules.

Other aspects of the ultrastructure of lung carcinoids can be useful in defining the criteria used by light microscopy to separate them from small cell and non-small cell carcinomas, and to explain light microscopic appearance such as oncocytic transformation. Morphometric studies have shown that the size of carcinoid tumor cells and their nuclei are of limited value in differential diagnosis as there is considerable overlap between carcinoids and the other lung tumors, but the quantity of cytoplasm and appearance of the nuclear chromatin are helpful. The variety of architectural patterns seen among lung carcinoids by light microscopy can be appreciated in low-magnification micrographs but they have little correlation with the fine structure of the component cells.

The classic paper by Arrigoni et al published in 1972 confirmed earlier indications in the literature that the presence of atypical microscopic features is significant with regard to the biologic behavior of lung carcinoids, and typical and atypical forms are now accepted, based on light microscopic criteria including mitotic activity. While cell and nuclear size and shape are more variable in the atypical tumors, and their granules may be more sparse than in typical tumors, the electron microscopist should not attempt to make this distinction.
 

Light microscopic and ultrastructural features of aypical carcinoid. Note fewer numbers of neurosecretory granules in the cytoplasm of the tumor cells.

Small Cell Carcinoma
Small cell carcinomas account for 20% to 25% of primary carcinomas of the lung and it is now widely acknowledged that they form the aggressive end of the spectrum of neuroendocrine lung tumors. This can be a difficult light microscopic diagnosis, particularly when the amount and quality of the material available for study are less than optimal, and electron microscopy is useful if an adequate specimen is available. Fine needle aspirates are suitable for ultrastructural study provided they have been properly procured and processed.

Some variation in cell and nuclear size and shape can be appreciated among small cell lung carcinomas by light microscopy and there have been attempts to define subtypes. However, these features generally vary within a particular tumor, and they are influenced by artefactual changes and are consequently not sufficiently reproducible for use as criteria in paraffin sections, nor is there evidence that the suggested subtypes have clinical relevance. Morphometry performed using low magnification electron micrographs has shown that there is an overlap in cell and nuclear size and shape with other lung tumors, but the nuclear-cytoplasmic ratio is significant. The scanty cytoplasm is often strikingly obvious to the electron microscopist. Organelles are generally sparse. Cell junctions range from a few small well-formed desmosomes to scattered tiny densities on the apposed cell membranes. The nuclear chromatin is customarily finely clumped and evenly distributed, obscuring small nucleoli, but in a few otherwise typical small cell carcinomas, the chromatin is fine and nucleoli prominent.

Neuroendocrine differentiation is manifested ultrastructurally by the presence of dense-core granules. They are consistently small in caliber in small cell lung carcinoma cells, measuring roughly 100 nm and resembling the granules in extrapulmonary neuroendocrine carcinomas and neuroblastomas. In my experience, they are never as plentiful as granules in a typical carcinoid, and are rarely numerous. More often, scattered granules are found in only some of the cells. In roughly one third of cases, granules are extremely difficult to find or seem to be absent, and the possibility that these are true neuroendocrine tumors can not be confirmed from their ultrastructure. Since the lower end of the range of granule size in carcinoid cells is of the order of 100 nm, this feature alone will not discriminate between carcinoids and small cell carcinomas when the granules are small. Most carcinoids have larger granules, however, and the other ultrastructural features make distinction an easy one.

In a small proportion of small cell lung carcinomas, a non-small cell component is present, and the ultrastructural of both parts of these combined variants is fairly typical. Cells of the two types can be connected by small desmosomes.

Light and electron microscopy of small cell carcinoma. Note the infrequency of neurosecretory granules in the cytoplasm.

 

Non-Small Cell Carcinoma With Neuroendocrine Features
At the present time, there is limited information on the ultrastructure of the tumors that fall in this category and more examples need to be studied with the electron microscope. By light microscopy, a large cell neuroendocrine carcinoma with many mitoses has been defined and recognized to be an aggressive tumor with a survival rate similar to that of small cell lung carcinoma. For its identification, neuroendocrine features in large cell carcinomas must be evident by light microscopy and their presence verified by immunoperoxidase studies (chromogranin and synaptophysin are the best markers) or electron microscopy.

Some non-small cell lung carcinomas which look like typical adenocarcinomas or less frequently squamous cell carcinomas have been documented to possess neuroendocrine properties by immunoperoxidase methods. The clinical relevance of this observation has not been adequately defined. In my experience, it is an unusual occurrence to encounter numerous dense bodies which are morphologically acceptable as neuroendocrine granules in cells of an adenocarcinoma, and they are distinctly rare in squamous carcinomas.

References.
Arrigoni MG, Woolner LB, Bernatz PE. Atypical carcinoid tumors of the lung. J Thoracic Cardivasc Surg 1972;64:413.

Mackay B, Fanning T, Bruner JM, Steglich MC. Diagnostic electron microscopy using fine needle aspiration biopsies. Ultrastruct Pathol 1987;11:659.

Mackay B, Ordonez NG, Bennington JL, Dugan CC. Ultrastructural and morphometric features of poorly differentiated and undifferentiated lung tumors. Ultrastruct Pathol 1989;13:561.

Travis WD, Linder J, Mackay B. Classification, Histology, Cytology, and Electron Microscopy. Chapter 23 in Lung Cancer - Principles and Practice (Pass HI, Mitchell JB, Johnson DH, Turrisi AT, eds). Lippincott-Raven, 1995.

Vuitch F, Sekido Y, Fong K, Mackay B, Minna JD, Gazdar AF. Neuroendocrine tumors of the lung. Pathology and molecular biology. Chest Surg Clin NA 1997;7:21.