Follicular Adenoma

Pathologic Features of Follicular Adenoma

Follicular adenomas are benign tumors of the thyroid gland that grow in glandular or follicular patterns. They can occur in any portion of the thyroid and in any age group; they are more common in young adults. Adenomas are usually solitary and less than 3 cm in size, although significant numbers of exceptions to these rules exist.6 The lesions tend to grow slowly within a capsule of surrounding compressed thyroid glandular tissue. Over time, they develop a dense capsule surrounding the lesion. Because of this they are more firm than the surrounding tissue. They become palpable when they reach 5 to 10 mm in size. On cut section, they vary from a soft grayish white tissue that bulges out above the cut surface to brown gelatinous tissue. On histologic examination, the follicular adenoma demonstrates the presence of follicles (Fig. 13-1). There can be marked variability in the follicles produced in one follicular adenoma compared with another. Some follicular tumors can be composed of nearly solid cords of tumor cells with rudimentary acinar formation. These are also sometimes called embryonal adenomas. Some follicular adenomas form extremely large dilated glandular structures with a large amount of colloid and only a very scant stroma. These follicular adenomas are sometimes called colloid nodules. Other patterns that are sometimes recognized include a variant with small well-formed acini very similar to normal thyroid tissue but with a large amount of hyalin collagenous fibrous tissue separating the follicles (fetal adenomas). Finally, adenomas with well-formed follicles but follicular cells that are considerably larger and more variable in size than usual thyroid follicular cells and that contain an abundant granular pink cytoplasm are called Hiirthle cell adenomas.7 Hurthle cell tumors are discussed in more detail in a separate chapter in this book.

The origin of follicular adenomas and the stimuli that maintain them are as yet not clear. Investigations have supported the idea that most, if not all, follicular adenomas are of a monoclonal origin and represent true neoplasms. The evidence suggesting that follicular adenomas are monoclonal

FIGURE 13-1. Follicular adenoma of the thyroid. Note the well-formed acini and the intact tumor capsule. Original magnification xl60.

neoplasms comes from cytogenetic or restriction fragment length polymorphism (RFLP) analysis.7"10 Studies have focused on the growth advantages conferred by these molecular changes. These include activating mutations of the thyroid-stimulating hormone (TSH) receptor.11"15

Clinical Features of Follicular Adenoma

Adenomas tend to grow slowly, be unchanged for years at a time, and become symptomatic only late and rarely. They are typically discovered by palpation by the patient or physician on directed physical examination. If they are very inconveniently placed or are allowed to grow to a large size, the tumors can occasionally cause local symptoms such as dysphasia, voice changes, stridor, or pain. These symptoms may be brought on by bleeding or necrosis of the center portion of the lesion that causes a sudden increase in size. In that situation, the symptoms may be temporary and a portion of the lesion may become cystic.

The vast majority of follicular adenomas are hypofunc-tional on radioiodine scan. If imaged in this fashion, they are seen as "cold" or "warm" (the same as normal thyroid) nodules. A small proportion of these nodules may be hyper-functional, concentrating iodine avidly, which may suppress function in the remainder of the thyroid. They may occasionally produce thyrotoxicosis (toxic adenoma).

Interestingly, once the neoplasm has differentiated as a follicular adenoma, it appears only rarely if ever to degenerate to carcinoma. There is little evidence in humans to suggest that adenomas transform into invasive carcinomas. Changes from hyperplasia to adenoma to invasive carcinoma are seen rarely in some people who have congenital goitrous hypothyroidism. This does not appear to be the typical course in most adults.

The indications for removing a follicular adenoma are (1) evaluation for possible carcinoma, (2) treatment of toxic adenoma, and (3) resolution of local compressive symptoms.

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