A follicular neoplasm of the thyroid refers to an abnormal growth of follicular cells within the thyroid gland, a butterfly-shaped organ located at the base of the neck. These follicular cells are responsible for producing thyroid hormones, which regulate various bodily functions. The term “neoplasm” indicates a new, abnormal growth of tissue. While the word “neoplasm” can sound concerning, it is a broad term that encompasses both non-cancerous (benign) and cancerous (malignant) growths. Further assessment is necessary to determine the specific nature of the growth.
Defining Follicular Neoplasm
The term “follicular” refers to the follicular cells of the thyroid, which are the primary cells forming the thyroid gland. These cells produce hormones like thyroxine (T4) and triiodothyronine (T3) and organize into small, spherical structures called follicles.
The crucial distinction for follicular neoplasms is whether they are benign or malignant. A benign follicular neoplasm is a follicular adenoma, while a malignant one is a follicular carcinoma. Follicular adenomas are well-defined and encapsulated, meaning they are surrounded by a thin fibrous tissue layer, and their cells do not grow beyond this capsule. In contrast, a follicular carcinoma invades beyond this capsule or into blood vessels.
Under a microscope, follicular adenomas and carcinomas can appear remarkably similar, making differentiation challenging based solely on cell appearance. Both types consist of cells resembling normal thyroid follicular cells and often form small follicles. The presence or absence of capsular or vascular invasion is the primary criterion pathologists use to classify the growth as benign or malignant after surgical removal. Therefore, a definitive diagnosis often requires examining the entire nodule, not just a sample of cells.
Identifying and Diagnosing Follicular Neoplasm
A follicular neoplasm is often discovered incidentally during imaging for other conditions or when an individual notices a palpable lump in their neck. Initial diagnostic steps include a physical examination, blood tests to assess thyroid hormone levels, and a thyroid ultrasound. Ultrasound imaging helps characterize the thyroid nodule, including its size, composition, and other features that might suggest malignancy.
Following the initial assessment, a fine needle aspiration (FNA) biopsy is performed. During an FNA, a thin needle collects cells directly from the thyroid nodule for microscopic examination. While FNA is effective for many thyroid nodules, it presents a challenge for follicular neoplasms. The limitation of FNA is that it cannot reliably distinguish between a benign follicular adenoma and a malignant follicular carcinoma based on cell appearance alone. This is because capsular or vascular invasion, the key diagnostic feature, cannot be assessed from a small sample of cells.
Due to this limitation, FNA results for follicular neoplasms are categorized as “follicular neoplasm” or “suspicious for follicular neoplasm,” indicating an indeterminate finding with a malignancy risk ranging from 15% to 30%. To refine this risk, newer molecular tests can be performed on the FNA sample. These tests analyze genetic mutations or gene expression patterns within the cells, which can help predict the likelihood of malignancy. However, even with molecular testing, a definitive diagnosis of follicular carcinoma still requires surgical removal of the nodule for thorough pathological examination.
Management and Outlook
Surgical removal is the standard approach for managing a follicular neoplasm, serving as both the definitive diagnostic step and primary treatment. A thyroid lobectomy, removing the half of the thyroid gland containing the nodule, is performed. In some cases, if the nodule is large or other suspicious features are present, a total thyroidectomy, removing the entire thyroid gland, may be considered as the initial surgery. The National Comprehensive Cancer Network (NCCN) guidelines recommend lobectomy with isthmusectomy as the initial surgery, with a prompt completion thyroidectomy if invasive follicular thyroid carcinoma is found.
After surgical removal, the excised thyroid tissue is sent to a pathologist. The pathologist examines the entire nodule under a microscope for signs of invasion, such as tumor cells extending through the capsule or into blood vessels. This examination determines whether the neoplasm is a benign follicular adenoma or a malignant follicular carcinoma. If the growth is confirmed as a follicular carcinoma, further treatment, such as radioactive iodine therapy, may be considered, especially for more invasive forms or if there is concern for residual disease.
The prognosis for most follicular neoplasms is favorable. For benign follicular adenomas, surgical removal is curative. Even for follicular carcinomas, the prognosis is excellent, particularly when detected early and completely removed. The 10-year survival rate for minimally invasive follicular carcinoma is 98%, while for invasive follicular carcinoma, it is 80%. Long-term follow-up care includes regular monitoring for recurrence and thyroid hormone replacement therapy if a significant portion or the entire thyroid gland has been removed.