|By Dr. Bhanumathy Pillay Pathologist
The thyroid gland is made up of lobules composed of small and large follicles lined by epithelial (follicular) cells. The size as well as the morphology of the follicular cells and the consistency of the colloid in the follicles depend on the activity of the gland.
Any thyroid disease can present as a nodule and generally it is not possible to differentiate benign nodules from malignant nodules with non-invasive procedures.
The vast majority of thyroid nodules are benign. The size and consistency of a thyroid mass may not always indicate whether it is benign or malignant - some tumours can be very minute (occult carcinoma) while nodular colloid goitre is noted to be extremely large. Benign nodules can be hard in texture due to calcification while cystic papillary carcinoma is usually soft. An irregular and fixed thyroid swelling may be seen in thyroiditis as well as in malignancy.
Clinical features such as nodule size, pain, hard consistency, irregularity, fixation, obstructive symptoms and even vocal cord paralysis may be caused by both benign as well as malignant lesions. The best clinical indicator for malignancy is the presence of a thyroid swelling and cervical lymphadenopathy.
The main goal of fine needle aspiration (FNA) of the thyroid is not the precise diagnosis of thyroid cancer but to identify nodules that require surgery from those that do not. Therefore FNA is mainly a triage procedure. But in the ideal set-up (full clinical details, good sampling and experienced evaluators) the procedure can be diagnostic for several lesions - papillary carcinoma, anaplastic carcinoma, medullary carcinoma, follicular neoplasm, Hashimoto's thyroiditis, colloid goitre and large cell lymphoma.
FNA can be done by a pathologist, endocrinologist, radiologist or a surgeon. A 25 gauge needle is best and can be used with or without aspiration. Since the thyroid is very vascular needle sampling without aspiration yields less bloody smears.
Adequate sampling is very important for correct interpretation. If sufficient material is obtained several smears can be made, some should be air-dried (for Romanowsky staining) and some wet fixed (spray or 95% ethyl alcohol) for Papanincolaou staining. Any remaining material in the syringe should be rinsed in a fixative solution and sent to the laboratory.
Except for caution in patients with a bleeding diathesis, there are no contraindications for thyroid FNA.
The criteria for adequacy of smears are variable but in general the presence of 5-6 clusters of well preserved follicular cells, each cluster containing about 10 cells, is accepted as suitable for evaluation. However, some lesions, due to their very nature, such as cysts and colloid nodules may yield scanty or no follicular cells.
Abundant colloid in a smear is almost always a sign of a benign lesion. A large quantity of diffuse watery or thick colloid is the hallmark of colloid goitre. Cysts yield numerous macrophages, haemosiderin-laden, if the cyst is haemorrhagic. Cysts, larger than 4cm should be evaluated after the initial aspiration. A residual lump should be reaspirated or other investigations must be considered to rule out a cystic papillary carcinoma.
Benign nodules include colloid nodule, multinodular goitre and cystic goitre.
Cellular smears are encountered in the hyperplastic phase of colloid goitre, in tumours and in Hashimoto's thyroiditis. Hypercellular smears with scanty or no colloid favour a diagnosis of neoplasm.
The need for full clinical and radiological (if available) data cannot be stressed enough in the interpretation of material from a thyroid FNA.
Figure 1. Watery colloid with no follicular cells
Figure 2. Cellular smear with no colloid
Figure 3. Lymphocytes & oncocytes in Hashimoto's thyroiditis
Figure 4. Papillary carcinoma
Figure 5. Anaplastic carcinoma
Figure 6. Cyst macrophages