Papillary carcinoma is the most common thyroid malignancy, followed by follicular, anaplastic and medulary carcinoma. Papillary carcinoma is commonly seen in the younger age group, and is usually occult. The young adult often presents with nodal metastases as the primary complaint, with ultrasound of the neck revealing the occult nodule in the thyroid. Papillary carcinoma has a good prognosis, and hence it should be treated aggressively.
Ultrasound is an effective imaging modality for thyroid malignancies because of the inherent high echogenicity of the gland and superficial location rendering high resolution scanning possible. Since many of these thyroid malignancies are occult, ultrasound provides easy detection of these masses which are usually hypoechoic contrasted against the relatively echogenic thyroid gland. At the same time ultrasound can also detect the involved cervical nodes which, if metastatic, become rounded and lose their central hilar echo complex.
Papillary carcinoma has a broad spectrum of sonographic appearances. Chan, et al reviewed the common features of papillary carcinoma. These include:
1. A hypoechoic mass with well-defined margins
2. Microcalcifications or no calcifications
4. Heterogeneity is also a common finding
The uncommon features of this malignancy include:
1. A hyperechoic appearance
2. Irregular margins of the mass [may predict tumor invasion]
3. Coarse calcifications [click here to view]
4. Cystic areas within the mass
If the mass is completely encapsulated, then the halo sign is usually seen.
There have been conflicting views regarding the reliability of microcalcifications as indicators for malignancy. However, most studies agree that small echogenic particles without acoustic shadowing mainly constitute microcalcifications which have a high specificity as a marker for malignancy. Accordingly, any thyroid mass detected with ultrasound should be carefully inspected for the presence of microcalcifications. Hatabu, et al described a new sonographic sign of ‘a calcified nodule in a cyst’, that should raise suspicion for a cystic papillary carcinoma. However, it should be noted that these echogenic areas may sometimes represent fibrous bands and condensed colloids.
Since many thyroid carcinomas present as occult masses, ultrasound guidance for FNA of these clinically non-palpable thyroid nodules is often necessary. Current literature recommends high resolution ultrasound imaging of the neck combined with fine needle aspiration as an initial diagnostic test in suspect thyroid malignancies.
Most thyroid nodules are benign, but for those that are not absolutely benign in ultrasound characteristics FNA biopsy is currently the best method to diagnose malignancy. Ultrasound also provides a convenient and reliable means for following benign lesions over time.