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Thyroid cysts not always benign

Issue: August 10, 2015

ByStephanie L. Lee, MD, PhD, ECNU

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A 44-year-old woman was self-referred for a growing right thyroid mass. Seven years before presentation, the patient was seen at another institution for a large right cystic thyroid nodule. By her report, aspiration of the nodule yielded “blood” and completely deflated the nodule to her exam. The result of the cytology is not known.

After 2 to 3 years, the right nodule recurred with a significant increase in size. The patient now had a constant, mild anterior pressure or globus sensation, especially when turning her neck, without dysphagia, cough or change in voice.

The patient had no history of head and neck radiation. She reported no family history of thyroid disease, including thyroid cancer. On exam, the patient was euthyroid with bilateral asymmetrically enlarged thyroid lobes. A firm, mobile and nontender 4-cm nodule could be felt in the right thyroid lobe.

Stephanie L. Lee

Nonshadowing linear hyperechoic foci

Ultrasound exam in the endocrine clinic revealed an enlarged goiter with a 7.6-cm right lobe and 4.6-cm left lobe in sagittal dimensions. Numerous spongiform nodules and colloid cysts (hypoechoic with linear hyperechoic colloid reverberations) were detected in both lobes (Figure 1).

In the right lobe, a 4.3-cm cystic mass was found in the mid-lobe (Figure 2). This nodule showed posterior acoustic enhancement, no visible solid tissue and no intranodular vascularity, but innumerable nonshadowing linear hyperechoic foci without reverberation artifact.

Figure 1. Transverse image of the left upper thyroid lobe. A small hypoechoic colloid nodule is located in the left upper lobe between the trachea and the carotid artery (CA). The typical colloid reverberation is seen with a stacked series of linear echoes (yellow arrow; 4 in this example). This structure is not cystic (fluid-filled cavity), but is solid tissue with large follicle containing large amounts of colloid. This type of nodule is called a colloid cyst or a hypoechoic nodule with a colloid clot.

Images reprinted with permission from Stephanie L. Lee, MD, PhD.

Adjacent to this cyst was a 2.8-cm isoechoic nodule in the inferior lobe without intranodular vascularity of microcalcifications. An ultrasound-guided fine needle aspiration of this lower solid isoechoic nodule showed mixed microfollicular and macrofollicular thyroid cells, watery colloid and macrophages consistent with a benign colloid nodule. The aspiration of the mid-lobe cyst removed 18 mL of translucent amber-colored fluid, which completely deflated the cyst.

Figure 2. Pathology of the cystic wall of the mid-right thyroid cyst. Along the wall of the cyst was a thin, mural lining of follicular cells that showed both papillary and follicular morphology with nuclear changes (nuclear overlap, nuclear crowding, nuclear clearing) consistent with a follicular variant of papillary thyroid carcinoma.

The cytology was insufficient for diagnosis because of the lack of viable follicular cells. A diagnostic right lobectomy was recommended because of the large size of the right lobe ( > 7.5 cm), progressive growth of the nodule by patient history and constant mild obstructive symptoms.

The pathology revealed a 4.3-cm x 4-cm x 2.5-cm cystic follicular variant of papillary thyroid carcinoma in an adenomatous goiter. The nodule was more than 98% cystic with a thin rim of tissue along the inferior edge with a mixed follicular and papillary arrangement of cells with the nuclear characteristics of papillary thyroid carcinoma (Figure 3).

Even after careful review of the cine loop images of her thyroid ultrasound exam, solid tissue was not detected within the cystic mass. The patient had a completion thyroidectomy that revealed an adenomatous goiter with no additional tumor.

Papillary thyroid carcinoma

Bonavita characterized the sonographic patterns of 500 benign and malignant thyroid nodules. As explained in a prior Imaging Analysis article (September 2014), a “cyst with colloid clot” described as cystic nodule with hyperechoic foci with linear echoes, no solid component and no vascularity detected by Doppler imaging is nearly always benign. The second confirmatory study of 811 nodules contained 28 “cysts with colloid clot.” The cytology of all these nodules was benign.

In a subsequent article, Bonavita said the benign cystic lesions must not contain any suspicious characteristics, such as solid hypoechoic tissue, calcification or increased vascular flow. If any of these characteristics are seen, the cystic nodule should be biopsied.

Figure 3. Right cystic nodule in the mid-right thyroid lobe. A Transverse image. B. Sagittal image. A 4.3-cm cystic nodule was located in the mid-right lobe. This cystic structure did not contain any solid material or  intranodular vascularity. The nodule had posterior acoustic enhancement consistent with a cystic structure. The ultrasound showed the cyst was filled with innumerable hyperechoic linear, nonshadowing foci. This was described as “indeterminate hyperechoic, nonshadowing foci that may represent cystic debris, colloid or calcifications” on the ultrasound report. CA: carotid artery.

In this case, this nodule was completely cystic by careful sonographic evaluation, but the presence of these atypical linear hyperechoic, nonshadowing foci that did not show reverberation artifact (stacks of linear lines; see Figure 1) should not be assumed to be colloid, especially if there is no solid tissue (to produce the colloid) within the nodule. Although these foci may represent cellular debris, this is not a typical appearance of a partially cystic nodule. Generally, the cystic areas of a partially cystic thyroid nodule are very hypoechoic with posterior acoustic enhancement without vascular flow or hyperechoic foci. Because the cyst is larger than 4 cm and obstructive symptoms are present, a diagnostic lobectomy is the appropriate diagnostic procedure.