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Ultrasound can predict extrathyroidal extension
A 72-year-old Albanian female was referred for the evaluation of a thyroid nodule. The nodule was found during an initial physical evaluation by her primary care physician.
Her thyroid function was normal with a thyroid-stimulating hormone measurement of 1.98 mU/L. She has no family history of thyroid disease or thyroid cancer. She has no prior history of thyroid disease or head and neck radiation. She did not have any symptoms of obstruction, including dysphagia, change in voice or cough.
Stephanie L. Lee
On exam, she appeared to be euthyroid with a hard palpable mass in the left isthmus and left thyroid lobe. The mass was very hard and fixed. The nodule did not move with swallowing. No adenopathy was felt on exam.
Ultrasound performed
An office ultrasound was performed that demonstrated a 1.5 cm right hypoechoic nodule that was hypoechoic and taller-than-wide (Figure 1A). In the isthmus and extending into the left lobe was a hypoechoic nodule with grade 3 to 4 vascular flow (low to moderate flow) by Doppler analysis. Both nodules contained non-shadowing hyperechoic foci consistent with microcalcifications. The left nodule had a well-delineated but undulating margin (Figures 1B, C).
Careful observation shows a mushroom extension of the tumor outside the thyroid capsule with apparent extension into an overlying strap muscle. No abnormal adenopathy was seen in the bilateral neck, levels II to VI.
An ultrasound fine-needle aspiration biopsy of both the left and right thyroid nodules showed follicular cells with mixed follicular and trabecular patterns with membrane-bound nucleoli suspicious for papillary thyroid carcinoma. The ultrasound appearance of the papillary thyroid carcinoma in the left lobe suggested extrathyroidal extension into the strap muscle.
A CT scan of the neck and chest was performed to assist in preoperative planning because of the increased risk for tracheal invasion, cervical adenopathy and distant metastatic disease. The CT of the neck without contrast showed the bilateral thyroid nodules with tracheal compression at the level of the thyroid with the narrowest anteroposterior diameter of 8 mm.
There is no evidence of invasion of the tumor through the trachea, laryngeal cartilages or strap muscles, although adherent disease to these adjacent structures cannot be excluded. No abnormal nodes were identified, but the exam was limited by lack of contrast. No pulmonary or mediastinal masses were seen.
The patient had a thyroidectomy with bilateral level VI node dissection. The left sternothyroid was adherent to the surface of the tumor and an en bloc resection of the affected portion of the left sternothyroid muscle was removed with the thyroid and central neck nodes.
Figure 1. Thyroid ultrasound. A. Transverse image of the hypoechoic, tall-than-wide right thyroid nodule (red arrow). B. Transverse image of the hypoechoic left thyroid nodule with anterior extrathyroidal extension (red arrow). C. Outline of the tumor showing undulating border showing invasion of the tumor through the anterior thyroid capsule into the strap muscle. D. Sagittal image of the left thyroid nodule showing grade 4-5 (low to moderate) intranodular vascular flow by Doppler analysis. Reprinted with permission from: Stephanie L. Lee, MD, PhD.
The pathology showed bilateral partially encapsulated classical PTC that was widely invasive with lymphovascular and capsular invasion. There was extensive extrathyroidal extension of the papillary thyroid carcinoma from the left lobe into the strap muscle, the sternohyoid muscle. Five of 26 nodes in the level VI (pretracheal, paratracheal and prelaryngeal/Delphian) lymph nodes were positive for metastatic PTC without extranodal invasion. The BRAF V600E mutation was presented in the dominant left tumor.
Determining surgery extent
The preoperative ultrasound provides the surgeon with important information that determines the extent of surgery. Abnormal nodes can easily be seen inferior and lateral to the thyroid.
Further, ultrasound is excellent at detecting invasion of the thyroid capsule and extrathyroidal extension. Anterior extension of the tumor outside the thyroid capsule was obvious in this patient (Figures 1B, C), although it was not detected on a CT scan of the neck (Figure 2).
Figure 2. CT scan of the neck. Axial image of the neck through the thyroid gland. The thyroid cancer (CA) is seen as a mass in the left side of the isthmus extending into the left lobe of the thyroid (red arrow). Although the mass is most prominent in the midline (anterior to the trachea), there was no evidence of extrathyroidal extension or invasion into the strap muscles. Trachea: TR.
Several studies have examined the usefulness of preoperative ultrasound. A study by Kwak showed that 40.3% of papillary thyroid carcinomas surgically removed had evidence of extrathyroidal extension. Although the mean size of the tumor was not associated with extrathyroidal extension (P=.527), extensive contact between the tumor and the thyroid capsule was predictive of extrathyroidal extension (P<.0001). The authors concluded that >25% of a nodule’s surface contacting the adjacent capsule is the most accurate measurement that predicted extrathyroidal extension.
Park suggested if >50% of the tumor abutted the thyroid capsule, the sensitivity, specificity and accuracy of sonography in predicting extrathyroidal invasion were 85.3%, 70% and 74.5%, respectively. As expected, Lee suggested that sonographic evidence of the tumor protruding outside the thyroid capsule had even a higher predictive value than the degree of abutting (P=.001) to predict extrathyroidal extension on final pathology.
Preoperative sonography is an important tool in staging. Clinicians should focus not only on lesion size, central and lateral nodal stage but also the amount of the nodule’s surface abutting the thyroid capsule, disruption of the thyroid capsule, extension of tumor outside the thyroid capsule and evidence of invasion into surrounding structures.
Identification of extrathyroidal extension of tumor increases the risk for local metastatic nodes, upgrades older patients to American Joint Committee on Cancer stage III but with gross macroscopic invasion, and increases the AJCC to stage IVB with a significant risk of death. If capsule invasion by thyroid cancer is suggested by preoperative sonographic ultrasound, a CT scan should be performed to assess tracheal invasion and cervical adenopathy. The minimal surgery for these patients is a total thyroidectomy and level VI (central neck) dissection.