Thyroid Cancer

The use of CT and MRI in the management of thyroid cancer can be considered in two situations: preoperative assessment and the follow-up of treated disease. The commonest situation preoperatively is that of a solitary or a dominant thyroid nodule. Cytology, ultrasound, and the clinical picture may indicate a likely diagnosis but often a definitive diagnosis is not available until formal histology can be performed on the operative specimen. In this situation CT and MRI have little to add and are not routinely indicated. However, one of these modalities may be useful when the limits of an enlarged thyroid cannot be determined with ultrasound or if there is evidence of a locally advanced tumour, for example clinically fixed in the neck or associated with hemoptysis [14].

In these patients the function of the imaging modality is not diagnosis but staging; particularly to advise the surgeon about extension into critical areas and structures, notably the adjacent muscles, the carotid arteries, the trachea, larynx, pharynx, esophagus, and the mediastinum. Small tumours visible on ultrasound (less than 10 mm diameter) may be completely overlooked on CT and MRI; ultrasound is preferred therefore if preoperative assessment of the presence of a multifocal tumor is required [15]. The appearance of thyroid malignancies is variable but generally they are intermediate signal on T1 weighted and high signal on T2 weighted sequences [16]. They may be ill defined but at least as often are well defined when they cannot confidently be distinguished from benign thyroid nodules. Different forms of thyroid malignancy cannot be confidently distinguished on imaging although some suggestive features have been described and may be observed (Table 28.1).

Papillary carcinoma is usually relatively small, well defined, and localized. A minority, however, are locally invasive and may invade throughout both lobes of the thyroid or out of the thyroid into adjacent structures, including the larynx and trachea and, less often, the esophagus. Areas of cystic necrosis are often present within the tumor (Figure 28.1). Punctate or cloudy calcific areas (psammoma bodies) may be apparent on CT and multifocal deposits are not uncommon (Figure 28.2). Lymph node

Table 28.1 Imaging features of thyroid carcinoma on CT and MRI

Calcification

Necrosis

Invasive

Papillary

+/-, punctate, cloudy

+++/-

—/+

Follicular

---/+

-----/+

--/+

Medullary

+/-, coarse

----/+

Undifferentiated

+++/-, amorphous

+++/-

+++/-

Lymphoma

-----/+

-----/+

Rare

European Thyroid

Figure 28.1 Transverse MRI scan through the thyroid (T2 weighted image) showing a relatively centrally placed papillary carcinoma of the thyroid (arrows) with central cystic change. Multiple abnormal lymph nodes are seen bilaterally (arrowheads) in the internal jugular and posterior cervical chains, also showing cystic change and representing metastatic disease.

Figure 28.1 Transverse MRI scan through the thyroid (T2 weighted image) showing a relatively centrally placed papillary carcinoma of the thyroid (arrows) with central cystic change. Multiple abnormal lymph nodes are seen bilaterally (arrowheads) in the internal jugular and posterior cervical chains, also showing cystic change and representing metastatic disease.

metastases are extremely common, up to 50% of cases at presentation, and may be bilateral. Lymph nodes may show calcification and vary from entirely solid and hypervascular to largely or entirely cystic.

Follicular carcinoma is also occasionally aggressive and locally invasive. They are rarely cystic and are much less commonly associated

Transverse Arrowheads
Figure 28.2 Transverse post-contrast CT demonstrating small bilateral papillary carcinomas, both showing substantial cystic change centrally. Small calcific foci are also discernible (arrowheads).

with lymph node metastases (roughly 10%). Anaplastic carcinomas are classically locally invasive into a variety of structures including the great vessels of the neck and the trachea and larynx (Figure 28.3). They often show substantial cystic necrosis and hemorrhage. Amorphous calcification is common. Lymph node metastases are common and up to a quarter involve the mediastinum. Medullary cell carcinoma is usually solid and may show coarse or psammomatous calcifications and local invasion [17]. Up to 50% are associated with neck and mediastinal lymph node involvement. Around one third of these tumours are familial and may be associated with multiple endocrine neoplasia, in which case they are often bilateral. Thyroid lymphoma is almost always primary and often associated with hashimoto's disease. It usually appears as a solitary mass, occasionally as multiple nodules. It rarely necroses (Figure 28.4).

Metastases to the thyroid constitute around 2% of thyroid cancer (although some series report up to 17% [18]). They most commonly arise from primary bronchial or breast carcinoma, sometimes from elsewhere including malignant melanoma and renal cell carcinoma. They have nonspecific malignant appearances and may demonstrate local extraglandular invasion. If they arise from squamous cell carcinoma then they may reflect the tendency of that tumor to demonstrate substantial necrosis (Figure 28.5).

Conventional tnm staging of thyroid cancer is primarily clinical, supported by ultrasound and other imaging findings. In day-to-day practice the radiologist is probably serving the surgeon best if he adequately describes the areas involved by tumor and indicates the degree of confidence he feels. Ultrasound appears to be the superior modality for the assessment of tumor extent within the thyroid gland but once extracapsular extension has occurred MRI or CT is indicated, particularly for the diagnosis of invasion of the aerodigestive tract (Figure 28.6) Which has been reported in around 6.5% of patients [15].

The British Thyroid Association recommends that all lymph nodes within the central compartment of the neck including the pre- and paratracheal nodes should be removed [14] (level VI node dissection) and that lymph nodes lying along the carotid sheath and internal

Thyroid Central Compartment Thyroid Central Compartment

Figure 28.3 Transverse MRI scan (T2 weighted) through the thyroid and neck. The remaining normal thyroid gland is seen as relatively low signal compared with the ill-defined mass of anaplastic carcinoma arising from the posterior aspect of the right lobe (A).The tumor extends posteriorly, coming to lie against the prevertebral muscles and laterally to encase the carotid artery (arrow). Posteromedial^ the tumor extends round the back of the trachea, which it invades posteriorly (arrowhead), and abuts the esophagus (arrowhead), which is also probably invaded. For comparison a transverse post-contrast CT scan (B) on the same patient demonstrates the irregular tumor enhancing poorly compared with the intensely enhancing normal thyroid. Once again carotid artery encasement is seen (arrow) and also invasion of the sternocleidomastoid muscle (arrowheads). Further inferiorly at the level of the thoracic inlet (C) the trachea is grossly narrowed by extensive tumor, the airway (arrowheads) reduced to a narrow slit.

jugular vein (levels II to IV) should be palpated; suspicious nodes should be sent for frozen section with a view to selective neck dissection of levels proving positive. Radical neck dissection is therefore not routinely recommended. As indicated above, routine preoperative CT or MRI is only recommended for clinically advanced disease and then principally to stage the primary tumor. Ultrasound should have been performed and neither CT or MRI is likely to improve the detection of lymph node metastases substantially, except arguably for some relatively inaccessible lymph nodes, particularly inferior to the thyroid and beyond, into the superior mediastinum. Normal lymph nodes are routinely visible on CT and MRI and may have a maximum diameter anywhere between 2 and 25mm. Lymph node metastases are common with thyroid cancer, especially papil lary carcinoma, being present in up to 50% or more in papillary carcinoma at presentation. The lymph nodes, however, will often remain small. Therefore conventional size criteria for abnormal nodes (greater than 10 mm transverse diameter in the upper neck, 9 mm elsewhere) are insensitive for the detection of metastases. Attempts to increase sensitivity by reducing the accepted upper limit of normal to 7 mm in the jugulodigastric and 6 mm at other levels in the neck leads to substantial numbers of false positives. Nodes may change shape with involvement, becoming rounded instead of the normal well-defined elliptical/bean-shape, and the number of lymph nodes may increase substantially. These signs, however, can be subjective and give rise to considerable overlap with benign lymph nodes. There does appear to be a strong tendency for the normal

Long Right Thyroid

Figure 28.4 Transverse MRI (T2 weighted image) demonstrating a homogeneous mass of lymphoma arising from the right lobe of an atrophic thyroid (long-standing Hashimoto's disease) and extending widely in the right supraclavicular fossa and posterior to the thyroid.The patient was elderly and has a marked thoracic kyphosis, hence the appearance of the upper ribs in this plane.

architectural pattern (focal fat at the hilum; appearing on CT as low density and on MRI as high signal on T1 and T2 weighted sequences, low signal on fat suppression sequences) to disappear early. Cystic change in the lymph nodes is common (Figure 28.7), reported in

Figure 28.4 Transverse MRI (T2 weighted image) demonstrating a homogeneous mass of lymphoma arising from the right lobe of an atrophic thyroid (long-standing Hashimoto's disease) and extending widely in the right supraclavicular fossa and posterior to the thyroid.The patient was elderly and has a marked thoracic kyphosis, hence the appearance of the upper ribs in this plane.

Necrotic Posterior Thoracic Mass

Figure 28.6 Coronal MRI scan (STIR sequence) showing enormous enlargement of the thyroid gland by lymphoma (A). Tumor extends in all directions, including into the mediastinum but also superomedially into the larynx and pharynx (arrow-head).Tumor can be seen on the transverse T2 weighted image (B) extending into the posterior aspect of the right vocal cord and the hypopharynx (arrowheads).

Thyroid Metastasis Mri

Figure 28.5 Coronal MRI (STIR sequence) demonstrating squamous cell carcinoma metastasis to the right lobe of thyroid showing the characteristic necrotic appearance of this process. There is a large right upper cervical nodal metastasis (arrow) showing similar necrosis and a halo of high signal edema (arrowhead) indicating extranodal extension.

Figure 28.6 Coronal MRI scan (STIR sequence) showing enormous enlargement of the thyroid gland by lymphoma (A). Tumor extends in all directions, including into the mediastinum but also superomedially into the larynx and pharynx (arrow-head).Tumor can be seen on the transverse T2 weighted image (B) extending into the posterior aspect of the right vocal cord and the hypopharynx (arrowheads).

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Figure 28.7 Coronal MRI scans demonstrating papillary carcinoma lymph node metastases. In the first example there is a dominant markedly enlarged left level III lymph node (A, STIR sequence) showing loss of normal architectural pattern and considerable heterogeneity. The T1 weighted sequence (B) shows the classic high signal cystic areas within the diseased node mass; heterogeneous appearances with high signal cystic areas are also demonstrated on the T2 weighted sequence (C). The second patient shows more extensive bilateral lymph node metastases,especially on the right.They are easily visible on the STIR sequence (D) while the T1 weighted sequence (E) once again demonstrates the high signal cystic areas characteristic of this condition.

Figure 28.8 MRI scan 4 years after thyroidectomy for medullary thyroid carcinoma. The post-contrast transverse T1 weighted image (A) demonstrates a substantial enhancing mass of recurrent tumour (arrowheads) lying against the trachea at the thoracic inlet. This is seen as a heterogeneous but predominantly high signal mass on the STIR sequence (B), which also demonstrates recurrent disease in the lymph node drainage (arrows).

around 50% of patients on MRI [15]. These changes are likely to be due to thyroid protein (colloid or thyroglobulin), hemorrhage, or tumor necrosis. Papillary thyroid carcinoma may also give rise to calcification within involved lymph nodes, best seen on ultrasound or CT.

Following treatment life-long follow-up is required. This will be primarily clinical and biochemical with an appropriate isotope scan if recurrence or metastatic disease is suspected (radioactive iodine for papillary or follicular thyroid cancer) [18]. Ultrasound, potentially with guided aspiration biopsy, is likely to be first line for clinically or isotopi-cally evident local recurrence. CT or MRI is indicated to evaluate local recurrence in the neck if this is inadequately delineated with ultrasound or if it extends into the thorax. They may also be used to evaluate thoracic metastases, CT being the modality of choice for this purpose.

In contrast to most other head and neck cancer surgery, there is usually only modest postsurgical change in the neck on scanning. Often a neat scar at the thyrodectomy site is seen on CT or MRI. In the acute stage this will have a fairly active appearance. Loss of tissue planes, edema, distortion/swelling of normal structures, and engorgement of lymphatics can be identified on CT and MRI. High signal on T2W and STIR sequences is seen in edematous fat and other tissues on MRI. Tissue that has been traumatized by surgery will also show enhancement with gadolinium. However, it is rare that the neck needs to be imaged in the first few weeks after surgery and most of these acute changes will resolve by 6 weeks, although in some patients a stable postoperative appearance may not be apparent until 12-18 months following intervention. As the tissue matures the signal spectrum will generally fall towards intermediate to low on all sequences with the disappearance of enhancement. The stereotypical scar tissue of more than 6 months' age is therefore fairly inert in appearance on MRI. By comparison deposits of recurrent tumor have a more active appearance (Figure 28.8) with enhancement following intravenous contrast and elevation of signal on the T2 weighted sequence [5].

Figure 28.8 MRI scan 4 years after thyroidectomy for medullary thyroid carcinoma. The post-contrast transverse T1 weighted image (A) demonstrates a substantial enhancing mass of recurrent tumour (arrowheads) lying against the trachea at the thoracic inlet. This is seen as a heterogeneous but predominantly high signal mass on the STIR sequence (B), which also demonstrates recurrent disease in the lymph node drainage (arrows).

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