Assessment and Staging

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The majority of patients with differentiated thyroid cancer will present with a palpable goiter and a clinically negative neck (within both the central and lateral compartments). The treatment rationale for such patients is discussed later but in general, since there is no evidence that elective surgery for the N0 (no nodes palpable) neck improves survival, there are currently few indications for elective imaging. In the future, increased use of ultrasound at the primary site may involve lymph node assessment in level VI and while it provides a guide to which patients need elective surgery, more of these patients are now being treated by total thy-roidectomy and level VI neck dissection.

Table 13.4 Modalities used to evaluate patients with differentiated thyroid cancer

The evaluation of patients with differentiated thyroid cancer involves several modalities (Table 13.4). Clinical examination of the neck has a variable reliability with inevitable false-positive and false-negative rates of around 20-30% [22]. This is compounded by the fact that many patients have micrometastases which are often small and therefore impalpable. The central and lateral compartments of the neck may be evaluated using the modalities listed in Table 13.4.

The range of normal or nonpathological cervical neck nodes is from 3 mm to 3 cm, but for squamous cell carcinoma of the neck, nodes greater than a centimeter in size on CT scanning usually contain metastatic disease. However, for papillary thyroid cancer, the size criteria are different and metastatic nodes are usually smaller [23]. There is little evidence in the literature to justify routine elective imaging of the N0 neck. Levels I to VII should be clinically evaluated and, those patients with either palpable or suspected neck disease, as well as those with proven recurrence, should be imaged anatomically. When assessing recurrent disease, it is important to evaluate both the retropharyngeal and parapha-ryngeal spaces since patterns of drainage can be altered by previous treatment with either surgery or external beam radiotherapy.

The CT criteria for malignancy include cystic and hemorrhagic change, calcification, contrast enhancement, and a hypoplastic appearance [21]. Imaging can be done with or without contrast, but the iodine load from the former will interfere with subsequent treatment with radioactive iodine for up to 3 months or sometimes longer. MRI uses similar staging characteristics to CT with regard to malignancy but usually takes longer and is inferior to CT when imaging the chest.

Ultrasound is becoming more important in the primary evaluation of lymph node metastases and in the follow-up of patients treated with differentiated thyroid cancer. Lymph node metastases as small as 2 to 3 mm can now be detected when ultrasonography is performed with a high frequency probe [24]. Some units suggest that neck ultrasonography should be routinely performed in all patients with differentiated thyroid carcinoma at presentation [25] and this not only includes level VI, but the "at risk levels" in the lateral compartments (levels III, IV, and V).

Suspicious nodes tend to be round, hypoe-choic, and devoid of an echoic central line, with microcalcifications or cystic components, and are hypervascular on Doppler ultrasonography [25].

Following initial treatment, patients are followed up with TSH suppression and sequential thyroglobulin monitoring. In the presence of a truly positive elevated thyroglobulin, recurrent disease is assessed with whole-body 131I scanning and anatomical imaging with either CT or ultrasound. The role of PET in the evaluation of cervical lymphadenopathy in differentiated thyroid cancer is controversial. There is currently no role in using PET to detect occult metastases in the N0 neck but it maybe useful in detecting recurrent neck disease in the presence of an elevated serum thyroglobulin with negative 131I scans and anatomical images.

All tumors should be TNM staged. In the past, the UIIC-AJCC TNM Classification of Malignant Tumours (5th edn) staged the regional lymph nodes as described in Table 13.5.

The latest UICC-AJCC TNM classification of malignant tumors (6th edn) has amended this as shown in Table 13.6.

Reasons for these changes are that in the past, little prognostic significance was given to level VI regional lymphadenopathy, and an emphasis was placed on prognostic differences between unilateral and bilateral neck disease, for which there is now thought to be little evidence (Professor J. Shah, personal communication).

Table 13.5 Lymph node staging according to the 5th edition of the UICC-AJCC TNM classification [26]

N

Regional lymph nodes

NX

Regional lymph nodes cannot be assessed

N0

No regional lymph node metastasis

N1

Regional lymph node metastasis

• Clinical examination

• Ultrasound

• Positron emission tomography (PET)

Table 13.6 Lymph node staging according to the 6th edition of the UICC-AJCC TNM classification [27]

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