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INTRODUCTION

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Thyroid cancer is considered to be the most common endocrine malignancy, with estimated new cases exceeding 60,000 in 2013.1 Although most patients who present with thyroid cancer have well-differentiated intrathyroidal tumors that carry an excellent prognosis, these tumors have a distinct tendency for multifocal involvement and regional lymph node metastasis. The adverse prognostic factors in thyroid cancer have been well-established and include patient age, tumor histology, primary tumor size, extrathyroidal extension, and distant metastasis.2 The greatest negative impact on prognosis, from a surgical standpoint, is extrathyroidal extension.3 Up to 15% of patients with differentiated thyroid cancer (DTC) exhibit aggressive behavior, hallmarked by extrathyroidal extension, treatment resistance, and increased mortality.3,4

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Locally advanced DTC may involve the central and lateral neck compartments, or the mediastinum by direct primary tumor invasion or from extracapsular extension of the involved lymph nodes.5,6 The invasion of regional structures is uncommon; however, when invasion occurs, the structures most frequently involved are the strap muscles (53%), recurrent laryngeal nerves (47%), trachea (37%), esophagus (21%), larynx (12%), followed less frequently by the thoracic duct and carotid sheath contents.7 Patients with locally advanced disease tend to have an increased incidence of local recurrence, regional spread, and distant metastases.6,8-11

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Surgical resection is the primary treatment for patients with locally advanced DTC, with the fundamental goal of complete resection and negative margins. However, such resection may be associated with significant morbidity, specifically when gross disease involves critical structures of the neck such as the recurrent laryngeal nerve, trachea and esophagus. The significant morbidity and subsequent decrease in quality of life has led some surgeons to pursue conservative approaches using peeling or shaving techniques aimed at preserving function, but this must be counterbalanced against oncologic control. The successful management of locally invasive thyroid cancer depends on a thorough understanding of the patterns of invasion, preoperative evaluation, and techniques of surgical resection and reconstruction. Moreover, the appropriate use of adjuvant therapy with radioactive iodine (RAI) and external beam radiation therapy (EBRT) is key to optimize management results.

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PREOPERATIVE EVALUATION

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Although locally advanced thyroid cancer is uncommon, clinical evidence of local invasion should be sought on the initial evaluation of any patient in whom thyroid cancer has been diagnosed or suspected. This will allow for better planning and optimal treatment strategies. Physical examination findings may raise the suspicion of local invasion in patients with thyroid cancer. Large size, firmness, fixation to surrounding structures, or tenderness of a mass may suggest extrathyroidal extension. Multiple or large bulky lymph nodes that are palpable in the central or lateral neck compartments should also prompt concern for extension of tumor into the soft tissue. Dysphonia or hoarseness resulting from recurrent laryngeal nerve dysfunction is often the first sign of extrathyroidal extension, but sometimes this can be absent due to the accommodation of the ...

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