The AJCC classification of differentiated thyroid cancer (DTC) upstages patients with extrathyroidal extension (ETE) to T3 disease if primary tumor size is less than 4cm. Furthermore, the presence of ETE is an indication for adjuvant radioactive iodine (RAI) therapy. This is based on the assumption that ETE may be a marker of aggressive disease. Intraoperatively, surgeons resect the sternothyroid and/or sternohyoid muscles in the presence of gross ETE to achieve clear surgical margins and allow better mobilization of the thyroid gland. However in the absence of gross ETE, the resection of the sternothyroid muscle is surgeon dependent. The aim of this study is to determine patient and tumor factors associated with the presence of microscopic ETE.
An institutional database of 3664 previously untreated patients with DTC operated between 1986 and 2010 was reviewed. Patients with less than total thyroidectomy, residual disease at end of surgery, M1 at presentation, tumor size greater than 4cm, or gross ETE were excluded. 2304 patients were included for analysis (Figure 1). Patient demographics and pathological features were retrospectively recorded. Pearson's chi-squared analysis was used to compared patients and tumor characteristics.
The median age of the cohort was 47.1 years (range 12-87). Table 1 shows a comparison of patient and tumor characteristics between those with and without microscopic ETE. Patients with microscopic ETE are more likely to be male (p=0.002), more likely to exhibit papillary tall cell variant (p<0.001), and associated with tumor size between 1-3cm (p<0.001). Patients with microscopic ETE were also more likely to receive adjuvant RAI therapy (p<0.001). Microscopic ETE was present in 16.4%, 33.5%, 32.2% and 21.3% of patients with primary tumor less than 1cm, 1-2cm, 2-3cm, and 3-4cm respectively.
Greater than 30% of patients with no evidence of gross ETE at time of surgery are subsequently found to have microscopic ETE on histopathology if the primary tumor is between 1 and 3cm. With the incidence of microscopic ETE being above 30%, due consideration should be given for the resection of the sternothyroid muscle for satisfactory oncologic purposes.