Occult invasion of sternothyroid muscle by differentiated thyroid cancer

To determine the incidence of occult invasion of sternothyroid by differentiated thyroid cancer (DTC) and identify clinico-pathological features associated with the same.

Retrospective study of a consecutive series of DTC patients undergoing surgery, with preoperative ultrasound showing no evidence of strap muscle invasion. All had en bloc excision of sternothyroid muscle along with thyroidectomy. Incidence of microscopic invasion of sternothyroid and clinicopathologic features associated with the same, were studied.

A total of 76 patients with DTC (2010-2014) were identified, of whom 62 met the inclusion criteria and were included in this study. Of these, 22 (36%) had no extrathyroidal extension (ETE), 30 (48%) had minimal ETE without sternothyroid invasion and 10 (16%) had minimal ETE with microscopic sternothyroid invasion. The mean tumor sizes of the three sub-groups were 1.9, 3.1 and 4.9 cm, respectively, with a significant difference between no ETE and sternothyroid invaded sub-groups (p = 0.03). Out of the 40 cases with minimal ETE, 3 (7.5%) had positive tumor microscopic margin. Retaining sternothyroid in situ would have theoretically increased this proportion to 27.5%. Over a median follow-up of 52 months, 58 (94%) patients remained structurally disease free, with only 1 local recurrence.

Occult invasion of sternothyroid muscle occurred in 16% of DTC in this series. Excision of the muscle en bloc with thyroidectomy, particularly in larger tumors, may confer benefit in accurately staging the disease, encompassing occult ETE and achieving clear microscopic margins.