Papillary thyroid carcinoma with rare lymph node metastasis between the non-ipsilateral sternocleidomastoid and striate muscles: two case reports and literature review.

[BACKGROUND] Differentiated thyroid cancer (DTC) has a high incidence but a generally good prognosis. However, lymph node metastasis is likely to occur. It usually metastasizes from the central group lymph nodes to the deep cervical lymph nodes and less frequently to the sternocleidomastoid-sternohyoid muscle. The lymph nodes between the sternocleidomastoid and sternohyoid muscles (LNSS) is easily overlook. Ignoring the preoperative assessment and dissection of level LNSS, especially in the contralateral neck level LNSS, may lead to incomplete surgery and thus require reoperation. The metastatic relevant factors and pathway for LNSS remains inconclusive require further investigation. There is a lack of reports of contralateral or bilateral cervical level LNSS metastasis in thyroid cancer. We hope to arouse attention to the level LNSS through our two case reports.

[CASE DESCRIPTION] We report two cases of non-ipsilateral LNSS metastases. The patients were diagnosed with thyroid cancer by fine-needle aspiration (FNA), and ultrasound examination showed enlarged lymph nodes at the LNSS level. After surgical treatment and postoperative paraffin pathology, both patients were diagnosed with papillary thyroid carcinoma (PTC) and LNSS-level lymph node metastasis. Case 1 was a 63-year-old woman admitted to our hospital with a 15-day history of an untreated thyroid nodule and preoperative euthyroidism. This patient underwent total thyroidectomy, central and left neck LNSS dissection. Her prognosis was good, and there were no signs of recurrence at her 6-month follow-up appointment; Case 2 was a 24-year-old woman admitted to our hospital for a physical examination of an anterior cervical mass that had been present for 1 year and preoperative euthyroidism. This patient underwent total thyroidectomy, central and bilateral neck LNSS dissection. Her prognosis was good, and there were no signs of recurrence at her 12-month follow-up appointment.

[CONCLUSIONS] The occurrence of contralateral and bilateral simultaneous LNSS metastasis in thyroid cancer is relatively rare. However, in clinical practice, surgeons should focus on the evaluation and clearance of LNSS, especially in patients with cancer foci located in the lower pole, cancer foci invading the anterior cervical band muscle, extensive metastasis in the lateral cervical lymph nodes or stages T3/4 and to reduce postoperative recurrence.