The rate of unexpected thyroid cancers found at the time of thyroidectomy varies widely, while the notion of increased cancer incidence in retrosternal multinodular goiters(MNGs) compared with cervical MNGs is still controversial.

A retrospective study was conducted to evaluate the prevalence of preoperatively undiagnosed cancer in patients undergoing thyroidectomy for cervical or substernal MNG, by comparing the rates of cancer detected on final pathology. 411 patients who underwent thyroidectomy for substernal (114) or cervical (297) MNG were identified between January 2019 and October 2022

Patients with substernal MNGs were older (58.2 vs. 50.6; p<0.001) and more frequently men (34.2 vs. 22.9 %; p=0.014). Overall, thyroid cancer was found in 49.5% of cervical and in 35.1% of substernal MNG specimens(p=0.006) respectively, while cancer >1 cm was found in 17.4 % of substernal MNG specimens and in 37.4% of cervical MNG specimens (p<0.001); 60% of the carcinomas >1 cm found in retrosternal MNGs were unexpected(10.5% of all retrosternal MNGs). At multivariable analysis tracheal deviation (OR 0.47, 0.237-0.958 95% CI, p=0.038) and preoperative Fine-Needle Aspiration Cytology (FNAC) (OR 6.4, 4-10.2 95% CI, p<0.001) were factors for cancer diagnosis.

In this study the rate of unexpected thyroid cancer discovered postoperatively in patients with substernal MNGs was lower than in cervical MNGs, though still significant from a clinical standpoint. This may be partially attributed to the difficulty in performing FNAC in the mediastinum. Surgeons should counsel patients prior to surgery regarding the risk of unexpected thyroid cancer to set appropriate expectations of outcome.