Clinical significance of gross extrathyroidal extension to only the strap muscle according to tumor size in differentiated thyroid cancer: a systematic review and metaanalysis.

Objective: The presence of extrathyroidal extension (ETE) in differentiated thyroid cancer (DTC) patients is an important predictor of their prognosis. Therefore, the tumor stage of DTC is divided into extensive ETE and gross ETE only affecting the strap muscle (Gross strap muscles invasion; gSMI). However, insufficient evidence exists regarding the association of gSMI with the prognosis, especially in relation to tumor size.

Methods: Related literature was searched in MEDLINE, Embase, Cochrane Library and KoreaMed. All processes were performed in accordance with PRISMA guidelines, and the process was conducted by two independent reviewers. The meta-analysis was performed using a random-effect model considering the diversity of studies. RoBANS version 2.0, an evaluation tool for non-randomized studies, was used to evaluate the quality of the selected research. Clinical data from observational studies that analyzed the relationship between the degree of ETE and prognosis were collected, and a meta-analysis was performed.

Results: Eighteen observational studies were included in this study. A subgroup analysis was performed for each outcome. The recurrence rate (odds ratio [OR]: 2.498), disease-specific mortality (risk ratio [RR]: 2.984), overall mortality (RR: 1.361) and lymph node (LN) metastasis (OR: 5.355) were significantly higher in patients with gSMI than in those with no ETE. However, in an analysis limited to tumors 4 cm or smaller, no significant difference in prognostic outcomes was seen, except for LN metastasis.

Conclusion: gSMI has a negative impact on the prognosis; however, this correlation diminishes when the tumor size is small. Thus, a more cautious approach is warranted during the treatment process.