In this study, sarcopenia was a significant risk factor for major organ invasion in PTC, and consequently a significant risk factor for more locally advanced disease. Sarcopenia was not significantly associated with disease-free survival in PTC.
Although significant associations between sarcopenia and adverse cancer outcomes have been suggested,4 published literature about the potential impact of sarcopenia on PTC is scarce. Our study showed that despite the higher risk of locally advanced disease in PTC patients with sarcopenia, sarcopenia by itself was not significantly associated with disease-free survival. This may be explained in part by advancements in surgical techniques that increase the rate of complete resection, even in advanced ETE.16,17 The low incidence of tumor recurrence in our study may also be a contributing factor.
Our results showed that sarcopenia was significantly associated with major organ invasion, but not with LN metastasis. The association between sarcopenia and potential LN metastasis in various types of cancers remains controversial.18–21 Sarcopenia has been shown to increase the risk of LN metastasis in colorectal cancer and advanced urothelial carcinoma, while no significant association has been found with resectable bile duct cancer or recurrent pancreas adenocarcinoma.18–21 As with the association between sarcopenia and adverse cancer outcomes, the association between sarcopenia and LN metastasis also seems to depend on the specific type of cancer. Preoperative diagnosis of sarcopenia may raise the suspicion of the clinician, radiologist, as well as the surgeon, for a more locally advanced disease and direct prompt and appropriate management.
In our study, BMI was not significantly associated with major organ invasion in PTC. While a previous Korean study suggested BMI as a significant risk factor for microscopic ETE and higher TNM stage,22 another Korean study suggested that higher BMI was a significant risk factor for tumor multiplicity, but not for higher T stage, positive LN metastasis, nor ETE.23 In a United States study, higher BMI was significantly associated with higher incidence of PTC, but with less tumor invasion and LN metastasis.24 Evidently, any potential association between BMI and aggressive clinicopathologic features of PTC is variable. Differences in ethnicity, culture, and lifestyle that may influence body composition,2 as well as the application of previous versions of American Joint Committee on Cancer (AJCC) staging criteria may be the cause for these discrepant results.
We utilized BIA to assess body composition and to determine sarcopenia using SMI. BIA estimates skeletal muscle mass using whole-body electrical conductivity, and incorporates an affordable, widely available, and portable instrument with reproducible results.2,25 Unlike other malignancies in which abdomen and pelvic CT scans are often included during routine staging work-up, measuring SMI on CT scans to assess sarcopenia in thyroid cancer may unnecessarily burden patients with more radiation exposure. Alternatively, BIA may be an easier, more affordable tool to diagnose sarcopenia in PTC, as demonstrated in our study. In addition, the assessment of sarcopenia may be of importance in treatment decisions for advanced or refractory thyroid cancer because sarcopenia is considered a contraindication or discouraging factor in tyrosine kinase inhibitor treatment.15,26
There are several limitations to this study. First, an inherent bias owing to the retrospective analysis of prospectively obtained data was inevitable. Second, the low incidence of sarcopenia in our study was a major limiting factor in statistical analysis. Last, body composition analysis by BIA may yield inconsistent or discrepant results depending on different instrument brands, as well as different population characteristics.2 Our study utilized cutoff values previously obtained with identical equipment in young, healthy Korean subjects.27 Therefore, the findings of this study may be limited to Korean patients. Further research that includes patients from more diverse populations may help clarify the potential impact of sarcopenia on the outcomes and clinicopathologic features of PTC patients.
In conclusion, sarcopenia is significantly associated with a higher risk of major organ invasion in conventional PTC. Sarcopenia in PTC patients should raise clinical, radiological, and surgical suspicion for a more locally advanced disease and direct appropriate management.