Thyroid microcarcinoma was the thyroid tumors ≤1 cm in maximum diameter; in view of the fact that the vast majority of thyroid cancers were papillary thyroid carcinomas (PTCs), thyroid microcarcinomas were often referred to as PTMCs [10]. In this study, there was no significant difference between MTMCs and PTMCs in age, sex and size. Previous researches had showed MTCs patients were more common in females and most were single nodule [11, 12]. In accord with the earlier reports, there was a slightly higher number of women than men in MTMCs patients in this study. Only 5 cases had two MTMCs nodules and the rest were single. The youngest patient with MTMCs (MEN II B) was a 14-year-old boy with one lesion on each side. Because of the family history of MTCs, regular examinations were performed. The maximum diameter of the nodule in the left thyroid lobe was only 2 mm. Our results showed that there was significantly different in lymph node metastasis between MTMCs and PTMCs. In PTMCs group, 21 (20.19%) patients had lymph node metastasis; the prevalence of which was similar to that in previous reports [13–15]. MTMCs were more likely to have lymph node metastases, which indicated the strong invasive characteristics of MTCs. This also suggests that although the incidence of MTCs is very low, early diagnosis of MTCs is of great significance.
Zhou et al [8] found that MTMCs had no significant difference from PTMCs in internal composition, calcifications, echogenicity, margin, and shape (P > 0.05). In our study, there were no significant differences in the location, margin, boundary, echogenicity, peripheral halo ring, the presence of calcifications between the two groups (P > 0.05). But there were statistic differences in the characteristics as follows: internal composition, shape, the type of calcifications and vascularity, which were not exactly consistent with the results in Zhou’s. The possible reasons may be the different sample size, imaging instrument and evaluation standard of the ultrasonic characteristic.
Cystic change was not common in PTMCs and MTMCs [ 8, 12]. Our results also showed that the majority of cases in both PTMCs (98.53%) and MTMCs (86.96%) exhibited solid composition, >50% solid composition was significantly more frequent in MTMCs than in PTMCs. In 46 MTMCs, >50% solid composition was observed in 6 (13.04%) lesions. Although the mechanism of cystic change is unclear, solid-cystic nodules might result from degeneration of solid nodules, with the accumulation of fluid probably due to intranodular necrosis. MTCs tended to display less differentiated and grow faster than PTCs, and cystic change in MTCs was most likely induced by necrosis as a result of cell proliferation that exceeds the available blood supply. Lee et al [12] also reported that cystic change was significantly more common in MTCs compared with in PTCs, and the tumor size had no significant relationship with cystic changes.
The PTCs nodules usually had a large intensive fibrosis, and its compressibility was reduced compared to benign tumors, resulting in its standing-like morphology [16]. A taller-than-wide shape had been regarded as an ultrasongraphic feature for PTMCs [17, 18]. In our study, it was also a common sign in PTMCs group, and the proportion was as high as 59.56%. However, there was only 9 (19.57%) cases with a taller-than-wide shape in MTMCs, which indicated that it did not apply to MTMCs. Previous studies have shown that, regardless of size, the shape of MTCs was mostly round or ovoid rather than taller than wide [19, 20]. Similar results were also observed in our series. The MTMCs nodules were more likely to show ovoid to round comparing with PTMCs, and the difference was statistically significant (P = 0.000).
Calcifications, especially microcalcifications, had been regarded as a specific sign associated with thyroid malignancy. Kim et al [21] found that calcifications were not significantly different between PTCs and MTCs. The results of this study showed that calcifications were not common in MTMCs and PTMCs, and the incidence of calcification was similar (P = 0.674). However, the type of calcification was different between the two groups. We found that microcalcification was frequently seen in PTMCs, and macrocalcification appeared more commonly in MTMCs. This might be related to the different mechanisms of calcification formation in MTCs and PTCs. Calcification in MTCs was mainly due to the deposition of local calcium salts surrounded by amyloid substances, leading to the formation of coarse calcification. However, calcifications in PTCs are mainly caused by psammoma bodies with the smaller diameter of 10~100 μm, which are commonly manifested in round or concentric under the light microscope.
To the best of our knowledge, there were very limited literatures reported the blood supply of MTCs. Trimboli et al [22] reported that the percentage of intranodular vascularization in MTCs (25%) was higher than that in PTCs (15%), but with no significant difference. However, in the present study, MTMCs were more likely to show hypervascularity than PTMCs (p = 0.000). PTMCs were predominantly characterized by reduced blood supply, which might be due to the incompletely developed neoangiogenetic vascular bed in comparison with the uncontrolled cell proliferation typical of carcinoma [23]. In the present series, 80.15% of PTMCs were deficient in vascularity. According to the study reported by Lai et al [24], 72.4% of MTCs showed hypervascularity, which might be induced by the rapid division and growth of MTCs cells in patients, resulting in a faster rate of proliferation than PTCs. In this study, abundant blood supply was also commonly detected in MTMCs (58.70%), but the proportion was not as high as reported in the literature. This may be due to the small size of the tumor and its relatively small number of blood vessels.
There were some limitations in our study. First, this was a retrospective study with statistical bias. Second, due to the low incidence of MTCs and the collection of case take too much time, the development of ultrasound technology may have an impact on the evaluation of certain ultrasound features. In addition, although the sample size had increased compared with previous studies, it still belonged to small sample studies, and the error is unavoidable. The larger series is very useful to define the US presentation of MTMCs.