Histopathological Evaluation of Diffusion Weighted Magnetic Resonance Imaging in Differentiation between Benign and Malignant Thyroid Nodules

DISCUSSION

Thyroid nodules are common and might be difficult to treat clinically. Malignant nodules have atypical nuclei, atypical parenchyma, and high cellular numbers than benign thyroid nodules, resulting in crowded cells and subsequently decreased extracellular space. ²

According to research, routine imaging studies result in a high rate of incidental thyroid nodule discovery. Thyroid nodules percentage have raised from 20% to 67% in some groups of individuals as a result of the extensive use of sonography in clinical practice. ¹

The initial modality employed in order to diagnose a thyroid enlargement is ultrasound. It is, however, ineffective at distinguishing benign from malignant tumors. ⁴

The use of FNAB to discriminate such lesions is common and very successful. Even with expert hands, however, 28 percent of patients were found to be inconclusive. A novel non-invasive pre-surgical diagnostic tool is needed to reduce the risk of unnecessary surgery. ⁴

However, FNAC have some limitations in thyroid nodules such as insufficient samples, unspecified cell shapes, sampling mistakes, the possibility of hemorrhage, pain, and not differentiating between benign follicular adenomas and adenocarcinomas. In those nodules, confirmatory results rely on permanent post-surgical histopathological examination results. ⁴

An MRI technique that can be used to assess the nature of a thyroid nodules is diffusion weighted imaging.⁵

The flow of water in intracellular and extracellular regions, as well as intravascular areas, produces a diffusion weighted signal. The MRI image appears based on the degree of cellularity.  The extracellular gaps in malignant tumors have decreased, resulting in a decrease in the apparent diffusion coefficient. ⁵

Thyroid lesions' MRI signal intensity criteria may help in differentiation between various types of thyroid pathologies, thereby enhancing therapeutic care. DWI has been used in some trials to elaborate malignant thyroid lesions. Malignant lesions are found to have lower ADC rates than benign thyroid nodules. ¹

Although T1 weighted images were ineffective in elaboration of malignant and benign thyroid lesions, it might be useful in detection of bleeding or fat inside the nodules when combined with T2 weighted images. ¹

Water diffusion is restricted in malignant tumors because of hypercellularity and increased nuclei, according to research. Low ADC values are caused by an increase in cell counts, a restriction of intracellular spaces, and diffusion of water. ⁴

In this study, a total of 34 people with 34 thyroid nodules were enrolled. Among them 28 females (82.4%) and 6 males (17.6%). They were between the ages of 25 and 70. The maximum diameter of the nodules sized from 14 to 62 mm (mean diameter of 31.29 mm). The left lobe of the thyroid gland had 23 nodules, whereas the right lobe had 11 lesions.

There was no statistical difference in the size of the nodule, its location, the ages, or the sexes of the individuals studied. Thyroid lesions were more common in females than males, regardless of malignancy, which is consistent with. ^1,4,5

In this research, DW MRI was done for all individuals at b value of 600 s/mm². Thyroid nodule mean ADC values were determined. Thyroid nodule ADC values calculated 0.40 to 2.40 x 10^-3 mm²/s in benign nodules (mean: 1.61, SD: 0.53). Thyroid nodule ADC values in the malignant group calculated 0.50 to 0.93 x 10^-3 mm²/s (mean: 0.75, SD: 0.18).

ADC values differs significantly from benign and malignant thyroid nodules (p-value=0.000**). DWI and ADC mapping revealed 24 nodules to be benign with ADC rates greater than 0.93 x 10^-3 mm²/s while 10 nodules to be malignant with ADC rates lower than 0.921 x 10^-3 mm²/s.

According to histopathological results, the ADC value for three nodules calculated 0.93 x 10^-3 mm²/s, with two of them being malignant and one being benign (area of overlap).

The results of the histopathological examination were used as the highest standard for malignancy in this investigation. All of the nodules were histopathologically examined, and the results yielded that 26 nodules were benign (76.5%) and 8 nodules were malignant (23.5%), respectively.

The results of histopathologic examinations were compared to DWI and ADC values derived by MR Imaging.

In order to distinguish benign from malignant thyroid lesions, an ADC of 0.9 10^-3 mm²/s was chosen as a cutoff rate from the ROC curve. For the b value 600 s/mm², the sensitivity, specificity, PPV, NPV, and p value were 100, 92.3, 80, 100, and = 0.000**, respectively.

Sensitivity was found to be 100% as the ADC rate of all benign nodules were ranging from 0.40 and 2.40 x 10^-3 mm²/s in other words no false negative results for malignancy were detected.

According to histopathological data, the specificity was 92.3%, with ADC rates of 0.32 and 0.35 x 10^-3 mm²/s for two benign lesions, respectively. In other words, out of 26 benign nodules, two false positive results for malignancy were detected.

Our findings were similar to those of Mutlu H. et al. ⁶, who discovered that the specificity, sensitivity, NPV, PPV, , and accuracy rates for ADC values in distinguishing between benign and malignant thyroid lesions were calculated as 97%, 80%, 96%, and 97% respectively, for b values 50, 400, and 1000 s/mm² when using a cut off value of 1.0 x10^-3. ⁴

Our findings differed from those of Abdel-Rahman et al. ⁴ study, who chose a higher ADC rates of 1.5 x10^-3 mm²/s as a cutoff rate for distinguishing benign from malignant solitary thyroid nodules. For b value of 1000 s/mm², the specificity, sensitivity, NPV, PPV and p value were 100%, 100%, 100%, 100%, 100, and 0.001, respectively. Similarly; Nakahira et al. ⁸ discovered that for P value 1000 s/mm², a higher cutoff value of 1.61 x 10^-3 mm²/s resulted in specificity and sensitivity of 82.60%, 94.73% respectively, and accuracy of 88.09%, for malignant nodules. ⁴

In 2014, a meta-analysis of seven identical studies on the use of diffusion weighted imaging to distinguish malignant and benign nodules was published, with the conclusion that DWI can be used as an evolving technique to elaborate pathologically different thyroid lesions (benign or malignant) by calculating the ADC rates. In a further meta-analysis published in 2016, eight additional references were added that were not included in the previous one. It addressed at the technical elements of DWI as well as its potential for tumor characterization. According to this finding, DWI is a reliable approach for identifying malignant thyroid lesions. ²

Due to the differences in MRI systems, coils, and pulse sequences, the specific threshold rate for detecting thyroid cancer should be determined by each MRI device. ⁴

here are some inherent limitations to our study that should be noted when interpreting our findings. To begin with, this study was retrospective as well as with a small sample size, which could have resulted in selection bias, which is a problem in diagnostic studies because it can lead to an overestimation of diagnostic accuracy. To validate our quantitative findings, more clinical investigations are needed. Second, the micro-cystic component of thyroid nodules may alter our end results, despite the fact that we carefully escaped cystic parts of thyroid nodules in signal intensity assessments. Third, even if they are uncommon, alternative histologic types should be studied in future trials. Finally, case follow-up is challenging because, currently, the first line of treatment is usually total surgical thyroidectomy, and some investigations have shown that benign thyroid nodules can include concealed thyroid cancer. ^{1; 2}

CONCLUSION

Quantitative DWI has a significant sensitivity and specificity, suggesting that it could be a reliable, painless, and non-radiating imaging approach for thyroid nodule elaboration. The ADC readings appear to have the power to distinguish between nodules of benign nature and those of malignant nature. So, adding to thyroid US as well as US guided-FNAC for visualization of thyroid nodules cellular structure, DWI appears to have considerable utility as a diagnostic modality, and these data may potentially play a part in determination of nodules that will need FNAC. Diffusion-weighted MRI might be included in the usual modalities used for elaborating malignant and benign thyroid nodules, but more research is needed.