Head-to-head comparison of FNA cytology vs. calcitonin measurement in FNA washout fluids (FNA-CT) to diagnose medullary thyroid carcinoma. A systematic review and meta-analysis

The diagnosis of MTC can be challenging and the significant limitations of FNA-cytology in detecting this neoplasm are recognized. As reported in a meta-analysis published in 2015 [6], only 56% of histologically proven MTC lesions can be correctly detected by cytological evaluation. The results of that meta-analysis were not particularly different compared to single studies enrolling large or small series of MTC. Elisei et al. [17] found a 45% sensitivity in 44 MTCs, Bugalho et al. [18] recorded a 63% sensitivity in 67 cases, and Essig et al. [19] collected 245 MTCs from 12 institutions over 29 years and 112 (45.7%) were correctly diagnosed. In this context, two historical studies with different design must be mentioned [20, 21], as they found a positive predictive value regarding FNA-cytology of 89% [20] and 81% [21], respectively. The present meta-analysis found FNA-cytology to have a 54% sensitivity, thus perfectly confirming the previous results obtained in a different collection of studies [6]. In recent years, the use of FNA-CT in patients with suspicious MTC (i.e., high serum CT levels) has gained momentum and this strategy was introduced in 2015 ATA guidelines [2]. Since its first description [11], FNA-CT showed high sensitivity and an apparently higher diagnostic performance than FNA-cytology. The main advantage of FNA-CT is that it can be used in any institution with a laboratory department. Even if the conventional assays for CT are not developed for this use, no specific concerns regarding the measurement of CT in FNA fluids have been reported in the literature. The major potential limitation of FNA-CT should be the lack of a fixed threshold to be adopted in the clinical practice [22]. Nevertheless, a three-point discussion has to be addressed about this debated issue: (1) as intrinsically showed in the data of the herein included studies, adopting an institutional cut-off can solve this problem; (2) the complete absence of heterogeneity we observed in the pooled sensitivity of FNA-CT could intrinsically demonstrate that the different technical approaches used to prepare and measure FNA-CT (Table 3) did not influence the final result; (3) the FNA-CT values recorded in MTC lesions are generally very high enabling us to correctly diagnose it. Other potential weaknesses of FNA-CT are fully shared with alternative diagnostic options (i.e., immunocytochemistry for CT). Mainly, C cell hyperplasia may result as a false positive case for both FNA-CT and immunocytochemistry [22]. However, while immunocytochemistry cannot be performed in a suboptimal sample collected by FNA, FNA-CT can be perfectly evaluated in these cases. All in all, it has to be underlined that we need a high value of serum CT before FNA to better select the cases in which the use of FNA-CT might be appropriate, being this clinical strategy debatable [23]. Moreover, measuring serum CT value represents a key-information to better enhance the morphological evaluation of FNA specimens. In order to improve the performance of both FNA-CT and FNA-cytology, testing for serum CT in patients undergoing FNA should be considered in clinical setting. The present meta-analysis demonstrates with strong statistical power that the sensitivity of FNA-CT is significantly higher than that of FNA-cytology. The use of FNA-CT has been recommended by ATA guidelines with Grade B recommendation [2]. With the present new evidence-based data, the use of FNA-CT should be upgraded to Grade A recommendation (strong evidence) in the future revised guidelines. In addition, the use of FNA-CT should be included in next revised guidelines for thyroid cytology (i.e., Bethesda system, etc.).

A critical discussion about the herein recorded results should be addressed. As showed by our sub-analysis, the pooled sensitivity of FNA-cytology was negatively influenced by its poor performance in prospective studies; this corroborates that the cytological diagnosis of MTC during the clinical practice is more difficult than its detection when reviewing the cytological specimens for research purpose. Furthermore, the FNA-cytology sensitivity of the most recent studies [15, 16] was improved with respect that of the previously published ones; we can speculate that cytopathologists have recently become more aware of this difficult diagnosis and, maybe, have been positively influenced by the concomitant use of FNA-CT. In addition, one study [12] showed a very poor sensitivity of cytology; this finding might be due to several reasons such as the prospective enrollment of cases and the potential difficult sampling in small nodules (i.e., median size 9.5 mm). Furthermore, it should be relevant to underline the role of different preparation for cytological evaluation adopted by the six studies (Table 3). Overall, the use of the conventional smears is the most widely used method while immunohistochemistry can be crucial to detect MTC [20‐22]. Regarding FNA-CT, the low sensitivity recorded by De Crea et al. [14] might be due to the prospective design and the enrollment of thyroidectomized patients only; some patients of that series might be operated-upon following suspicious serum CT and/or FNA-cytology despite of low CT value in the washout fluids (these cases were described as false-negative FNA-CT cases). Anyway, the heterogeneous ultrasound presentation of MTC has to be taken into account when critically analyzing the limitations of FNA procedure in detecting it. In fact, as recently reported by Matrone et al. [24], <50% of MTCs is correctly identified as suspicious by ultrasound systems for risk stratification of thyroid nodule. The Fig. 4 illustrates the ultrasound presentation of one MTC case.

Due to the aim and conceptualization of the present study, the specificity of FNAs could not be known. However, in most cases, patients with negative FNAs were not operated upon and we had no strong reference of standard to calculate the true negatives and false positives cases with high reliability.

As usual, some potential limitations may affect the present meta-analysis. First, no large-sample studies were found and this influenced the pooled sample size. This obviously depends on the infrequent incidence of MTC. The herein recorded difference in sensitivity between FNA-CT and FNA-cytology was significant (i.e., non-overlapping 95% CIs) and this data suggests to perform, as a further proof, prospective multicenter studies with direct comparison between FNA-CT and FNA-cytology. Second, some studies were retrospective. This could influence the findings of FNA-cytology (with potential over-estimate) and explain the heterogeneity observed only in FNA-cytology. Third, the series of patients should be not consecutive and a potential selection bias should be taken into account. Fourth, we must be aware that a different use of serum CT testing could be adopted by different institutions. This might have influenced in clinical practice the selection of patients undergoing FNA with or without measurement of CT in washout fluids. However, the major strength of the present data is that FNA-CT showed excellent sensitivity with no discordant data among studies (i.e., I₂ = 0).

In conclusion, our meta-analysis (1) confirms that FNA-cytology can only detect approximately a half of histologically proven MTC lesions, and (2) demonstrates that FNA-CT is significantly more sensitive than FNA-cytology. Accordingly, FNA-CT is the standard method to use in patients with suspicious MTC lesions and it has to be combined with FNA-cytology in all cases.