Endobronchial ultrasound elastography for the differentiation of benign and malignant lymph nodes

We read with great interest the article by Korrungruang et al. wherein the authors report the utility of endobronchial ultrasound (EBUS) elastography in assessment of mediastinal lymphadenopathy and differentiating benign from malignant mediastinal lymph nodes. The authors concluded that a strain ratio >2.5 and the presence of a non-type 1 elastographic pattern had 100% sensitivity in the differentiation of benign versus malignant lymph nodes. Ninety-five of the 101 lymph nodes with type 3 architecture (predominantly blue) turned out to be malignant. We recently performed EBUS elastography in a 26year-old lady with history of treated mediastinal lymph node tuberculosis (TB) 2 years back. The patient presented with weight loss for 2 months and computed tomography (CT) scan of thorax demonstrated right paratracheal lymphadenopathy (Fig. 1A). On EBUS, the lymph node was heterogeneous with the presence of coagulation necrosis sign. EBUS elastography demonstrated predominantly a blue pattern, type 3 (Fig. 1B) and the strain ratio was 22. Cytological examination of the EBUS aspirates demonstrated necrotizing granulomatous inflammation consistent with TB. Mycobacterium tuberculosis was detected on Xpert MTB/RIF test and rifampicin resistance was not detected. This case highlights the presence of a benign diagnosis with a non-type 1 pattern and a high strain ratio. The type 3 features on elastography may be attributed to previous history of lymph node TB, as the previous healing process might have led to fibrosis in and around the lymph node causing hardness. This highlights the fact that elastography may not be an accurate modality in patients with healed/active TB, especially in high TB prevalence settings. Our observation aligns with signs of coagulation necrosis in lymph nodes on EBUS. While earlier studies found this to be an independent predictor of malignancy, the same finding in a TB endemic setting was found as a characteristic feature in tuberculous lymph nodes. The possibility of selection bias cannot be ruled out in the study by Korrungruang et al., as there is a predominance of patients with a malignant diagnosis. These observations suggest that newer modalities such as elastography need to be evaluated in TB endemic populations with a higher number of lymph nodes associated with a potentially benign diagnosis, to exclude the possibility of selection bias. Another point of concern is the diagnostic value of strain ratio. In various studies, multiple cut-off values for strain ratio have been suggested, varying from 2.5 to 32, to differentiate malignant from benign lymph nodes. Apparently, the method of calculating strain ratio is also not well accepted and the area of comparison does not seem standardized. Some authors have used surrounding mediastinal tissue of the same area for comparison, while others have used the tissue in between node and bronchial cartilage. Similarly, the effect of tracheal cartilage in the assessment of strain ratio is unclear. Elastography needs further evaluation in order to standardize the procedural and technical aspects.