Can we differentiate between primary Sjögren’s syndrome and idiopathic multicentric Castleman disease based on the features of lung cysts?

<bold>Background:</bold> The diagnosis of cystic lung diseases (CLDs) is a challenge, but the radiological features of cysts and the associated radiologic findings can provide helpful clues. Primary Sjögren’s syndrome (pSS) and idiopathic multicentric Castleman disease (iMCD) can present as CLDs when lung involved, but no study has focused on the features of lung cysts in them. <bold>Aims:</bold> To identify radiological features that could help differentiate CLDs between pSS and iMCD. <bold>Method:</bold> Patients with pSS or iMCD who had cysts were enrolled. Cyst features and other accompanying findings were compared between them. <bold>Results:</bold> Eleven patients with pSS and 25 patients with iMCD were enrolled. Eleven patients with pSS (100.0%) and 23 patients with iMCD (92.0%) had regular cysts. None of patients with pSS had irregular cysts, but 21 (84.0%) patients with iMCD had irregular cysts (p=0.005). Smooth-walled cysts presented in 11 patients with pSS (100.0%) and 18 patients with iMCD (72.0%). Only one patient with pSS (9.1%) exhibited non-smooth-walled cysts, while 23 patients with iMCD (92.0%) had non-smooth-walled cysts (p=0.003). Nodules were common in both groups (p=1.000). However, the nodules were more likely to be larger and more numerous in iMCD (p<0.001). Cysts with mural nodules (52.2%) and central nodules (47.8%) were only observed in iMCD (p=0.007). <bold>Conclusion:</bold> Although regular and smooth-walled cysts were common in the two diseases, irregular and non-smooth-walled cysts were more often associated with iMCD than pSS. Nodules in iMCD tended to be larger and more numerous, and a close positional relationship between nodules and cysts was only observed in iMCD.