FDG PET/CT signs of proven pulmonary hydatid cyst: is there any clue?

Pulmonary hydatid cyst (PHC) can imitate many diseases. Sometimes, positron emission tomography/computed tomography (PET/CT) is performed in terms of malignancy exclusion for complicated cysts. Although some specific findings (doughnut sign) have been identified in hydatid cyst of the liver, there is no specific sign described for PHC. The aim of this study is to investigate the presence of a common finding in PHC patients scanned with PET/CT inadvertently.From January 2015 to 2020, patients proven to have PHC were analyzed retrospectively. From all the patients, only 17, having a previous PET/CT, were included the study. Lesions were evaluated in three groups according to FDG uptake: A, negative; B, focal; C, doughnut sign.The total number of patients was 17. Nine of the patients were male and the median age was 41.94 + 14.68 (16-65) years. SUV max of the lesions ranged from 0.5 to 15.8 (mean ± SE: 4.68). According to the FDG uptake of the lesions, five were in Group A, two in Group B, and the remaining ten (58.8%) in Group C with doughnut sign. To correlate the CT findings with PET/CT findings, doughnut sign, which is a typical finding of hydatid cysts of liver, is seen in only four patients in Group 1-classified cysts which are non-complicated. But in Group 2 (n = 3) and 3(n = 4), the finding of doughnut sign is three in both groups.PET/CT is not a recommended imaging technique for PHC, but in cases where a definitive diagnosis is difficult, interpreting PET/CT findings is significant. This study demonstrates that previously described doughnut sign for liver hydatid cysts is also common for perforated pulmonary cysts. According to our knowledge, this is the first largest series of determining PET/CT findings of PHC. Further larger series will contribute to the literature.