Materials and methods
This retrospective, observational, descriptive longitudinal study was approved by our institutional Research Ethics Board. We performed a free word search of our radiological report database using the terms ”subpleural cysts”, “peripheral cysts” or ”subpleural blebs” over a 21-year period (2001-2022) at a tertiary level pediatric hospital. We limited the search to children under 3 years of age at their initial chest CT. Patient demographics, clinical information and laboratory work-up including genetics and histopathology were extracted from patients’ charts. Echocardiographic and cardiac catherisation reports were reviewed for the documentation of underlying congenital heart disease (CHD) and/or pulmonary artery hypertension (PAH).
All CT scans, including studies acquired at external institutions, were performed on multi-detector scanners. Scanners differed with upgraded technology due to a long study duration of over 22 years. All cases were performed as helical scans, reconstructed to 2.5mm thickness for review of mediastinal/soft tissue windows using approximate window width 350, level 40 and reconstructed to 0.625-2.5mm thickness for review on a lung algorithm (approximate window-width 1500, level -500). Intravenous contrast was given in most cases, depending on the clinical question. Our primary endpoint was the presence or absence of multiple 1-4 mm air-filled subpleural cysts.
All studies performed at our institution utilised a CT dose-adjusted algorithm according to the body weight of the child. More recent studies were performed using CT scanners that provided additional automatic exposure control (AEC) with modulation of the tube-current depending on the thickness of the body part examined. As a result, mAs varied between 80-150 with a 120 kVp. As all patients were under the age of 4, scans were performed either using a “feed and sleep” approach, sedation or under general anaesthesia (GA). Non-GA cases were acquired in quiet breathing and GA cases were acquired in full inspiration, either via endotracheal tube or positive pressure mask. Prospective ECG-gated triggering was done in cases performed with a cardiac CT protocol.
All CT images were qualitatively analysed for the presence and distribution of subpleural cysts. The size of the largest cyst was recorded. The heart and mediastinal structures were evaluated for structural cardiac abnormalities, in correlation with data from echocardiographic examinations. Ancillary findings of mosaic attenuation, ground glass opacities and pleural effusion or pneumothorax were also recorded. Follow-up CT’s in the available cases were compared for the interval status of cyst size and distribution. All images were analysed independently by three pediatric radiologists with a specialty interest in pulmonary imaging (33 years’ experience, 3 years’ experience, and a fellow in pediatric imaging). The radiologists were blinded to the clinical information and to each other’s interpretations, with a consensus on final image interpretation.
Subpleural cysts were differentiated from subpleural lines seen in children with lymphatic distension due to lymphangiectasia by set criteria consisting of the lack of fissural thickening, lack of pleural effusions, lack of basal septal thickening creating typical hexagonal opacities, and lack of ‘Kerley’ lines in our cohort of children with peripheral cystic disease.
Demographic characteristics were summarized using descriptive statistics.