Optimizing prone CT use for suspected interstitial lung abnormalities

Abstract

Objectives: We investigated whether supine chest CT alone suffices for diagnosing ILAs, thereby reducing the need for prone chest CT.

Materials and methods: Patients who underwent prone chest CT for suspected ILAs from January 2021 to July 2023, with matching supine CT within 1 year, were retrospectively evaluated. Five multinational thoracic radiologists independently rated ILA suspicion and fibrosis scores (1 to 5-point) and ILA extent (1-100%) using supine CT first, then combined supine-prone CT after a 1-month washout. We categorized ILA suspicion and fibrosis scores into four diagnostic groups; normal, non-fibrotic, indeterminate-type, and fibrotic ILAs. The areas under the receiver operating characteristic curve (AUCs) of ILA suspicion scores, inter-reader agreement on diagnostic categories, and intra-reader/inter-reader reliability for ILA extent were evaluated.

Results: This study included 69 patients (mean age 67.2 ± 7.2 years; 36 women), with 23 age- and sex-matched patients in each group: normal, non-fibrotic ILAs, and fibrotic ILAs. The pooled AUC for ILA suspicion and inter-reader agreement on diagnostic categories improved for non-fibrotic ILAs with prone CT (AUC 0.76 to 0.92, p < 0.001; Fleiss kappa 0.25 to 0.51, p = 0.004), but not for fibrotic ILAs (AUC 0.94 to 0.99, p = 0.06; Fleiss kappa 0.63 to 0.72, p = 0.08). ILA extent was 1-2% smaller with prone CT for both ILA types (p < 0.001).

Conclusion: For fibrotic ILAs, supine CT alone exhibited substantial diagnostic accuracy and inter-reader agreement, while the diagnosis of non-fibrotic ILAs benefited from adding prone CT. Supine CT alone slightly overestimated extent regardless of ILA type.

Key points: Question Prone CT is recommended when interstitial lung abnormalities (ILAs) are suspected on supine CT, but its benefits remain underexplored. Findings Supine CT alone sufficed for diagnosing fibrotic ILAs, while prone CT improved non-fibrotic ILA diagnosis and reduced extent overestimation for both types. Clinical relevance Omitting prone CT reduces extra time, space, and radiation exposure without compromising the diagnosis of fibrotic ILAs, which have higher rates of progression and mortality risks, enhancing patient comfort and simplifying patient management.

Keywords: Interstitial lung abnormality; Lung diseases (Interstitial); Prone CT; Pulmonary atelectasis; Tomography (X-ray computed).

Fig. 1

Flowchart of study population selection

Fig. 2

Pooled receiver operating characteristic curves of all five readers’ ILA suspicion scores for diagnosing ILA in non-fibrotic and fibrotic ILA subgroups

Fig. 3

Chest CT images of exemplary cases of a normal subject (a, b), a patient with non-fibrotic ILA (c, d), and a patient with fibrotic ILA (e, f). Subtle ground-glass opacities in the posterior subpleural portion are seen on (a) supine CT and were initially evaluated as positive for ILA by the majority of readers using supine CT alone. The lesions disappeared on (b) prone CT, indicating dependent atelectasis. Subtle ground-glass opacities in the posterior subpleural portion are seen on (c) supine CT and (d) persist in prone CT, indicating non-fibrotic ILA. Mild reticular opacities with traction bronchiolectasis in the subpleural portion are noted in both (e) supine and (f) prone CTs. The majority of readers identified fibrotic ILAs in supine CT alone and supine-prone CTs

Fig. 4

Distributions of readers’ diagnostic categories. NF ILA, non-fibrotic ILA; ID ILA, indeterminate-type ILA; FB ILA, fibrotic ILA