Impact of section thickness on classification of pulmonary nodules into solid, part-solid, and non-solid: an observer study

PURPOSE Recently published recommendations by the Fleischner Society differentiate between solid, part-solid, and non-solid nodules. A section thickness of 1mm is recommended for evaluation. It is, however, common practice to reconstruct thicker (3mm or 5mm) sections to reduce the number of sections to evaluate. Purpose of this study was to evaluate the impact of section thickness on nodule classification agreement. METHOD AND MATERIALS 20 part-solid, 10 non-solid and 10 solid nodular lesions were randomly selected from the NELSON screening trial. A reference standard was established using the consensus reading of two experienced chest radiologists. Data had been acquired using a low dose (16x0.75mm, 120-140 kVp, 30 mAs) protocol. Complete CTs were shown with axial and coronal projections with either 1mm, 3mm or 5mm section thickness, the latter two with 1mm overlap. Readers could interactively scroll through the scans, use magnification, windowing and manual calibre measurements as warranted. Four readers of varying experience were asked to classify the lesions as solid (1), part-solid (2), or non-solid (3). All readings were done twice in six sessions, in which all permutations of nodules and section thicknesses were presented in different random orders. We report percentage agreement between observers and the consensus reference. All results stated are averaged over all reading sessions. RESULTS Mean agreement rate with the reference standard decreased from 85% (range 78-95%) to 77% (range 68-84%) and 75% (range 68-84%), for 1mm, 3mm, and 5mm section thickness, respectively. Readers were affected differently by increasing section thickness. The most experienced reader was influenced the least (agreement = 84-82-80%). Two readers demonstrated a major decrease in performance already for 3mm (81-72-70% and 91-78-81%). One reader showed a stepwise performance decline (86-77-69%). CONCLUSION Nodule classification is affected by section thickness. The degree of loss of accuracy appears to be reader dependent. CLINICAL RELEVANCE/APPLICATION Nodule classification is impaired by increasing section thickness which may have consequences for patient management. Visual classification therefore requires acquisition and storage of 1mm sections.