Inter- and intrascanner variability of pulmonary nodule volumetry on low-dose 64-row CT: an anthropomorphic phantom study

Objective:

To assess inter- and intrascanner variability in volumetry of solid pulmonary nodules in an anthropomorphic thoracic phantom using low-dose CT.

Methods:

Five spherical solid artificial nodules diameters 3, 5, 8, 10 and 12 mm; CT density +100 Hounsfield units (HU)] were randomly placed inside an anthropomorphic thoracic phantom in different combinations. The phantom was examined on two 64-row multidetector CT (64-MDCT) systems (CT-A and CT-B) from different vendors with a low-dose protocol. Each CT examination was performed three times. The CT examinations were evaluated twice by independent blinded observers. Nodule volume was semi-automatically measured by dedicated software. Interscanner variability was evaluated by Bland–Altman analysis and expressed as 95% confidence interval (CI) of relative differences. Intrascanner variability was expressed as 95% CI of relative variation from the mean.

Results:

No significant difference in CT-derived volume was found between CT-A and CT-B, except for the 3-mm nodules (p<0.05). The 95% CI of interscanner variability was within ±41.6%, ±18.2% and ±4.9% for 3, 5 and ≥8 mm nodules, respectively. The 95% CI of intrascanner variability was within ±28.6%, ±13.4% and ±2.6% for 3, 5 and ≥8 mm nodules, respectively.

Conclusion:

Different 64-MDCT scanners in low-dose settings yield good agreement in volumetry of artificial pulmonary nodules between 5 mm and 12 mm in diameter. Inter- and intrascanner variability decreases at a larger nodule size to a maximum of 4.9% for ≥8 mm nodules.

Advances in knowledge:

The commonly accepted cut-off of 25% to determine nodule growth has the potential to be reduced for ≥8 mm nodules. This offers the possibility of reducing the interval for repeated CT scans in lung cancer screenings.Lung cancer is the primary cancer in males and the second most common cancer in females worldwide, causing 18% of the total number of deaths 1]. Many lung cancers are found at a relatively late stage, resulting in a 5-year survival of only 15% or less 2]. Low-dose CT is a promising screening method for early detection of lung cancer 3–7]. The first result indicates that CT lung cancer screening can reduce lung cancer-specific mortality 8].In lung cancer screening, treatment decisions usually depend on pulmonary nodule size for the nodules at first detection and on the growth rate at follow-up 4]. Therefore, it is essential to assess the nodule size and growth rate accurately and reproducibly 9,10]. Variability has been found in CT-derived nodule size assessment 11,12]. In view of the current practice of patients frequently undergoing follow-up examinations, sometimes not on the same scanner, reliable inter- and intrascanner reproducibility of nodule volumetry is important.However, previous studies reported inconsistent results regarding the reproducibility of nodule volumetry. Some in vitro studies have been performed in which artificial nodules were placed at known locations in a thoracic phantom without pulmonary vessels 13–15]. Some of these studies were based on older generation CT scanners 13,16]. These studies generally showed a small margin of variability in nodule volumetry for software from different vendors. On the other hand, in vivo studies have shown that variability can be considerable, with variability up to 25% for 15 to 500 mm³ nodules 11,17–19]. A study to investigate inter- and intrascanner variability under optimally controlled conditions, yet resembling human lungs, using a more realistic phantom, has not been performed. Nowadays, 64-row multidetector CT (64-MDCT) scanners are most commonly utilised, as well as in lung cancer screenings. The variability of nodule volumetry of these scanners impacts nodule management, e.g. the interval of repeated CT scanning. As an extension to our recent study on observer detection and accuracy of manual and semi-automated volumetry 10], the focus of this study is on reproducibility between and within 64-MDCT systems. We assessed the inter- and intrascanner variability of pulmonary nodule volumetry on low-dose 64-MDCT, using randomly placed solid nodules in an anthropomorphic thoracic phantom with a background of pulmonary vasculature.