Defining growth in small pulmonary nodules using volumetry: results from a "coffee-break" CT study and implications for current nodule management guidelines.

Objectives: An increase in lung nodule volume on serial CT may represent true growth or measurement variation. In nodule guidelines, a 25% increase in nodule volume is frequently used to determine that growth has occurred; this is based on previous same-day, test-retest (coffee-break) studies examining metastatic nodules. Whether results from prior studies apply to small non-metastatic nodules is unknown. This study aimed to establish the interscan variability in the volumetric measurements of small-sized non-metastatic nodules.

Methods: Institutional review board approval was obtained for this study. Between March 2019 and January 2021, 45 adults (25 males; mean age 65 years, range 37-84 years) with previously identified pulmonary nodules (30-150 mm) requiring surveillance, without a known primary tumour, underwent two same-day CT scans. Non-calcified solid nodules were measured using commercial volumetry software, and interscan variability of volume measurements was assessed using a Bland-Altman method and limits of agreement.

Results: One hundred nodules (range 28-170 mm; mean 81.1 mm) were analysed. The lower and upper limits of agreement for the absolute volume difference between the two scans were - 14.2 mm and 12.0 mm respectively (mean difference 1.09 mm, range - 33-12 mm). The lower and upper limits of agreement for relative volume difference were - 16.4% and 14.6% respectively (mean difference 0.90%, range - 24.1-32.8%).

Conclusions: The interscan volume variability in this cohort of small non-metastatic nodules was smaller than that in previous studies involving lung metastases of varying sizes. An increase of 15% in nodule volume on sequential CT may represent true growth, and closer surveillance of these nodules may be warranted.

Key Points: • In current pulmonary nodule management guidelines, a threshold of 25% increase in volume is required to determine that true growth of a pulmonary nodule has occurred. • This test-retest (coffee break) study has demonstrated that a smaller threshold of 15% increase in volume may represent true growth in small non-metastatic nodules. • Closer surveillance of some small nodules growing 15-25% over a short interval may be appropriate.