Radiographs Miss Enlarged Thoracic Lymph Nodes in Dogs—CT Proves Far More Reliable

Radiographic Detection of Intrathoracic Lymphadenomegaly in Dogs: How Useful Is It?

Gordon Lye, Rachel E. Pollard, Angela Hartman, Sarah Pemberton, Michael S. Kent, Allison L. Zwingenberger

Background

Intrathoracic lymph node (LN) enlargement in dogs may indicate inflammatory or neoplastic disease. While computed tomography (CT) allows accurate visualization and measurement of sternal (STLN), cranial mediastinal (CrMLN), and tracheobronchial (TBLN) lymph nodes, thoracic radiographs are more commonly used in clinical practice. The authors hypothesized that intrathoracic lymph node enlargement cannot be consistently identified on radiographs, even when assessed by experienced radiologists, and sought to determine whether a CT-derived volume threshold exists above which enlargement is reliably detected radiographically.

Methods

This retrospective, multicenter cross-sectional study included 74 dogs that underwent thoracic CT and three-view thoracic radiography within 9 days (2012–2023). Thirty-six dogs had CT-reported lymphadenomegaly, and 38 served as controls. CT volumes for each LN group were calculated using the ellipsoid formula, and volumes were summed per region. Three board-certified radiologists independently graded each LN group on radiographs using a 5-point scale (1 = cannot assess; 5 = marked enlargement). Interobserver agreement was assessed using Gwet’s AC1 statistic. Associations between radiographic grades and CT-measured volumes were evaluated using Spearman’s rank correlation.

Results

Interobserver agreement on radiographic grading was almost perfect (AC1: STLN 0.88, CrMLN 0.93, TBLN 0.95). However, correlations between radiographic grades and CT-measured volumes were weak or nonsignificant. For STLN and CrMLN, Spearman’s coefficients ranged from −0.16 to 0.10 and −0.03 to 0.04, respectively, without statistical significance. For TBLN, only one observer demonstrated a weak but statistically significant positive correlation (ρ = 0.27, p = 0.02). No CT-derived volume threshold resulted in unanimous radiographic classification of enlargement (grade ≥ 3). Radiographs frequently failed to detect enlarged lymph nodes and occasionally overestimated enlargement in normal-sized nodes.

Limitations

The retrospective, multicenter design introduced potential selection and information bias. CT protocols and radiographic techniques varied across institutions and over time. The CT reviewer was not blinded and cases were not formally randomized. Radiograph–CT intervals of up to 9 days may have allowed interval change. Histopathologic confirmation of lymphadenomegaly was uncommon, limiting classification of discordant findings as true false positives or negatives. Contrast administration parameters were inconsistently recorded.

Conclusions

Thoracic radiography demonstrated poor alignment with CT-measured lymph node volume despite high interobserver agreement. No reliable CT volume threshold predicted consistent radiographic detection of intrathoracic lymphadenomegaly. When accurate lymph node assessment is expected to influence clinical decision-making, thoracic CT should be considered over radiography.

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