CT reconstruction with thick slices not only underestimates lymph node size but also reduces data reproducibility in colorectal cancer.

Background: Reliable size measurement of lymph node (LN) metastases is important for the evaluation of cancer treatment. However, image analyses without proper settings may result in inappropriate diagnoses and staging.

Purpose: To investigate whether reconstruction slice thickness in computed tomography (CT) affects measurements of LN size and reproducibility.

Material And Methods: We analyzed 48 patients with histological diagnoses of sigmoid colon and rectal cancer who underwent contrast-enhanced CT colonography as part of a surgical treatment preparation. A board-certified radiologist selected 106 LNs whose short-axis diameter was ≥5 mm on 1-mm-thick images; the short-axis diameters were measured on 1- and 5-mm-thick images by the radiologist and residents and compared using Wilcoxon matched-pairs signed rank test. Data variation and reproducibility were evaluated using the F test and Bland-Altman analysis. <0.05 was considered significant.

Results: Short-axis diameters measured on 5-mm-thick images were significantly lower than those measured on 1-mm-thick images (<0.01), even in the LNs whose short-axis diameters were over twice the slice thickness (<0.05). Of the 106 LNs, 57 showed short-axis diameter <5 mm on 5-mm-thick images; the maximum short-axis diameter was 6.7 mm on a 1-mm thick image. Data variation was significantly larger on 5-mm thick images than 1-mm-thick images in small LNs (<0.05) and reproducibility on 5-mm-thick images was inferior to that on 1-mm-thick images.

Conclusion: Thick reconstruction slices in CT can result in an underestimation of LN size and reduce data reproducibility. When measuring LN size, a thin reconstruction slice would be recommended based on targeted LN size.