Appearances of screen-detected versus symptomatic colorectal cancers at CT colonography

Abstract

Objectives: The aim of this study was to compare the morphology, radiological stage, conspicuity, and computer-assisted detection (CAD) characteristics of colorectal cancers (CRC) detected by computed tomographic colonography (CTC) in screening and symptomatic populations.

Methods: Two radiologists independently analyzed CTC images from 133 patients diagnosed with CRC in (a) two randomized trials of symptomatic patients (35 patients with 36 tumours) and (b) a screening program using fecal occult blood testing (FOBt; 98 patients with 100 tumours), measuring tumour length, volume, morphology, radiological stage, and subjective conspicuity. A commercial CAD package was applied to both datasets. We compared CTC characteristics between screening and symptomatic populations with multivariable regression.

Results: Screen-detected CRC were significantly smaller (mean 3.0 vs 4.3 cm, p < 0.001), of lower volume (median 9.1 vs 23.2 cm³, p < 0.001) and more frequently polypoid (34/100, 34 % vs. 5/36, 13.9 %, p = 0.02) than symptomatic CRC. They were of earlier stage than symptomatic tumours (OR = 0.17, 95 %CI 0.07-0.41, p < 0.001), and were judged as significantly less conspicuous (mean conspicuity 54.1/100 vs. 72.8/100, p < 0.001). CAD detection was significantly lower for screen-detected (77.4 %; 95 %CI 67.9-84.7 %) than symptomatic CRC (96.9 %; 95 %CI 83.8-99.4 %, p = 0.02).

Conclusions: Screen-detected CRC are significantly smaller, more frequently polypoid, subjectively less conspicuous, and less likely to be identified by CAD than those in symptomatic patients.

Key points: • Screen-detected colorectal cancers (CRC) are significantly smaller than symptomatic CRC. • Screening cases are significantly less conspicuous to radiologists than symptomatic tumours. • Screen-detected CRC have different morphology compared to symptomatic tumours (more polypoid, fewer annular). • A commercial computer-aided detection (CAD) system was significantly less likely to note screen-detected CRC.

Keywords: CT colonography; Colorectal neoplasms; Computer-assisted diagnosis; Mass screening; Occult blood.

Fig. 1

Bar charts showing tumour features that were recorded as binary variables; all charts show the percentage of tumours with (dark grey) or without (light grey) a given imaging feature. Asterisks indicate statistical significance at the 5 % level. “Advanced T stage” refers to either a T4 or T3 tumour with ≥5 mm of extramural spread. Scr = screen-detected tumours, Symp = symptomatic tumours

Fig. 2

Examples of different cancers; subtle screening (a), subtle symptomatic (b), obvious screening (c), and obvious symptomatic (d) tumours (arrows). In each case, the left panel shows the supine image and the right panel shows the prone series. The subtle tumours were assigned a mean conspicuity score of 10 (screening case) and 12 (symptomatic case); the screening case (a) was not detected by CAD. The obvious tumours were assigned a mean conspicuity score of 97.5 (screening case) and 100 (symptomatic case)

Fig. 3

Example of an upper rectal tumour missed by CAD in a screening patient. Image quality was judged to be good by both readers, with a small amount of residual fluid which was well-tagged