Digital image analysis of pan-cytokeratin stained tumor slides for evaluation of tumor budding in pT1/pT2 colorectal cancer: Results of a feasibility study

Tumor budding is an independent prognostic factor in colorectal cancer. However, varying degrees of interobserver agreement and reproducibility challenges the use of tumor budding in diagnostics. Immunohistochemical staining of tumor slides with pan-cytokeratin visualizes the budding tumor cells and has been suggested to improve reproducibility. Here we demonstrate the methodology of tumor budding assessment using digital image analysis based on tumor slides stained for pan-cytokeratin, and investigate interobserver agreement, agreement between manual and digital assessment methods and digital reproducibility between users. Tumor slides from 126 patients with pT1/pT2 colorectal cancer were stained with pan-cytokeratin and tumor budding at the invasive tumor front was assessed by conventional manual microscopy. A digital image analysis algorithm for identification and quantification of budding tumor cells was developed and tested on the pan-cytokeratin stained slides. Manual assessment of tumor budding using pan-cytokeratin stained tumor slides exhibited high correlations (Spearman Rank 0.84-0.89, p < 0.001),excellent agreement between observers (Intra-class correlation coefficient (ICC): 0.86 -0.87) and 2.20 higher odds for regional metastases with increasing budding counts (p = 0.017). Digital image analysis correlated well to manual assessment (Spearman Rank 0.71-0.88) and agreement between the two methods was good (ICC 0.62-0.82). However, only a trend towards increased odds for metastatic progression was found for the adjusted digital estimates (p = 0.076). Digital estimates were higher than manual estimates, demonstrated by a systematic median difference of 3-4.5 buds. Image analysis was highly reproducible between users of the algorithm (ICC 0.98). In conclusion, assessment of tumor budding using pan-cytokeratin stained tumor slides is a method with high correlation and agreement between observers. Digital image analysis quantifies budding tumor cells in high agreement with manual estimates, but approval of the digital slides by a pathologist is mandatory. The method qualifies for further investigation.