Intra-Tumoral Genomic Heterogeneity in Rectal Cancer: Mutational Status Is Dependent on Preoperative Biopsy Depth and Location

SIMPLE SUMMARY: A subset of patients with rectal cancer are treated before surgery with chemoradiation. Unfortunately, this neoadjuvant chemoradiotherapy does not have the preferred effect of tumor downstaging in all patients, but does bring substantial side effects and possible complications. A pre-treatment prediction based on available parameters might provide a means to better select therapy for individual patients. Genomic mutational status of pre-treatment biopsies may provide prognostic information, however, it also might be influenced by tumor heterogeneity. This study investigates whether pre-treatment biopsy material is a reliable way of defining mutational status in rectal cancer. ABSTRACT: Neoadjuvant therapy before surgical resection is indicated for patients with locally advanced rectal cancer. However, a significant number of patients show minimal or no response to neoadjuvant therapy. Unfortunately, we are currently unable to predict response and identify non-responding patients before neoadjuvant treatment is given. Genomic mutational status might provide valuable prognostic information. However, it is unclear whether predictions based on genomic mutational status in single preoperative biopsies are reliable due to intra-tumoral heterogeneity. In this study we aim to investigate the reliability of genomic mutations found in single pre-operative biopsies by comparing genomic mutations to four other locations within the same tumor using next generation sequencing. Rectal cancer patients undergoing primary resection without neoadjuvant therapy were included. From each patient, one biopsy, two deep and two superficial samples were obtained and sequenced using a targeted next generation sequencing gene panel. Concordance between these five samples was assessed. In this feasibility study we included 11 patients. In 7 out of 11 (64%) patients, all 5 samples showed concordant mutations. In 4 out of 11 patients (36%) discordant mutations were observed. In conclusion, assessment of mutational status on a single pre-operative biopsy shows discordance with tumor tissue from other locations in 36% of cases. These results warrant careful interpretation of biopsy material analysis, as these might be influenced by tumor heterogeneity.