Morphological change of a laterally spreading rectal tumor over a short period

BACKGROUND Laterally spreading tumors (LSTs) are generally defined as superficial lesions ≥10 mm in diameter that typically extend laterally rather than vertically along the colonic wall. LSTs are usually categorized into 2 subtypes: granular type and nongranular type. Large nodules or depressed areas in granular-type LSTs (LST-Gs) are endoscopic findings of a cancerous component and sometimes represent submucosal invasion. However, the lateral growth and development of LST-Gs remains unclear. CASE PRESENTATION This case report describes a case of 79-year-old woman who underwent total colonoscopy due to a positive fecal occult blood test and was detected a LST-G, about 30 mm in diameter in the lower rectum. The lesion consisted of not only aggregated small and large nodules typically seen in LST-Gs but also the hardly elevated flat parts. In the flat part, there were dilated round pits and no evident capillary vessels. Three months later, the flat part increased in height, the dilated round pits were partly replaced by type IIIL pits, and capillary vessels were evident. The lesion was removed by endoscopic submucosal dissection, and diagnosed pathologically as tubular adenoma. We performed the sequence analyses on KRAS, BRAF, NRAS and PIK3CA genes in flat part and nodular part separately, and a mutation of KRAS gene at codon 146 was observed at only nodular part, suggesting probable that nodular part be a precancerous lesion. CONCLUSION This is a unique and suggestive case, providing information on progression of LST-Gs at the very early stage to carcinogenesis.