Microscopic gastrointestinal stromal tumours: a clinical and molecular study of 13 cases.

Recent literature suggests that clinically silent, microscopic gastrointestinal stromal tumours (micro-GISTs) are common incidental findings. The aim of this study was to examine the histological, immunohistochemical and molecular characteristics of these tumours, which we have defined as measuring ≤20 mm, in order to determine whether the rate and spectrum of mutations are similar to those of clinically symptomatic gastrointestinal stromal tumours (GISTs).,Thirteen micro-GISTs identified as incidental findings in patients undergoing management of concomitant disease were tested for KIT/PDGFRA mutations. Ten micro-GISTs (77%) were located in the stomach, two (15%) in the duodenum, and one (8%) in the rectum. The mean tumour size was 9.3 mm (range 2-19 mm). All tumours were well-circumscribed lesions showing a predominantly spindle-cell morphology and a very low mitotic rate. Twelve of 13 (92%) tumours carried mutations in either KIT (83%) or PDGFRA (17%), a rate higher than in other published series. A high mutation rate (80%) was also seen in lesions measuring ≤5 mm.,Our results suggest that KIT/PDGFRA mutation is a very common early event in GIST development, that tumour size does not reliably predict the presence of mutation, and that one or more subsequent mutations are required for clinical manifestation.