Multiple sporadic gastrointestinal stromal tumours arising at different gastrointestinal sites: pattern of involvement of the muscularis propria as a clue to independent primary GISTs.

Multifocal sporadic gastrointestinal stromal tumours (GISTs) may be misinterpreted as recurrent or metastatic disease, leading to inappropriate treatment. As molecular analysis is generally not available in routine practise, histological criteria that would facilitate diagnosis of multiple primary GISTs in routine slides are needed. We studied 14 GISTs (mean size, 2.7 cm) from six men and one woman (mean age, 70 years) applying morphological features and direct sequencing of KIT, PDGFRA, BRAF, and KRAS. Diagnosis was synchronous in five and metachronous in two patients. Paired tumours originated in stomach/small bowel (n = 5), duodenum/jejunum (n = 1), and stomach/oesophagus (n = 1) and revealed spindle (n = 10) and mixed spindle and epithelioid (n = 4) phenotype. Tumours were well circumscribed and have involved the muscularis propria in a pattern typical of primary GISTs. Different somatic KIT mutations were found in tumours from four patients. One patient had a KIT-mutated and a BRAF-mutated (V600E) tumour. Two patients had wild-type tumours. No PDGFRA or KRAS mutations were detected. Our results underscore the molecular heterogeneity of sporadic multifocal GISTs. The characteristic involvement of the muscularis propria and the site-typical morphology and immunophenotype facilitated the diagnosis of primary GISTs in all cases and correlated with molecular findings, emphasising the value of conventional histology in recognising independent primary GISTs.