Journal Article

P318 Diffuse omental epithelioid GIST- a triple jeopardy in recognising our ‘man from Istanbul’

Dasgupta, Kaushiki, Dasgupta, Kaushik, Dildey, Petra, Wilson, David, Dear, Thomas, Fleming, Katherine

2025

Overview

IntroductionThe differentials of diffuse omental mass include metastasis and mesothelioma. We describe the challenges in recognising our man from Istanbul (known man in an unexpected location) masquerading as mesothelioma, misconstrued as PEComa and the truncated immune-phenotype was finally salvaged by NGS.Case Description77 year old man presented with diarrhoea and weight loss, underwent a CT colonogram that revealed a diffuse omental mass. In the absence of any other primary, this was radiologically suggestive of mesothelioma although there was no history of exposure.A laparoscopic biopsy was performed as CT biopsy was scanty for exhaustive IHC. The tumour was nested, epithelioid with prominent vessels. The cells had pericellular space with spidery cytoplasmic processes. Although distinctive, this was not diagnostic. Extended panel for lineage excluded mesothelial/epithelial nature (Calretinin, D2-40, CAM5.2, CK5/6, 34BE12, WT1, P40, Pan CK, GATA3, GLUT1, PAX8), neuroendocrine carcinoma (Chromogranin, INSM1, Synaptophysin), melanoma (S100, HMB45, Melan A, BRAFV600E, PRAME), sarcoma (CD34, Desmin, Myogenin, Myo D1, ERG) and lymphoma (CD45/LCA, MUM1). The SWI-SNF complex proteins INI1 and SMARC A4 were retained and so was FH with no accumulation of the metabolite 2 SC. NGS and MDM2 FISH was not available but Cathepsin K showed diffuse strong expression although the focal weak expression of SMA and TFE3 were interpreted as negative but TFE 3 was interpreted as focally positive by the external referral pathologist. A probable diagnosis of malignant PEComa (Perivascular Epithelioid Cell tumour) pending MDM2 FISH, molecular confirmation and GPNMB IHC (never received) was made. Referral to the external soft tissue pathologist confirmed lack of expression of other lymphoma/germ cell (CD30, ALK), macrophage (CD68) and more specific soft tissue sarcoma (EMA,MUC4) markers. MDM2 FISH was negative for liposarcoma which was another significant differential. Archer FusionPlex sarcoma panel V2 was negative. Crucially this does not screen for TSC1/2 abnormalities in PEComa but only TFE3 which was negative. Hence the probability of malignant PEComa was retained along with differentials of pseudo-endocrine sarcoma and GLI 1 altered sarcoma. At the regional sarcoma MDT, CD117 and DOG1 were performed - this was diffusely positive for DOG1 (Discovered on GIST1- K9 clone) but not CD117. The possibility of an unusual GIST with PDGFRA mutation was considered and then confirmed on NGS (V546E and Y849C activating mutations in exon 11 and 18 of PDGFRA gene).Our patient reports symptomatic relief from ascites (that developed later) following treatment with 2 cycles of imatinib and awaits interval scans.Abstract P318 Figure 1ConclusionThis case illustrates that GIST must be considered in all difficult to classify intra-abdominal tumours irrespective of cross-sectional imaging and morphology.

• Publisher: BMJ Publishing Group LTD
• Subject: Ascites / Biopsy / Calretinin / Cathepsin K / CD30 antigen / CD34 antigen / CD45 antigen / Computed tomography / Desmin / Diarrhea / GATA-3 protein / Imatinib / Laparoscopy / Lymphoma / Macrophages / MDM2 protein / Melanoma / Mesothelioma / Metastases / Mutation / Myogenin / Neuroendocrine tumors / Pax8 protein / Phenotypes / Sarcoma / Soft tissue sarcoma