[661] Microscopic Gastrointestinal Stromal Tumors (mGIST): A Series of 79 Cases

S Rossi, L Toffolatti, G Gallina, C Sartor, L Messerini, I Bearzi, G Mazzoleni, AP Dei Tos. Regional Hospital, Treviso, Italy; School of Medicine, Florence, Italy; School of Medicine, Ancona, Italy; General Hospital, Bolzano, Italy

Background: mGIST are common incidental findings in the stomach of elderly population (20-35%). Conversely, very few cases of mGIST have been reported in small intestine, large bowel and rectum. KIT/PDGFRA mutations have been detected in less than half of gastric mGIST analyzed, but whether this reflects the occurrence of other unknown initiating molecular events or it is rather the result of technical problems is unclear.
Design: 79 GIST with a diameter < 1 cm were retrieved from the files of Pathology Department of Treviso and from other 4 Italian centers. Follow-up was available for 26 patients (1 to 15 years). Cell type, cellularity, regressive changes and mitoses were evaluated. KIT immunostain was performed in all cases.In addition, 14 cases were stained for DOG1,PKCtheta,nestin,L1. KIT and PDGFRA genes status was assessed in 19 cases. DNA extraction was preceded by tumor dissection.
Results: Of 79 mGIST, 58 were located in the stomach, 16 in the small intestine, 3 in the large bowel and 2 in the rectum. Size ranged between 0.2 and 1 cm (median 0.75). The morphology was spindled in 64 cases (81%), mixed in 11 (14%) and epithelioid in 4 (5%). Cellularity was low in 39 cases (49%), moderate in 25 (31.5%) and high in 15 (19.5%). Regressive changes, as hyalinization either alone or associated to calcification, were seen in 45 cases (57%). 19 cases showed mitotic activity, with 4 cases displaying >5 mitoses (mean 0.8).Of the 21 non gastric mGIST,only 2 cases were hypocellular and hyalinized and mitotic count was significantly higher than in gastric mGIST (mean 1.3). KIT was positive in 70 cases (88.6%) and negative in 9 (all gastric). DOG1, nestin and L1 were expressed in all the 14 cases studied, whereas PKCtheta was expressed 9/14 cases. KIT mutation was detected in 15 of 19 cases and involved exon 11 in 13 cases and exon 9 in 2. PDGFRA mutation was detected in 2 cases, involving exon 18 in 1 case and exon 12 in 1 case. One of 19 cases was WT for all exons and in 1 case the results were not conclusive. Interestingly, 550-558del involving the intron 10-exon 11 boundary was over-represented (3 of 13 cases). Furthermore, one of the 2 exon 9 mutations was a point mutation (F504L), never reported before. Clinically 1 of 26 patients progressed.
Conclusions: 1. Non gastric mGIST appear as morphologically distinct from gastric ones. 2. In contrast with previous reports, KIT/PDGFRA mutations represent a common genetic event.
Category: Gastrointestinal

Wednesday, March 11, 2009 9:30 AM

Poster Session V # 88, Wednesday Morning