The coexistence of adenocarcinoma and GIST is uncommon, and GIST has often been detected incidentally in the gastric mucosa or serosa, or, occasionally, intramurally, at surgery [
1‐
4]. Further, gastric tumors are derived from various other carcinomas and a precise pre- and post-operative diagnosis is important. However, when the GIST is submucosal or subserosal, the gastric mucosa may not be invaded, and endoscopic biopsies can be normal. In fact, in our case, the pre-operative biopsy fragments showed only adenocarcinoma, and the GIST was detected only in the resected stomach. Therefore, it may be difficult to diagnose preoperatively.
Approximately 20% of patients with GIST develop other cancers and various hypotheses have been proposed regarding the simultaneous development of GIST and adenocarcinoma [
7‐
9]. It is not clear whether this is a simple incidental coexistence or the two lesions are connected by a causal relationship. Gene mutations may underlie tumor predisposition in patients harboring a double neoplasia. However, at present, no data are available to support such a hypothesis.
GIST is the most common mesenchymal tumor [
10], accounting for about 0.1 to 3% of all GI tumors [
11]. IHC staining, such as is for CD34, smooth muscle actin (SMA), and S100, as well as c-kit (CD117), is necessary to make an accurate diagnosis of GIST [
13]. It was revealed that c-kit and CD34 showed diffuse, strongly positive expressions in GIST. Rabin et al. [
11] reported that 40 to 70% of GIST's were positive for CD34, 20 to 30% were positive for SMA, and 10% were positive for S100 protein [
12,
13]. Therefore, immunostaining of CD34, c-kit, SMA, and S100 is useful, and we could confirm the histological diagnosis using these markers. Accordingly, immunohistochemical as well as clinical information may be required in order to diagnose GIST appropriately. Although it is not easy to speculate on the coexistence of adenocarcinoma and GIST, pre- and post-operative diagnoses may be essential. Further the adjuvant therapy and lymphadenectomy are important. The refinement of risk stratification systems [
14] will increase the precision of these systems for predicting recurrence, which may facilitate improvements in individual disease management.