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What is Most common type of gastric tumor in adults: and explain in detail?
About 85% of stomach cancers are adenocarcinomas, with 15% due to lymphomas, gastrointestinal stromal tumors (GISTs), and leiomyosarcomas. 
Gastric adenocarcinomas may be subdivided into two pathologically defined categories:

Diffuse type, in which cell cohesion is absent, so that individual cells infiltrate and thicken the stomach wall without forming a discrete mass.
Intestinal type, characterized by cohesive neoplastic cells that form gland-like tubular structures. 

The diffuse carcinomas occur more often in younger patients, develop throughout the stomach (including the cardia), result in a loss of distensibility of the gastric wall (so-called linitis plastica, or “leather bottle” appearance), and carry a poorer prognosis.
Reference: Harrison’s P R I N C I P L E S   O F I N T E R N A L M E D I C I N E 20th edition page no 568