Gastric lymphoma is not cancer, but a malignant sarcoma. We usually classify malignant tumours into two types, cancer and sarcoma, depending on the tissue source of the tumour cells.
Malignant tumours originating from epithelial tissues, called cancers, commonly found in the lung, stomach, bladder, liver and head of the pancreas.
Malignant tumours originating from mesenchymal tissue are called sarcomas, of which gastric lymphoma is one type, and common sarcomas include osteosarcoma, lymphoma and leukaemia.