The spread of gastric adenocarcinoma may follow three main patterns: hematogenic, lymphatic and intraperitoneal. A GTV should be considered in preoperative or exclusive radiation therapy. After non-radical surgery, a "residual GTV" will be defined with the help of the surgeon. The CTV encompasses three intricated volumes. a) A "tumor bed" volume. After radical surgery, local recurrences appear as frequent as distant metastases. The risk depends upon the depth of parietal invasion and the nodal status. Parietal infiltration may extend beyond macroscopic limits of the tumor, especially in "linitis plastica". Therefore this volume will include: the tumor and the remaining stomach or their "bed of resection", a part of the transverse colon, the duodenum, the pancreas and the truncus of the portal vein. In postoperative RT, this CTV also includes the jejuno-gastric or jejuno-esophageal anastomosis. b) A peritoneal volume. For practical purposes, two degrees of spread must be considered: (1) contiguous microscopic extension from deeply invasive T3 and T4 tumors, that remain amenable to local sterilization with doses of 45-50 Gy, delivered in a CTV including the peritoneal cavity at the level of the gastric bed, and under the parietal incision; (2) true "peritoneal carcinomatosis", with widespread seeds, where chemotherapy (systemic or intraperitoneal) is more appropriate. c) A lymphatic volume including the lymph node groups 1 to 16 of the Japanese classification. This volume must encompass the hepatic pedicle and the splenic hilum. In proximal tumors, it is possible to restrict the lower part of the CTV to the lymphatic volume, and therefore to avoid irradiation of large intestinal and renal volumes. In distal and proximal tumors, involvement of resection margins is of poor prognosis--a radiation boost must be delivered at this level. The CTV in tumors of the cardia should encompass the lower part of the thoracic esophagus and the corresponding posterior mediastinum. In tumors invading the distal esophagus, a more complete coverage of mediastinal lymph nodes should be considered, especially in patients in good general condition. In tumors of the gastric fundus, most of the left hemidiaphragm should be included, as well as the spleen and its hilum (or their resection bed). In proximal tumors without involvement of the lesser curvature, a full coverage of the hepatic pedicle is not necessary. In contrast, for distal tumors, the hepatic pedicle and the hepatoduodenal ligament should be included whereas the splenic area could be spared.
Conclusion: Planning the treatment of gastric cancer remains difficult; target volumes must be customized by experienced radiation oncologists according to tumoral and clinical situation.