Table 1. Primary Tumor (T)a
| TX | Primary tumor cannot be assessed. |
| T0 | No evidence of primary tumor. |
| Tis | Carcinoma in situ. |
| T1a | Tumor invades lamina propria. |
| T1b | Tumor invades submucosa.b |
| T2 | Tumor invades muscularis propria. |
| T3 | Tumor invades through the muscularis propria into the subserosa or into the nonperitonealized perimuscular tissue (mesentery or retroperitoneum) with extension ≤2 cm.b |
| T4 | Tumor perforates the visceral peritoneum or directly invades other organs or structures (includes other loops of small intestine, mesentery, or retroperitoneum >2 cm, and abdominal wall by way of serosa; for duodenum only, invasion of pancreas or bile duct). |
| aReprinted with permission from AJCC: Small intestine. In: Edge SB, Byrd DR, Compton CC, et al., eds.: AJCC Cancer Staging Manual. 7th ed. New York, NY: Springer, 2010, pp 127-32. |
| bThe nonperitonealized perimuscular tissue is, for jejunum and ileum, part of the mesentery and, for duodenum in areas where serosa is lacking, part of the interface with the pancreas. |
