Purpose: To clarify the radiotherapy clinical target volume (CTV) margin needed for esophageal squamous-cell carcinoma (SCC) and gastroesophageal junction (GEJ) adenocarcinoma. Methods and Materials: Surgical specimens of esophageal SCC (n = 34) and GEJ adenocarcinoma (n = 32) were prospectively collected and analyzed for microscopic spread along the esophagus and GEJ both proximally and distally from gross tumor and for lymph node ILN) metastasis. Results: For SCC, the mean microscopic spread beyond the gross tumor was 10.5 +/- 13.5 mm proximally (< 30 mm in 32 of 34 cases) and 10.6 +/- 8.1 mm distally (< 30 mm in 33 of 34 cases). For GEJ adenocarcinoma, the spread was 10.3 +/- 7.2 mm proximally (< 30 mm in 29 of 29 cases) and 18.3 +/- 16.3 mm distally (< 30 mm in 27 of 32 cases). The extent of microscopic spread of cancer was significantly associated with pathologic T stage (P = 0.012). LN metastases were observed in 12 (35%) of 34 patients with middle and lower esophageal SCC and 15 (47%) of 32 patients with GEJ adenocarcinoma. Conclusions: The extent of microscopic spread within esophagus (recommended CTV margin) was < 30 mm in about 94 of cases of esophageal cancer, except for distal microscopic spread in GEJ adenocarcinoma, in which 50 mm was needed to cover about 94% of cases. (c) 2007 Elsevier Inc.