We retrospectively analyzed 20 patients with stage I-IV esophageal carcinoma treated by limited external radiotherapy (EXR) combined with high dose rate intracavitary brachytherapy (ICBT) using 60Co as boost therapy. In 10 of 20 patients treated by combined therapy, we used endoscopic ultrasonography (EUS) before ICBT and measured the thickness of the residual tumor following external beam therapy. Of the patients treated by combined therapy, the local control rate of esophageal carcinoma with a thickness under 10 mm before ICBT was significantly better than that with a thickness over 10 mm (confidence limit 95%). Therefore, when administering high dose rate intracavitary brachytherapy after EXR, we should measure the thickness of esophageal carcinoma with millimeter unit precision. One of the most precise methods currently available for measuring thickness is EUS. Our results indicate that esophageal carcinoma under 10 mm in thickness after EXR is the most favorable indication for ICBT.