The goal of this clinical trial is to learn if adding high-dose-rate (HDR) brachytherapy can improve outcomes in patients with locally advanced esophageal squamous cell carcinoma (ESCC) who have already received external beam radiotherapy (EBRT), chemotherapy, and the immune therapy drug nivolumab.
The main questions it aims to answer are:
* Does HDR brachytherapy reduce the chance of the cancer coming back in the esophagus or nearby areas within 12 months?
* What side effects or safety issues occur when HDR brachytherapy is given after EBRT, chemotherapy, and nivolumab?
Participants will:
* Receive 1-2 sessions of HDR brachytherapy delivered through a thin tube placed inside the esophagus, within three weeks after starting nivolumab.
* Continue nivolumab and be monitored with regular follow-up visits, imaging tests, and blood samples to check treatment response and safety.