Final answer:
The principal diagnosis for a patient undergoing radiation and chemotherapy for distal esophageal carcinoma is the carcinoma itself. Radiation therapy may include external beam radiation or brachytherapy, and these treatments are usually combined with chemotherapy for better results.
Step-by-step explanation:
The appropriate principal diagnosis for a patient receiving radiation and chemotherapy for distal esophageal carcinoma is the distal esophageal carcinoma itself. As part of the treatment process, radiation therapy, including external beam radiation therapy (EBT) and brachytherapy, is used to deliver high-energy radiation to damage the DNA of cancer cells, either from a machine outside the body or from a radioactive substance inside the body. Chemotherapy uses chemical substances to target and kill cancer cells and may be administered alongside radiation therapy to increase treatment efficacy.