Ninety-three cancer patients about to undergo radiotherapy were studied. These patients were affected by cancer of the head and neck, breast cancer, and abdominopelvic cancer. They were randomly distributed into control and experimental groups. The patients from the control group were allowed to eat ad libitum, whereas those from the experimental group followed a strict nutritional protocol that covered their needs while satisfying their particular tastes and habits. Data were collected the day before radiotherapy commenced and the day treatment ended. Anthropometric, analytical, and clinical parameters were determined. The results showed the benefit of dietetic treatment in experimental patients with head and neck or abdominopelvic cancer.