In external beam radiation therapy, the treatment field must always be:

In external beam radiation therapy, it is crucial for the treatment field to be adequately defined to cover the tumor. This ensures that the radiation dose is effectively delivered to the tumor while sparing surrounding healthy tissues as much as possible. Proper delineation of the treatment field involves accurately identifying the tumor's size, shape, and location, taking into consideration its movement due to respiration or other factors.

An adequately defined treatment field not only helps in targeting the tumor specifically but also plays a vital role in treatment planning and management. Accurate coverage of the tumor minimizes the risk of under-treatment, which could compromise the treatment's effectiveness, and helps in limiting the radiation exposure to adjacent healthy structures, thereby reducing potential side effects.

When setting up the treatment field, considerations also include the tumor type, stage, and individual patient anatomy. Any changes in the patient's positioning or tumor size during the course of treatment may require adjustments to the treatment field to ensure continued effectiveness of the therapy. Thus, the emphasis on adequate definition directly relates to achieving optimal treatment outcomes.