The physical field size is defined at the ______% isodose line?

The physical field size in radiation therapy refers to the area that is defined by a specific isodose line. In this context, the 50% isodose line is commonly used to delineate the physical dimensions of the treatment field. This is because the 50% isodose line represents a level of dose that reflects the effective delivery of radiation to the target area while minimizing exposure to surrounding healthy tissues.

Using this line allows clinicians to determine the treatment boundaries effectively. In many cases, the 100% isodose line indicates the maximum dose delivered at the center of the treatment field, but to account for variations in dose distribution and to guide treatment planning, the 50% line is utilized to define the physical field size.

Understanding this concept is essential for radiation therapists as it directly impacts treatment planning and the accuracy of dose delivery to the intended area.