What is the relationship between electron energy (E) and the depth of the 90% isodose line?

The correct answer reflects that the depth of the 90% isodose line is directly proportional to the energy of the electrons being used in the treatment. As the energy increases, the electrons penetrate deeper into the tissue, resulting in a greater depth where the dose distribution reaches the 90% threshold. This principle is a fundamental aspect of radiation physics and dosimetry.

Higher energy electrons travel further before losing their energy, allowing them to deposit therapeutic doses at deeper locations in the tissue. This characteristic is crucial for treatment planning in radiation therapy, as it enables practitioners to adjust the electron energy to ensure the desired treatment area receives the appropriate dose while minimizing exposure to surrounding healthy tissues.

In contrast, lower energy electrons have limited penetration ability, resulting in a shallower depth for achieving the same percentage of the isodose line. Therefore, the relationship established here is essential for determining the most effective energy setting for various clinical scenarios, ensuring optimal therapeutic outcomes while safeguarding healthy tissue.