What happens to the effective primary radiation incident on a patient as the field size increases?

As the field size increases, the effective primary radiation incident on a patient increases due to several factors related to the dynamics of radiation beam characteristics and interactions with the treatment area.

When using external beam radiation therapy, the radiation is delivered in a cone-shaped beam from the linear accelerator. As the field size expands, it allows for a greater volume of tissue to receive radiation. This is because a larger field size captures more of the radiation beam, leading to an increased dose delivered to the target area. Consequently, a larger treatment field encompasses a broader range, thereby increasing the amount of primary radiation exposure that reaches the patient's tissue.

Additionally, a larger field size can minimize the influence of scatter radiation from bone and soft tissue, ensuring that the majority of the radiation comes directly from the primary source. Therefore, as the field size increases, patients receive a higher amount of the effective primary radiation, which can be essential for adequately covering the tumor volume while ensuring that critical surrounding organs are protected by using advanced radiation techniques.

This understanding helps guide dosimetrists in planning and delivering effective treatment while balancing the need to maximize tumor dose and minimize exposure to healthy tissues.