How does surface dose change with increasing field size?

As field size increases, the surface dose tends to increase due to a variety of factors associated with radiation interactions and the geometry of the treatment beam. A larger field size results in a greater number of radiation particles delivering their energy to the surface area of the irradiated tissue.

When considering radiation therapy, a larger field allows for a more uniform distribution of the radiation dose across the target area, which often leads to an increase in the dose delivered to the skin or surface tissue. Additionally, the broader spread of the radiation beam reduces the percentage of scatter radiation that is directed away from the surface as opposed to a smaller field, where a larger proportion may be lost.

Moreover, with increasing field sizes, the contribution from the scattered radiation within the treatment area becomes more significant, further adding to the dose at the surface. These combined effects account for the observed increase in surface dose as field size expands, making it an important consideration in treatment planning to ensure that the dose delivered matches therapeutic goals while minimizing excess exposure to surrounding healthy tissues.