What is used to define and measure field flatness in radiation therapy?

The uniformity index is a crucial metric in radiation therapy used to define and measure field flatness. It quantifies how evenly the radiation dose is distributed across the treatment field. An ideal radiation field should deliver a consistent dose to the tumor while minimizing exposure to surrounding healthy tissues, and the uniformity index helps assess this characteristic. By providing a numerical value or ratio, it indicates whether the dose remains within acceptable limits across different areas of the field, allowing dosimetrists to evaluate and optimize the delivery of radiation for effective treatment.

Central axis measurement, while important in determining the characteristics of the radiation beam, primarily focuses on a single central point rather than the overall flatness of the field. The distance to isodose line is more associated with the geometry of the treatment plan and the spatial dose distribution rather than specifically addressing flatness. Surface dose correlation can provide insights into how the dose behaves at or near the skin surface, but it does not specifically quantify field flatness as the uniformity index does. Thus, the uniformity index is the most direct and relevant measure for assessing flatness in the context of radiation therapy.