What ratio is typically defined as P80/20 in radiation therapy?

The P80/20 ratio in radiation therapy is defined as the distance between the dose values of 80% and 20% of the prescribed dose delivered by a radiation treatment beam. This measurement provides valuable information regarding how quickly the dose falls off with increasing depth in tissue, essentially helping to illustrate the dose distribution characteristics of a beam.

In practice, this ratio is important for understanding the dose gradient, which is essential for optimizing treatment plans, ensuring that the targeted tumor receives the adequate dose while minimizing exposure to surrounding healthy tissues. A narrow P80/20 ratio indicates a steep dose fall-off, suggesting that there is a quick decrease in dose outside the targeted area, while a wider ratio indicates a more gradual decrease.

The other options refer to different concepts in radiation therapy. The ratio of superficial to deep tissue dose focuses on the comparative dosimetry at varying tissue depths, while measurement of skin dose versus target dose looks at the dose delivered to the skin in comparison to the dose at the tumor site. The percentage depth dose relates to the dosimetry at specific depths but does not specifically describe the difference in doses at the 80% and 20% levels. These concepts are important but distinct from the definition of the P80/20 ratio