Describe why surface dose increases with field size in megavoltage photon beams?.

Expert Solution

A larger field means that there is an increase in the amount of scatter from the treatment head as well as an increase in the amount of scatter for interactions in air. This leads to electron contamination and is an important factor in field size changes. Very small field sizes may cause disequilibrium of electron scatter in the central part of the beam, leading to loss of the flat portion. This limits the utility of very small field sizes.

In general as a field size increases, surface dose will increase due to increased electron contamination of the beam.

As field size increases, zmax will move closer to the surface due to electron contamination and scatter from the outer parts of the beam.

genius_generous answered 2 years ago

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