For a 6 MV photon beam used in SRS, what is the minimum required beam size for electronic equilibrium?

The minimum required beam size for electronic equilibrium in a 6 MV photon beam used in Stereotactic Radiosurgery (SRS) is crucial because it determines the effective delivery of radiation to the target volume while minimizing dose to surrounding healthy tissues.

For high-energy photon beams like 6 MV, electronic equilibrium occurs when the dose distribution is not significantly affected by the scatter and absorption of photons within the tissue. At smaller beam sizes, the impact of the surface and internal tissue interactions becomes more pronounced, leading to a gradient in dose distribution that can result in inadequate targeting of the tumor.

Specifically, a beam size of at least 30 mm is typically mandated to achieve electronic equilibrium because it ensures sufficient lateral scattering is occurring to provide a uniform dose distribution. This helps in avoiding hot or cold spots that could detrimentally influence treatment outcomes. In SRS, achieving precise dose delivery is essential due to the sensitivity of surrounding healthy tissues.

Thus, a 30 mm beam size provides the necessary conditions for electronic equilibrium to be met, allowing for effective treatment planning and dose distribution to the target while maintaining the safety of surrounding normal tissues.