High-energy photons in IMRT: uncertainties and risks for questionable gain.

For a start, although integral dose is generally assumed to be improved by the use of higher energy photons, there is concern that some treatment planning systems might not effectively model the “beam narrowing” at depth observed with high energy photons in the small radiation beamlets encountered in IMRT. This phenomenon is caused by lateral beam degradation due to penumbra widening (8); when very small fields are used, there is greater loss of electron equilibrium laterally with higher energy photons (8, 9). This is especially problematic at high gradients, leading to dose reduction near the beam edge and along the central axis (10). The clinical solution to this problem is to use wider beams – which negates the hypothetical gain from high-energy photons as far as integral dose is concerned.