Attenuation in very narrow photon beams.

When one measures the half-value layer (HVL) or the attenuation coefficient (mu) in a high-energy photon beam, it is necessary to use a narrow beam to eliminate the scattered photons produced in the attenuator. However, lateral electron equilibrium will be compromised if the beam is too small. If the HVL and mu are based on measurements of absorbed dose, the results will then depend on field size for a polyenergetic photon spectrum. The measured values also become sensitive to detector properties. This has been examined by experiments and Monte Carlo calculations. The field size should be sufficient for lateral electron equilibrium to prevent ambiguities in the resulting HVL or mu, which are of the order of 10% for 6-MV X rays.