ABSTRACT
The complexity of small-field dosimetry has been compounded by the radiation-source size that plays an important role in dosimetry. Small-field dosimetry is performed ad-hoc based on comparison with many detectors using a daisy chain or other intercomparison approaches without understanding radiation transport and knowing the characteristics of the detector in nonequilibrium conditions. The International Atomic Energy Agency (IAEA) and American Association of Physicists in Medicine (AAPM) realized the importance of small-field dosimetry and formed a task group to tackle this issue. The primary source occlusion effect becomes important when the field diameter is comparable to or smaller than the size of the primary photon source. In small fields, the photon spectrum becomes harder with depth. This is different from larger fields where the beam-hardening effect is offset by an increasing amount of scattered radiation, which, depending on the field size, may lead to the effective softening of photon spectrum with depth.
