2. Field size definition

In general, two types of field sizes have been defined by the International Electrotechnical Commission. The first is called the geometric field size; it is defined as two-dimensional projection by the source of radiation of the collimator opening on a plane orthogonal to the central axis of the primary photon beam. The second is called the irradiation field size; it is defined as the two-dimensional area bounded by specific isodose lines in a plane orthogonal to the central axis of the radiation beam. An alternative way to define irradiation field size is by using full width at half maximum (FWHM) of radiation beam profiles obtained along the lateral direction (in-line or crossline profiles) at isocenter depth. This FWHM is equal to the opening of the collimating jaws at the isocenter. Therefore, at isocenter the geometrical and irradiation fields are in consonance with each other. Hence, this agreement can be

Prospective Monte Carlo Simulation for Choosing High Efficient Detectors for Small-Field… DOI: http://dx.doi.org/10.5772/intechopen.89150

verified by FWHM measurement of the beam profile along the lateral direction. However, in the small beam condition due to the partial blockage of primary source of photon and reduction in the head scatter along the central axis, the output of radiation is reduced. As a consequence the condition of lateral charged particle equilibrium (LCPE) is not fulfilled. Hence, due to the reduction in the radiation output along the central axis, the value of maximum dose also gets reduced, and agreement between the geometrical and irradiation fields is lost. Figure 1 illustrates the overlapping of penumbra region with decrease in field size, as it leads to decrease in output and increase in penumbra width. Hence, the parameters like FWHM breaks down for small-field sizes [3, 10]. In case of small beams due to the overlap of penumbra region, the FWHM of the profile gets broader relative to the collimator settings, and this effect is called apparent field widening.
