Large discrepancies between calculated Dmax and diode readings for small field sizes and small SSDs of 15 MV photon beams.

Clinical observations have revealed that diode readings from 15 MV photon beams with small field sizes and small SSD values were consistently 5%-10% higher than doses calculated at the maximum depth. Dose at depth depends on the collimator scattering factor and the phantom scattering factor--both field size dependent. Diodes are placed on the skin surface, and therefore the phantom scatter factor is different than at depth. In addition, inverse square corrections should be calculated with respect to the location of the diode itself rather than the location of Dmax within the tissue. This may account for about 8% difference between calculated Dmax and diode readings for very small radiation fields and small SSDs, e.g., a prostate boost.