LARGE DISCREPANCIES BETWEEN CALCULATED D-MAX 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 pho ton beams with small field sizes and small SSD values were consistentl y 5%-10% higher than doses calculated at the maximum depth. Dose at de pth depends on the collimator scattering factor and the phantom scatte ring factor-both field size dependent. Diodes are placed on the skin s urface, and therefore the phantom scatter factor is different than at depth. In addition, inverse square corrections should be calculated wi th respect to the location of the diode itself rather than the locatio n of D-max within the tissue. This may account for about 8% difference between calculated D-max and diode readings for very small radiation fields and small SSDs, e.g., a prostate boost. (C) 1998 American Assoc iation of Physicists in Medicine.