Jg. Wierzbicki et Ds. Waid, LARGE DISCREPANCIES BETWEEN CALCULATED D-MAX AND DIODE READINGS FOR SMALL-FIELD SIZES AND SMALL SSDS OF 15 MV PHOTON BEAMS, Medical physics, 25(2), 1998, pp. 245-246
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.