Methods for beam data acquisition offered by a mini-phantom

Mini-phantoms are an important tool for measurement of basic head scatter p arameters in high-energy photon beams, and recently they have also been use d for beam quality specification. Therefore the feasibility and reliability of basic beam parameter acquisition using only a mini-phantom is checked i n 6, 18 and 25 MV photon beams. These parameters include head scatter corre ction factors, phantom scatter correction factors, total scatter correction factors, wedge factors, off-axis ratios, as well as beam attenuation coeff icients and beam hardening coefficients. In order to specify beam quality v ariations and beam quality modifications by a wedge, two different methods are compared: the first method uses a constant source to chamber distance o f 1 m, the second method refers to narrow beam geometry. mu values derived with two different beam quality specification methods show a systematic dev iation. However, relative variations of the attenuation coefficient within the beam and the associated beam quality modifications observed with the tw o methods show good agreement in open and wedged beams. Phantom scatter cor rection factors are calculated from measured head scatter correction factor s and total scatter correction factors as well as from attenuation coeffici ents. Measured and calculated phantom scatter correction factors agree with in 1% with the values given in literature. For 18 and 25 MV photon beam, we dge factors measured in water or in the mini-phantom agree within 0.5%, but maximum deviations of similar to 1.5% are observed at 6 MV for the largest field sizes, it is demonstrated that the determination of several beam dat a related to full scatter conditions does not necessarily require the avail ability of a full scatter phantom. The mini-phantom is a reliable but very cheap and simple tool. it offers versatile possibilities to measure, check and verify basic beam parameters in high-energy photon beams.