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Technical Note: Correction for intra-chamber dose gradients in reference dosimetry of flattening-filter-free MV photon beams.

PURPOSE
To estimate and correct for the volume averaging effect which results from the intra-chamber dose gradients when a Farmer ionization chamber (IC) is used for reference dosimetry in flattening-filter-free (FFF) MV photon beams.


METHODS
An intra-chamber dose gradients correction factor (kicdg) to the charge reading of a Farmer IC is estimated by comparison to a small volume IC (∼0.1 cm(3)), in the FFF beams of a TrueBeam™ (Varian, Inc.) linear accelerators. An independent estimate of the correction for the volume averaging effect (pvol) is deduced using the ratio of the active length (L) of the Farmer IC to the integral of a high-resolution FFF radial dose profile over this same length.


RESULTS
Mean (sd) values for kicdg equal to 1.0025 (0.0025) for 6 MV-FFF, and equal to 1.0057 (0.0025) for 10 MV-FFF, were estimated based on four dosimetry sessions, performed in a time interval of six months. Similarly, pvol (Farmer) equal to 1.0030 (0.0003) for 6 MV-FFF, and equal to 1.0063 (0.0005) for 10 MV-FFF, were computed.


CONCLUSIONS
The systematic bias which results from intra-chamber dose gradients when a Farmer IC is used for reference dosimetry in FFF MV photon beams is estimated to be -0.6% for 10 MV-FFF, and -0.3% for 6 MV-FFF, based on the obtained values of the factor kicdg. This bias can be corrected, within 0.1%, by the simple measure of pvol at the beginning of the dosimetry session.