The effect of off-axis quality changes on zero area TAR for megavoltage beams

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Abstract
An analytical technique was employed to extract the primary component of dose at off-axis points from broad-beam water phantom data for a Clinac-18 Clinac-6 (100), Clinac-4 (Pb filter) and Philip's cobalt unit. The methodology presented here does not require separate off-axis HVL measurements and guarantees the accurate description of the data within the radiation field. It is shown that the off-axis reduction in the primary dose component for linacs does lead to significant (5-10%) errors in dose calculations if it is not taken into account in extended field external beam calculations. A method is described in which the off-axis variation in zero field TAR can be easily extracted from broad-beam scan data using the framework established by Cundiff et al (1973) as modified by Hanson and Berkeley (1980) for the dosimetry of large irregular fields. The technique is self-consistent and leads to parameters which have simple physical interpretations and which guarantee an accurate description of the beam.