Clinical implementation of photon beam flatness measurements to verify beam quality

This work describes the replacement of Tissue Phantom Ratio (TPR) measurements with beam profile flatness measurements to determine photon beam quality during routine quality assurance (QA) measurements. To achieve this, a relationship was derived between the existing [Formula: see text] energy metric and beam flatness, to provide baseline values and clinically relevant tolerances. The beam quality was varied around two nominal beam energy values for four matched Elekta linear accelerators (linacs) by varying the bending magnet currents and reoptimizing the beam. For each adjusted beam quality the [Formula: see text] was measured using an ionization chamber and Solid Water phantom. Two metrics of beam flatness were evaluated using two identical commercial ionization chamber arrays. A linear relationship was found between [Formula: see text] and both metrics of flatness, for both nominal energies and on all linacs. Baseline diagonal flatness ([Formula: see text]) values were measured to be 103.0% (ranging from 102.5% to 103.8%) for 6 MV and 102.7% (ranging from 102.6% to 102.8%) for 10 MV across all four linacs. Clinically acceptable tolerances of [Formula: see text] for 6 MV, and [Formula: see text] for 10 MV, were derived to equate to the current [Formula: see text] clinical tolerance of [Formula: see text]. Small variations in the baseline diagonal flatness values were observed between ionization chamber arrays; however, the rate of change of [Formula: see text] with diagonal flatness was found to remain within experimental uncertainty. Measurements of beam flatness were shown to display an increased sensitivity to variations in the beam quality when compared to TPR measurements. This effect is amplified for higher nominal energy photons. The derivation of clinical baselines and associated tolerances has allowed this method to be incorporated into routine QA, streamlining the process whilst also increasing versatility. In addition, the effect of beam adjustment can be observed in real time, allowing increased practicality during corrective and preventive maintenance interventions. PACS number: 87.56.Fc