SU‐E‐T‐285: Flatness as a Measure of Changes in Photon Energy for Megavoltage X‐ Ray Radiotherapy

Purpose: To compare the sensitivity of flatness based energy metrics versus an attenuation based metric (percent depth dose) as a function of energy changes from the linac's nominal clinical energy. Methods: Energy changes were accomplished by adjusting the bending magnet current (BMI) ±15% away from the value used clinically. Two metrics for flatness, relative flatness in the central 80% of the field (flatness) and average maximum dose along the diagonals normalized by central axis (CAX) dose (diagonal normalized flatness, DNF), were measured using a commercially available ion chamber array (IC profiler, Sun Nuclear, Melbourne FL). Percent depth dose was measured in water for depths of 5 and 10 cm in 3×3 and 10×10 cm̂2 field sizes. Results:The sensitivity of percent depth dose (PDD) and flatness to energy changes were smaller than changes in DNF. For 6X the changes in PDD were from −1+ to +3% and the changes in flatness were from ‐5% to +2% for energy changes of ±15% from the nominal energy, for 18X the PDD changed from −0.5% to +2.5% and the flatness from ‐5% to +2% over this energy range. DNF changed from +12% to −8% for 6X and from +8% to ‐6% for 18X over a ±15% change from the nominal energy and showed a near linear correlation with energy. In addition DNF was the only metric that was found to be sensitive to both increases and reductions of energy for both the 6 and 18 MV beams. Conclusions: Diagonal normalizedflatness was found to be the most sensitive metric to energy changes for photon beams of 6 and 18 MV. The IC profiler allows this metric to be conveniently measured as part of routine linac quality assurance. Sun Nuclear Corporation provided the IC profiler.