Flatness as a Measure of Changes in Photon Energy for Megavoltage X-Ray Radiotherapy
S Gao1*, M Rose2, W Simon3, P Balter4, (1) MD Anderson Cancer Ctr., Houston, TX, (2) Sun Nuclear Inc., Melbourne, FL, (3) Sun Nuclear Corp, Melbourne, FL, (4) UT MD Anderson Cancer Center, Houston, TXSU-E-T-285 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall
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 3x3 and 10x10 cm^2 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 normalized flatness 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.