• DOI:10.1118/1.4735353
• Corpus ID: 25020463

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

@article{Gao2012SUET285FA,
  title={SU-E-T-285: Flatness as a Measure of Changes in Photon Energy for Megavoltage X- Ray Radiotherapy.},
  author={S Gao and Mark Rose and William E. Simon and Peter A. Balter},
  journal={Medical physics},
  year={2012},
  volume={39 6Part14},
  pages={
          3769
        }
}

• S. Gao, M. Rose, +1 author P. Balter
• Published 1 June 2012
• Medicine, Physics
• Medical physics

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.