Gao S, Rose M, Simon W, Balter P
MD Anderson Cancer Ctr., Houston, TX.
Sun Nuclear Inc., Melbourne, FL.
Med Phys. 2012 Jun;39(6Part14):3769. doi: 10.1118/1.4735353.
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.
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.
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.
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.
比较基于平坦度的能量指标与基于衰减的指标（百分深度剂量）随直线加速器标称临床能量变化的灵敏度。
通过将弯曲磁铁电流（BMI）从临床使用值调整±15%来实现能量变化。使用市售的离子室阵列（IC轮廓仪，Sun Nuclear，佛罗里达州墨尔本）测量两种平坦度指标，即射野中心80%区域的相对平坦度（平坦度）和沿对角线的平均最大剂量除以中心轴（CAX）剂量进行归一化（对角线归一化平坦度，DNF）。在水模中测量3×3和10×10 cm²射野尺寸下5 cm和10 cm深度处的百分深度剂量。
百分深度剂量（PDD）和平坦度对能量变化的灵敏度小于DNF的变化。对于6X射线，从标称能量变化±15%时，PDD的变化为-1%至+3%，平坦度的变化为-5%至+2%；对于18X射线，在此能量范围内PDD从-0.5%变化至+2.5%，平坦度从-5%变化至+2%。从标称能量变化±15%时，6X射线的DNF从+12%变化至-8%，18X射线的DNF从+8%变化至-6%，且与能量呈现近似线性相关。此外，DNF是唯一被发现对6 MV和18 MV射束的能量增加和减少均敏感的指标。
发现对角线归一化平坦度是6 MV和18 MV光子束能量变化最敏感的指标。IC轮廓仪使该指标能够作为直线加速器常规质量保证的一部分方便地进行测量。Sun Nuclear公司提供了IC轮廓仪。
