6MV-15MV下物理楔形因子和动态楔形因子的比较

目的:探讨不同能量下,Varian21EX直线加速器中物理楔形因子和动态楔形因子受照射野大小和深度的影响。方法:在固体水膜体中利用0.6 cc电离室对6 MV和15 MV射线束下不同角度物理楔形板和动态楔形板分别测量加和不加楔形滤片时的剂量率来计算楔形因子。通过测量不同角度的物理楔形板和动态楔形板在固定照射野(10 cm×10 cm)的不同深度下的楔形因子来研究楔形因子随深度的变化规律。同时,对于楔形因子随射野大小的变化规律,还测量了不同角度的物理楔形板和动态楔形板在固定深度(d=10 cm)下的不同射野大小的楔形因子。为了更好地分析物理楔形因子与动态楔形因子的差异,引入了相对楔形因子NWF。结果:深度对于物理楔形板的楔形因子较为明显,深度增加时楔形因子增大,且随着楔形角的增大变化更明显。对于150、300、450、600的物理楔形板,当深度由最大深度增加到20 cm时对于6 MV能量楔形因子分别增加了1.86%、3.79%、4.99%、7.95%;对于15 MV能量1.29%、1.35%、1.49%、2.03%。而动态楔形因子随深度变化不明显,最大变化不到1%。射野大小对于物理楔形因子也有一定的影响,楔形因子随射野增加而增加,但是增加幅度不大;而对于动态楔形板,在6 MV和15 MV射线束下楔形因子受射野的增大都有明显的减小。对于100、150、200、250、300、450、600的动态楔形板,从参考射野(10 cm×10 cm)到最大射野,楔形因子分别减少了7.91%、11.04%、14.08%、16.96%、19.7%、28.03%、35.89%对于6 MV和5.72%、8.17%、10.41%、12.85%、15.08%、21.82%、30.59%对于15 MV能量。结论:对于物理楔形板,深度和射野大小都对物理楔形因子有影响,所以临床剂量计算时必须考虑深度和射野大小对物理楔形因子的影响并对它进行修正。对于动态楔形板,深度对动态楔形因子影响较小,在临床剂量计算时可以忽略;而射野大小对动态楔形因子影响比较明显,在临床剂量计算时只须考虑相对射野楔形因子。

Comparison of Physical and Enhanced Dynamic Wedge Factors for 6MV-15MV Photon Beams

Objective： To investigate variation in Varian21EX＇s physical and enhanced dynamic wedge factors （WF） as a func- tion of depth and field size in tWO difference energy. Methods： For 6 MV and 15 MV,different physical wedge factor and en- hanced dynamic wedge Factor measurements were performed in solid water phantom using cylindrical 0.6 cc ionization cham- ber. Wedge factors（WF） were measured by taking the ratio of wedge and open field ionization data. In order to study depth de- pendency of Physical and enhanced dynamic wedge factors （WF）,measurements were carried at various depths at fixed field size （10 cm×10 cm）. While for field size dependency of WF the measurements were made for different field sizes at a fixed depth of 10 cm. A normalized wedge factor （NWF） was introduced to circumvent large differences between wedge factors for different wedge angles. Results： A strong linear dependence of PW factor （PWF） with depth was observed. For 15°,30°,45°. 60° PW, depth from the maximum to 20 cm ,PWF increases were 1.86% ,3.79% ,4.99% ,7.95% at 6 MV and 1.29%, 1.35 %, 1.49 % ,2.03 % for 15 MV photon beams respectively. The variation in EDW factor （EDWF） with depth was almost negligible and less than one per cent. A little linear dependence of PW factor （PWF） with field size was observed. PWF in- creases with field size increase but no obvious increasing trend. While, the EDW revealed strong dependency between field size and NWF for all wedges at both photon energies. EDW seems a smooth and continuous decrease in the NWF with in- creasing field size. For 10°. 15°,20°,25°,30°,45°,60° EDW ,from the maximum variation relative to reference field size of（10×10） cm2 EDWF decreases were 7.91% ,11.04% ,14.08% ,16.96% ,19.7% ,28.03% ,35.89% at 6 MV and 5.72% ,8.17% 10.41%, 12.85 %, 15.08 % ,21.82 % ,30.59 % for 15 MV photon beams respectively. Conclusions： depth and field size are all ef- fect on the PW, so in clinical dose calculation, it was significant to adjust the effect of depth and filed size on physical wedge Fac- tor. While for EDW, the influence from depth is small and can be neglect, yet the influence from field size is great and cannot be neglect. So in clinical dose calculation only field size dependency of WF is to be taken into account on enhance wedge factor.