Shoalhaven Cancer Care Centre; Prince of Wales Hospital.
J Appl Clin Med Phys. 2016 May 8;17(3):223-235. doi: 10.1120/jacmp.v17i3.6059.
Flattening filter-free (FFF) beams are becoming the preferred beam type for stereotactic radiosurgery (SRS) and stereotactic ablative radiation therapy (SABR), as they enable an increase in dose rate and a decrease in treatment time. This work assesses the effects of the flattening filter on small field output factors for 6 MV beams generated by both Elekta and Varian linear accelerators, and determines differences between detector response in flattened (FF) and FFF beams. Relative output factors were measured with a range of detectors (diodes, ionization cham-bers, radiochromic film, and microDiamond) and referenced to the relative output factors measured with an air core fiber optic dosimeter (FOD), a scintillation dosimeter developed at Chris O'Brien Lifehouse, Sydney. Small field correction factors were generated for both FF and FFF beams. Diode measured detector response was compared with a recently published mathematical relation to predict diode response corrections in small fields. The effect of flattening filter removal on detector response was quantified using a ratio of relative detector responses in FFF and FF fields for the same field size. The removal of the flattening filter was found to have a small but measurable effect on ionization chamber response with maximum deviations of less than ± 0.9% across all field sizes measured. Solid-state detectors showed an increased dependence on the flattening filter of up to ± 1.6%. Measured diode response was within ± 1.1% of the published mathematical relation for all fields up to 30 mm, independent of linac type and presence or absence of a flattening filter. For 6 MV beams, detector correction factors between FFF and FF beams are interchangeable for a linac between FF and FFF modes, providing that an additional uncertainty of up to ± 1.6% is accepted.
在立体定向放射外科（SRS）和立体定向消融放疗（SABR）中，逐渐采用不带准直器滤过的（FFF）射束，因为它可以提高剂量率并缩短治疗时间。本研究评估了 Elekta 和 Varian 医用直线加速器产生的 6MV 射线在不带准直器滤过和带准直器滤过两种情况下，对小射野输出因子的影响，并确定了两种射线在探测器响应方面的差异。使用一系列探测器（二极管、电离室、光致变色胶片和 microDiamond）测量相对输出因子，并与在 Chris O'Brien Lifehouse 悉尼研发的空气芯光纤剂量计（FOD）测量的相对输出因子进行参照。生成了两种射线的小射野校正因子。比较了二极管测量的探测器响应与最近发表的数学关系式，以预测小射野中二极管响应的校正。使用同一射野大小的 FFF 和 FF 射野的相对探测器响应比值，量化了去除准直器滤过片对探测器响应的影响。去除准直器滤过片对电离室响应有很小但可测量的影响，在所有测量的射野尺寸中，最大偏差小于±0.9%。固态探测器的响应与准直器滤过的依赖性增加，最大偏差高达±1.6%。在 30mm 以内的所有射野中，测量的二极管响应与发表的数学关系式相差在±1.1%以内，与直线加速器类型以及是否存在准直器滤过片无关。在 6MV 射线中，在 FF 和 FFF 模式之间切换直线加速器时，FFF 和 FF 射线之间的探测器校正因子可以互换，前提是接受高达±1.6%的附加不确定性。
