Patwe Parimal T, Deshpande Sudesh S, Mahajan Gajanan R
School of Physical Sciences, Swami Ramanand Tirth Marathwada University, Nanded, Maharashtra, India.
Department of Radiation Oncology, P.D. Hinduja Hospital and Medical Research Centre, Mumbai, Maharashtra, India.
J Med Phys. 2024 Apr-Jun;49(2):279-284. doi: 10.4103/jmp.jmp_136_23. Epub 2024 Jun 25.
Modern generation linear accelerator (linac) either generates X-rays with a flattening filter (WFF beam) or without flattening filter free (FFF beam). The FFF beams are associated with a significantly higher dose per pulse compared to WFF beams due to the absence of a flattening filter and the corresponding attenuation caused by it. This results in increased ion recombination and a larger saturation correction factor (k). In accordance with the IAEA TRS 398 dosimetry protocol, k is necessary for the accurate measurement of absorbed dose at a point in water. The objective of this study was to evaluate the k for the indigenous FAR 65-GB ion chamber (IC) for the FFF X-rays.
The study was carried out on TrueBeam linac (Varian, A Siemens Healthineers company) which offers 6 MV WFF, 6 MV FFF, 10 MV WFF and 10 MV FFF beams. The two-voltage method was employed to measure k in a solid water phantom at a depth of 10 cm for a FAR 65-GB and SNC 600c and 0.6cc PTW 30013 Farmer chambers at 100 cm and 150 cm source-to-chamber distances for a 10 cm × 10 cm field size.
The k values for the FAR 65-GB, PTW 30,013, and SNC 600c were 1.0055 (1.0113), 1.0051 (1.0071), and 1.0033 (1.0066) for the 6 MV WFF (FFF) beams, respectively, and 1.0066 (1.0178), 1.0061 (1.0137), and 1.0035 (1.0119) for the 10MV WFF (FFF) beams, respectively. The k values calculated by two-voltage method matches with k values obtained from Jaffe's plot. The chamber exhibited a linear dose-response up to 3000 cGy, beyond which a saturation effect was observed.
Our study reveals that this chamber is suitable for the reference dosimetry for the FFF beams.
现代直线加速器（linac）可产生带有均整器的X射线（宽均整野射线束，WFF束）或无均整器的X射线（无均整野射线束，FFF束）。由于没有均整器及其相应的衰减，FFF束与WFF束相比，每个脉冲的剂量显著更高。这导致离子复合增加以及饱和校正因子（k）增大。根据国际原子能机构（IAEA）TRS 398剂量测定协议，k对于准确测量水中某点的吸收剂量是必要的。本研究的目的是评估国产FAR 65 - GB电离室（IC）对FFF X射线的k值。
本研究在TrueBeam直线加速器（瓦里安公司，西门子医疗集团旗下公司）上进行，该加速器提供6 MV的WFF、6 MV的FFF、10 MV的WFF和10 MV的FFF束。采用双电压法在固体水模体中10 cm深度处测量FAR 65 - GB、SNC 600c以及0.6 cc的PTW 30013 Farmer电离室在源至电离室距离为100 cm和150 cm、射野尺寸为10 cm×10 cm时的k值。
对于6 MV的WFF（FFF）束，FAR 65 - GB、PTW 30013和SNC 600c的k值分别为1.0055（1.0113）、1.0051（1.0071）和1.0033（1.0066）；对于10 MV的WFF（FFF）束，k值分别为1.0066（1.0178）、1.0061（1.0137）和1.0035（1.0119）。通过双电压法计算的k值与从贾菲图获得的k值相符。该电离室在剂量高达3000 cGy时呈现线性剂量响应，超过此剂量则观察到饱和效应。
我们的研究表明，该电离室适用于FFF束的参考剂量测定。
