- 作者：Yuenan Wang ; Ph.D.&lowast ; ; yuenan.wang@gmail.com ; Mohammad K. Khan ; M.D. ; Ph.D.&dagger ; ; Joseph Y. Ting ; Ph.D.&lowast ; ; Stephen B. Easterling ; M.S.&lowast ;
- 关键词：Flattening filter free ; Surface dose ; Buildup region ; Skin dose
- 刊名：International Journal of Radiation Oncology*Biology*Physics
- 出版年：2012
- 期刊代码：136_03603016
- 类别：med
- 出版时间：1 June, 2012
- 卷：83
- 期：2
- 页码：e281-e285
- 文件大小：516 K
Purpose
Flattening filter-free (FFF) x-rays can provide more efficient use of photons and a significant increase of dose rate compared with conventional flattened x-rays, features that are especially beneficial for stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). The available data on the entrance doses of the FFF photon beams remain limited. The purpose of this study was to investigate the entrance dose of FFF photons in the buildup region and to compare it with that of conventional flattened photons.Methods and Materials
A Varian TrueBeam linear accelerator has been in full clinical operation with 6-MV and 10-MV FFF and flattened x-ray photons. Entrance dose at the surface was measured using a parallel plate ionization chamber in a solid water phantom with buildup depth = 0鈭?5 mm for 6X and 0鈭?5 mm for 10X. Different field size (FS) patterns were created in the Eclipse Treatment Planning System by multileaf collimator (MLC) rather than jaws (FS = 2 脳 2, 3 脳 3, 4 脳 4, 6 脳 6, and 10 脳 10 cm2 by MLC and jaw size = 2.2 脳 2.2, 3.2 脳 3.2, 4.2 脳 4.2, 6 脳 6, and 10 脳 10 cm2). The smallest FS was about four times larger than the ion chamber dimension. All buildup dose measurements were normalized to FS = 10 脳 10 cm2 at the depth of dose maximum (dmax).
Results
Good repeatability was demonstrated and surface dose increased linearly with FS for both flattened and FFF photons. The entrance dose of the FFF photons was modestly larger than that of the corresponding flattened photons for both 6X and 10X for different FS ranging from 2 脳 2 cm2 to 10 脳 10 cm2.
Conclusions
The FFF photons have a higher entrance dose than that of the corresponding flattened photons for FS smaller than 10 脳 10 cm2. However, the difference is not substantial and may be clinically insignificant.