背景:水体模需要执行参考剂量测定和光束质量测量,但没有关于此类体模的尺寸要求的公开研究。
目的:调查,使用蒙特卡罗技术,用于参考剂量测定和/或测量光束质量说明符%d(10)x$\\%dd(10)_{\\sfx}$和TPR1020$TPR^{20}_{10}$的水体模的尺寸要求。
方法:EGSnrc应用程序DOSXYZnrc用于计算D(10)$D(10)$,在入射10×10cm2$10的水模型中,在10cm深度处的每个入射通量的剂量,\\次\\,10\\,{\\rm{cm}}^{2}$光束,含60个Co$^{60}{\\rm{Co}}$或6个MV光子。水幻影尺寸从30×30×40厘米3$30不等,\\次\\,30\\,\\次\\,40\\,{\\\rm{cm}}^3$至15×15×22cm3$15\\,\\次\\,15\\,\\次\\,22\\,{\\rm{cm}}^3$,偶尔较小。%dd(10)x$\%dd(10)_{\\sfx}$和TPR1020$TPR^{20}_{10}典型的统计不确定性为0.03%。
结果:对于大于20×20×25cm3$20\\的幻影,幻影尺寸仅有较小的影响,\\次\\,20\\,\\次\\,25\\,{\\rm{cm}}^3$。10×10cm2$10$10中10cm深度剂量校正表,\\次\\,10\\,{\\rm{cm}}^{2}$束60Co$^{60}{\\rm{Co}}$束或6个MV光子,对于60Co$^{60}{\\rm{Co}}$束入射到20×20×15cm3$20\\\\次\\,20\\,\\次\\,15\\,{\\rm{cm}}^3$幻影。使用方法A或方法B测量的TPR1020$TPR^{20}_{10}$值可能存在明显差异,特别是对于较小的幻影。它明确表明,在±$\\pm$0.15%内,TPR1020$TPR^{20}_{10}$30×30×30cm3$30\\,\\次\\,30\\,\\次\\,30\\,{\\rm{cm}}^3$使用方法A或B测量的体模与40到200cm之间的源探测器距离无关。
结论:TG-51和IAEATRS-398参考剂量测定方案中推荐的体模尺寸足以进行准确的参考剂量测定,在某些情况下甚至是保守的。校正因子对于在较小体模中10cm深度处的剂量的精确测量是必要的,并且提供了这些因子。参考剂量测定本身不需要非常精确的光束质量说明符,但是为了指定光束稳定性和特性,重要的是指定体模尺寸以及用于TPR1020$TPR^{20}_{10}$测量的方法。
BACKGROUND: Water phantoms are required to perform reference dosimetry and beam quality measurements but there are no published studies about the size requirements for such phantoms.
OBJECTIVE: To investigate, using Monte Carlo techniques, the size requirements for water phantoms used in reference dosimetry and/or to measure the beam quality specifiers % d d ( 10 ) x $\\%dd(10)_{\\sf x}$ and T P R 10 20 $TPR^{20}_{10}$ .
METHODS: The EGSnrc application DOSXYZnrc is used to calculate D ( 10 ) $D(10)$ , the dose per incident fluence at 10 cm depth in a water phantom irradiated by incident 10 × 10 cm 2 $10\\,\\times \\,10 \\, {\\rm {cm}}^{2}$ beams of 60 Co $^{60}{\\rm {Co}}$ or 6 MV photons. The water phantom dimensions are varied from 30 × 30 × 40 cm 3 $30 \\,\\times \\, 30 \\,\\times \\, 40 \\, {\\rm {cm}}^3$ to 15 × 15 × 22 cm 3 $15 \\,\\times \\, 15 \\,\\times \\, 22 \\, {\\rm {cm}}^3$ and occasionally smaller. The % d d ( 10 ) x $\\%dd(10)_{\\sf x}$ and T P R 10 20 $TPR^{20}_{10}$ values are also calculated with care being taken to distinguish T P R 10 20 $TPR^{20}_{10}$ results when using Method A (changing depth of water in phantom) and Method B (moving entire phantom). Typical statistical uncertainties are 0.03%.
RESULTS: Phantom dimensions have only minor effects for phantoms larger than 20 × 20 × 25 cm 3 $20 \\,\\times \\, 20 \\,\\times \\, 25 \\, {\\rm {cm}}^3$ . A table of corrections to the dose at 10 cm depth in 10 × 10 cm 2 $10 \\,\\times \\, 10 \\, {\\rm {cm}}^{2}$ beams of 60 Co $^{60}{\\rm {Co}}$ or 6 MV photons are provided and range from no correction to 0.75% for a 60 Co $^{60}{\\rm {Co}}$ beam incident on a 20 × 20 × 15 cm 3 $20 \\,\\times \\, 20 \\,\\times \\, 15 \\, {\\rm {cm}}^3$ phantom. There can be distinct differences in the T P R 10 20 $TPR^{20}_{10}$ values measured using Method A or Method B, especially for smaller phantoms. It is explicitly demonstrated that, within ± $\\pm$ 0.15%, T P R 10 20 $TPR^{20}_{10}$ values for a 30 × 30 × 30 cm 3 $30 \\,\\times \\, 30 \\,\\times \\, 30 \\, {\\rm {cm}}^3$ phantom measured using Method A or B are independent of source detector distance between 40 and 200 cm.
CONCLUSIONS: The phantom sizes recommended in the TG-51 and IAEA TRS-398 reference dosimetry protocols are adequate for accurate reference dosimetry and in some cases are even conservative. Correction factors are necessary for accurate measurement of the dose at 10 cm depth in smaller phantoms and these factors are provided. Very accurate beam quality specifiers are not required for reference dosimetry itself, but for specifying beam stability and characteristics it is important to specify phantom sizes and also the method used for T P R 10 20 $TPR^{20}_{10}$ measurements.