Departamento de Física de Partículas, Universidad de Santiago de Compostela, Spain.
Med Phys. 2012 Apr;39(4):1964-70. doi: 10.1118/1.3692181.
Recently, an international working group on nonstandard fields presented a new formalism for ionization chamber reference dosimetry of small and nonstandard fields [Alfonso et al., Med. Phys. 35, 5179-5186 (2008)] which has been adopted by AAPM TG-148. This work presents an experimental determination of the correction factors for reference dosimetry with an Exradin A1SL thimble ionization chamber in a TomoTherapy unit, focusing on: (i) machine-specific reference field, (ii) plan-class-specific reference field, and (iii) two clinical treatments.
Ionization chamber measurements were performed in the TomoTherapy unit for intermediate (machine-specific and plan-class-specific) calibration fields, based on the reference conditions defined by AAPM TG-148, and two clinical treatments (lung and head-and-neck). Alanine reference dosimetry was employed to determine absorbed dose to water at the point of interest for the fields under investigation. The corresponding chamber correction factors were calculated from alanine to ionization chamber measurements ratios.
Two different methods of determining the beam quality correction factor k(Q,Q(0) ) for the A1SL ionization chamber in this TomoTherapy unit, where reference conditions for conventional beam quality determination cannot be met, result in consistent values. The observed values of overall correction factors obtained for intermediate and clinical fields are consistently around 0.98 with a typical expanded relative uncertainty of 2% (k = 2), which when considered make such correction factors compatible with unity. However, all of them are systematically lower than unity, which is shown to be significant when a hypothesis test assuming a t-student distribution is performed (p=1.8×10(-2)). Correction factors k(Q(clin),Q(pcsr) ) (f(clin),f(pcsr) ) and k(Q(clin),Q(msr) ) (f(clin),f(msr) ), which are needed for the computation of field factors for relative dosimetry of clinical beams, have been found to be very close to unity for two clinical treatments.
The results indicate that the helical field deliveries in this study (including two clinical fields) do not introduce changes on the ion chamber correction factors for dosimetry. For those two specific clinical cases, ratios of chamber readings accurately represent field output factors. The values observed here for intermediate calibration fields are in agreement with previously published data based on alanine dosimetry but differ from values recently reported obtained via radiochromic dosimetry.
最近，一个非标准场的国际工作组提出了一种新的 formalism 用于小尺寸和非标准场的电离室参考剂量学[Alfonso 等人，医学物理学 35,5179-5186(2008)]，该 formalism 已被 AAPM TG-148 采用。本工作通过在 TomoTherapy 装置中使用 Exradin A1SL 测径器电离室，对参考剂量学的校正因子进行了实验测定，重点研究了：（i）机器特定参考场，（ii）计划类特定参考场，以及（iii）两种临床治疗。
根据 AAPM TG-148 定义的参考条件，在 TomoTherapy 装置中对中间（机器特定和计划类特定）校准场进行了电离室测量，并对两种临床治疗（肺部和头颈部）进行了测量。采用丙氨酸参考剂量学来确定感兴趣点的水吸收剂量。通过对测径器和丙氨酸测量值的比值，计算相应的腔室校正因子。
对于 TomoTherapy 装置中的 A1SL 电离室，有两种不同的方法可以确定束质校正因子 k(Q,Q(0) )，因为无法满足常规束质确定的参考条件，所以得到的结果是一致的。对于中间和临床场获得的整体校正因子的观测值，其值约为 0.98，典型的扩展相对不确定度为 2%（k=2），这使得这些校正因子与 1 相兼容。然而，所有这些值都系统地低于 1，当使用假设 t 分布的假设检验（p=1.8×10(-2)）时，这是显著的。对于临床束的相对剂量学计算场因子所需的校正因子 k(Q(clin),Q(pcsr) )(f(clin),f(pcsr) )和 k(Q(clin),Q(msr) )(f(clin),f(msr) )，对于两种临床治疗，发现其非常接近 1。
结果表明，在本研究中（包括两种临床场），螺旋场输送不会对电离室剂量学校正因子产生影响。对于这两个特定的临床病例，腔室读数的比值准确地代表了场输出因子。这里观察到的中间校准场的值与基于丙氨酸剂量学的先前发表的数据一致，但与最近通过放射色剂量学获得的值不同。
