Chaudhary Rahul Kumar, Pathan Munir, Kumar Rajesh, Sharma S D, Sapra B K
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India.
Homi Bhabha National Institute, Mumbai, Maharashtra, India.
J Med Phys. 2021 Jan-Mar;46(1):26-32. doi: 10.4103/jmp.JMP_94_20. Epub 2021 May 5.
Modern radiotherapy modalities, such as Intensity-Modulated Radiotherapy and Volumetric Modulated Arc Therapy involve complex dose delivery. The dose delivery is complex as it involves beam modulation, hence, manual dose calculations for these techniques are not possible. Film dosimetry is commonly used method of dose verification for these modalities because of the advantages associated with it. The quantification of uncertainty associated with a film dosimetry system under clinical use becomes important for accurate dosimetry. The spread in the distribution of the pixel values (PV) of the irradiated film contributes to the uncertainty. The probability distribution (PD) of the PV was studied for the clinical photon beam energies of 6, 10, and 15 MV.
Gafchromic EBT3 film and EPSON 10000XL flatbed scanner were used for this purpose and using the resulting PD, the uncertainty budgets for these energies in the red, green and blue color channels were estimated.
The PV of exposed films for the energies studied follows t-distribution, the sum of the squares of the deviation of the measured data from the fitted value was of the order of 10, this indicates the goodness of fit. The "" value corrected combined standard uncertainty (CSU) at 1σ confidence level for exposed film and dose measurement at 200 cGy were 1.42%, 1.48%, and 1.63% and 1.99%, 3.23%, and 5.08% for 6, 10, and 15 MV energies, respectively, in the red colour channel.
In the case of the limited number of measurements of a quantity, the SU values must be corrected using the "t" value to get the correct CSU.
现代放射治疗方式，如调强放射治疗和容积调强弧形治疗，涉及复杂的剂量输送。剂量输送很复杂，因为它涉及射束调制，因此，无法对这些技术进行手动剂量计算。由于与之相关的优势，胶片剂量测定法是这些治疗方式常用的剂量验证方法。对于准确的剂量测定而言，临床使用的胶片剂量测定系统相关不确定性的量化变得很重要。照射后胶片像素值（PV）分布的离散性导致了不确定性。研究了6、10和15兆伏临床光子束能量下PV的概率分布（PD）。
为此使用了Gafchromic EBT3胶片和爱普生10000XL平板扫描仪，并利用所得的PD估计了这些能量在红、绿和蓝颜色通道中的不确定性预算。
所研究能量下照射后胶片的PV遵循t分布，测量数据与拟合值偏差的平方和约为10，这表明拟合良好。在红色通道中，对于200厘戈瑞的照射后胶片和剂量测量，6、10和15兆伏能量在1σ置信水平下的“t”值校正合并标准不确定度（CSU）分别为1.42%、1.48%和1.63%，以及1.99%、3.23%和5.08%。
在对一个量的测量次数有限的情况下，必须用“t”值校正标准不确定度（SU）值以得到正确的CSU。
