Godson Henry Finlay, Manickam Ravikumar, Saminathan Sathiyan, Ganesh Kadirampatti Mani, Ponmalar Retna
Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Dr. M.H. Marigowda Road, Bangalore, 560 029, India.
Department of Radiotherapy, Christian Medical College, Vellore, India.
Radiol Phys Technol. 2017 Jun;10(2):195-203. doi: 10.1007/s12194-016-0385-y. Epub 2016 Dec 1.
Intensity-modulated radiation therapy (IMRT) requires a patient-specific quality assurance (QA) program to validate the treatment plan and a high level of dosimetric accuracy in the treatment delivery. Dosimetric verification generally consists of both absolute- and relative-dose measurements in a phantom using ionization chambers. Measurements were carried out with three different ionization chambers (Scanditronix FC 65G, Exradin A18, and PTW PinPoint 31014) to assess the effects of influence quantities such as the stability, pre- and post-irradiation leakage, stem effect, polarity, and ion recombination on the IMRT point-dose verification with two different orientations. The Exradin A18 and PTW PinPoint ion chambers demonstrated noticeable leakage to magnitudes of 0.6 and 1.2%, whereas negligible leakage was observed with FC 65G ion chamber. Maximum deviations of 0.5 and 0.6% were noticed for the smallest field owing to the ion recombination effect with the PTW PinPoint ion chamber in the parallel and perpendicular orientations, respectively. The calculated total uncertainties of all influence quantities for the FC 65G, A18, and PTW PinPoint ion chambers were 0.5, 0.7, and 1.3%, respectively. The uncertainties determined for each chamber were incorporated into the point-dose measurements of 30 head and neck patient-specific QA plans, and the variation was found to be within ±3%. The magnitude of the leakage in a small-volume ion chamber indicated the significance of incorporating the correction factors in the absolute-dose measurement. A paired t test analysis indicated that the influence quantities significantly affect the point-dose measurements in the patient-specific IMRT QA.
调强放射治疗（IMRT）需要一个针对患者的质量保证（QA）程序来验证治疗计划，并在治疗实施过程中确保高水平的剂量准确性。剂量验证通常包括使用电离室在模体中进行绝对剂量和相对剂量测量。使用三种不同的电离室（Scanditronix FC 65G、Exradin A18和PTW PinPoint 31014）进行测量，以评估诸如稳定性、辐照前后泄漏、杆效应、极性和离子复合等影响量对两种不同方向的IMRT点剂量验证的影响。Exradin A18和PTW PinPoint电离室的泄漏量分别达到了0.6%和1.2%，较为明显，而FC 65G电离室的泄漏量可忽略不计。由于在平行和垂直方向上PTW PinPoint电离室存在离子复合效应，最小射野的最大偏差分别为0.5%和0.6%。FC 65G、A18和PTW PinPoint电离室所有影响量的计算总不确定度分别为0.5%、0.7%和1.3%。将每个电离室确定的不确定度纳入30个头部和颈部患者特定QA计划的点剂量测量中，发现变化在±3%以内。小体积电离室中的泄漏量表明在绝对剂量测量中纳入校正因子的重要性。配对t检验分析表明，这些影响量对患者特定IMRT QA中的点剂量测量有显著影响。
