Renkamp C K, Eulenstein D, Sebald M, Schlüter F, Buchele C, Rippke C, Debus J, Klüter S
Department of Radiation Oncology, Heidelberg University Hospital, Heidelberg, Germany; Heidelberg Institute of Radiation Oncology (HIRO), National Center for Radiation Oncology (NCRO), Heidelberg, Germany.
PTW Freiburg GmbH, Freiburg, Germany.
Z Med Phys. 2024 Jan 5. doi: 10.1016/j.zemedi.2023.12.003.
Patient specific quality assurance (QA) in MR-Linacs can be performed with MR-compatible ion chamber arrays. However, the presence of a static magnetic field can alter the angular response of such arrays substantially. This works investigates the suitability of two ion chamber arrays, an air-filled and a liquid-filled array, for patient specific QA at a 0.35 T MR-Linac using a static phantom.
In order to study the angular response, the two arrays were placed in a static, solid phantom and irradiated with 9.96 × 9.96 cm fields every 10° beam angle at a 0.35 T MR-Linac. Measurements were compared to the TPS calculated dose in terms of gamma passing rate and relative dose to the central chamber. 20 patient specific quality assurance plans were measured using the liquid-filled array.
The air-filled array showed asymmetric angular response changes of central chamber dose of up to 18% and down to local 3 mm / 3% gamma rates of 20%, while only minor differences within 3% (excluding parallel irradiation and beams through the couch edges) were found for the liquid-filled ion chamber array without rotating the phantom. Patient plan QA using the liquid-filled array yielded a median local 3 mm / 3% 3D gamma passing rate of 99.8% (range 96.9%-100%).
A liquid-filled ionization chamber array in combination with a static phantom can be used for efficient patient specific plan QA in a single measurement set-up in a 0.35 T MR-Linac, while the air-filled ion chamber array phantom shows large angular response changes and has its limitations regarding patient specific QA measurements.
磁共振直线加速器（MR-Linac）中的患者特异性质量保证（QA）可通过与磁共振兼容的电离室阵列来执行。然而，静磁场的存在会显著改变此类阵列的角响应。本研究使用静态体模，调查了两种电离室阵列（一种充气式和一种充液式阵列）在0.35 T MR-Linac中用于患者特异性QA的适用性。
为了研究角响应，将这两种阵列置于静态固体体模中，并在0.35 T MR-Linac上以每10°束角用9.96×9.96 cm的射野进行照射。根据γ通过率和中心电离室的相对剂量，将测量结果与治疗计划系统（TPS）计算的剂量进行比较。使用充液式阵列测量了20个患者特异性质量保证计划。
充气式阵列显示中心电离室剂量的不对称角响应变化高达18%，局部3 mm/3%γ通过率低至20%，而充液式电离室阵列在不旋转体模的情况下，仅在3%以内发现微小差异（不包括平行照射和穿过治疗床边缘的射束）。使用充液式阵列进行患者计划QA时，局部3 mm/3%三维γ通过率的中位数为99.8%（范围96.9%-100%）。
充液式电离室阵列与静态体模相结合，可用于在0.35 T MR-Linac的单次测量设置中高效地进行患者特异性计划QA，而充气式电离室阵列体模显示出较大的角响应变化，并且在患者特异性QA测量方面存在局限性。
