Gazdag-Hegyesi Szilvia, Gáldi Ádám, Koszta Enikő, Stelczer Gábor, Szegedi Domonkos, Major Tibor, Pesznyák Csilla
Center of Radiotherapy, National Institute of Oncology, Budapest, Hungary.
Doctoral School of Physical Sciences, Budapest University of Technology and Economics, Budapest, Hungary.
Rep Pract Oncol Radiother. 2025 Mar 21;30(1):79-87. doi: 10.5603/rpor.104733. eCollection 2025.
The aim of this study is to investigate the beam width and beam profile of kilovoltage cone beam computed tomography (kV CBCT) using different measurement techniques on an O-ring linear accelerator. The effect of the imaging beam on the dosimetric parameters was analysed.
The uncertainty of field size adjustment, the dependence of beam width on field size, and the effect of deflection from the isocenter on the beam profile were investigated by ionization chamber detector matrices. The 2D beam profile of the CBCT was analysed by relative ionization chamber measurements.
The average setup uncertainties of the field sizes were 0.3 mm ± 0.02 mm. The dependence of beam width on field size investigation revealed that the largest discrepancies occurred for small field sizes, which are important in determining computed tomography dose index (CTDI) values of the kV CBCT. The beam width deviation between measured and vendor-based data was larger than 1 mm below 40 mm field of view. The pelvis protocol demonstrated the smallest CTDI value difference of 2.3%, yet presented the largest effective dose deviation of 0.12 mSv.
The measured CTDI coefficients were higher than predicted by the manufacturer for all cases. The currently internationally recommended CTDI measurement protocols for CBCT contain no reference to the determination of the beam width as a basic element of the calculations. Based on our measurement results, the beam width parameters affect CTDI: therefore, it would be advisable to apply this type of correction.
本研究旨在使用不同测量技术，在一台O型环直线加速器上研究千伏锥束计算机断层扫描（kV CBCT）的射束宽度和射束轮廓。分析了成像射束对剂量学参数的影响。
通过电离室探测器矩阵研究了射野大小调整的不确定性、射束宽度对射野大小的依赖性，以及与等中心的偏移对射束轮廓的影响。通过相对电离室测量分析了CBCT的二维射束轮廓。
射野大小的平均设置不确定性为0.3 mm±0.02 mm。射束宽度对射野大小的依赖性研究表明，小射野大小出现的差异最大，这在确定kV CBCT的计算机断层扫描剂量指数（CTDI）值时很重要。在视野小于40 mm时，测量数据与供应商数据之间的射束宽度偏差大于1 mm。骨盆扫描方案的CTDI值差异最小，为2.3%，但有效剂量偏差最大，为0.12 mSv。
在所有情况下，测量得到的CTDI系数均高于制造商的预测值。目前国际上推荐的CBCT的CTDI测量方案未提及将射束宽度的确定作为计算的基本要素。根据我们的测量结果，射束宽度参数会影响CTDI；因此，建议应用此类校正。
