Pivot Odran, Pittet Patrick, Clackdoyle Rolf, Desbat Laurent, Rit Simon
Univ. Grenoble Alpes, CNRS, UMR 5525, VetAgro Sup, Grenoble INP, TIMC, F-38000 Grenoble, France.
Institut des Nanotechnologies de Lyon (INL), CNRS UMR5270, Université de Lyon, Université Claude Bernard Lyon 1, F-69100, France.
Phys Med Biol. 2023 Apr 26;68(9). doi: 10.1088/1361-6560/acc925.
. Patient-specific Quality Assurance (QA) measurements are of key importance in radiotherapy for safe and efficient treatment delivery and allow early detection of clinically relevant errors. Such QA processes remain challenging to implement for complex Intensity Modulated Radiation Therapy (IMRT) radiotherapy fields delivered using a multileaf collimator (MLC) which often feature small open segments and raise QA issues similar to those encountered in small field dosimetry. Recently, detectors based on long scintillating fibers have been proposed to measure a few parallel projections of the irradiation field with good performance for small field dosimetry. The purpose of this work is to develop and validate a novel approach to reconstruct MLC-shaped small irradiation fields from six projections.. The proposed field reconstruction method uses a limited number of geometric parameters to model the irradiation field. These parameters are iteratively estimated with a steepest descent algorithm. The reconstruction method was first validated on simulated data. Real data were measured with a water-equivalent slab phantom equipped with a detector made of 6 scintillating-fiber ribbons placed at 1 m from the source. A radiochromic film was used to acquire a reference measurement of a first dose distribution in the slab phantom at the same source-to-detector distance and the treatment planning system (TPS) provided the reference for another dose distribution. In addition, simulated errors introduced on the delivered dose, field location and field shape were used to evaluate the ability of the proposed method to efficiently identify a deviation between the planned and delivered treatments.. For a first small IMRT segment, 3%/3 mm, 2%/2 mm and 2%/1 mm gamma analysis conducted between the reconstructed dose distribution and the dose measured with radiochromic film exhibited pass rates of 100%, 99.9% and 95.7%, respectively. For a second and smaller IMRT segment, the same gamma analysis performed between the reconstructed dose distribution and the reference provided by the TPS showed pass rates of 100%, 99.4% and 92.6% for the 3%/3 mm, 2%/2 mm and 2%/1 mm gamma criteria, respectively. Gamma analysis of the simulated treatment delivery errors showed the ability of the reconstruction algorithm to detect a 3% deviation between the planned and delivered doses, as well as shifts lower than 7 mm and 3 mm when considering an individual leaf and a whole field shift, respectively.. The proposed method allows accurate tomographic reconstruction of IMRT segments by processing projections measured with six scintillating-fiber ribbons and is suitable for water-equivalent real-time small IMRT segments QA.
患者特异性质量保证（QA）测量在放射治疗中对于安全有效地进行治疗以及早期发现临床相关误差至关重要。对于使用多叶准直器（MLC）进行的复杂调强放射治疗（IMRT）放射治疗野，此类QA过程的实施仍然具有挑战性，这些野通常具有小的开放段，并且会引发与小野剂量学中遇到的问题类似的QA问题。最近，基于长闪烁光纤的探测器已被提出用于测量照射野的一些平行投影，在小野剂量学方面具有良好的性能。这项工作的目的是开发并验证一种从六个投影重建MLC形状的小照射野的新方法。所提出的野重建方法使用有限数量的几何参数来模拟照射野。这些参数通过最速下降算法进行迭代估计。该重建方法首先在模拟数据上进行了验证。使用配备有由6条闪烁光纤带组成的探测器的水等效平板模体在距源1 m处测量实际数据。使用放射变色胶片在相同源到探测器距离下获取平板模体中首次剂量分布的参考测量值，并且治疗计划系统（TPS）提供另一个剂量分布的参考。此外，在输送剂量、野位置和野形状上引入的模拟误差用于评估所提出的方法有效识别计划治疗与实际治疗之间偏差的能力。对于第一个小IMRT段，在重建剂量分布与用放射变色胶片测量的剂量之间进行的3%/3 mm、2%/2 mm和2%/1 mm伽马分析的通过率分别为100%、99.9%和95.7%。对于第二个更小的IMRT段，在重建剂量分布与TPS提供的参考之间进行的相同伽马分析对于3%/3 mm、2%/2 mm和2%/1 mm伽马标准的通过率分别为100%、99.4%和92.6%。对模拟治疗输送误差的伽马分析表明，重建算法能够检测出计划剂量与实际剂量之间3%的偏差，以及在考虑单个叶片和整个野移位时分别低于7 mm和3 mm的移位。所提出的方法通过处理用6条闪烁光纤带测量的投影，允许对IMRT段进行精确的断层重建，并且适用于水等效实时小IMRT段的QA。
