Pisaturo Olivier, Miéville Frédéric, Tercier Pierre-Alain, Allal Abdelkarim Said
Department of Radiation Oncology, HFR - Hôpital Fribourgeois, Chemin des Pensionnats, Fribourg, Switzerland.
Med Phys. 2018 Jan;45(1):438-447. doi: 10.1002/mp.12672. Epub 2017 Dec 5.
TransitQA is an innovative method for Tomotherapy transit dosimetry using the on-board detector (OBD). Our previously published model for Tomotherapy treatment plan verification (AirQA) has been enhanced to take into account patient and couch transmission. AirQA estimates the OBD signal during irradiation with nothing in the beam path from the leaf control sinogram, allowing us to check whether the planned treatment is correctly delivered by the machine. TransitQA allows us to check the treatment delivery with the patient on the couch, potentially showing the effects of changes in the patient anatomy and delivery errors.
Patient and couch transmission have been added to the model using the OBD projections of pretreatment megavoltage computed tomography (MVCT). The difference in the energy spectra between the imaging and treatment beams has been corrected by an exponent from the MVCT projections consisting of the ratio of the mass attenuation coefficients. This exponent has been found to not vary significantly with the atomic number Z, allowing us to apply this procedure to heterogeneous media, such as patients. The attenuated OBD projections acquired during the treatment are compared to the model via a signed global γ-index analysis. The dose criterion was 5% of the 95 percentile of the dose distribution, and the distance to agreement (DTA) was 4 mm.
Our method has been applied to a heterogeneous phantom with 98.1% of the points passing the γ-evaluation test, showing that the model can predict the attenuated OBD projection. The method has been applied to two representative patients throughout the whole treatment, highlighting variations in the signal transmission and γ-index.
This paper establishes the proof-of-concept of transit dosimetry for all patients treated by Tomotherapy. Moreover, this method can be used as a surrogate for in vivo dosimetry.
TransitQA是一种利用机载探测器（OBD）进行螺旋断层放疗传输剂量测定的创新方法。我们之前发表的用于螺旋断层放疗治疗计划验证的模型（AirQA）已得到改进，以考虑患者和治疗床的透射情况。AirQA通过叶控制正弦图在射束路径中无任何物体时估计OBD信号，使我们能够检查机器是否正确执行了计划的治疗。TransitQA使我们能够在患者位于治疗床上时检查治疗执行情况，有可能显示患者解剖结构变化和治疗误差的影响。
利用治疗前兆伏级计算机断层扫描（MVCT）的OBD投影将患者和治疗床的透射情况添加到模型中。成像束和治疗束之间能谱的差异已通过由质量衰减系数之比组成的MVCT投影的指数进行校正。已发现该指数不会随原子序数Z显著变化，这使我们能够将此程序应用于异质介质，如患者。通过有符号全局γ指数分析将治疗期间获取的衰减OBD投影与模型进行比较。剂量标准为剂量分布第95百分位数的5%，一致性距离（DTA）为4毫米。
我们的方法已应用于一个异质体模，98.1%的点通过了γ评估测试，表明该模型可以预测衰减的OBD投影。该方法已应用于两名代表性患者的整个治疗过程，突出了信号传输和γ指数的变化。
本文确立了螺旋断层放疗所有患者传输剂量测定的概念验证。此外，该方法可作为体内剂量测定的替代方法。
