The Physical Model Validation And Clinical Application Study Of The 3D In Vivo Dose Monitor System

With the rapid progress of the radiation therapy technology, precision radiotherapy has become the current stat-of- art means of radiation treatment. The exceptionally high standard for precision and accuracy of the treatment brings much higher risk of potential errors in dose delivering of treatment, thus requires a higher standard for quality control and quality assurance of radiotherapy. Based on EPID(Electronic Portal Image Device),a 3D in vivo dose monitor system was proposed to effectively reduced risk of potential radiotherapy errorsby checking the dose difference between the planed dose with the actual dose that may be caused bythe errors from hardware and softwareor variationsfrom patients.The main contents of this study include: 1) establishment of the physical model of the 3D in vivo dose monitor system based on EPID, and comparing the data obtained from this model with those data measured from 3D water tank; 2)examination and verification of 3D dose reconstruction accuracy of this 3D in vivo dose monitor system based on EPID in homogeneous and inhomogeneous phantoms; 3)the3D in vivo dose monitor has been performed in clinical IMRT treatment. The results were analyzed; 4) theclinical application of this 3D in vivo dose monitor system was explored in the adaptive radiotherapy of nasopharyngeal carcinoma.Compared with the absolute dose measured by ionization chamber, the deviation of the reconstructed by 3D in vivo dose monitor system based on EPID was(0.12±0.91)%in a homogeneous phantom.In a Delta4 phantom,the dose distributionsmeasured by Delta4 detectorsand reconstructed by this 3D in vivo dose monitor systemcompared withcalculated by TPS(Treatment planning system), the γ pass rates, with aγcriteria of 3%/3mm(a 3% dose-difference and a 3 mm distance-to-agreementacceptance) were(94.18±1.69)% and(93.82±1.19)% respectively. No significant difference in theγ pass rates,(t=0.295,p=0.783). In inhomogeneous phantom, for the absolute dose measured by ionization chamber and reconstructed by 3D in vivo dose monitor system based on EPID, the deviation of doses was(0.03±0.85)%. For the dose distributions measured by film compared with reconstructed by 3D in vivo dose monitor system based on EPID, the γ pass rate, with aγcriteria of 3%/3mm, was(95.24±1.62)%.Therefore, the 3D dose reconstruction accuracy by this 3D in vivo dose monitor systembased on EPID was reliable and it can be used for clinical purposes.For the in vivo dose verification in the clinical treatment of 12 head and neck cases, the deviation between the absolute dose reconstructed by 3D in vivo dose monitorsystem and calculated by TPS was(0.75±1.53)%. The γ pass rates of the dose distributions reconstructed by the 3D in vivodose monitor system compared with calculated by TPS, with aγcriteria of 3%/3mm and 5%/3mm, were(89.11±3.24)% and(96.40±1.47)% respectively.The in vivo dose verification of the clinical treatment was relatively complex, thus the dose distributions difference and the goodness of the plan cannot be evaluated only based on γ pass rate. The DVH based on patient’s structures can more effectively evaluate the influence of the dose difference between the planed doses with the actual measurements.To our knowledge, in china this is the very first study to use the 3D in vivo dose monitor system based on EPID in the adaptive radiotherapy of nasopharyngeal carcinoma. Our results showed that the best time for the adaptive radiotherapy of nasopharyngeal carcinoma is NO.20 fraction of treatments.The 3D in vivo dose monitor system based on EPID can accurately detected the actual dose changes in the OARs(Organs at Risk) and treatment targets, and thus may aid in determining the time for adaptive intervention.