My study

Development of log file based Dose evaluation method for patient-specific QA in carbon-ion radiotherapy
  

1. Introduction 

 In radiotherapy that enables irradiation of complex shapes such as Intensity modulated radiation therapy(IMRT) and particle beam therapy, a patient-specific quality assurance (QA) to validate the treatment planning system (TPS) is important on view point of quality assurance and quality control. In carbon ion radiotherapy (CIRT) with a scanning delivery system at National Institutes for Quantum and Radiological Science and Technology(QST) and National Institute of Radiological Sciences (NIRS), As patient-specific QA, The dose distribution calculated by replacing the treatment planning beam calculated on patient CT with water phantom is compared with the measurement using the 2D ion chamber(2D-array) by 𝛾 index analysis with criteria 3% and 3mm and pass rate of 90%. However, Since it takes time and effort for patient-specific QA, it is necessary to carry out the measurement efficiently while ensuring the quality of treatment as the number of patient increases. In order to solve that, in proton and photon therapy, patient-specific QA using Monte Carlo (MC) calculation using irradiation log files has been proposed, but not CIRT.  The aim of this study is to develop for the log file based MC calculation method for patient-specific QA in carbon ion radiotherapy with a scanning delivery system at QST/NIRS. In this study, the calculation method was established and experimentally verified using CIRT for ocular choroidal malignant melanoma with a small irradiation field as an example.

2. Materials and Methods 

Particle and Heavy-ion Transport Code System (PHITS) version 3.20 was used in this study. The source parameters in the MC calculation were determined from beam energy, beam position, beam fluence at each beam spot recorded in the irradiation log file. In addition, the beam spread (intensity distribution: f(x,y) ) for each spot was determined based on the three-component Gaussian model from radiation source data on treatment planning system. 4) f(x,y) is described as

 𝑓(𝑥, 𝑦) = 𝐼𝑐𝑜𝑢𝑛𝑡 ∑𝑤𝑖 3 𝑖=1 1 2𝜋𝜎𝑖𝑥𝜎𝑖𝑦 𝑒𝑥𝑝(− (𝑥 − 𝑥𝑏) 2 2𝜎𝑖𝑥 2 )𝑒𝑥𝑝(− (𝑦 − 𝑦𝑏) 2 2𝜎𝑖𝑦 2 )   (1) 

 where 𝐼𝑐𝑜𝑢𝑛𝑡 is the weight of each spot energy calculated form the monitor count, 𝑥𝑏and 𝑦𝑏 are the beam position, 𝜎1, 𝜎2 and 𝜎3 are the beam size of the three components.𝑤 is the ratio of 𝜎1 , 𝜎2 and 𝜎3 from the source data. In this study, the calculated lateral dose and depth dose distribution were compared with measurement. The experiment was implemented in treatment room G of the new treatment facility, and the lateral dose distribution was measured by the GAF chromic EBT3 films and pinpoint chamber, and the depth dose distribution was measured by the Bragg peak chamber.The irradiation fields used in this study were shown in table1.

3. Results and Discussion 

The comparison of Off-center ratio(OCR) on F-190 were shown in Fig 1(a). There were good agreement between phits and measurement on lateral dose distribution. The comparison of relative dose were shown in Fig 1(b). There were good agreement between phits and measurement on depth dose distribution. Furthermore, the peak dose position were compared. Highly accurate agreement was confirmed for the peak position difference under all conditions.

 4. Conclusion 

 In this study, We experimentally verified the constructed calculation method for the development of MC calculation method using the irradiation logfile for patient0specific QA in CIRT. The all calculation have a good agreement with measurement, indicating that the developed calculations may be used for patient QA in the future. As a next challenges, it is necessary to compare with the planned dose distribution (RT-dose) and to compare by the γ analysis method.