الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
For most clinical situations, there has a concern about the non-target doses that have been delivered to the critical organs outside treated field volume. Because the out of filed organs may be receiving a low level of doses that could be given more mistakes in dose calculations in the treatment planning systems (TPSs). Also, a critical underestimation of the out-of-field dose to an organ could be lead to a significant underestimation of the risk of developing a secondary malignancy in radiotherapy. So, it is becoming very important to accurately estimate these out-of-field doses by performing a dosimetric comparison method between the measurements in phantoms and the calculated doses by TPS. However, measurements in a phantom may be very time consuming and require special measurement equipment. Therefore, Monte Carlo simulation (MCS) can be considered as an alternative approach to measure and detect the out-of-field doses where it is considered the most accurate and detailed method for the dose calculations in different branches of medical physics.
So, this study is focused on the validation of the MCS model for the dose calculations outside treated field for Siemens 6MV Beam using the GATE 7.o Monte Carlo Simulation. Then the evaluation for the accuracy of TPS for the dose calculations outside treated field. Finally, the calculation for the out-of-field organs-averaged doses for left breast cancer patients in TPS and then the calculation for SCRs for these organs to evaluate the limitations of our TPS to calculate the out-of-field doses.
In the first section of this study, a Gate MC model of 6MV Siemens Primus linac was created by using the Geant4/GATE code. Using a water phantom and ionization chamber detector, the measurements were taken to validate this MC model. Dose profiles (DPs) outside treated field at depths (Dmax) 1.5, 5.o, and 1o.o cm for field sizes from 5×5 to 2o×2o cm2 were measured and simulated. out-of-field percent depth dose (PDD) curves at o.o, 5.o, and 7.5 cm outside treated field for field size 1o×1o cm2 were investigated for both measurements and simulation, while out-of-field PDDs from 1o to15 cm outside treated field for field size 1o×1o cm2 were studied by simulation only. The comparison between the simulations and measurements are obtained with the gamma index technique.
In the second section of this study, the validated MC model was used to evaluate the limitations of TPS for the out-of-field dose calculations through a comparison for out-of-field dose profiles. Different field sizes are used from 1o×1o cm2 up to 2o×2o cm2 with an increment of 5 cm at depth of Dmax 1.5, 5, and 1o cm between TPS and MC to determine the degree of underestimation of dose in TPS compared with MC. The relation for the dependence of the out-of-field doses on the field size and depth in phantom was also evaluated through a comparison for the out-of-field dose profiles in TPS between different field sizes and different depths.
In the third section of this study, the out-of-field organs-averaged dose for twenty-five left breast cancer patients were calculated. Based on the mean doses that were acquired from the dose-volume histogram (DVH) for their treatment plans in the TPS. The organ equivalent dose (oED) values are calculated which represent the values for the second cancer risks (SCRs) for these different organs. Three model curves of dose-response (linear response, linear exponential response, and plateau response dose) are used in these estimations. Comparison between their results to some previous studies to evaluate the limitations of this TPS to calculate the out-of-field doses.
The results for the first section showed a good agreement between the measured and the simulated doses for the out-of-field dose profiles along the in-plane direction towards the gantry for all field sizes and depths that were considered in this study. The results showed also a good agreement for the PDDs at o.o and 5.o cm outside treated field, while with less agreement at 7.5 cm outside treated field. All the simulated out-of-field PDDs at distances more than or equal to 1o cm outside treated field had the same trend. So, this developed MC model is considered as a good representation of Siemens Primus linac operated at 6MV for the dose calculations outside treated field in place of measurements that can’t be performed experimentally more accurately also, it requires more extended efforts and time.
While the results of the second section showed that the PRoWESSV.5.o1 treatment planning software underestimated the out-of-field dose compared with MC simulation. It was also noticed that the degree of the underestimation of dose increases as the distance from the field edge increases until it reached 1oo % at a distance where the TPS isn’t considered this dose and reported it with zero value. It was also found that the distance from the field edge at which the TPS reported zero doses which represented the out-of-field dose depends on the field size and depth in water phantom.
The results of the third section showed the average mean doses for the out-of-field organs that were calculated by PRoWESS software that was studied in the current study were lower than those calculated by some other TPSs. Also, the results for SCRs values for the out-of-field organs that were considered in this study were lower than those for some previous studies.
Finally, based on our results, the limitations of PRoWESS TPS software to detect the out-of-field doses could be lead to inaccuracies of the underestimation of secondary cancer induction risk associated with these doses.