Comparison Of Monitor Unit Calculation Methods For High Energy Photon Beams

Currently, there are mainly two kinds of monitor unit calculation methods for high energy photon beams. The representative of methods using dmax as the reference depth is that introduced in the textbook of Faiz Khan and that of Yimin Hu. The representative of methods using 10cm as the reference depth is that introduced in ESTRO publications of ESTRO Booklet No 3 and Booklet No 6. dmax methods are gradually replaced by d10 ones in developed countries, but they are still widely used in China. The main aim of this research work is to introduce the d10 methods to Chinese radiation therapy community and to compare the differences between the two kinds of methods.In the work, the differences were investigated by comparing between the calculation results using the both methods and the measurements results of a same group of test fields. During the beam data collection, a tool easy to use and various phantoms including buildup caps for head scatter measurement were designed and developed. The work checked the characters of data collection systems as well as the reliablity of the collected data. The advantages and disadvantages of the both methods in different kinds of data collection were also reviewed. During the calculations and the measurements, the deviations grouped by beam conditions were discussed; the factors influencing the dmax method calculation results were studied independently. At the same time, the reason why the d10 method has the same accuracy with dmax method in conventional open fields was explored too.The results show that MUs calculated with d10 method tallied with the measurement much better than those calculated with dmax method for complex fields. After necessary correction, dmax method fit the measurement well too. The regularity of head scatter factors varying with phantom sizes was also studied. The work shows that the phantom scatter factors listed in published papers cannot be directly used with the dmax methods. And this work confirms the influence of the collimator jaw exchange effect in wedged fields, as well as the deficiency in MU calculation for irregular blocked fields by area-perimeter-ratio method comparing with Day method. The normalization depth doses not show its improvement on the calculation accuracy in d10 method.In conclusion, the d10 method has the advantages over the dmax method. It provides a systematic solution to the problem of monitor-unit calculations in clinic. It considers multiple factors affecting the accuracy of MU calculations, and has a comprehensive protocol for data collection and measurements.