| ObjectMeasure 6MV Varian accelerator's Sc, Sp, transmission factor of shadow tray, transmission factor of shielding blocks using ESTRO rules, and obtain the A, Sc, Sp and data fitting function, which are basis of designing clinical dose calculation software.Methods1. Obtain Sc and ScpSc and Scp are measured for the full range of rectangular collimator settings corresponding to those generally used in the clinic, with the X- and Y-jaws set independently at, for example, 4, 5, 6, 7, 8, 10, 12, 15, 20, 25, 30, 35 and 40 cm, record the data of static electricity measuring instrument. In order to obtain Sc and Scp for square collimator settings, the field size is changed from 4cm ~* 4cm up to the maximum field size, usually 40cm ~* 40cm, with lcm interval. Then measure 10cm ~*10 cm square field, with 10 minutes interval.Sc is the ratio of the time series data to the square field data, which is gaincd in mini-phantom, Scp is the same ratio but gained in water phantom. 2. Obtain phantom scatter correction factor SpThe ratio of Scp and Sc is Sp of the field.3. Measurement of tray factor in air and in water phantomWith 4cm ~* 4cm square field, 5cm interval, up to 35cm '35cm, record data with tray and without tray by static electricity measuring instrument. The ratio of them is tray factor. The tray factor in air is the one measured in mini-phantom, and the one measured in water phantom is the factor in water.4. Measurement of transmission factor of shield block in air and in water phantomShield block transmission factor is the ratio of the data obtained without shield and the other one obtained with shield block of 3cm ~*3cm, 6cm~*6cm, 10cm~*10cm, 14cm~*14cm, up to 19cm ~* 19cm square field.5. Data processAll the data is fitted by VB software. First, obtain all Sc data of rectangular field, with 0.1cm interval, then, register square field Ce of rectangular field with the same Sc to obtain A.Result1. Sc: the main influent factor to Sc is the scatter of primary collimator and conical flattening filter.(1) Sc increases with the augment of field.(2) For the rectangular field of the same area, the Sc of lower jaws X is smaller than upper jaws Y, and increases with the field augment. Exchanging X and Y, the Sc is different. It is called the collimator effect, which caused by inner structure of VARIAN accelerator.2. The change of kt gained in mini-phantom and that one in water phantom is little.3. The parameter k_b(c,s_b) increases with the enlargement of field. Because of the shield block, the radial scatter in medicine and the scatter in phantom are changed. So the Sp in phantom increases with the field enlarges, kb(C,S_b) is same rise as Sp. Above all, the radial scatter in medicine and the scatter in phantom can process individually, kb(C,S_b) can be gained by measuring V(Z_R,S_b)/V(Z_R,c)4. The mean square error of Sc is the smallest by 6 rank fitting, which is used for searching A. At last, A is equal 1.862.5. Analysis by Matlab:(1) five group of square fields are given, six rank function is used to fitting measure data. The square field numbers is 8, 13, 16, 17 and 22 respectively. The maximal error is 0.58 percent, 0.47 percent, 0.56 percent, 0.33 percent, 0.27 percent separately. The number of square field is proposed to 13 or 17.(2) The influence of accurate of A is little to Sc. 0.1 is adequate. Kim's method is proposed to calculate A using Sc.ConclusionWe gives the method of measuring 6MV Varian accelerator's Sc, Sp, transmission factor of shadow tray, transmission factor of shielding blocks using ESTRO rules, and obtain the A, Sc, Sp and data fitting function, which can be used for practical dose calculation. |
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