Journal of Medical Physics
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Year : 2003  |  Volume : 28  |  Issue : 1  |  Page : 12-17

Collimator Exchange Effect Of Multileaf Collimator System

Correspondence Address:
Sudhir Kumar

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Source of Support: None, Conflict of Interest: None

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In high energy x-ray beam therapy, number of monitor units for treatment of a patient is Calculated using empirical functions of central axis dosimetry parameters, such as percentage depth dose (PDD), tissue maximum ratio (TMR), and quantities that express the field size dependence of scattered radiation - the collimator scatter correction factor (Sc), the phantom scatter correction factor (SP) and the total scatter correction factor (Scp). For rectangular fields, the interchange of upper and lower collimator jaws affects the numerical value of collimator scatter factor which is commonly known as collimator exchange effect (CEE). Collimator exchange effect of Siemens MLC for 6 and 15 MV open photon beams was determined experimentally. The measurements were carried out using a PMMA mini phantom and an ionization chamber at the depth of dose maximum (dmax=1.5 cm), 5 and 10 cm for 6 MV photon beams and at the depth of dose maximum (dmax = 3.0 cm) and 10 cm for 15 MV photon beams, respectively. Results of these measurements indicate that the secondary collimator of Primus linac exhibits the collimator exchange effect. It is also observed that the value of Sc for those rectangular fields, the longer side of which are defined by y-jaws are higher than the value of corresponding rectangular field, the longer side of which are defined by x-jaws. The collimator exchange effect of this collimator system may therefore be attributed to the radiation backscattered to the monitor chamber by upper jaws of the secondary collimator. Further, as the elongation ratio (ratio of length and width of rectangular field) of the rectangular field increases, the percentage difference of Sc values between the two corresponding rectangular fields also increases in the beginning and then tends towards saturation. The maximum percentage difference between the Sc of two corresponding rectangular fields at 1.5,5.0 and 10.0 crn depths for 6 MV photons are 1.66,1.44 and 1.1 9%, respectively. While the maximum percentage difference between the Sc values of corresponding rectangular fields at 3.0 and 10.0 cm depths for 15 MV photons are 1.74 and 1.38%, respectively.

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