Determination of effective doses in image-guided radation therapy system

Abstract

This study determined the imaging doses starting from the planning to the verification process involved in IGRT for clinical protocols of head, chest and pelvis regions. The dose calculation from Varian Acuity simulator and on-board image were performed to obtain the entrance surface air kerma (ESAK). The product of ESAK with the conversion coefficient was resulted as the effective dose. The organ doses from electronic portal imaging device (EPID) were calculated by Varian Eclipse treatment planning system. The tissue weighting factors were applied according to the ICRP 103 for calculating the total effective dose. For the 3D imaging system, computed tomography dose index (CTDI) and cone-beam CT dose index (CBDI) measurements were performed using a 100 mm pencil ionization chamber with Accu-Pro dosimeter and CTDI phantoms. Then, effective doses were calculated by ImPACT software. Moreover, the organ and effective doses from CBCT pelvic protocol was measured by using the thermoluminescent dosimeters inserted in Rando phantom to verify the accuracy of ImPACT calculation. The total effective doses from planning imaging modalities for head, chest and pelvic protocols were 0.03, 0.31 and 0.43 mSv, respectively, from the 2D conventional simulator and 3.3, 13 and 7.2 mSv, from the 3D CT. From the verification imaging modalities, the total effective doses for head, chest and pelvis protocols were 0.018, 0.12 and 0.16 mSv from OBI system and 3.54, 6.24 and 4.86 mSv, respectively from EPID. In kV CBCT, the total effective doses were 0.14 mSv for head, 2.4 mSv for chest and 4.99 mSv for pelvis from the ImPACT calculation and 5.17 mSv for pelvis protocol from TLD measurement. Among the 2D imaging modalities, EPID effective doses were higher than the others because of using the treatment megavoltage beam. In 3D, planning CT doses were higher than those from the CBCT. For CBCT, ImPACT dose calculation result showed 35 % of variation from TLD measurement and it can be applied with acceptable result. Based on 30 fractions of treatment course, total effective doses (3D CT, 2D setup verification and 6 times CBCT) of head, thorax and pelvis were 4.17, 27.71and 37.57mSv, respectively. Therefore, IGRT should be applied carefully to optimize the dose.