Implementation of Convolution/Superposition Model of Photon Dose Calculation
The implementation, accuracy and performance of a collapsed cone convolution/superposition dose calculation algorithms is presented. The primary fluence or TERMA (total energy released per unit mass) is calculated using a polyenergetic spectrum which is m
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School of Information Science & Engineering, Northeastern University, Shenyang, China Radiotherapy Product Department, Neusoft Medical System Co., Ltd., Shenyang, China
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Abstract — The implementation, accuracy and performance of a collapsed cone convolution/superposition dose calculation algorithms is presented. The primary fluence or TERMA (total energy released per unit mass) is calculated using a polyenergetic spectrum which is modified with depth to account for water effective depth hardening and with off axis position to account for softening of the beam. An Inverse square law correction for kernel tilting is applied at the dose deposition site The algorithm uses energy deposited kernels generated by use of EGSnrc Monte Carle Code. In addition, this work also reports on some of the initial test that was conducted during the commissioning of a commercially available 3D treatment planning system. And the algorithm was verified by using AAPM phantom data with open, unwedged, and tilting fields for both symmetric and asymmetric collimator settings with variant energy (4MV, 18MV). Also measured data from variant linac and practical clinical data are used for verifying applicability and feasibility of the algorithm. Results presented in this paper show good agreement between measured data and calculated results in homogeneous and homogeneous media for different fields, and demonstrate the correct behaviour of collapsed cone convolution/superposition dose calculation algorithms.
II. MATERIALS AND METHODS The dose equation for momoenergetic irradiation of a homogeneous phantom with a parallel beam is given by [5, 6]: K D(r )
K
K
3
K rc
(1)
V
Where T (rKc) is TERMA describing total energy released per mass of primary photon in the volume eleK K 3K ment d r c A(r r c) is energy deposit kernel generated in water. The kernels are generated by use of EGSnrc Monte Carlo code. The geometry of the basic theory of convolution is given in Figure 1. source
K r0
K r '
K r
Keywords — TPS, photon beam, dose calculation, convolution/superposition, EGSnrc
K K r r ' D (r )
I. INTRODUCTION Radiotherapy using high x-ray is an important modality on clinic. The goal of accurate dose calculation is delivering a prescribed dose to a target volume while minimizing radiation damage to the surrounding healthy tissue. Three dimensional dose calculations for radiation therapy using convolution/superposition algorithms have been investigated by many researchers [1, 2], and been applied to many commercial treatment planning systems [3, 4]. In this paper, implementation, accuracy and performance of a collapsed cone convolution/superposition dose calculation algorithms is presented. The basic theory of the model was discussed. Calculated and measured dose in homogeneous and inhomogeneous media were compared for wedged and unwedged fields. Results using AAPM test package and practical data show agreement of 2% or 3mm in most case. These tests demonstrate the correct behaviour of the collapsed cone implementation of the algorithm.
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