Journal of Medical Physics
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Year : 2003  |  Volume : 28  |  Issue : 2  |  Page : 46-49

Beam Intensity Scanner System For Three Dimensional Dose Verification Of IMRT



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Young W Vahc


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Patient dose verification is clinically one of the most important parts in the treatmerit delivery of radiation therapy. The three dimensional (3D) reconstruction of dose distribution delivered to target volume helps to verify patient dose and determine the physical characteristics of beams used in IMRT. Here we present Beam Intensity Scanner (BlnS) System for the pre-treatment dosimetric verification of two dimensional photon intensity. The BlnS is a radiation detector with custom-made software for dose conversion of fluorescence signals from scintillator. The scintillator is used to produce fluorescence from the irradiation of 6 MV photons on a Varian Clinac 21 EX. The digitized fluoroscopic signals obtained by digital video camera based scintillator(DVCS) will be processed by our custom made software to reproduce 3D- relative dose distribution. For the intensity modulated beam (lMB), the BlnS calculates absorbed dose in absolute beam fluence which is used for the patient dose distribution. Using BlnS, we performed various measurements related to IMRT and found the following: (1) The 3D-dose profiles of the lMBs measured by the BlnS demonstrate good agreement with radiographic film, pin type ionization chamber and Monte Carlo simulation. (2) The delivered beam intensity is altered by the mechanical and dosimetric properties of the collimation of dynamic and/or step MLC system. This is mostly due to leaf transmission, leaf penumbra, scattered photons from the round edges of leaves, and geometry of leaf. (3) The delivered dose depends on the operational detail of how to make multileaf opening. These phenomena result in a fluence distribution that can be substantially different from the initial and calculated intensity modulation and therefore, should be taken into account by the treatment planning for accurate dose calculations delivered to the target volume in IMRT.


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