A Monte Carlo investigation of contaminant electrons due to a novelin vivotransmission detector

Abstract

A novel transmission detector (IBA Dosimetry, Germany) developed as an IMRT quality assurance tool, intended for in vivo patient dose measurements, is studied here. The goal of this investigation is to use Monte Carlo techniques to characterize treatment beam parameters in the presence of the detector and to compare to those of a plastic block tray (a frequently used clinical device). Particular attention is paid to the impact of the detector on electron contamination model parameters of two commercial dose calculation algorithms. The linac head together with the COMPASS transmission detector (TRD) was modeled using BEAMnrc code. To understand the effect of the TRD on treatment beams, the contaminant electron fluence, energy spectra, and angular distributions at different SSDs were analyzed for open and non-open (i.e. TRD and block tray) fields. Contaminant electrons in the BEAMnrc simulations were separated according to where they were created. Calculation of surface dose and the evaluation of contributions from contaminant electrons were performed using the DOSXYZnrc user code. The effect of the TRD on contaminant electrons model parameters in Eclipse AAA and Pinnacle(3) dose calculation algorithms was investigated. Comparisons of the fluence of contaminant electrons produced in the non-open fields versus open field show that electrons created in the non-open fields increase at shorter SSD, but most of the electrons at shorter SSD are of low energy with large angular spread. These electrons are out-scattered or absorbed in air and contribute less to surface dose at larger SSD. Calculated surface doses with the block tray are higher than those with the TRD. Contribution of contaminant electrons to dose in the buildup region increases with increasing field size. The additional contribution of electrons to surface dose increases with field size for TRD and block tray. The introduction of the TRD results in a 12% and 15% increase in the Gaussian widths used in the contaminant electron source model of the Eclipse AAA dose algorithm. The off-axis coefficient in the Pinnacle(3) dose calculation algorithm decreases in the presence of TRD compared to without the device. The electron model parameters were modified to reflect the increase in electron contamination with the TRD, a necessary step for accurate beam modeling when using the device.

Characterization of a novel two dimensional diode array the “magic plate” as a radiation detector for radiation therapy treatment

“…At smaller field sizes, the lack of phantom scatter resulted in a much lower field size factor, whereas for field size >10 Â 10 cm 2 , the higher contribution of the collimator scatter due to the shorter SDD resulted in a higher field size factor. Asuni et al 63 performed Monte Carlo studies on the contaminant electrons due to transmission detector. They found that the transmission detector increased the contaminant electrons at shorter SSD of 70 cm and increasing with larger field sizes.…”

Investigation of the spatial resolution of an online dose verification device

“…BEAMnrc=EGSnrc Monte Carlo code 19 was used to model the head of the linear accelerator based on a detailed description provided by the manufacturer for the 6 MV photon beam. This model has been previously validated 20 and found to be accurate. For this work, two phase-space files were obtained as shown in Fig.…”

10-MV X-ray dose calculation in water for MLC and wedge fields using a convolution method with X-ray spectra reconstructed as a function of off-axis distance

“…For phantoms constructed of water-like media, we express as , (6) where is the relative electron density averaged between point P and the ∆S center (however, it has been found [13][14][15] that the contamination dose does not vary simply in proportion to the beam water collision kerma of ).…”

“…(d) h wedge and k wedge (used as θ wedge <π/2) in equations 1 and 2 are, respectively, , (13) , (14) where F wedge_p and F wedge_s are the correction factors, respectively, for the calculation of the primary and scatter dose components, not as a function of E N . We express them as ,…”