The influence of a novel transmission detector on 6 MV x-ray beam characteristics

Abstract

The purpose of this work was to investigate the influence of a new transmission detector on 6 MV x-ray beam properties. The device, COMPASS (IBA Dosimetry, Germany), contains 1600 plane parallel ionization chambers with a detector spacing of 6.5 mm and an active volume of 0.02 cm3. Surface dose measurements were carried out using a Markus chamber and radiochromic film for a range of field sizes and source-to-surface distances (SSDs). The surface dose and dose in the build-up region for COMPASS fields were compared to open fields. For moderately narrow beam geometric conditions, the increase in surface dose was small. For the largest field size investigated (20x20 cm2) at a 90 cm SSD, the surface dose with the detector was 34.9% versus 26.8% in the open field. However, the increase in surface dose in COMPASS fields was less than that observed with a standard block tray in the field (38.7% in the above example). It was found that beyond dmax, the difference in relative dose (profiles and PDDs) between open and COMPASS fields was insignificant. The mean transmission factor of the detector was 0.967 (standard deviation=0.002) measured over a range of field sizes from 3x3 to 20x20 cm2 at SSDs from 70 cm to 90 cm. In summary, the transmission detector was found to increase the relative dose in the buildup region but had a negligible effect on the beam parameters beyond dmax.

Pretreatment quality assurance of volumetric modulated arc therapy on patient CT scan using indirect 3D dosimetry system

“…To address this issue, alternate QA techniques has been developed to verify the 3D dose distribution by measuring fluence at different gantry angle using ion chamber matrix or EPID in a patient computed tomography (CT) scan. [13][14][15][16][17][18] The COMPASS QA system (IBA Dosimetry, Germany) is one such technique which uses MatriXX Evolution along with gantry angle sensor. It consists of (i) a measurement-based dose reconstruction, and (ii) model-based dose calculation.…”

Beam perturbation characteristics of a 2D transmission silicon diode array, Magic Plate

“…Most of the currently used transmission‐type detectors such as the COMPASS detector (IBA Dosimetry), (31) the integral quality monitoring system (IQM), (30) and the DAVID system 29 , 32 are based on pixelated ionization chambers and these systems have been shown to significantly increase the patient surface dose due to additional electron contamination. The presence of these transmission devices also causes beam attenuation up to 7% for a 6 MV photon beam for doses beyond ${\mathrm{d}}_{\text{max}}$ 30 , 31 , 33 .…”

“…The presence of these transmission devices also causes beam attenuation up to 7% for a 6 MV photon beam for doses beyond ${\mathrm{d}}_{\text{max}}$ 30 , 31 , 33 . Recently, wireless transmission detectors, such as Delta 4 Discover (ScandiDos) based on a 2D diode array (34) and Dolphin (IBA Dosimetry) based on a 2D ionization chamber array, (35) have been introduced.…”

A new transmission methodology for quality assurance in radiotherapy based on radiochromic film measurements

“…All of these studies are examples of transmission QA methodologies; they present the main advantage of coupling the measurement device to the linac treatment head, making the beam always perpendicular to the device's sensitive volume, eliminating variations in its response as a function of the gantry's angular displacement. Another important advantage is that, depending on the detector, this methodology can be used for in vivo evaluation 15 , 16 , 17 , 18 …”