The DOSIMAP, a high spatial resolution tissue equivalent 2D dosimeter for LINAC QA and IMRT verification

Abstract

The continual need for more accurate and effective techniques in radiation therapy makes it necessary to devise new control means combining high spatial resolution as well as high dose accuracy. Intensity modulated radio therapy (IMRT) allows highly conformed fields with high spatial gradient and therefore requires a precise monitoring of all the multileaf positions. In response to this need, the authors have developed a new 2D tissue equivalent dosimeter with high spatial resolution. A plastic scintillator sheet is sandwiched between two polystyrene blocks and the emitted light is captured by a high resolution camera. A newly developed procedure described herein allows efficient discrimination of the scintillation from the parasitic Cerenkov radiation. This processing is applied on the cumulated image from a sequence of images taken during an irradiation field at a rate of 10 images/s. It provides a high resolution mapping of the cumulated dose in quasireal time. The dosimeter is tissue equivalent (ICRU-44) and works both for electrons and photons without complex parameter adjustment since phantom and detector materials are identical. Instrument calibration is simple and independent of the irradiation conditions (energy, fluence, quality, ...). In this article, the authors present the principle of the dosimeter and its calibration procedure. They compare the results obtained for photons and electron beams with ionization chamber measurements in polystyrene. Technical specifications such as accuracy and repeatability are precisely evaluated and discussed. Finally, they present different IMRT field measurements and compare DOSIMAP measurements to TPS simulations and dosimetric film profiles. The results confirm the excellent spatial resolution of the instrument and its capacity to inspect the leaf positions for each segment of a given field.

Performance characterization of a 3D liquid scintillation detector for discrete spot scanning proton beam systems

“…High water-equivalency for plastic scintillators, in addition to their energy independence, and flexibility in measuring both x-ray and electron radiation fields make them a good fit for precision dosimetry applications (Beddar 1992). Thin plastic scintillation sheets have been used for performing high resolution and rapid beam measurements for 2D photon (Petric et al 2006, Collomb-Patton et al 2009) and proton dosimetry (Torrisi 2000). …”

Projection imaging of photon beams using Čerenkov-excited fluorescence

“…In the context of profiling static megavoltage linear accelerator (LINAC) beams for routine quality assurance (QA), typically only a small subset of the full volumetric dose is directly measured by virtue of an ionization chamber measurement in water. Several methods have been proposed to move towards 2D and/or full 3D beam profiling without the use of ionization chambers, including techniques that use plastic or liquid scintillators, as well as gel-based dosimetry (Beddar et al , 1992a; Beddar et al , 1992b; Frelin et al , 2008; Collomb-Patton et al , 2009; Guillot et al , 2011; Kirov et al , 2000; Ponisch et al , 2009; Beddar et al , 2009; Archambault et al , 2012; Maryanski et al , 1994; Maryanski et al , 1996; Kelly et al , 1998; McJury et al , 2000). However, both methods have limitations in that they are not truly water equivalent.…”

Three-dimensional Čerenkov tomography of energy deposition from ionizing radiation beams

“…Alternative modalities to overcome the prohibitive spatial profiling capabilities of ionization chambers have been proposed, including plastic or liquid scintillation and gel dosimetry, in which the number of emitted scintillation photons or a chemical change in a polymer gel is assumed to scale with imparted dose [5–7]. Although the techniques offers several advantages, each requires a material other than water to record the deposited dose, an unfavorable requirement which introduces dosimetric inaccuracies due to material property differences.…”