Dose response evaluation of a low-density normoxic polymer gel dosimeter using MRI

Abstract

A low-density (approximately 0.6 g cm(-3)) normoxic polymer gel, containing the antioxidant tetrakis (hydroxymethyl) phosponium (THP), has been investigated with respect to basic absorbed dose response characteristics. The low density was obtained by mixing the gel with expanded polystyrene spheres. The depth dose data for 6 and 18 MV photons were compared with Monte Carlo calculations. A large volume phantom was irradiated in order to study the 3D dose distribution from a 6 MV field. Evaluation of the gel was carried out using magnetic resonance imaging. An approximately linear response was obtained for 1/T2 versus dose in the dose range of 2 to 8 Gy. A small decrease in the dose response was observed for increasing concentrations of THP. A good agreement between measured and Monte Carlo calculated data was obtained, both for test tubes and the larger 3D phantom. It was shown that a normoxic polymer gel with a reduced density could be obtained by adding expanded polystyrene spheres. In order to get reliable results, it is very important to have a uniform distribution of the gel and expanded polystyrene spheres in the phantom volume.

Basic investigation on performance of low-density polymer gel dosimeter

“…It is in accordance with De Deene et al results (13) but is in contrast with Haraldsson et al (9) , who reported a linear dose-response just for doses between 2 to 8Gy. The results of our experiment showed a sharp increase in R2 values from 0 to 2Gy, but Haraldsson reported inhibition of polymerization in the low-dose region (≤2Gy) (9) . Although gel preparation, irradiation and imaging were followed according to Haraldsson study, the differences can be due to different concentration of foam beads or 2itting method and threshold application in data processing.…”

“…While, compared to Haraldsson et al study (9) , the gel density and CT number were decreased signi2icantly with addition of more polystyrene spheres, still they were higher than the corresponding values for normal human lung tissue, which ranges from −770 to −875 Houns2ield units (12) . The gel density could potentially be further lowered by adding of smaller polystyrene spheres.…”

“…For UD polymer gel dosimeters, the prepared solution was poured into testing vials and for preparation of LD gels, the solution was injected into the vials 2illed with polystyrene spheres (StyrofoamTM spheres, Isopan, Regensburg, Germany). For further density reduction compared to Haraldsson study (9) , the vials were completely 2illed with foam beads.…”

3D Dosimetric Tools in Radiotherapy for Photon Beams

“…In this context, dosimetry based on gel has shown a great potential for measured dose-distribution in 3D [4][5][6][7].…”

Magnetic resonance imaging approaches to gel dosimetry for validation of conformal radiotherapy treatment plans

“…One of the main reasons that has spurred on the development of new gel dosimeters has been the problem of oxygen poisoning (see section on disadvantages of polymer gels) faced by the PAG gel dosimeter which requires the gel to be manufactured under strict hypoxic conditions (usually by bubbling with inert gases such as argon). A new generation of gel dosimeters (normoxic gel dosimeters-permit manufacture under normal atmospheric conditions) [16,45,46,49] contain an anti-oxidant that binds to the oxygen in the gel and consequently prevents free radical scavenging by the oxygen. Despite the advantage of being able to make these gels 9 under normoxic conditions, there have been reports of post irradiation instabilities (both temporal and spatial) due to long-lived polymer radicals and high acidity content as in the case of MAGIC gel dosimeters [16].…”