Optical scanner for 3D radiotherapy polymer gel dosimetry

Abstract

Sophisticated techniques employed in radiotherapy for irradiation of tumours require comprehensive dosimetry allowing for precise, high resolution measurements of radiation dose distribution in three dimensions and verification of treatment planning systems. Polymer gel dosimetry has been shown to be a unique technique for three-dimensional high resolution measurements of absorbed radiation dose distributions. If exposed to ionizing radiation, radical polymerisation and crosslinking of monomeric components take place in a 3D polymer gel dosimeter, leading to the formation of large polymeric structures that scatter visible light. This feature allows for optical observation of the effects of the absorbed dose and its distribution. Presently, magnetic resonance imaging is employed the most often for analysis of 3D polymer gel dosimeters. However, much attention is also being given to the development of optical computed tomography since this technique is hoped to serve as a substitute for expensive and not easily available magnetic resonance imaging. The optical scanner presented in this work consists of a laser diode, a scanning system and a signal detector. A 3D polymer gel dosimeter is measured in an immersion liquid in order to reduce deflection of the light from the dosimeter phantom. The very first results were obtained with the newly constructed scanner for PABIG^nx 3D polymer gel dosimeter, which was inhomogeneously irradiated with ¹⁹²Ir brachytherapy source. The results have been contrasted with those for magnetic resonance imaging and are presented in this work together with the description of the optical scanner. Currently, optimization of the optical scanner is performed.

Figure 1

PABIG^nx polymer gel dosimeter after irradiation with ¹⁹²Ir brachytherapy source (conditions: 10 Gy at 10 mm distance from the source). The opaque region in the middle of the sample corresponds to the polymerisation and crosslinking of monomeric components which occurred after irradiation.

Figure 2

Components of the optical scanning system for measurements of 3D polymer gel dosimeters. A: The measurement units and a tank with an immersion liquid and the PABIG^nx vial inside (¹⁹²Ir irradiated); B: A side view of the PABIG^nx vial in the tank.

Figure 3

3D dose distribution in PABIG^nx polymer gel dosimeter after irradiation with brachytherapy source, ¹⁹²Ir, placed centrally inside the dosimeter as described in Section 2.2. The scale on the left represents the absorbed dose range of 0.5-40 Gy. The catheter position through which ¹⁹²Ir was introduced in PABIG^nx is indicated with a dot line. The source position was roughly in the middle of this line

Figure 4

A: Reconstruction of a single plane as relative dose distribution across the longer axis of the source and crossing it at the centre of an active part. B: A profile obtained along X axis centrally from the plane presented in A, as in dictated with a dot line.

Figure 5

The measured area that was not irradiated, reconstruction (a), profile (B)

Figure 6

Cross-view of example projection

Figure 7

Reconstruction obtained with optical scanner (A), Profile from optical scanner (B)