In brachy therapy treatment, as well as in treatment with external beams, it is of crucial importance to thoroughly determine the absorbed dose in the tumour, in surrounding normal tissue and in risk organs.
Several kinds of gel dosimeters have been, or are about to be, developed in order to get a three dimensional dosimeter, which would be very useful, especially in the context of brachy therapy. The need for high spatial resolution is raised by the fact that the absorbed dose decreases very fast with the distance from a brachy therapy source. The steep dose gradient also requires a dosimeter material with a wide dose range and no signal diffusion.
Fe(II)/Fe(III) gel and polymer gels such as BANANA and BANG. These systems are analysed with magnetic resonance imaging (MRI) which gives a detailed picture with very high resolution (~0.5 mm) without the need to cut out samples and thereby destroy the geometry of the gel. One of the drawbacks for MRI-gels is that inhomogeneities in the magnetic field make it difficult to calibrate the gel in absolute values of absorbed dose.
The Fe(II)/Fe(III) gel is the most well known of the gel dosimeters mentioned. The working principle is that it contains Fe(II)-ions that are oxidised to Fe(III)-ions when irradiated. The differences in paramagnetic properties between the ions can then be used to make an MRIimage of the dose distribution. The dose response is linear up to 40 Gy.
The problem with this dosimeter type is the rapid diffusion of the Fe-ions which makes it necessary to image the gel immediately after irradiation to maintain the high resolution.
In 1993 Maryanski et al. (Maryanski, 1993) reported a tissue-equivalent gel based on agarose, acrylamide and N,N´-methylene-bis-acrylamide (bis) in a de-aerated aqueous solution. The gel is called BANANA and works as a dosimeter due to the radiation induced polymerisation of the monomers acrylamide and bis. Later on, the agarose was replaced by gelatin because of its lower background signal. It is also more transparent which makes it easy to optically see the dose distribution since the polymerised gel volume changes to a white colour. This new gel is called BANG, and when further improved by substituting the acrylamide with acrylic acid it got the name BANG-2.
The BANG-2 gel can measure doses down to 0.1 Gy which is much below the limit for both alanine gel and Fe(II/III) gel, but the dose response is only linear up to 6 Gy. Another drawback is the difficulties in preparing the gel. The preparation has to be made absolutely oxygen free since oxygen inhibits the polymerisation, and the gel must be stored in glass containers since most plastics are oxygen permeable. This puts great requirements on the preparation equipment, or the gel has to be bought, already cast in a predetermined shape. The glass container might also give some dosimetric effects since it has a higher atomic number than the gel itself.
We have instead used a stiff agarose gel, heavily doped with alanine. The gel is heated and over saturated with alanine which recrystallizes when the gel is cooled down.
When crystalline alanine is irradiated, radicals are formed which can be detected by means of electron spin resonance (ESR) spectroscopy. A signal proportional to the amount of radicals is then obtained. Since the amount of radicals is proportional to the absorbed dose, the substance may serve as a dosimeter material. The radicals in alanine are unusually stable because of the crystalline form, and in pure dry crystals the signal remains almost unchanged for several years.
When the alanine is added to an agarose gel, the crystals are trapped in the gel which prevents signal diffusion. After irradiation, samples can be cut out at positions of interest. For the gel composition used in this work a sample weight of ~0.16 g is needed, which corresponds to a volume of ~0.12 cm3 (density: 1.28 g/ cm3). The shape of the sample can be chosen as convenient for the situation.
The ESR analysis does not destroy the signal and thereby repeated read-outs of one sample are allowed.
Alanine has a linear dose response from well below 1 Gy up to 104 Gy. The sensitivity when used in a gel allows doses down to ~3 Gy, as will be shown later on in this report.
To make absolute dose measurements possible, as well as relative, the alanine/agarose gel requirescalibration.
Absolute dose measurements are for example needed to verify Monte Carlo calculations experimentally. Dose planning systems used today do not take into account scattering effects at interfaces between materials of different atomic numbers, or scattering effects in a larger volume due to inserted shielding material. To verify that these simplifications do not set the outcome of the treatment at risk, and if possible to correct for the introduced errors, experimental measurements in such critical situations are needed.
The aim of this report is to indicate a way of calibrating the alanine/agarose gel, and to examine the radical stability in the obtained calibration samples.
Linköping: Linköping University Electronic Press , 1998. , 18 p.