Speaker
Description
Proton therapy uses protons to irradiate tumors and the way they deposit energy, characterised by the well-known Bragg peak, allows to conform dose better than in traditional techniques. Accuracy is very important to reach the maximal potential of this technique, which currently cannot be achieved due to the existence of uncertainties in the calculation of the proton range. In-vivo dose verification techniques would help to reach this objective. Different techniques such as PET image, PG image guiding and ultrasound have been proposed to verify the dose during a proton treatment. All these techniques have their own drawbacks. For example, PET is subject to wash-out, half-life and the spatial distribution of the isotopes generated; and PG has to deal with the large amount of gamma emitted and with the detectors efficiency of the high energy photon.
In order to overcome these drawbacks we propose the use of contrast agents. These are elements that must be introduced in the target previously to the irradiation and must present useful properties when protons interact with them. In PET we could look for elements that generate very short life isotopes in the Bragg peak region, which would solve the wash-out effect, or isotopes that do not suffer from wash-out. Instead, in PG imaging we would look for isotopes which produce some low energy gammas in the BP region improving the system efficiency.
Three experiments have been carried out using the multiple energy proton beams delivered by up to 10 MeV protons at CMAM tandetron accelerator (Centro de Microanálisis de Materiales, Madrid, Spain) to calculate the cross section up to 10 MeV of some of the studied contrast agents. The most promising reactions and those studied in these experiments are ¹²⁷I(p,n)¹²⁷*Xe (t1/2=69.2 s) for SPECT purpose, ²³Na(p,n)²³Mg (t1/2=11.3 s) and ¹⁸O(p,n)¹⁸F (t1/2=109 min) for PET and also PGs of the ¹⁸O and ²³Na. Figure 1 shows the Cross Section obtained for the ²³Na(p,n)²³Mg and the corresponding theoretical activation for the first minute after the irradiation of a PMMA phantom with a 5% of this contrast agent, using a 100 MeV proton beam. A sharp peak is generated just under the Bragg peak due to the contrast agent, which could help to dose reconstruction. In figure 2 the spectrums of the PGs of a water and a water-18 (93% H2O+ 7% H2¹⁸O) target irradiated during the experiments are shown. Three PGs in the water-18 clearly stand out from the spectrum of normal water and also have a lower energy than the most typical PGs of natural tissues, which facilitates their detection.
