Proton-Induced X-ray Emission From Metal Targets For Dose Delivery Optimization in Proton Therapy

Proposers: Vanessa La Rosa, Adam Gibson, Gary Royle, Pablo Cirrone, Francesco Romano and Francesco Paolo Romano

Abstract

This project investigates the feasibility of implementing a tool for the real time proton beam range verification and dose assessment in proton therapy. The evaluation, during the treatment, of the dose actually delivered to the tumour, will improve targeting accuracy and help to protect vital organs and healthy tissues as much as possible. The study is in particular addressed to eye proton therapy treatments, during which the main problem is to obtain a good local control on the tumour, sparing the optical nerve and preserving the functionality of the eye. Our approach suggests the surgically implanting of a non toxic metal marker in the back of the eye in front of the optic nerve and exploits the detection of the proton induced x-ray emissions (PIXE) on the high-Z target in order to verify the beam positioning and the delivered dose.

Figure 1- Application description: gold marker sutured around the sclera close to the optic nerve. The interaction of the proton beam with the metal marker will result in x-ray emissions that can be used to retrieve information on the proton beam position and proton fluence.

Background

Proton Induced X-ray Emissions (PIXE) have received considerable attention in recent years [1]. When an incident beam hits the target, its atoms can be ionized e.g. an inner shell electron can be ejected. This vacancy is filled with an electron from the outer shells and the difference in energy between the two shells is released by the emission of a characteristic x-ray. Proton beams for cancer treatment can achieve a dose distribution, which is better confined to the tumour target volume than is possible by using conventional photon irradiation [2]. Patient positioning and tumour volume definition are hence two particularly important issues in this kind of treatments. Charged particle beams are ideal for treating intra-ocular cancers. A large number of proton treatments of uveal melanoma, the most common ocular tumour, achieved a local control rate of about 95% [3]. The full potential of external beam radiation therapy, indeed, can only be realized if the radiation field is accurately targeted on diseased tissue. Poor targeting accuracy may compromise local control of the tumour and increase risk or severity of complications in normal tissues [4]. During the treatment-planning phase, axial eye length and tumour height are estimated by ultrasound. Then most uveal melanoma patients, undergo surgical tumour localisation, during which tantalum clips (typical sizes: 2.5 mm diameter, 0.2 mm thickness) are sutured to the sclera around the perimeter of the tumour [5]. Finally, during the treatment the patient fixates an external light source at a chosen angle, so that

the beam enters the eye as much as possible through the sclera, thus reducing the direct irradiation of the cornea, retina and lens [6].

Motivation for the present proposal

Proton-induced x-ray emissions (PIXE) were computed by Monte Carlo (MC) simulation. The patient anatomy was taken from a CT image and the emitted x-rays were detected outside the simulated phantom. Some preliminary experimental tests with x-rays (in our laboratory and at the Diamond synchrotron) and proton beams (at Clatterbridge Centre for Oncology) were performed to validate the MC simulation. A good agreement between the two sets of data was found. The main aim of the experiment at Diamond was to detect x-ray fluorescence from two different metals (gold and tantalum) embedded in a gelatine phantom and check whether it was possible to discriminate between the two. Finally the L counts of the two different metals were used to image the phantom, obtaining promising results.

Figure 2: Gold and tantalum wires were embedded in a phantom and imaged at the Diamond Synchrotron facility, from their characteristic L x-rays. Zn is present as a contaminant in the detection module. The amount of the emitted x-rays increased with proton energy and is proportional to the proton fluence and mass of the metal. The optimal thickness for gold markers is 0.4 mm. Above that threshold produced x-rays photons will be self attenuated by the target itself. The next step of this research will be reproducing a real proton treatment on an eye phantom and see the amount of characteristic photons that can be detected for different doses, marker positions and eye sizes. This requires the reproduction of a real eye treatment, using a 62 MeV proton beam. Therapy with hadron beams still represents a pioneer technique, and only a few centres worldwide can provide this advanced specialized modality [7]. Experimental procedure

Proton induced x-ray emissions from these metal markers (tantalum, gold and silver) will be detected with a HPGe detector and used to localise the beam and shut it down when it starts generating fluorescence (RANGE VERIFICATION). Then a further analysis on the acquired spectrum will be performed in order to retrieve information on the dose delivered to the target (DOSIMETRY).

RANGE VERIFICATION

PIXE from gold and silver for different positions in the SOBP for marker (Requested BTUs: 1)

The aim of this measurement is to verify the possible use of this technique to spare the optic nerve from unwanted proton dose. The detector is placed right after the metal marker in order to avoid any possible attenuation of the emitted photons due to the slabs. Thereby the variation on the produced K x-rays moving the position of the marker with respect to the SOBP will be analysed. Negligible detected counts,

when the gold marker is beyond the Bragg peak, would mean negligible dose to the optic nerve. The residual dose at the marker exit will be monitored with TLD.

DOSIMETRY

Dose-PIXE calibration curve (Requested BTUs: 1)

The aim of this measurement is to verify if this tool can be used to assess in real time the dose delivered to the tantalum markers (i.e. to the perimeter of the tumour). We will fix the position of the marker and the residual energy of the beam and we will change at each measurement the delivered dose. A calibration curve of the produced yield against the measured dose will be performed.

Real treatment on an eye phantom and optimal metal choice (Requested BTUs: 2)

Using an eye gelatine phantom with tantalum markers around the tumour and another metal close to the optic nerve, we will fire it with a clinical dose to reproduce the final application. Increasing thickness of PMMA slabs (1 mm each) will be added to account for different eye-sizes and correct for them in order to obtain a tool that is independent on the particular treated patient. As a matter of fact, in real cases, the ultrasound estimation of the eye length will be used to compensate the acquired spectra. This experiment will be repeated with different metals in order to understand what is the optimal choice in terms of ionisation cross section and photons attenuation. From theoretical calculations silver (Kα1=22.162 keV Kα2=21.99 keV) seems to be a good trade-off. Though the percentage of transmission through 23 mm of water at this energy is 22% (for gold is about 60%), its K-shell ionisation cross-section 10 MeV protons is 3.784E+1 barns [8], almost two orders of magnitude higher than gold.

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