1. AQUA-ADVANCED QUALITY ASSURANCE FOR CNAO
U. Amaldi, S. Iliescu, N. Malakhov, J. Samarati, F. Sauli and D. Watts
TERA Foundation
The program AQUA (Advanced Quality Assurance) of the TERA foundation [1] aims to develop diagnostic instruments in hadrotherapy for CNAO (Centro Nazionale di Adroterapia Oncologica):
PRR: Proton Range Radiography
IVI: Interaction Vertex Imaging
NST: Nuclear Scattering Tomography
Crystal-PET and RPC-PET: In-beam dose monitoring on ion-induced PET emitters
PROTON RANGE RADIOGRAPHY
Measuring position and residual energy (or range) of a beam of mono-energetic protons after an absorber one can deduce the integrated density along the trajectory and build its 2-D density map in the target. The technique has the advantage of reflecting the true density distribution in the target, as against the more complex dependence of absorption on atomic composition for X-rays [2]: E E ER L
(l) and S(l,El) are density and stopping power of the medium after a penetration l and residual energy El
The AQUA PRR system makes use of a scintillator stack to measure directly the residual proton range, a method developed some time ago at PSI [3]. A pair of position-sensitive detectors (Gas Electron Multipliers, GEM) record position and direction of protons emerging from the target.
The Proton Range Telescope includes 30, 3 mm thick plastic scintillators read out with wavelength shifter fibres by solid state sensors (Multi - Pixel Photon Counters). Two thicker scintillators in front are used for triggering. For each event, the full pulse height profile is recorded on a PC with a system of fast ADC readout.
GEM1
GEM2
Scintillators stack
Target
(Left): Average Bragg energy loss profile recorded for 93 and 126 MeV protons at the Paul-Sherrer-Institute (PSI-Villigen). The penetration is given in scintillation counters units (3 mm).
(Right): Using a simple recognition of the rightmost scintillator with signal over threshold, one obtains the “digital” range determination shown for 72 and 99 MeV; a gaussian fit to the distribution provides a resolution of about 0.52 units of scintillator thickness, or 1.4 mm rms.
(Left): The plot summarizes the results of the PSI test beam exposures, providing the measured range and statistical errors in the scintillator stack as a function of beam energy. Points shown in red, with larger error bars, have been measured with a beam degrader close to the PRR and include a larger beam energy spread.
The expected relative density resolution in the target is given approximately by [2, 3]:
where L is the target length, R the range resolution and N the number of events per image pixel.
With R ~ 2 mm, L= 200 mm, a density resolution of ~ 0.1% is obtained with 100 counts per pixel; with 2x2 mm2 pixels and an image size of 20x20 cm2 this requires the recording of 106 proton tracks, or 10 seconds of data taking with the present setup. A faster electronics, under development, will reduce the acquisition by a factor of ten, permitting to study organ motions or to realize tomographic images by rotation of the patient.
REFERENCES:
[1] Sistema AQUA, “Advanced Quality Assurance” per il Centro Nazionale di Adroterapia Oncologica (TERA Proposal )
[2] K. M. Hanson et al, Phys. Med. Biol. 26 (1981) 965
[3] P. Pemler et al, Nucl. Instr. and Meth. A432 (1999) 483