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Proton Computed Tomography

PROTON COMPUTED TOMOGRAPHY (pCT): A GENERAL OVERVIEW

 

What is the pCT?

 

Proton beams have important advantages compared to other radiation treatment options. Thanks to their specific depth dose curve, when they penetrate inside matter (a relatively low entrance dose followed by a high-dose peak) they can deliver the prescribed dose precisely to the tumor mass, leaving surrounding healthy tissues almost undamaged. On the other hand, a precise radiation modality requires very accurate Quality Assurance (QA) of the treatment procedures. In proton therapy QAs are mainly related to the patient positioning and to the dose calculations with a treatment planning software. In existing proton treatment centers, positioning and dose calculations are currently based on x-ray radiographs and x-ray computed tomography (xCT), respectively.

A new challenge, in the next years, will be the direct use of the proton beam for the tomographic imaging of the patient body developing a proton Computed Tomography ( pCT ) system. This will make the proton radiation procedure more precise

by defining the position of the Bragg peak more accurately, enabling the verification of patient and tumor position with respect to the proton beam and by permitting a direct measurements of the electron densities (and hence of the stopping powers values) of the involved tissues [1].

 

Principle of pCT

 

The basic idea of pCT is the reconstruction of the electron density distribution map by measuring the proton energy loss along their path Most Likely Path ( MLP ) L

through the traversed tissues, on the basis of the fundamental equation derived from the Bethe-Bloch formula.

 

Our activities

 Our group is involved in the Italian PRIMA (PR ton IMAging) project [2]. PRIMA is founded by INFN and its aim is the development of a prototype of 2D proton imaging system. i.e. a device able to reconstruct a proton transmission image of an object. The 2D image will represent the first step on the realization of a complete 3-dimentional tomographic image with proton beams.

At LNS, a dedicated application using the GEANT4 Monte Carlo libraries was developed by our group.It has permitted us to study some aspects useful of pCT:

  • Specification of the detector main characteristic
  • Estimation of the Most Likely Path ( MLP ) inside an homogeneous and inhomogeneous medium (Fig 1) and its comparison with a semi-analitycal calculation for of MLP [3] and experimental data [4] (Fig 2);
  • Preliminary studies of the algorithms for the tomographic image reconstruction

 

References:

[1] R. W. Schulte et al., Density resolution of proton computed tomography

Med.Phys., vol. 32, no. 4, April 2005.

[2] G.A.P. Cirrone et al. The Italian project for a proton imaging device

Nucl. Instrum. Meth. A., 576 (2007) 194 - 197.

[3] D. C. Williams, The most likely path of an energetic charged particle through a uniform medium

Phys. Med. Biol., vol.49, 2004.

[4] G.A.P. Cirrone et al. Monte Carlo Studies of a Proton Computed Tomography System accepted for publication submitted on IEEE Trans. Nucl. Sci., 2006 C
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