57. International Winter Meeting on Nuclear Physics

The Dose Profiler tracker: an online Particle Therapy monitor optimised for the detection of charged fragments produced by the ion beams interactions with matter.

22 Jan 2019, 18:20

20m

Bormio, Italy

Short Contribution Tuesday Afternoon Session

Speaker

Ms Micol De Simoni (Università di Roma "La Sapienza", Scienze di Base e Applicate per l'Ingegneria, Rome, Italy)

Description

The use of C, He and O ions in Particle Therapy (PT) exploits the enhanced Relative Biological Effectiveness and Oxygen Enhancement Ratio of such projectiles to improve the treatment efficacy in damaging the cancerous cells. To fully profit from the increased tumor control probability and ballistic precision of the projectiles, an accurate online monitor of the dose release spatial distribution is required to spare the healthy tissues surrounding the tumor area, preventing unwanted damage due to, for example, morphological changes in the patient during treatment with respect to the initial CT scan. A monitoring technology capable of monitoring online PT treatments is still missing in the clinical routine. Several studies are underway to develop beam range verification systems exploiting the detection of the emitted secondary radiation produced by the primary beam interactions with the patient body along the path towards the target volume. An interesting opportunity for C, He and O treatments is represented by the detection of charged particles that can be performed with high efficiency in a nearly background free environment. The Dose Profiler (DP) detector, developed within the INSIDE project, is a scintillating fibre tracker that allows an online charged fragments reconstruction and backtracking. The unfolding of the different matter interaction effects (absorption and multiple scattering inside the patient) on the measured shape represent a crucial task when trying to correlate the measured emission profile with the beam range. Several strategies, based on MC methods, are currently being explored to accomplish such task. In this contribution the preliminary tests performed on the DP, using the $^{12}$C ions beam of the CNAO treatment centre and an anthropomorphic phantom (RANDO®) as target, will be reported together with the implications for the treatment monitoring applications. The first DP clinical trial is scheduled to start in early 2019 at the CNAO center aiming to study the fragments production in the treatment of patients with different clinical conditions and expected treatment toxicity.

Summary

Accelerated light ion beams (C, He,O) in Particle Therapy (PT) are used to treat solid tumors, exploiting their capability to achieve a high localized dose distribution, allowing to concentrate most of the energy release in the tumor volume. Moreover, the different physics processes involved in the energy loss in PT lead to a higher Relative Biological Effectiveness and Oxygen Ratio Enhanced, especially for $^{12}$C ions treatments, make such therapy particularly suited for the treatment of radio resistant tumors.
PT treatments are highly sensitive to beam range uncertainties because of their peculiar (peaked) dose release profile. Any shift in the Bragg peak position with respect to the planned position leads to an under-dosage of the tumor volume and, at the same time, to an over-dosage of healthy tissue surrounding the tumor reducing significantly the advantages of the intrinsic PT precision capabilities. The sources of beam range uncertainty with respect to the predictions include anatomical changes (organ motion, Tumour regression, weight loss), physiological variations and uncertainties in CT Hounsfield Units (HU) calibration. All these effects can be summed up resulting into an overall uncertainty of the order of few millimeters.
At present, an online monitoring system, capable of checking the target volume dose conformity during the treatment, is still missing.
During PT treatments a large amount of secondary particles is produced, as an
outcome of the interaction between the beam and the crossed tissues. Secondary photons, neutrons and different types of charged particles (mainly protons, at large angles wrt the incoming beam direction) are emitted. Light charged particles are produced by the projectile fragmentation and are mainly forward peaked.
However, the proton component shows a non negligible tail at large emission angles. This experimental evidence has recently (since ~2010) triggered several studies on the possibility to exploit charged fragments for monitoring purposes and in particular the large angles production as it is the most interesting for monitoring applications, because of the relative advantageous proton reconstruction resolution.
The Dose Profiler detector is being developed in the framework of the INSIDE (INnovative Solution for In-beam DosimEtry in hadrontherapy) project as a beam range monitor for the CNAO therapy Center (Italy).
In 2017 a test of the Dose Profiler at the CNAO has been performed to study the fragments production in a realistic case and a first DP clinical trial is scheduled to start at the CNAO in early 2019.
The emission profiles obtained in a real treatment are strongly dependent on the patient geometry and tumor localization, since the emitted particles travel along different paths and cross materials with different densities .
Two different approaches aiming to make the Bragg peak depth evaluation independent from the treatment specific conditions are being pursued and are presented. The first method proceeds towards the emission profile reconstruction using a MC method to assign a weight to each particle accordingly the amount of crossed materials, extracted using the patient CT information.
The other approach under development is the MLEM iterative algorithm (Maximum Likelihood Expectation Maximization). The performances of the two methods and the relative impact on the uncertainty of the beam range will be reviewed and discussed.

Presentation materials

Slides

5-desimoni.pdf