[Proton imaging applications for proton therapy: state of the art]

Proton therapy allows a highly precise tumour volume irradiation with a low dose delivered to the healthy tissues. The steep dose gradients observed and the high treatment conformity require a precise knowledge of the proton range in matter and the target volume position relative to the beam. Thus, proton imaging allows an improvement of the treatment accuracy, and thereby, in treatment quality. Initially suggested in 1963, radiographic imaging with proton is still not used in clinical routine. The principal difficulty is the lack of spatial resolution, induced by the multiple Coulomb scattering of protons with nuclei. Moreover, its realization for all clinical locations requires relatively high energies that are previously not considered for clinical routine. Abandoned for some time in favor of X-ray technologies, research into new imaging methods using protons is back in the news because of the increase of proton radiation therapy centers in the world. This article exhibits a non-exhaustive state of the art in proton imaging.

An indirect in vivo dosimetry system for ocular proton therapy

Secondary radiation, particularly neutron radiation, is a cause of concern in proton therapy. However, one can take advantage of its presence by using it to retrieve useful information on the primary proton beam. At the Centre Antoine Lacassagne the secondary radiation in the treatment room has been studied in function of the beam modulation. A strong correlation was found between the secondary ambient dose equivalent per proton dose H*(10)/D and proton dose rate D/MU. A large volume ionisation chamber fixed on the wall at 2.5 m from the nozzle was used with an in-house computer interface to retrieve the value of D/MU derived from the measurement of photon H*(10) integrated over treatment time, using the correlation curve. This system enables the verification of D and D/MU to be made independently of the monitoring of the primary beam and represents a first step towards an alternative in vivo dosimetry in proton therapy.

Secondary neutron doses in proton therapy treatments of ocular melanoma and craniopharyngioma

Monte Carlo simulations were used to assess secondary neutron doses received by patients treated with proton therapy for ocular melanoma and craniopharyngioma. MCNPX calculations of out-of-field doses were done for ∼20 different organs considering realistic treatment plans and using computational phantoms representative of an adult male individual. Simulations showed higher secondary neutron doses for intracranial treatments, ∼14 mGy to the salivary glands, when compared with ocular treatments, ∼0.6 mGy to the non-treated eye. This secondary dose increase is mainly due to the higher proton beam energy (178 vs. 75 MeV) as well as to the impact of the different beam parameters (modulation, collimation, field size etc.). Moreover, when compared with published data, the assessed secondary neutron doses showed similar trends, but sometimes with sensitive differences. This confirms secondary neutrons to be directly dependent on beam energy, modulation technique, treatment configuration and methodology.

Secondary neutron doses received by paediatric patients during intracranial proton therapy treatments

This paper's goal is to assess secondary neutron doses received by paediatric patients treated for intracranial tumours using a 178 MeV proton beam. The MCNPX Monte Carlo model of the proton therapy facility, previously validated through experimental measurements for both proton and neutron dosimetry, was used. First, absorbed dose was calculated for organs located outside the clinical target volume using a series of hybrid computational phantoms for different ages and considering a realistic treatment plan. In general, secondary neutron dose was found to decrease as the distance to the treatment field increases and as the patient age increases. In addition, secondary neutron doses were studied as a function of the beam incidence. Next, neutron equivalent dose was assessed using organ-specific energy-dependent radiation weighting factors determined from Monte Carlo simulations of neutron spectra at each organ. The equivalent dose was found to reach a maximum value of ∼155 mSv at the level of the breasts for a delivery of 49 proton Gy to an intracranial tumour of a one-year-old female patient. Finally, a thorough comparison of the calculation results with published data demonstrated the dependence of neutron dose on the treatment configuration and proved the need for facility-specific and treatment-dependent neutron dose calculations.

Quantification of dose perturbations induced by external and internal accessories in ocular proton therapy and evaluation of their dosimetric impact

PURPOSE

Proton scattering on beam shaping devices and protons slowing down on media with different densities within the treatment volume may produce dose perturbations and range variations that are not predicted by treatment planning systems. The aim of this work was to assess the dosimetric impact of elements present in ocular proton therapy treatments that may disturb the prescribed treatment plan. Both distal beam shaping devices and intraocular elements were considered.

METHODS

A wedge filter, tantalum fiducial marker, hemispherical compensator, two intraocular endotamponades (densities 0.97 and 1.92 g cm(-3)) and an intraocular eye lens (IOL) were considered in the study. For these elements, longitudinal dose distributions were measured and∕or calculated in water in beam alignment for a clinical spread-out Bragg peak. Under the same conditions, the unperturbed dose distributions were similarly measured and∕or calculated in the absence of the element. The dosimetric impact was assessed by comparison of unperturbed and perturbed dose distributions. Measurements and calculations were carried out with two methods. Measurements are based on EBT3 films with dedicated software, which makes use of a calibration curve and correction for the quenching effect. Calculations are based on the Monte Carlo (MC) code MCNPX and reproduce the experimental conditions. Both dose maps are obtained with a resolution of 300 dpi.

RESULTS

The degree of disturbance of distal beam shaping devices is low for the wedge filter (2% overdose ripple all along the central axis) and moderate for the hemispherical compensator (two bands of variable overdose of up to 10% downstream the compensator lateral edges and -5% underdose on the plateau at off-axis distance of 5 cm). Tantalum clips produce important dose shadows (-20% behind the clip parallel to the beam and range reduction of 1.1 mm) and bands of overdose (15%). The presence of endotamponades modifies the dose distribution very significantly (-5% underdose on the plateau and 3 mm range prolongation for the tamponade with density 0.97 g cm(-3) and -15% underdose on plateau and 8 mm range reduction for that with density 1.92 g cm(-3)). No dose perturbations were found for the IOL. The high performance of EBT3 film and MC tools used was confirmed and good agreement was found between them (percentage of pixels passing the gamma test >87%).

CONCLUSIONS

The degree of disturbance by external beam shaping devices remains low in ocular proton therapy and can be reduced by bringing accessories closer to the eye. Tantalum fiducial markers must be located in such a way that dose perturbation is not projected on the tumor. The treatment of patients with intraocular endotamponades must be carefully managed. It is fundamental that radiation oncologists and medical physicists are informed about the presence of such substances prior to the treatment.

Spread-out Bragg peak and monitor units calculation with the Monte Carlo code MCNPX

The aim of this work was to study the dosimetric potential of the Monte Carlo code MCNPX applied to the protontherapy field. For series of clinical configurations a comparison between simulated and experimental data was carried out, using the proton beam line of the MEDICYC isochronous cyclotron installed in the Centre Antoine Lacassagne in Nice. The dosimetric quantities tested were depth-dose distributions, output factors, and monitor units. For each parameter, the simulation reproduced accurately the experiment, which attests the quality of the choices made both in the geometrical description and in the physics parameters for beam definition. These encouraging results enable us today to consider a simplification of quality control measurements in the future. Monitor Units calculation is planned to be carried out with preestablished Monte Carlo simulation data. The measurement, which was until now our main patient dose calibration system, will be progressively replaced by computation based on the MCNPX code. This determination of Monitor Units will be controlled by an independent semi-empirical calculation.

Chronic dietary alpha-linolenic acid deficiency alters dopaminergic and serotoninergic neurotransmission in rats

This study examined the effects of dietary alpha-linolenic acid [18:3(n-3)] deficiency on dopaminergic serotoninergic neurotransmission systems in 60-d-old male rats. Rats were fed semipurified diets containing either peanut oil [the (n-3)-deficient group] or peanut plus rapeseed oil (control group). We measured the densities of the serotonin-2 (5-HT2) receptors and the dopamine-2 (D2) receptors by autoradiography and membrane-binding assays in relation to the fatty acid composition and levels of endogenous monoamines in three cerebral regions: the frontal cortex, the striatum and the cerebellum. Long-term feeding of the (n-3)-deficient diet induced a significantly higher 5-HT2 receptor density in the frontal cortex compared with the control rats without any difference in the endogenous serotonin concentrations. The results also showed some modification of dopaminergic neurotransmission specifically in the frontal cortex in the rats deficient in alpha-linolenic acid, with a significantly lower density of D2 receptors and a significantly lower concentration of endogenous dopamine than in control animals. Moreover, there were lower levels of (n-3) fatty acids in all the regions studied in the deficient rats, balanced by greater levels of (n-6) fatty acids. These results suggest that chronic consumption of an alpha-linolenic acid-deficient diet could induce modifications of the neurotransmission pathways; this might induce the behavioral disturbances previously described in this fatty acid-deficient animal model.

Monte Carlo simulation of a medical linear accelerator for radiotherapy use

A Monte Carlo code MCNPX (Monte Carlo N-particle) was used to model a 25 MV photon beam from a PRIMUS (KD2-Siemens) medical linear electron accelerator at the Centre Antoine Lacassagne in Nice. The entire geometry including the accelerator head and the water phantom was simulated to calculate the dose profile and the relative depth-dose distribution. The measurements were done using an ionisation chamber in water for different square field ranges. The first results show that the mean electron beam energy is not 19 MeV as mentioned by Siemens. The adjustment between the Monte Carlo calculated and measured data is obtained when the mean electron beam energy is approximately 15 MeV. These encouraging results will permit to check calculation data given by the treatment planning system, especially for small fields in high gradient heterogeneous zones, typical for intensity modulated radiation therapy technique.

Monte Carlo simulation of a protontherapy platform devoted to ocular melanoma

Patients with ocular melanoma have been treated since June 1991 at the medical cyclotron of the Centre Antoine Lacassagne (CAL). Positions and sizes of the ocular nozzle elements were initially defined based on experimental work, taking as a pattern functional existing facilities. Nowadays Monte Carlo (MC) calculation offers a tool to refine this geometry by adjusting size and place of beam modeling devices. Moreover, the MC tool is a useful way to calculate the dose and to evaluate the impact of secondary particles in the field of radiotherapy or radiation protection. Both LINAC and cyclotron producing x rays, electrons, protons, and neutrons are available in CAL, which suggests choosing MCNPX for its particle versatility. As a first step, the existing installation was input in MCNPX to check its aptitude to reproduce experimentally measured depth-dose profile, lateral profile, output-factor (OF), and absolute dose. The geometry was defined precisely and described from the last achromatic bending magnet of our proton beam line to the position of treated eyes. Relative comparisons of percentage depth-dose and lateral profiles, performed between measured data and simulations, show an agreement of the order of 2% in dose and 0.1 mm in range accuracy. These comparisons, carried out with and without beam-modifying device, yield results compatible to the required precision in ocular melanoma treatments, as long as adequate choices are made on MCNPX input decks for physics card. Absolute dose and OF issued from calculations and measurements were also compared. Results obtained for these two kinds of data, carried out in the simplified situation of an unmodulated beam, indicate that MC calculation could effectively complement measurements. These encouraging results are a large source of motivation to promote further studies, first in a new design of the ocular nozzle, and second in the analysis of the influence of beam-modifying devices attached to the final patient collimator, such as wedge or compensators, on dose values.