Accurate estimation of dose distributions inside an eye irradiated with 106Ru plaques.

BACKGROUND: Irradiation of intraocular tumors requires dedicated techniques, such as brachytherapy with (106)Ru plaques. The currently available treatment planning system relies on the assumption that the eye is a homogeneous water sphere and on simplified radiation transport physics. However, accurate dose distributions and their assessment demand better models for both the eye and the physics. METHODS: The Monte Carlo code PENELOPE, conveniently adapted to simulate the beta decay of (106)Ru over (106)Rh into (106)Pd, was used to simulate radiation transport based on a computerized tomography scan of a patient's eye. A detailed geometrical description of two plaques (models CCA and CCB) from the manufacturer BEBIG was embedded in the computerized tomography scan. RESULTS: The simulations were firstly validated by comparison with experimental results in a water phantom. Dose maps were computed for three plaque locations on the eyeball. From these maps, isodose curves and cumulative dose-volume histograms in the eye and for the structures at risk were assessed. For example, it was observed that a 4-mm anterior displacement with respect to a posterior placement of a CCA plaque for treating a posterior tumor would reduce from 40 to 0% the volume of the optic disc receiving more than 80 Gy. Such a small difference in anatomical position leads to a change in the dose that is crucial for side effects, especially with respect to visual acuity. The radiation oncologist has to bring these large changes in absorbed dose in the structures at risk to the attention of the surgeon, especially when the plaque has to be positioned close to relevant tissues. CONCLUSION: The detailed geometry of an eye plaque in computerized and segmented tomography of a realistic patient phantom was simulated accurately. Dose-volume histograms for relevant anatomical structures of the eye and the orbit were obtained with unprecedented accuracy. This represents an important step toward an optimized brachytherapy treatment of ocular tumors.

Comparison between PENELOPE and electron Monte Carlo simulations of electron fields used in the treatment of conjunctival lymphoma.

For the treatment of conjunctival lymphoma in the early stages, external beam radiotherapy offers a curative approach. Such treatment requires the use of highly conformed small radiation beams. The beam size is so small that even advanced treatment planning systems have difficulties in calculating dose distributions. One possible approach for optimizing the treatment technique and later performing treatment planning is by means of full Monte Carlo (MC) simulations. In this paper, we compare experimental absorbed dose profiles obtained with a collimator used at the University Hospital Essen, with MC simulations done with the general-purpose radiation transport code PENELOPE. The collimator is also simulated with the hybrid MC code electron Monte Carlo (eMC) implemented in the commercial treatment planning system Eclipse (Varian). The results obtained with PENELOPE have a maximum difference with experimental data of 2.3%, whereas the eMC code differs systematically from the experimental data about 7% in the penumbra tails. We also show that PENELOPE simulations are able to obtain absorbed dose maps with an equivalent statistical uncertainty to the one found with eMC in similar CPU times.

Comparison of adjoint and forward Monte Carlo based treatment planning for a realistic head phantom.

BNCT treatment planning can be improved by having the adjoint technique available in the Monte Carlo transport code. In adjoint MC, the simulated particles travel backwards instead of 'forward'. By speeding up the calculations, more beam positions can be investigated and thus a better plan can be composed. In a realistic head phantom with 10 disseminated lesions in the brain, the adjoint method is more favourable than the forward calculations whenever larger beam diameters are applied.

Extension of the calibration curve for the PGRA facility in Petten.

At the boron neutron capture therapy (BNCT) facility in Petten, the Netherlands, (10)B concentrations in biological materials are measured with the prompt gamma ray analyses facility that is calibrated using certified (10)B solutions ranging from 0 to 210 ppm. For this study, newly certified (10)B solutions ranging up to 1972 ppm are added. MCNP simulations of the setup range to 5000 ppm. A second order polynomial (as already used) will fit (10)B-concentrations less than 300 ppm. Above 300 ppm a fitted third order polynomial is needed to describe the calibration curve accurately.

Early clinical trial concept for boron neutron capture therapy: a critical assessment of the EORTC trial 11001.

BNCT causes selective damage to tumor cells by neutron capture reactions releasing high LET-particles where (10)B-atoms are present. Neither the (10)B-compound nor thermal neutrons alone have any therapeutic effect. Therefore, the development of BNCT to a treatment modality needs strategies, which differ from the standard phase I-III clinical trials. An innovative trial design was developed including translational research and a phase I aspect. The trial investigates as surrogate endpoint BSH and BPA uptake in different tumor entities.

Calculation of boron neutron capture cell inactivation in vitro based on particle track structure and x-ray sensitivity.

The Monte-Carlo technique was used to perform quantitative microdosimetric model calculations of cell survival after boron neutron capture irradiations in vitro. The high energy 7Li and alpha-particles resulting from the neutron capture reaction 10B (n,alpha)7Li are of short range and are highly damaging to cells. The biophysical model of the Monte-Carlo calculations is based on the track structure of these a-particles and 7Li-ions and the x-ray sensitivity of the irradiated cells. The biological effect of these particles can be determined if the lethal effect of local doses deposited in very small fractional volumes of the cell nucleus is known. This lethal effect can be deduced from experimental data of cell survival after x-ray irradiation assuming a Poisson distribution for lethal events. The input data used in a PC-based computer program are the radial dose distribution inside the track of the released particles, cell survival after x-ray irradiation, geometry of the tumor cells, subcellular 10B concentration, and thermal neutron fluence. The basic concept of this Monte-Carlo computer model is demonstrated. Validations of computer calculations are presented by comparing them with experimental data on cell survival.