M.J. García-Fusté

Estimation of neutron-equivalent dose in organs of patients undergoing radiotherapy by the use of a novel online digital detector

Neutron peripheral contamination in patients undergoing high-energy photon radiotherapy is considered as a risk factor for secondary cancer induction. Organ-specific neutron-equivalent dose estimation is therefore essential for a reasonable assessment of these associated risks. This work aimed to develop a method to estimate neutron-equivalent doses in multiple organs of radiotherapy patients. The method involved the convolution, at 16 reference points in an anthropomorphic phantom, of the normalized Monte Carlo neutron fluence energy spectra with the kerma and energy-dependent radiation weighting factor. This was then scaled with the total neutron fluence measured with passive detectors, at the same reference points, in order to obtain the equivalent doses in organs. The latter were correlated with the readings of a neutron digital detector located inside the treatment room during phantom irradiation. This digital detector, designed and developed by our group, integrates the thermal neutron fluence. The correlation model, applied to the digital detector readings during patient irradiation, enables the online estimation of neutron-equivalent doses in organs. The model takes into account the specific irradiation site, the field parameters (energy, field size, angle incidence, etc) and the installation (linac and bunker geometry). This method, which is suitable for routine clinical use, will help to systematically generate the dosimetric data essential for the improvement of current risk-estimation models.

On line neutron dose evaluation in patients under radiotherapy

Current improvements in radiotherapy require methods to evaluate their costs and benefits. A possible counterpart of the benefit is the creation of a secondary, radiation induced cancer. A new procedure is presented to assess the peripheral dose delivered to the patients due to photo-neutrons by means of a new on line digital detector. The events in the monitor have been correlated with the neutron dose by Monte Carlo simulations and experimental measurements using CR39 and TLD. This digital detector was employed at 6 different linacs, with energies ranging from 6 to 23 MV, obtaining the doses received in each organ of the patient. Additionally, the ambient dose equivalent was determined finding values from 0 to 470 mSv for complete treatments.

Neutron spectrometry and determination of neutron ambient doses in radiotherapy treatments under different exposure conditions

A project has been set up to study the effect on a radiotherapy patient of the neutrons produced around the LINAC accelerator head by photonuclear reactions induced by the gamma radiation above ~8 MeV. These neutrons may reach directly the patient, or they may interact with the surrounding materials until they become thermalised, scattering all over the treatment room and affecting the patient as well, contributing to the peripherical dose. Spectrometry was performed with a set of Bonner spheres at 50 cm from the isocenter and at the place where a digital device for measuring neutrons will be located the treatment room. Exposures have taken place in six linac accelerators with different energies (from 6 to 23 MV). A summary of the spectrometry results and of the neutron doses received by the patient is presented.