M. R. Expósito

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.

Neutron contamination in radiotherapy: Estimation of second cancers based on measurements in 1377 patients

PURPOSE Second cancer, as a consequence of a curative intent radiotherapy (RT), represents a growing concern nowadays. The unwanted neutron exposure is an important contributor to this risk in patients irradiated with high energy photon beams. The design and development by our group of a neutron digital detector, together with the methodology to estimate, from the detector readings, the neutron equivalent dose in organs, made possible the unprecedented clinical implementation of an online and systematic neutron dosimetry system. The aim of this study was to systematically estimate neutron equivalent dose in organs of a large patient group treated in different installations. PATIENTS AND METHODS Neutron dosimetry was carried out in 1377 adult patients at more than 30 different institutions using the new neutron digital detector located inside the RT room. Second cancer risk estimates were performed applying ICRP risk coefficients. RESULTS Averaged equivalent dose in organs ranges between 0.5 mSv and 129 mSv depending on the type of treatment (dose and beam-on time), the distance to isocenter and the linac model. The mean value of the second cancer risk for our patient group is 1.2%. Reference values are proposed for an overall estimation of the risks in 15 linac models (from 2.8 × 10(-5) to 62.7 × 10(-5)%/MU). CONCLUSIONS The therapeutic benefit of RT must outweigh the second cancer risk. Thus, these results should be taken into account when taking clinical decisions regarding treatment strategy choice during RT planning.

Neutron spectra inside an adult and children anthropomorphic phantoms in high energy radiotherapy

This work presents the results of the Monte Carlo simulation of the neutron fluence energy spectra at 16 representative points inside three anthropomorphic phantoms mimicking an adult and two children. Simulations were carried out using the MCNPX-2.6.0 code, and the calculation of the neutron spectra for each beam incidence was made in a single simulation file, including the treatment room, each phantom and the linac. Results show that pediatric patients are exposed to higher neutron fluence than adults. Consequences in terms of an increased secondary cancer risk should be analysed.

Characterization of the neutron induced single event upset in SRAM around high megavoltage clinical accelerators

Linear accelerators for medical purposes represent a source of photoneutron radiation at high energy photon therapy modalities. In this clinical scenario, measurement of the neutron radiation field is a demanding task due to the high in-room photon fluence and its pulsed time structure. The interaction of neutrons with SRAM memory cells can produce upsets in the memory state. This effect has been exploited to produce a device composed of this type of memories used as a neutron detector. This device is mainly sensitive to low energy neutrons, and has been characterized previously in different dedicated experiments in calibrated beam and neutron sources. It has been proposed to estimate the neutron production strength of a medical linac and subsequently the patient neutron exposure during a treatment. The purpose of this work is to study the SRAM device dependence with gantry position and detector location inside a treatment room. These results reflect the neutron fluence variation inside a radiotherapy installation.

Neutron Contamination in Medical Linear Accelerators Operating at Electron Mode

Nowadays, neutron contamination in high energy photon beams normally used in radiotherapy treatments is an issue of interest from the radioprotection point of view. However, neutron production when using electron beams to treat superficial tumors has usually been ignored. The aim of this paper was to study such contamination and its effect on patients. In order to do that, experimental measurements in a radiotherapy environment were carried out using a digital device sensitive to thermal neutrons. Besides, Monte Carlo simulations were performed to estimate the difference in number of particles between photon and electron operational modes, required to deposit the same dose at a certain depth. Results show that neutron production is lower for electron beams than photon ones but not as low as previously expected.

1499 poster PERIPHERAL GAMMA DOSE AND THERMAL NEUTRON FLUENCIES EVALUATION FOR IMRT ON ADULT, TEEN AND CHILD

NEUTRON FLUENCE DISTRIBUTION STUDY IN A PROTON THERAPY FACILITY BUNKER J. I. Lagares1, P. Arce2, F. Sansaloni1, J. A. Terron3, M. R. Exposito4, J. J. Nieto-Camero5, A. Korf5, M. Loubser5, F. Sanchez-Doblado63 1 CIEMAT, Medical Applications Unit, Madrid, Spain 2 CIEMAT, Basic Research, Madrid, Spain 3 HOSPITAL UNIVERSITARIO VIRGEN MACARENA, Radiotherapy Department, Sevilla, Spain 4 UNIVERSIDAD DE SEVILLA, Fisiologia Medica y Biofisica, Seville, Spain 5 ITHEMBA LABS, Medical Radiation Department, Somerset West, South Africa 6 UNIVERSIDAD SEVILLA FACULDAD MEDICINA, Fisiologia Medica y Biofisica, Sevilla, Spain

Neutron Distribution in Radiotherapy Treatment Rooms

Neutrons have a high biological effectiveness. Therefore, patient exposition to them represents a relevant issue and consequences to this exposure, regarding secondary cancer induction, must be clarified. The aim of this work was to make a complete study on the production of photoneutrons in radiotherapy facilities. Detailed Monte Carlo simulation, using MCNPX code, was performed to characterize the neutron generation in a Siemens PRIMUS linac operating at 15 MV and to determine the fluence energy distributions inside the treatment room. This information is mandatory to understand the response, under radiotherapy conditions, of a new neutron digital detector developed by our group.

Evaluation of Neutron Component in Patients under High Energy Radiotherapy By Means of an On Line and In Vivo procedure

The use of improved radiotherapy methods has raised the concern about second cancer induction. Epidemiological studies have shown a major incidence of secondary cancer in radiotherapy patients compared to patients subjected to another type of treatment. In this regard, it is important to determine the peripheral dose received by the patient during the treatment. While photon doses have been deeply contemplated, neutron contamination in high energy photon beams is still a subject of research and discussion. In the present work, we introduce a new procedure based on a digital device that allows real time neutron contamination evaluation. Several irradiations of an anthropomorphic phantom have been carried out in a variety of facilities and treatments. The purpose was to correlate the measurements from the digital detector with the neutron doses obtained in the phantom by Monte Carlo simulations and experimental measurements. A model has been designed to calculate the organ equivalent dose and risk estimates...

The best prostate plan ever: A quantification of the therapeutic window

Finding out the best treatment plan that could be designed for a specific patient could be a difficult task. Benefits on terms of better tumor coverage have to be balanced against the drawbacks of poorer organ at risk sparing in order to make the decision on the optimum treatment technique. A large variety of photon beam radiotherapy (RT) techniques are currently available. The relative merits of different 34 treatment modalities for a prostate cancer, using the same CT data set and contours, have been analyzed in terms of the area under the Uncomplicated Control Probability curve. IMRT forward techniques have resulted to be the best ranked treatment modalities.