Monte carlo clinical dosimetry

The choice of the most appropriate strategy for radiotherapy treatment is mainly based on the use of a planning system. With the introduction of new techniques (conformal and/or small fields, asymmetrical and non coplanar beams, true 3D calculation, IMRT) the trustworthiness of the algorithms used is questioned. An alternative verification procedure has become increasingly more necessary to warranty treatment delivery.The reliability of the Monte Carlo method is generally acknowledged. However, its clinical use has not been practical due to the high CPU time required. During the last few years our objective has decreased CPU time by means of a new process distribution technique. This reduction has made it feasible, not only to apply physical dosimetry under special conditions, but also to use it in numerous clinical cases employing photon and electron conformal fields, in radiosurgery, and IMRT.The procedure carried out is presented. Furthermore, conventional Treatment Planning System calculations are compared with the Monte Carlo simulations

Peripheral organ equivalent dose estimation procedure in proton therapy

The aim of this work is to present a reproducible methodology for the evaluation of total equivalent doses in organs during proton therapy facilities. The methodology is based on measuring the dose equivalent in representative locations inside an anthropomorphic phantom where photon and neutron dosimeters were inserted. The Monte Carlo simulation was needed for obtaining neutron energy distribution inside the phantom. The methodology was implemented for a head irradiation case in the passive proton beam of iThemba Labs (South Africa). Thermoluminescent dosimeter (TLD)-600 and TLD-700 pairs were used as dosimeters inside the phantom and GEANT code for simulations. In addition, Bonner sphere spectrometry was performed inside the treatment room to obtain the neutron spectra, some relevant neutron dosimetric quantities per treatment Gy, and a percentual distribution of neutron fluence and ambient dose equivalent in four energy groups, at two locations. The neutron spectrum at one of those locations was also simulated so that a reasonable agreement between simulation and measurement allowed a validation of the simulation. Results showed that the total out-of-field dose equivalent inside the phantom ranged from 1.4 to 0.28 mSv/Gy, mainly due to the neutron contribution and with a small contribution from photons, 10% on average. The order of magnitude of the equivalent dose in organs was similar, displaying a slow reduction in values as the organ is farther from the target volume. These values were in agreement with those found by other authors in other passive beam facilities under similar irradiation and measurement conditions

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

Valoración de la contaminación de neutrones producidos por un acelerador lineal de electrones (18 MV) de uso clínico

Los haces de radiación de altas energías ( 10 MEV) están generalmente contaminados con neutrones resultantes de la reacción (y n) producida por la interacción de los fotones con los elementos constituyentes del acelerador (blanco blindaje colimadores bobinas de deviación...). Si bien el exceso de dosis añadido al paciente por la presencia de esta partícula puede ser despreciable no ocurre igual desde el punto de vista de la protección radiológica del personal ya que aparecen nuevos isotopos debido al fenómeno de la activación de los diferentes materiales presentes en la sala de tratamientos. En este trabajo se ha valorado el flujo de neutrones existente en diferentes puntos del bunker en torno a nuestro acelerador saturno-20. La valoración ha sido realizada mediante la activación de chapas de indio y posterior lectura en un analizador multicanal. La energía nominal utilizada es de rx-18MV por ser la que produce mayor flujo de neutrones de los haces disponibles. Así mismo mediante el procedimiento teórico del método de Montecarlo hemos obtenido el espectro neutrónico junto al acelerador. Por último una dosimetría ambiental de la sala nos ha permitido valorar el riesgo radiológico que supone este fenómeno para el personal profesionalmente expuesto.

Editorial: Out-of-field second primary cancer induction: Dosimetry and modelling

Second primary cancer induction is a growing concern, particularly for the younger cancer patient population with a longer life expectancy, as demonstrated by the increasing number of publications on the topic. Still, there is much work to do (1), such as assessing problems associated with the dosimetry under no reference conditions (particularly in proton treatments) or the presence of mixed-fields. Additionally, due to the poor performance of commercial treatment planning systems (TPS) in stray dose calculations for photon (2) and proton radiotherapy (RT), the development and implementation of computational tools are needed for out-of-field dose estimation in a systematic way. Thus, dosimetric information might be part of databases for cancer patients treated with modern RT techniques together with detrimental outcomes such as second primary cancers. The latter will improve existing risk models, which should also be considered during RT plan optimization. (extract

Evaluación en tiempo real de dosis de neutrones en pacientes en tratamiento con radioterapia y detector para llevar a cabo el procedimiento

Evaluación en tiempo real de dosis de neutrones en pacientes en tratamiento con radioterapia y detector para llevar a cabo el procedimiento.Procedimiento y detector para calcular las dosis equivalentes de neutrones en órganos de un paciente y el riesgo a

Intensity-modulated radiation therapy and volumetric modulated arc therapy versus conventional conformal techniques at high energy: Dose assessment and impact on second primary cancer in the out-of-field region

The aim of this work was to estimate peripheral neutron and photon doses associated with the conventional 3D conformal radiotherapy techniques in comparison to modern ones such as Intensity modulated radiation therapy and volumetric modulated arc therapy. Assessment in terms of second cancer incidence ought to peripheral doses was also considered. For that, a dosimetric methodology proposed by the authors has been applied beyond the region where there is no CT information and, thus, treatment planning systems do not calculate and where, nonetheless, about one third of second primary cancers occurs

Peripheral organ equivalent dose estimation procedure in proton therapy

The aim of this work is to present a reproducible methodology for the evaluation of total equivalent doses in organs during proton therapy facilities. The methodology is based on measuring the dose equivalent in representative locations inside an anthropomorphic phantom where photon and neutron dosimeters were inserted. The Monte Carlo simulation was needed for obtaining neutron energy distribution inside the phantom. The methodology was implemented for a head irradiation case in the passive proton beam of iThemba Labs (South Africa). Thermoluminescent dosimeter (TLD)-600 and TLD-700 pairs were used as dosimeters inside the phantom and GEANT code for simulations. In addition, Bonner sphere spectrometry was performed inside the treatment room to obtain the neutron spectra, some relevant neutron dosimetric quantities per treatment Gy, and a percentual distribution of neutron fluence and ambient dose equivalent in four energy groups, at two locations. The neutron spectrum at one of those locations was also simulated so that a reasonable agreement between simulation and measurement allowed a validation of the simulation. Results showed that the total out-of-field dose equivalent inside the phantom ranged from 1.4 to 0.28 mSv/Gy, mainly due to the neutron contribution and with a small contribution from photons, 10% on average. The order of magnitude of the equivalent dose in organs was similar, displaying a slow reduction in values as the organ is farther from the target volume. These values were in agreement with those found by other authors in other passive beam facilities under similar irradiation and measurement conditions