External photon radiation treatment for prostate cancer: Uncomplicated and cancer-free control probability assessment of 36 plans.

PURPOSE: To perform a systematic and thorough assessment, using the Uncomplicated and Cancer-Free Control Probability (UCFCP) function, of a broad range of photon prostate cancer RT treatments, on the same scenario (a unique pelvic CT set). UCFCP considers, together with the probabilities of local tumour control (TCP) and deterministic (late) sequelae (NTCP), the second primary cancer risk (SPCR) due to photon and neutron peripheral doses. METHODS AND MATERIALS: Thirty-six radiotherapy plans were produced for the same CT. 6, 10, 15 and 18 MV 3DCRT, IMRT and VMAT (77.4 Gy in 43 fractions) and 6 and 10 MV SBRT (36.25 Gy in 5 fractions with flattened and FFF beams) for Elekta, Siemens and Varian Linacs plans were included. DVH and peripheral organ dosimetry were used to compute TCP, NTCP, and SPCR (the competition and LNT models) for further plan ranking. RESULTS: Biological models (and parameters) used predicted an outcome which is in agreement with epidemiological findings. SBRT plans showed the lowest SPCR and a below average NTCPrectal. High energy plans did not rank worse than the low energy ones. Intensity modulated plans were ranked above the 3D conformal techniques. CONCLUSIONS: According to UCFCP, the best plans were the10 MV SBRTs. SPCR rates were low and did not show a substantial impact on plan ranking. High energy intensity-modulated plans did not increase in excess the average of SPCR. Even more, they ranked among the best, provided that MU were efficiently managed.

10-MV SBRT FFF IRRADIATION TECHNIQUE IS ASSOCIATED TO THE LOWEST PERIPHERAL DOSE: THE OUTCOME OF 142 TREATMENT PLANS FOR THE 10 MOST COMMON TUMOUR LOCATIONS.

There is a growing interest in the combined use of Stereotactic Body Radiation Therapy (SBRT) with Flattening Filter Free (FFF) due to the high local control rates and reduced treatment times, compared to conventionally fractionated treatments. It has been suggested that they may also provide a better radiation protection to radiotherapy patients as a consequence of the expected decrease in peripheral doses. This work aims to determine this reduction in unattended out-of-field regions, where no CT information is available but an important percentage of second primary cancers occur. For that purpose, ten different cases suitable for SBRT were chosen. Thus, 142 different treatment plans including SBRT, as well as 3D-CRT, IMRT and VMAT (with standard fractionation) in low and high energies for Varian (FF and FFF), Siemens and Elekta machines were created. Then, photon and neutron peripheral dose in 14 organs were assessed and compared using two analytical models. For the prostate case, uncomplicated and cancer free control probability estimation was also carried out. As a general behavior, SBRT plans led to the lowest peripheral doses followed by 3D-CRT, VMAT and IMRT, in this order. Unflattened beams proved to be the most effective in reducing peripheral doses, especially for 10 MV. The obtained results suggest that FFF beams for SBRT with 10 MV represent the best compromise between dose delivery efficiency and peripheral dose reduction.

Peripheral equivalent neutron dose model implementation for radiotherapy patients.

PURPOSE: Neutron peripheral contamination in high-energy radiotherapy implies an increase of secondary radiation-induced cancer risk. Although peripheral neutron dose (PND) has been evaluated in organs, few studies have been performed regarding patient size. This work aims to improve an existing methodology for adult patient PND estimations to generalize it to young and children, for its implementation in treatment planning systems (TPS). METHODS: As a first step, we aimed to generalize the previous model to be usable with any thermal neutron detector. Then, taking into account total neutron spectra and dose-to-point thermal neutron fluence measurements for three phantom sizes (adult, teen and child) and two common treatment locations (H&N and abdomen), the new model was proposed. It represents an upgraded parameterization and extension of the existing one, including patient anatomy. Finally, comparison between estimations and measurements, as well as validation against the original model, was carried out for 510 measured patients. RESULTS: Concordance found between experimental and theoretical estimations makes us confident about later implementation in treatment planning systems. Comparison among the previous and upgraded models shows no significant differences for the adult case. However, an important underestimation (34.1% on average) can be observed regarding child case for the original one. CONCLUSIONS: An improved generalization of an existing PND model, considering patient anatomy has been validated and used in real patients. The final methodology is easily implementable in clinical routine and TPS thanks to the ready availability of input parameters (patient height and weight, high-energy MU and facility characterization).

Neutron measurements in radiotherapy: A method to correct neutron sensitive devices for parasitic photon response.

One of the major causes of secondary malignancies after radiotherapy treatments are peripheral doses, known to increase for some newer techniques (such as IMRT or VMAT). For accelerators operating above 10MV, neutrons can represent important contribution to peripheral doses. This neutron contamination can be measured using different passive or active techniques, available in the literature. As far as active (or direct-reading) procedures are concerned, a major issue is represented by their parasitic photon sensitivity, which can significantly affect the measurement when the point of test is located near to the field-edge. This work proposes a simple method to estimate the unwanted photon contribution to these neutrons. As a relevant case study, the use of a recently neutron sensor for "in-phantom" measurements in high-energy machines was considered. The method, called "Dual Energy Photon Subtraction" (DEPS), requires pairs of measurements performed for the same treatment, in low-energy (6MV) and high energy (e.g. 15MV) fields. It assumes that the peripheral photon dose (PPD) at a fixed point in a phantom, normalized to the unit photon dose at the isocenter, does not depend on the treatment energy. Measurements with ionization chamber and Monte Carlo simulations were used to evaluate the validity of this hypothesis. DEPS method was compared to already published correction methods, such as the use of neutron absorber materials. In addition to its simplicity, an advantage of DEPs procedure is that it can be applied to any radiotherapy machine.

Improving the neutron-to-photon discrimination capability of detectors used for neutron dosimetry in high energy photon beam radiotherapy.

The increasing interest of the medical community to radioinduced second malignancies due to photoneutrons in patients undergoing high-energy radiotherapy, has stimulated in recent years the study of peripheral doses, including the development of some dedicated active detectors. Although these devices are designed to respond to neutrons only, their parasitic photon response is usually not identically zero and anisotropic. The impact of these facts on measurement accuracy can be important, especially in points close to the photon field-edge. A simple method to estimate the photon contribution to detector readings is to cover it with a thermal neutron absorber with reduced secondary photon emission, such as a borated rubber. This technique was applied to the TNRD (Thermal Neutron Rate Detector), recently validated for thermal neutron measurements in high-energy photon radiotherapy. The positive results, together with the accessibility of the method, encourage its application to other detectors and different clinical scenarios.

Experimental evaluation of neutron dose in radiotherapy patients: Which dose?

PURPOSE: The evaluation of peripheral dose has become a relevant issue recently, in particular, the contribution of secondary neutrons. However, after the revision of the Recommendations of the International Commission on Radiological Protection, there has been a lack of experimental procedure for its evaluation. Specifically, the problem comes from the replacement of organ dose equivalent by the organ-equivalent dose, being the latter "immeasurable" by definition. Therefore, dose equivalent has to be still used although it needs the calculation of the radiation quality factor Q, which depends on the unrestricted linear energy transfer, for the specific neutron irradiation conditions. On the other hand, equivalent dose is computed through the radiation weighting factor wR, which can be easily calculated using the continuous function provided by the recommendations. The aim of the paper is to compare the dose equivalent evaluated following the definition, that is, using Q, with the values obtained by replacing the quality factor with wR. METHODS: Dose equivalents were estimated in selected points inside a phantom. Two types of medical environments were chosen for the irradiations: a photon- and a proton-therapy facility. For the estimation of dose equivalent, a poly-allyl-diglicol-carbonate-based neutron dosimeter was used for neutron fluence measurements and, additionally, Monte Carlo simulations were performed to obtain the energy spectrum of the fluence in each point. RESULTS: The main contribution to dose equivalent comes from neutrons with energy higher than 0.1 MeV, even when they represent the smallest contribution in fluence. For this range of energy, the radiation quality factor and the radiation weighting factor are approximately equal. Then, dose equivalents evaluated using both factors are compatible, with differences below 12%. CONCLUSIONS: Quality factor can be replaced by the radiation weighting factor in the evaluation of dose equivalent in radiotherapy environments simplifying the practical procedure.

Using a Tandem Pelletron accelerator to produce a thermal neutron beam for detector testing purposes.

Active thermal neutron detectors are used in a wide range of measuring devices in medicine, industry and research. For many applications, the long-term stability of these devices is crucial, so that very well controlled neutron fields are needed to perform calibrations and repeatability tests. A way to achieve such reference neutron fields, relying on a 3 MV Tandem Pelletron accelerator available at the CNA (Seville, Spain), is reported here. This paper shows thermal neutron field production and reproducibility characteristics over few days.

Commissioning the neutron production of a Linac: development of a simple tool for second cancer risk estimation.

PURPOSE: Knowing the contribution of neutron to collateral effects in treatments is both a complex and a mandatory task. This work aims to present an operative procedure for neutron estimates in any facility using a neutron digital detector. METHODS: The authors' previous work established a linear relationship between the total second cancer risk due to neutrons (TR(n)) and the number of MU of the treatment. Given that the digital detector also presents linearity with MU, its response can be used to determine the TR(n) per unit MU, denoted as m, normally associated to a generic Linac model and radiotherapy facility. Thus, from the number of MU of each patient treatment, the associated risk can be estimated. The feasibility of the procedure was tested by applying it in eight facilities; patients were evaluated as well. RESULTS: From the reading of the detector under selected irradiation conditions, m values were obtained for different machines, ranging from 0.25 × 10(-4)% per MU for an Elekta Axesse at 10 MV to 6.5 × 10(-4)% per MU for a Varian Clinac at 18 MV. Using these values, TR(n) of patients was estimated in each facility and compared to that from the individual evaluation. Differences were within the range of uncertainty of the authors' methodology of equivalent dose and risk estimations. CONCLUSIONS: The procedure presented here allows an easy estimation of the second cancer risk due to neutrons for any patient, given the number of MU of the treatment. It will enable the consideration of this information when selecting the optimal treatment for a patient by its implementation in the treatment planning system.

A new online detector for estimation of peripheral neutron equivalent dose in organ.

PURPOSE: Peripheral dose in radiotherapy treatments represents a potential source of secondary neoplasic processes. As in the last few years, there has been a fast-growing concern on neutron collateral effects, this work focuses on this component. A previous established methodology to estimate peripheral neutron equivalent doses relied on passive (TLD, CR39) neutron detectors exposed in-phantom, in parallel to an active [static random access memory (SRAMnd)] thermal neutron detector exposed ex-phantom. A newly miniaturized, quick, and reliable active thermal neutron detector (TNRD, Thermal Neutron Rate Detector) was validated for both procedures. This first miniaturized active system eliminates the long postprocessing, required for passive detectors, giving thermal neutron fluences in real time. METHODS: To validate TNRD for the established methodology, intrinsic characteristics, characterization of 4 facilities [to correlate monitor value (MU) with risk], and a cohort of 200 real patients (for second cancer risk estimates) were evaluated and compared with the well-established SRAMnd device. Finally, TNRD was compared to TLD pairs for 3 generic radiotherapy treatments through 16 strategic points inside an anthropomorphic phantom. RESULTS: The performed tests indicate similar linear dependence with dose for both detectors, TNRD and SRAMnd, while a slightly better reproducibility has been obtained for TNRD (1.7% vs 2.2%). Risk estimates when delivering 1000 MU are in good agreement between both detectors (mean deviation of TNRD measurements with respect to the ones of SRAMnd is 0.07 cases per 1000, with differences always smaller than 0.08 cases per 1000). As far as the in-phantom measurements are concerned, a mean deviation smaller than 1.7% was obtained. CONCLUSIONS: The results obtained indicate that direct evaluation of equivalent dose estimation in organs, both in phantom and patients, is perfectly feasible with this new detector. This will open the door to an easy implementation of specific peripheral neutron dose models for any type of treatment and facility.

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.

SU-E-T-391: Modelling Peripheral Photon Dose in TomoTherapy Treatments.

PURPOSE: The delivery of the therapeutic radiation dose to the tumour in photon radiotherapy, also implies dose deposition in distant organs (peripheral dose) related to secondary cancers induction (Hall and Wuu, Int J Radiat Oncol Biol Phys 56:83-88, 2003). Therefore, peripheral dose estimation in MU-demanding techniques, such as Helical TomoTherapy (HT), becomes relevant. TLD measurements and Monte Carlo modelling were compared by D'Agostino (Strahlenther Onkol 187:693, 2011). The purpose of this work was to find out experimental models predicting the equivalent photon dose as a function of the distance to the isocenter for different treatment types. The prostate case is presented here. METHODS: A HT prostate plan was delivered to an anthropomorphic phantom mimicking a male adult. The phantom was made of polyethylene blocks whereas light wood was used for lungs. 16 points distributed along the phantom, covering different depths, were selected (Sánchez-Doblado IFMBE, World Congress Med Phys & Biomed Eng, 259-261, 2009). Additionally, a polyethylene sheet was inserted in the phantom to measure the off-axis dose profile at midplane depth. Measurements were carried out with standard TLD-100 pairs of dosimeters (calibrated in a 137Cs source). RESULTS: Two-exponential-terms curve fitting was carried out to model separately the scatter and leakage contribution (f=a*exp(-b*x)+c*exp(-d*x)). The former resulted predominant in the proximal region (10=x=14cm) and the latter in the distal re gion (x=14cm). Both components equate at 18cm. Scatter contribution becomes negligible for x=23cm. Points at 5cm were not used for the model as they are too close to the isocenter to be considered as peripheral dose. Model fits well experimental data (13% mean deviation). Only depths behind the build-up region could be properly modelled. CONCLUSIONS: Peripheral photon dose profiles in HT treatments have been modelled by a two-exponential-terms curve modelling separately scatter and leakage.

SU-E-T-466: TCP and NTCP: Is That All?

PURPOSE: Concerns about the secondary cancer risks associated to the peripheral neutron and photon contamination in photon modern radiotherapy (RT) techniques (e.g., Intensity Modulated RT -IMRT- or Intensity Modulated Arc Therapy -IMAT) have been widely raised. Benefits in terms of better tumor coverage have to be balanced against the drawbacks of poorer organ at risk sparing and secondary cancer risk in order to make the decision on the optimum treatment technique. The aim of this study was to develop a tool which estimates treatment success taking into consideration the neutron secondary cancer probability. METHODS: A methodology and benchmark dataset for radiotherapy real time assessment of patient neutron dose and application to a novel digital detector (DD) has been carried out (submitted to PMB, 2011). Our DD provides real time neutron equivalent dose distribution in relevant organs along the patient. This information, together with TCP and NTCP estimated from the DVH of target and organs at risks, respectively, have been built into a general biological model which allows us to evaluate the success of the treatments (Sánchez-Nieto et al., ESTRO meeting 2012). This model has been applied to make estimation of treatment success in a variety of treatment techniques (3DCRT, forward and inverse IMRT, RapidArc, Volumetric Modulated Arc Therapy and Helical Tomotherapy) to low and high energy. RESULTS: MU-demanding techniques at high energies were able to deliver treatment plans with the highest complicated-free tumour control. Nevertheless, neutron peripheral dose must be taken into consideration as the associated risk could be of the same order of magnitude than the usually considered NTCPs. CONCLUSIONS: The methodology developed to provide an online organ neutron peripheral dose can be successfully combined with biological models to make predictions on treatment success taking into consideration secondary cancer risks.

The determination of beam quality correction factors: Monte Carlo simulations and measurements.

Modern dosimetry protocols are based on the use of ionization chambers provided with a calibration factor in terms of absorbed dose to water. The basic formula to determine the absorbed dose at a user's beam contains the well-known beam quality correction factor that is required whenever the quality of radiation used at calibration differs from that of the user's radiation. The dosimetry protocols describe the whole ionization chamber calibration procedure and include tabulated beam quality correction factors which refer to 60Co gamma radiation used as calibration quality. They have been calculated for a series of ionization chambers and radiation qualities based on formulae, which are also described in the protocols. In the case of high-energy photon beams, the relative standard uncertainty of the beam quality correction factor is estimated to amount to 1%. In the present work, two alternative methods to determine beam quality correction factors are prescribed-Monte Carlo simulation using the EGSnrc system and an experimental method based on a comparison with a reference chamber. Both Monte Carlo calculations and ratio measurements were carried out for nine chambers at several radiation beams. Four chamber types are not included in the current dosimetry protocols. Beam quality corrections for the reference chamber at two beam qualities were also measured using a calorimeter at a PTB Primary Standards Dosimetry Laboratory. Good agreement between the Monte Carlo calculated (1% uncertainty) and measured (0.5% uncertainty) beam quality correction factors was obtained. Based on these results we propose that beam quality correction factors can be generated both by measurements and by the Monte Carlo simulations with an uncertainty at least comparable to that given in current dosimetry protocols.

The change of response of ionization chambers in the penumbra and transmission regions: impact for IMRT verification.

Significant deviations from the expected dose have been reported in the absolute dosimetry validation of an intensity modulated radiation therapy treatment when individual segments are analyzed. However, when full treatment is considered and all segment doses are added together, these discrepancies fade out, leading to overall dose deviations below a 5% action level. This contradictory behavior may be caused by a partial compensation between detector over-responding and under-responding for measurement conditions far from radiation equilibrium. We consider three treatment verification scenarios that may lead to ionization chamber miss-responding, namely: narrow beam irradiation, field penumbra location and multi-leaf collimator transmission contribution. In this work we have analyzed the response of three different ionization chambers with different active volume under these conditions by means of Monte Carlo (MC) simulation methods. Correction factors needed to convert the detector readout into actual dose to water were calculated by inserting the specific detector geometry (carefully modeled) into the simulations. This procedure required extensive use of parallel computing resources in order to achieve the desired level of uncertainty in the final results. The analysis of the simulations shows the relative contribution of each of the three previously mentioned miss-responding scenarios. Additionally, we provide some evidence on dose deviation compensation in multi-segment radiotherapy treatment verification.

A new method for output factor determination in MLC shaped narrow beams.

A new method for the measurement of output factors of narrow beams is presented in this work. By combining a new large area parallel plane ionization chamber (PTW model T34070) with a relative film dosimetry the output factors of small square fields of a 6 MV beam shaped by a MLC were measured. Several detectors (three ionization chambers, two solid state detectors and film) and Monte Carlo simulation were also employed to validate this new methodology and also to determine those detectors more suitable for narrow beam output factor determination. The proposed method for output factor measurement has shown to be in a very good agreement with diamond, diode and Monte Carlo results while it is insensitive to position displacements. Several uncertainties associated to the process of narrow beam dosimetry have also been addressed.

Efectos biomecánicos de los campos electromagnéticos en el hueso en crecimiento de conejos / Biomechanical effects of electromagnetic fields over growing rabbit bones

Introducción. Los campos electromagnéticos están en investigación desde hace décadas. Actúan, principalmente, mediante el fenómeno piezoeléctrico, factor clave del desarrollo normal y remodelado óseo. Nuestro objetivo fue la búsqueda de los efectos de dos tipos de campos electromagnéticos, variables en el tiempo, sobre las características biomecánicas del hueso en crecimiento en conejos sanos. Material y método. El estudio fue realizado siguiendo las normas de la Comunidad Europea sobre la investigación animal. Usamos 34 conejos de raza híbrida Grande Español, con cuatro meses de edad. Se dividieron en un grupo control y dos grupos de tratamiento, el primero con campos electromagnéticos mediante bobinas de Helmholtz dispuestas en paralelo, y el segundo en el interior de un solenoide. Se sometieron a 18 sesiones de una hora de duración cada una en días alternos. Las patas traseras, obtenidas tras sacrificar a los animales, fueron analizadas biomecánicamente. Estudiamos la carga, la extensión y el esfuerzo flector de rotura, módulo de elasticidad, esfuerzo de abandono de la primera zona elástica y relación entre extensión de la zona elástica y total hasta la rotura. Resultados. Se obtuvieron datos estadísticamente significativos para el esfuerzo de abandono de la primera zona elástica en los grupos primero y segundo, y para el esfuerzo flector de rotura en el primer grupo. Conclusiones. Nuestros datos indican la necesidad de una mayor carga para llegar al punto en el que el hueso presentara una deformación residual al retirar el esfuerzo ejercido tras la aplicación de campos electromagnéticos. Se podría afirmar que probablemente tras la aplicación de campos electromagnéticos pulsados al tejido óseo de animales en crecimiento conseguiríamos un hueso más elástico

Introduction. The electromagnetic fields have been under research for decades. They mainly act by piezoelectric phenomenon, a clear factor of normal development and bone remodelling. Our objective was to discover the effects of two types of electromagnetic fields, variables in time, on the biomechanical characteristics of the growing bone, in healthy rabbits. Material and methods. The study was conducted following the Europeas Comunity guidelines on animal research. We used 24 rabbits of the Spanish Giant hybrid race, with four months of age. They were divided into a control group and two treatment groups, the 1st with electromagnetic fields by Helmholtz coils arranged in parallel and the 2nd in the inside of an solenoid. They were subjected to 18 sessions of one hour long each one, on alternate days. The back lags, obtained after sacrificing the animals, were analyzed biomechanically. We studied: load, extension and flexural break stress, elasticity module, abandonment stress of the first elastic zone and relationship between extension of the elastic zone and total zone to the rupture. Results. Statistically significant data were obtained for the abandonment stress of the first elastic zone in groups 1 and 2 and for the flexural break stress in-group 1. Conclusions. Our data indicate the need of greater load to reach the point in which the bone would have a residual deformation on withdrawing force applied after the application of electromagnetic fields. It could be stated that we would probably achieve a more elastic bone after the application of pulsed electromagnetic fields to the bone tissue of growing animals

Verification of intensity modulated profiles using a pixel segmented liquid-filled linear array.

A liquid isooctane (C8H18) filled ionization chamber linear array developed for radiotherapy quality assurance, consisting of 128 pixels (each of them with a 1.7 mm pitch), has been used to acquire profiles of several intensity modulated fields. The results were compared with film measurements using the gamma test. The comparisons show a very good matching, even in high gradient dose regions. The volume-averaging effect of the pixels is negligible and the spatial resolution is enough to verify these regions. However, some mismatches between the detectors have been found in regions where low-energy scattered photons significantly contribute to the total dose. These differences are not very important (in fact, the measurements of both detectors are in agreement using the gamma test with tolerances of 3% and 3 mm in most of those regions), and may be associated with the film energy dependence. In addition, the linear array repeatability (0.27% one standard deviation) is much better than the film one ( approximately 3%). The good repeatability, small pixel size and high spatial resolution make the detector ideal for the real time profile verification of high gradient beam profiles like those present in intensity modulated radiation therapy and radiosurgery.

Monte Carlo correction factors for a Farmer 0.6 cm3 ion chamber dose measurement in the build-up region of the 6 MV clinical beam.

Reference dosimetry of photon fields is a well-established subject and currently available protocols (such as the IAEA TRS-398 and AAPM TG-51) provide methods for converting the ionization chamber (IC) reading into dose to water, provided reference conditions of charged particle equilibrium (CPE) are fulfilled. But these protocols cannot deal with the build-up region, where the lack of CPE limits the applicability of the cavity theorems and so the chamber correction factors become depth dependent. By explicitly including the IC geometry in the Monte Carlo simulations, depth-dependent dose correction factors are calculated for a PTW 30001 0.6 cm(3) ion chamber in the build-up region of the 6 MV photon beam. The corrected percentage depth dose (PDD) agrees within 2% with that measured using the NACP 02 plane-parallel ion chamber in the build-up region at depths greater than 0.4 cm, where the Farmer chamber wall reaches the phantom surface.

Ionization chamber dosimetry of small photon fields: a Monte Carlo study on stopping-power ratios for radiosurgery and IMRT beams.

Absolute dosimetry with ionization chambers of the narrow photon fields used in stereotactic techniques and IMRT beamlets is constrained by lack of electron equilibrium in the radiation field. It is questionable that stopping-power ratio in dosimetry protocols, obtained for broad photon beams and quasi-electron equilibrium conditions, can be used in the dosimetry of narrow fields while keeping the uncertainty at the same level as for the broad beams used in accelerator calibrations. Monte Carlo simulations have been performed for two 6 MV clinical accelerators (Elekta SL-18 and Siemens Mevatron Primus), equipped with radiosurgery applicators and MLC. Narrow circular and Z-shaped on-axis and off-axis fields, as well as broad IMRT configured beams, have been simulated together with reference 10 x 10 cm2 beams. Phase-space data have been used to generate 3D dose distributions which have been compared satisfactorily with experimental profiles (ion chamber, diodes and film). Photon and electron spectra at various depths in water have been calculated, followed by Spencer-Attix (delta = 10 keV) stopping-power ratio calculations which have been compared to those used in the IAEA TRS-398 code of practice. For water/air and PMMA/air stopping-power ratios, agreements within 0.1% have been obtained for the 10 x 10 cm2 fields. For radiosurgery applicators and narrow MLC beams, the calculated s(w,air) values agree with the reference within +/-0.3%, well within the estimated standard uncertainty of the reference stopping-power ratios (0.5%). Ionization chamber dosimetry of narrow beams at the photon qualities used in this work (6 MV) can therefore be based on stopping-power ratios data in dosimetry protocols. For a modulated 6 MV broad beam used in clinical IMRT, s(w,air) agrees within 0.1% with the value for 10 x 10 cm2, confirming that at low energies IMRT absolute dosimetry can also be based on data for open reference fields. At higher energies (24 MV) the difference in s(w,air) was up to 1.1%, indicating that the use of protocol data for narrow beams in such cases is less accurate than at low energies, and detailed calculations of the dosimetry parameters involved should be performed if similar accuracy to that of 6 MV is sought.

Radiocirugía: método de Monte Carlo / Radiosurgery: Monte Carlo method

Introducción: En algunos tratamientos especiales, tales como la radiocirugía, una diferencia de unos pocos milímetros en los contornos de las isodosis pueden conducir a tomar una estrategia de tratamiento diferente, por lo que un cálculo preciso resulta imprescindible. Esto es particularmente importante cerca de inhomogeneidades, debido a la ausencia de equilibrio electrónico. La simulación con Monte Carlo supera esta imprecisión pero a expensas de un elevado tiempo de CPU. Nuestro trabajo ha ido enfocado a la optimización de los recursos reduciendo el tiempo de computación.Material y Método: Se ha desarrollado un modelo de distribución para la simulación de tratamientos de radiocirugía sobre un cluster de PCs. Un PC servidor exporta el código MC y los datos de entrada a los nodos clientes, almacenando finalmente los resultados. Además, este modelo distribuye las tareas individuales desde un punto de vista clínico y físico por lo que, no solo se reduce el tiempo de CPU sino que se alcanza un análisis detallado del tratamiento. Para la simulación, se ha utilizado el código EGS4 operando bajo UNÍX e integrado en el paquete OMEGA-BEAM. En colaboración con la Escuela de Ingenieros de la Universidad de Sevilla, el modelo ha sido instalado en una red local de 36 PCs.Resultados: Tratamientos de radiocirugia complejos fueron simulados usando este modelo, empleando tiempo inferiores a la hora. Los resultados obtenidos con el planificador convencional y con MC coinciden en la mayoría de los casos convencionales. Sin embargo, muestran diferencias de hasta el 30 por ciento en algunas isodosis cuando entran en juego inhomogeneidades, implícitas en situaciones complejas.Conclusiones: Por tanto, una buena utilización de los recursos disponibles pueden convertir a la simulación con MC en una alternativa abordable por cualquier institución sin un gran coste adicional. Esta accesibilidad y operatividad gracias a la reducción sustancial en el tiempo de procesamiento, presentan a la simulación con MC como una herramienta eficaz para la verificación de tratamientos (AU)