A Probability Approach to the Study on Uncertainty Effects on Gamma Index Evaluations in Radiation Therapy

Two datasets of points of known spatial positions and an associated absorbed dose value are often compared for quality assurance purposes in External Beam Radiation Therapy (EBRT). Some problems usually arise regarding the pass fail criterion to accept both datasets as close enough for practical purposes. Instances of this kind of comparisons are fluence or dose checks for intensity modulated radiation therapy, modelling of a treatment unit in a treatment planning system, and so forth. The gamma index is a figure of merit that can be obtained from both datasets; it is widely used, as well as other indices, as part of a comparison procedure. However, it is recognized that false negatives may take place (there are acceptable cases where a certain number of points do not pass the test) due in part to computation and experimental uncertainty. This work utilizes mathematical methods to analyse comparisons, so that uncertainty can be taken into account. Therefore, false rejections due to uncertainty do not take place and there is no need to expand tolerances to take uncertainty into account. The methods provided are based on the rules of uncertainty propagation and help obtain rigorous pass/fail criteria, based on experimental information

Viscothermal effects in a two-dimensional acoustic black hole: A boundary element approach

[EN] The acoustic analog of the quantum black hole for airborne sound in two dimensions was denominated as an omnidirectional acoustic absorber by Climente et al. [see Appl. Phys. Lett., 100, 144103 (2012)], who characterized its absorbing properties without providing any theoretical support. The viscothermal losses of the underlying structure, which consists of an absorbing core and a surrounding gradient-index (GRIN) lens both made of periodic distributions of cylindrical rods, are here comprehensively studied by using the boundary element method (BEM) in two dimensions. It is shown that the numerical simulations in two dimensions reproduce fairly well the increase in absorption of the core when the GRIN lens is added and reveal that the discrepancy between measured and calculated values of absorbance is an artifact of the experimental setup. The possibility of independent calculation of viscous and thermal losses contributions in the two-dimensional (2D) BEM algorithm is employed for the comparison with a homogenization theory in which the cluster of cylinders is represented by a single fluidlike viscous cylinder with effective parameters. We conclude that viscous losses represent about 90% of the total energy dissipated in the core. The homogenization approach results are only 2% below the results calculated with 2D BEM, indicating that the effective parameters obtained by the homogenization are very accurate.Cutanda-Henríquez, V.; Sánchez-Dehesa Moreno-Cid, J. (2021). Viscothermal effects in a two-dimensional acoustic black hole: A boundary element approach. Physical Review Applied. 15(6):1-14. https://doi.org/10.1103/PhysRevApplied.15.064057S11415

Viscothermal Losses in Double-Negative Acoustic Metamaterials

[EN] The influence of losses in double-negative metamaterial slabs recently introduced by Graciá-Salgado et al. [Phys. Rev. B 88, 224305 (2013)] is comprehensively studied. Viscous and thermal losses are considered in the linearized Navier-Stokes equations with no flow. Despite the extremely low thicknesses of boundary layers associated with each type of losses, the double-negative behavior is totally suppressed for the rigid structures under analysis. In other words, almost 100% of the energy transmitted into the slab is dissipated by viscothermal effects, in agreement with experimental data. Simulations undertaken for larger structures, using scale factors of up to 20 times, show that double-negative behavior is never recovered. The huge dissipation obtained by these structures leads us to propose them as interesting alternatives to conventional absorbers for specific situations, e.g., when treating low frequencies or when the excitation is narrow banded.V. M. G.-C. and J. S.-D. acknowledge the support from the Spanish Ministerio de Economia y Competitividad (MINECO), and the European Union Fondo Europeo de Desarrollo Regional (FEDER) through Project No. TEC 2014-53088-C3-1-R.Cutanda-Henriquez, V.; Garcia Chocano, VM.; Sánchez-Dehesa Moreno-Cid, J. (2017). Viscothermal Losses in Double-Negative Acoustic Metamaterials. Physical Review Applied. 8(1):014029-1-014029-12. doi:10.1103/PhysRevApplied.8.014029S014029-1014029-128

Datos anómalos y regresión logística robusta en ciencias de la salud

La regresión logística tiene numerosas aplicaciones en Ciencias de la Salud. Existe una amplia literatura respecto a los métodos a seguir y al modo de hallar los estimadores de los parámetros a partir de las observaciones. Estos métodos están incorporados en todos los paquetes estadísticos usuales. Los estimadores son los llamados de �máxima verosimilitud�, es decir, son aquéllos que hacen que las observaciones obtenidas sean las más probables entre todos los posibles modelos que pudiéramos utilizar. Las buenas propiedades de los estimadores de máxima verosimilitud están ampliamente demostradas. Sin embargo, en la práctica existe una serie de circunstancias que pueden ocasionar la aparición de �datos anómalos�, es decir, observaciones que no corresponden al modelo logístico que utilizamos como hipótesis. En ocasiones, estas observaciones anómalas pueden tener un fuerte efecto sobre el ajuste y, por tanto, llevarnos a una conclusión equivocada. Las causas de estos datos anómalos dependen mucho del estudio en cuestión, pero pueden señalarse errores de clasificación, observaciones (sujetos) con características especiales que se han pasado por alto, incertidumbres en la medida de algunos parámetros, etc. El problema de los estimadores de máxima verosimilitud es que no son �robustos�, es decir, su sensibilidad a datos anómalos puede ser arbitrariamente grande, y una minoría de datos anómalos puede dar lugar a un modelo logístico erróneo. En este trabajo expondremos dos casos que ilustran las posibles consecuencias, y discutiremos la aplicación de métodos robustos

A quality index for equivalent uniform dose

Equivalent uniform dose (EUD) is the absorbed dose that, when homogeneously given to a tumor, yields the same mean surviving clonogen number as the given non-homogeneous irradiation. EUD is used as an evaluation tool under the assumption that two plans with the same value of EUD are equivalent, and their biological effect on the tumor (clonogen survival) would be the same as the one of a homogeneous irradiation of absorbed dose EUD. In this work, this assumption has been studied, and a figure of merit of its applicability has been obtained. Distributions of surviving clonogen number for homogeneous and non-homogeneous irradiations are found to be different even if their mean values are the same, the figure of merit being greater when there is a wider difference, and the equivalence assumption being less valid. Therefore, EUD can be closer to a uniform dose for some cases than for other ones (high α values, extreme heterogeneity), and the accuracy of the radiobiological indices obtained for evaluation, could be affected. Results show that the equivalence is very sensitive to the choice of radiobiological parameters, and this conclusion has been derived from mathematical properties of EUD

Impacto en el ámbito comunitario de una instalación de tomografía por emisión de positrones-tomografía computada (PET/CT)

Background: The PET/CT (Positron Emission Tomography/Computed Tomography) technique is currently expanding, with new facilities and indications appearing every year. Being mostly an outpatient technique, patients leave the facility when the study has been performed, although they still retain a certain amount of radiopharmaceutical. Therefore, setting up a PET/CT facility might involve a risk increase for the population. This study aims at estimating this risk. Methods: Comprehensive measurements to estimate dose levels have been carried out in the PET/CT facility at Hospital General Universitario Gregorio Marañón. The population has been distributed into five categories according to their involvement in the studies and their radiation exposure. A quantitative dose estimation has been carried out. Results: The risk associated with a PET/CT facility has been assessed and no risk increase has been detected. Staff dose is shown to be less than 2 μSv per patient, ˜20 μSv for an accompanying person and ˜40 μSv for a relative. Citizens walking along with patients or sharing public transportation with them receive a dose that is below Madrid´s background radiation. Conclusions: Values obtained are well below annual dose limits. Moreover, these results support the outpatient care of this kind of studies, because accompanying persons´ and relatives´ risk is negligible. The impact of a PET/CT facility does not involve an increase in risk for the general population.Fundamento: La técnica de PET/CT (Positron Emission Tomography/Computed Tomography) está actualmente en crecimiento, apareciendo nuevas unidades e indicaciones cada año. Al ser una técnica eminentemente ambulatoria, los pacientes que han terminado la prueba, y que retienen radiofármaco en su cuerpo en un nivel considerado seguro, abandonan la unidad. La instalación de una unidad de PET/CT podría, por tanto, suponer un incremento de riesgo a la población. El objetivo de este trabajo es estudiar este riesgo. Métodos: Se han realizado medidas exhaustivas en la unidad de PET/CT del Hospital General Universitario Gregorio Marañón, con las que se han elaborado las estimaciones. La población se ha dividido en cinco categorías según su implicación en los estudios y la intensidad de su exposición a la radiación y se ha realizado una estimación cuantitativa de dosis. Resultados: La dosis de los trabajadores resulta ser inferior a 2 µSv por paciente, la de un acompañante ˜20 μSv, y la de un familiar ˜40 μSv. Las dosis de ciudadanos que coinciden con pacientes en la calle o en transporte público resultan inferiores al fondo natural en Madrid. Conclusiones: Los valores obtenidos están muy por debajo de los límites anuales de dosis. Además, avalan el régimen ambulatorio de estos estudios, por el pequeño riesgo a familiares y acompañantes. El impacto de una unidad de PET/CT no supone incremento de riesgo para la población en general