Choice of a Suitable Dosimeter for Photon Percentage Depth Dose Measurements in Flattening Filter-Free Beams.

The International Atomic Energy Agency Technical Reports Series-398 code of practice for dosimetry recommends measuring photon percentage depth dose (PDD) curves with parallel-plate chambers. This code of practice was published before flattening filter-free (FFF) beams were widely used in clinical linear accelerators. The choice of detector for PDD measurements needs to be reassessed for FFF beams given the physical differences between FFF beams and flattened ones. The present study compares PDD curves for FFF beams of nominal energies 6 MV, 6 FFF, 10 MV, and 10 FFF from a Varian TrueBeam linear accelerator (Varian Medical Systems, Palo Alto, USA) acquired with Scanditronix photon diodes, two scanning type chambers (both PTW 31010 Semiflex), two small volume chambers (Wellhofer CC04 and PTW 31016 PinPoint 3D), PTW 34001 Roos, Scanditronix Roos, and NACP 02 parallel-plate chambers. Results show that parallel-plate ion chambers can be used for photon PDD measurements, although for better accuracy, recombination effects should be taken into account.

[Impact of a PET/CT facility in its community environment]. / 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.

Impacto en el ámbito comunitario de una instalación de tomografía por emisión de positrones-tomografía computada (PET/CT) / Impact of a PET/CT facility in its community environment

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(AU)

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(AU)

The use of a mixed Poisson model for tumour control probability computation in non homogeneous irradiations.

Tumour control probability (TCP) is the probability of destroying every clonogen in a tumour as a result of a Radiation Therapy treatment. Assuming absorbed dose homogeneity throughout the tumour volume, TCP can be easily derived from a cell survival model. If absorbed dose is non homogeneous, its distribution has to be taken into account, because survival fractions depend on dose. This work presents a method based on mixture probability distributions to introduce absorbed dose heterogeneity using dose volume histograms. Results are close to the ones provided by the standard voxel oriented method usually utilized, but the mixture method makes more robust assumptions about independence between voxels. Therefore, this method is more flexible, and could potentially deal with variations in survival fraction caused by other factors.

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.

Confidence intervals in dose volume histogram computation.

PURPOSE: Dose volume histograms (DVHs) are used in radiation therapy plan optimization and evaluation. Irradiation strategies are decided at the planning step, and an assessment of the reliability of computed dose distributions and DVHs is needed to ensure that decisions are made based on reliable information. This work describes a method used to assign confidence intervals to DVHs, caused by the uncertainty associated with dose computation. METHODS: A simple mathematical model has been developed, based on several alternative models for point dose uncertainty (rectangular, triangular, and Gaussian probability distributions). The result is an alpha-DVH, to be computed for two a values (0.25/0.75 or 0.10/0.90) to obtain a confidence interval. This method has been applied to several DVH curves. RESULTS: alpha-DVHs are dependent on the point uncertainty probability distribution, on the standard uncertainty, and on the choice of confidence level. Confidence intervals can be assigned either to the volume encompassed by an isodose level or to the dose corresponding to a given volume. CONCLUSIONS: alpha-DVHs provide bounds for DVH values while dose expected volume histograms can be used as central estimates for DVH when uncertainty is taken into account. They are particularly useful when an optimization approach is necessary.