RESUMEN
This work is devoted to studying the influence of chamber response functions on the standard IMRT verification for the different detector technologies available on commercial devices. We have tested three of the most used 2D detector arrays for radiotherapy dosimetry verification, based on air-ionization chambers and diode detectors. The response function has been carefully simulated using the Monte Carlo method and measured through slit and pinhole collimators. Although the response function of air-ionization detectors is considerably different with respect to that of standard diodes, the impact on a verification based in the gamma function with tolerances 3 mm and 3% is quite limited. The results show that the standard air-ionization detector arrays perform in a similar way whenever the tolerances for the gamma function are not lowered below 1.5 mm and 1.5%. Additionally, the sensitivity of these devices to fluence perturbations was measured by intentionally modifying some leaf positions in the multileaf collimator. The wider response function of air-ionization chamber arrays made them slightly more sensitive to random fluence perturbations, although silicon diode arrays are more accurate to describe the dose distribution in a point by point basis.
Asunto(s)
Radiometría/instrumentación , Radioterapia de Intensidad Modulada/instrumentación , Diseño de Equipo , Dosificación RadioterapéuticaRESUMEN
European regulations require the dose delivered to patients in CT examinations to be monitored and checked against reference levels. Dose estimation has traditionally been performed manually. This is time consuming and therefore it is typically performed on just a few patients and the results extrapolated to the general case. In this work an automated method to estimate the dose in CT studies is presented. The presented software downloads CT studies from the corporative picture archiving and communication system and uses the information on the DICOM headers to perform the dose calculation. Automation enables dose estimations to be performed on a larger fraction of studies, enabling more significant comparisons with diagnostic reference levels (DRLs). A preliminary analysis involving 5800 studies is presented with details of dose distributions for selected CT protocols in use at a university hospital. Average doses are compared with DRLs. Effective dose estimations are also compared with estimations based on the dose length product.
Asunto(s)
Algoritmos , Carga Corporal (Radioterapia) , Sistemas de Administración de Bases de Datos , Sistemas de Información Radiológica , Radiometría/métodos , Tomografía Computarizada por Rayos XRESUMEN
In the year 1997 Siemens introduced the virtual wedge in its accelerators. The idea was that a dose profile similar to that of a physical wedge can be obtained by moving one of the accelerator jaws at a constant speed while the dose rate is changing. This work explores the observed behaviour of virtual wedge factors. A model is suggested which takes into account that at any point in time, when the jaw moves, the dose at a point of interest in the phantom is not only due to the direct beam. It also depends on the scattered radiation in the phantom, the head scatter and the behaviour of the monitoring system of the accelerator. Measurements are performed in a Siemens Primus accelerator and compared to the model predictions. It is shown that the model agrees reasonably well with measurements spanning a wide range of conditions. A strong dependence of virtual wedge factors on the dosimetric board has been confirmed and an explanation has been given on how the balance between different contributions is responsible for virtual wedge factors values.