Introduction The aim of our work is to develop a transparent 2D detector dedicated to the control of the beam at the output of accelerator for IMRT treatments. This would permit firstly to achieve a ...constant quality control of the beam characteristics (UM stability, homogeneity, MLC leaves’ positions, etc.), and secondly to measure the fluence delivered to the patient. The retained solution is a pixelated parallel plate ionization chamber. Detection is indirect: a part of the photons interact in the upper wall of the chamber (‘‘converter”) creating Compton electrons that will ionize the sensitive volume of the chamber on their way. Several compromise result. On the one hand, the more thicker the converter is, the more important the attenuation of the beam is. But as there are many Compton electrons crossing the sensitive volume, the sensitivity is better. On the other hand, the more thicker the sensitive volume of the chamber is, the greater the sensitivity is. Compton electrons are not emitted in the direction of the incident photon, they can cross the sensitive volume with a large lateral path and degrade the spatial resolution. And this effect increases with the sensitive volume thickness. Materials and methods Monte Carlo simulations (Geant4 9.2.p04) were conducted to evaluate beam attenuation, the sensitivity and spatial resolution of the detector for different converter and sensitive volume heights (converter material: printed circuit board). The sensitivity and spatial resolution are obtained by analyzing the response to a step function (Heaviside): energy deposited in the exposed area and width 20–80%. The phase space Elekta Precise 6 MV from IAEA was used. Results The attenuation is of the order of ∼1% per mm of converter. The sensitivity is proportional to the thickness of the sensitive volume, but not to that of the converter: the proportion of electrons reaching the sensitive volume decreases with increasing thickness of the converter. We lose a factor of 3 on the spatial resolution between sensitive volume thickness of 0.5 mm or 3 mm. Conclusion Compromises expectedwere highlightedandeffectswere quantified. The determination of optimumparameters is dependent on the choice of the electronics, which sets the required sensitivity.
L’évaluation de la dose en curiethérapie prostatique par grains d’iode 125 est actuellement estimée à 1 mois sans tenir compte des mouvements prostatiques, de l’hétérogénéité des tissus ni de ...l’orientation des grains. Nous avons mis en place en logiciel pouvant prendre en considération l’orientation des grains dans le calcul de dose. L’objectif était de mesurer l’impact de cette orientation sur les distributions de dose dans la prostate et le rectum.
Une nouvelle méthodologie de traitement d’image a été développée, permettant de segmenter et de reconstruire les grains d’iode 125 à partir de scanographies effectuées un mois après l’implantation. Les répartitions de dose et les histogrammes dose-volume ont ensuite été calculés selon le formalisme du TG-43 proposé par l’AAPM (American Association of Physicists in Medicine). Les distributions calculées ont été comparées à celles obtenues selon la pratique actuelle, qui considère tous les grains alignés sur l’axe du scanographe. Cette première étude comparative a été menée sur 14 patients consécutifs implantés par grains libres.
L’ensemble des 14 dosimétries a pu être calculé par notre logiciel. La dose aux différents organes restait macroscopiquement la même, illustrée par l’absence de différence sur la D90 (dose reçue par 90 % de l’organe) et le V100 (volume ayant reçu 100 % de la dose prescrite). Cependant, des écarts de dose supérieurs à 5 % dans plus de 5 % du volume prostatique ont été observés. Ces écarts étaient principalement localisés à la périphérie de la glande. L’impact majeur se situait à l’interface prostate-rectum, où l’orientation des grains affectait directement les deux organes.
La prise en compte de l’orientation des grains d’iode 125 ne modifie pas les données dosimétriques de l’histogramme dose-volume à 1 mois. Néanmoins, ce travail a mis en évidence d’importants écarts de dose localisés qui pourraient avoir une importance dans les traitements focalisés ou de rattrapage.
Introduction External beam radiotherapy performance has been increased by the emergence of state-of-the-art techniques, which improve local irradiation while preserving surrounding organs. They ...require consistent Quality Assurance (QA) programs, supported by Electronic Portal Imaging Devices and Dynalog files. Nevertheless, they do not enable accurate dose-monitoring during online delivery sessions. In the framework of the INSPIRA project, the DAMe group, in collaboration with the Radiotherapy Department of CHUG, is developing an innovative Transparent Detector for Radiotherapy (TraDeRa). The beam modulated by the Multileaf Collimator will be monitored ahead of the patient, in real-time, without dead zone. Material and methods Current version of this detector is a 1:4 scale prototype, partially instrumented. Basically, it consists in a matrix of ionization chambers. First, we measured the dose with a PTW 0.3 cc ionization chamber, in standard conditions, with and without embedding TraDeRa, to quantify detector-induced attenuation. Then, the detector was irradiated using a 14 square cm beam. Inhouse front-end electronics allows the amplitude of each pulse to be extracted, providing dose rate at the pulse scale. Finally, TraDeRa was irradiated with a 3 mm sliding slit. A real-time map of the signal every 50 ms is provided. Results The detector induces a 0.6% attenuation of the beam. Global accuracy and stability of TraDeRa was verified. We highlighted dose rate noticeable variations during the irradiation. Amplitudes of delivered pulses vary around 5% of the mean value, and up to 25% for the first pulse. Accurate monitoring of leaf position was obtained. The interleaf leakage appears clearly and represents a non-negligible contribution to the dose. Recently, a specific integrated circuit has been designed by the LPSC microelectronics group. It represents a key-step for the forthcoming embedded electronics. Conclusion The current version of TraDeRa already shows promising results for IMRT-QA. It is quite transparent so that it does not hinder the irradiation, while keeping the beam upstream of the patient under constant control. It will offer both suitable precision on 2D-beam characterization and real-time measurements at the pulse scale or every 50 ms. The development of the final prototype, with all embedded electronics, is in progress and is expected by the end of 2013.
Purpose:
An innovative Transparent Detector for Radiotherapy (TraDeRa) has been developed. The detector aims at real‐time monitoring of modulated beam ahead of the patient during delivery sessions, ...with a field cover up to 40×40 cm 2.
Methods:
TraDeRa consists in a pixelated matrix of ionization chambers with a patented electrodes design. An in‐house designed specific integrated circuit allows to extract the signal and provides a real‐time map of beam intensity and shape, at the linac pulse‐scale. The measurements under irradiation are made with a 6 MV clinical X‐Ray beam. Dose calculations are performed with the Monte Carlo code PENELOPE, modeling the full accelerator head and the TraDeRa detector.
Results:
A 2 % attenuation of the beam was measured in the presence of TraDeRa and the PENELOPE dosimetric study showed no significant modification of the photon beam properties. TraDeRa detects error leaf position as small as 1 mm compared to a reference field, for both static and modulated fields. In addition, measurements are accurate over a large dynamic range from low intensity signals, as inter‐leaves leaks, to very high intensities as obtained on the medical line of the European Synchrotron Radiation Facility. The detector is fully operational for conventional and high dose rate beams as FFF modes (up to 2400 MU/min).
Conclusion:
The current version of TraDeRa shows promising results for IMRT quality assurance (QA), allowing pulse‐scale monitoring of the beam and high sensitivity for errors detection. The attenuation is small enough not to hinder the irradiation while keeping the beam upstream of the patient under constant control. A final prototype under development will include 1600 independent electrodes, half of them with a high resolution centered on the beam axis. This compact detector provides an independent set of measurements for a better QA.
Funding support : This work was supported by the LABEX PRIMES (ANR‐11‐LABX‐0063) of Universite de Lyon, within the program “Investissements d'Avenir” (ANR‐11‐IDEX‐0007) operated by the French National Research Agency (ANR) and within the project “INSPIRA” operated by the OSEO institution
