Characteristics of The Narrow Spectrum Beams Used in the Secondary Standard Dosimetry Laboratory at the Lebanese Atomic Energy Commission.

The Secondary Standard Dosimetry Laboratory at the Lebanese Atomic Energy Commission has different calibration methods for various types of dosimeters used in industrial, military and medical fields. The calibration is performed using different beams of X-rays (low and medium energy) and Gamma radiation delivered by a Cesium 137 source. The Secondary Standard Dosimetry laboratory in charge of calibration services uses different protocols for the determination of high and low air kerma rate and for narrow and wide series. In order to perform this calibration work, it is very important to identify all the beam characteristics for the different types of sources and qualities of radiation. The following work describes the methods used for the determination of different beam characteristics and calibration coefficients with their uncertainties in order to enhance the radiation protection of workers and patient applications in the fields of medical diagnosis and industrial X-ray. All the characteristics of the X-ray beams are determined for the narrow spectrum series in the 40 and 200 keV range where the inherent filtration, the current intensity, the high voltage, the beam profile and the total uncertainty are the specific characteristics of these X-ray beams. An X-ray software was developed in order to visualize the reference values according to the characteristics of each beam.

[Quality assurance of enhanced dynamic wedge using the aS500-II, EPID]. / Etudes et contrôle de qualité des filtres dynamiques à l'aide de l'imageur portal aS500-II (Varian).

PURPOSE: The work presented herein rests on the study of the Varian EPID aS500-II and the Image Acquisition system IAS3. We assessed the dosimetric performance of this EPID for measurements and quality assurance of enhanced dynamic wedge profiles and wedge factors. MATERIALS AND METHODS: We evaluated the dosimeter properties using the integrated asynchronous mode of acquisition in treatments with enhanced dynamic wedges (EDW). We studied the performance, stability and the reproducibility in measurements of the transmission factors and profiles of the fields with dynamic wedges. EPID profiles were compared to the "Profiler Sun Nuclear" diode array and PTW ion chamber. Analytical functions were developed in order to correct EDW profiles. The dependence of EPID measurements on wedge direction, beam dimensions and source to EPID distance was assessed. RESULTS: The backscatter produced by the "exact arm" was evaluated; EPID profiles depended on the EDW direction and on the detector source distance. Wedge factors were determined using this detector and compared to the ion chamber response, differences were all within 1 %. Two empirical correction functions were developed to produce EPID wedge profiles that correspond to diode for all wedge angles and energies depending on the wedge direction. CONCLUSION: The EPID is highly suited to regular measurement of EDW due to the reproducibility of the EPID-measured wedge factors and profiles.

[Impact of computed tomography (CT) and 18F-deoxyglucose-coincidence detection emission tomography (FDG-CDET) image fusion for optimisation of conformal radiotherapy in non-small-cell lung cancers]. / Tomographie par émission de positons et détection en coïncidence (TEDC) et recalage d'images de simulation virtuelle par tomodensitométrie. Impact sur la planification de la radiothérapie conformationnelle des cancers bronchiques non à petites cellules.

UNLABELLED: To report a retrospective study concerning the impact of fused 18F-fluorodeoxy-D-glucose (FDG)-hybrid positron emission tomography (PET) and computed tomography (CT) images on three-dimensional conformal radiation therapy (3D-CRT) planning for patients with non-small-cell lung cancer (NSCLC). PATIENTS AND METHODS: One hundred and one patients consecutively treated for stages I-III NSCLC were studied. Each patient underwent CT and FDG-hybrid PET for simulation treatment in the same radiation treatment position. Images were coregistered using five fiducial markers. Target volume delineation was initially performed on the CT images and the corresponding FDG-PET data were subsequently used as an overlay to the CT data to define target volume. RESULTS: FDG-PET identified previously undetected distant metastatic disease in 8 patients making them ineligible for curative CRT (one patient presented some positive uptakes corresponding to concomitant pulmonary tuberculosis). Another patient was ineligible for curative treatment because fused CT/PET images demonstrated excessively extensive intrathoracic disease. The gross tumor volume (GTV) was decreased by CT/PET image fusion in 21 patients (23%) and was increased in 24 patients (26%). The GTV reduction was > or = 25% in 7 patients because CT/PET image fusion reduced pulmonary GTV in 6 patients (3 patients with atelectasis) and mediastinal nodal GTV in 1 patient. The GTV increase was > or = 25% in 14 patients due to an increase of the pulmonary GTV in 11 patients (4 patients with atelectasis) and detection of occult mediastinal lymph node involvement in 3 patients. Among 81 patients receiving a total dose > or = 60 Gy at ICRU point, after CT/PET image fusion, the percentage of total lung volume receiving more than 20 Gy (VL20) increased in 15 cases and decreased in 22 cases. The percentage of total heart volume receiving more than 36 Gy increased in 8 patients and decreased in 14 patients. The spinal cord volume receiving at least 45 Gy (2 patients) decreased. After multivariate analysis, one single independent factor made significant effect of FDG/PET on the modification of the size of the GTV: tumor with atelectasis (P = 0.0001). Conclusion. - Our study confirms that integrated hybrid PET/CT in the treatment position and coregistered images have an impact on treatment planning and management of patients with NSCLC. FDG images using dedicated PET scanners with modern image fusion techniques and respiration-gated acquisition protocols could improve CT/PET image coregistration. However, prospective studies with histological correlation are necessary and the impact on treatment outcome remains to be demonstrated.

[Impact of computed tomography (CT) and 18F-deoxyglucose positron emission tomography (FDG-PET) image fusion for conformal radiotherapy in esophageal carcinoma]. / Tomographie par émission de positons et fusion d'images de simulation virtuelle par tomodensitométrie. Impact sur la planification de la radiothérapie conformationnelle des cancers de l'oesophage.

PURPOSE: To study the impact of fused (18)F-fluoro-deoxy-D-glucose (FDG)-hybrid positron emission tomography (PET) and computed tomography (CT) images on conformal radiation therapy (CRT) planning for patients with esophageal carcinoma. PATIENTS AND METHODS: Thirty-four patients with esophageal carcinoma were referred for concomitant radiotherapy and chemotherapy with radical intent. Each patient underwent CT and FDG-hybrid PET for simulation treatment in the same radiation treatment position. PET-images were coregistered using five fiducial markers. Target delineation was initially performed on CT images and the corresponding PET data were subsequently used as an overlay to CT data to define the target volume. RESULTS: FDG-PET identified previously undetected distant metastatic disease in 2 patients, making them ineligible for curative CRT. The Gross Tumor Volume (GTV) was decreased by CT and FDG image fusion in 12 patients (35%) and was increased in 7 patients (20.5%). The GTV reduction was >or=25% in 4 patients due to reduction of the length of the esophageal tumor. The GTV increase was >or=25% with FDG-PET in 2 patients due to the detection of occult mediastinal lymph node involvement in one patient and an increased length of the esophageal tumor in the other patient. Modifications of the GTV affected the planning treatment volume (PTV) in 18 patients. Modifications of delineation of GTV and displacement of the isocenter of PTV by FDG-PET also affected the percentage of total lung volume receiving more than 20 Gy (VL20) in 25 patients (74%), with a dose reduction in 12 patients and a dose increase in 13 patients. CONCLUSION: In our study, CT and FDG-PET image fusion appeared to have an impact on treatment planning and management of patients with esophageal carcinoma related to modifications of GTV. The impact on treatment outcome remains to be demonstrated.

[Dosimetric validation of compensator for their use in clinical routine, in conformation radiotherapy]. / Validation dosimétrique des compensateurs physiques pour leur utilisation clinique en radiothérapie conformationnelle.

PURPOSE: The aim of this work was to establish the acquisition, calculation and 3D compensator manufacturing optimum parameters. This methodology is based on virtual simulation and 3D dosimetry. MATERIALS AND METHODS: The material used is a helicoidal CT (PQ 5000-Marconi), a virtual simulation system (AcQsim version 4-Marconi), a Treatment Planning System (Dosigray), a linear accelerator Saturne 43 (Varian, CGR), an automated milling system for compensator filters (Autimo 2.5 D-HEK), a water tank phantom (wellhofer) and an homogeneous phantom with simple patterns in order to simulate the obliquity surface of patient body. The compensator was composed by granulate tin because this material ensures a good profile modulation. The compensation plane has been calculated at 80% to dose profile. The compensator thickness profile has been calculated with different acquisition (slice thickness, pitch factor), calculation (attenuation coefficient, bixel) and fabrication parameters (drill diameter, specification of milling system). RESULTS: After this preliminary study, we have defined the optimum parameters for the compensator realization. We have observed that the slice thickness, bixel size and drill diameter are the parameters that mainly affect the profiles homogeneity. The choice of parameters with smaller dimensions S = 3 mm; B = 3 mm, F = 3 mm, improve the profiles homogeneity. Though, for manufacture times compatible with the clinical routine, the selected parameters are S = 5 mm, B = 6 mm and F = 6 mm. Compensator can be used for any type of Linac. However, one must pay attention on their realization and their positioning on the beam central axis.