Original ArticleWe propose a process of quality assurance to validate and implement the single isocenter technique for breast cancer radiotherapy. We evaluated the dosimetric and temporal gains using the single isocenter technique compared to classic source to skin distance (SSD) technique. Methods: 6 patients of breast cancer localization were studied. For each patient 2 treatment plans were generated. In plan 1 the dose was calculated using SSD technique. In plan 2 the dose was calculated using single isocenter technique. To implement the plan 2 a dosimetric analysis including monitor units (MU), isodose curves, cumulative and differential dose volume histograms cDVH, dDVH respectively, coverage index, conformity index for planning target volume were used. The measurements using a PMMA phantom consist of measuring point dose by an ionization chamber and 2D dose distributions using 2D diodes arrays. Wilcoxon signed rank and Spearman's tests were used to calculate p-value and correlation coefficient, respectively. Results: The single isocenter technique reduced the MU by average on -30.1 ± 13.6%, (p = 0.03). We observed an improvement with statistical significance between the two techniques for the mean dose, minimum dose and volume receiving 95% of the prescribed dose without over-dosage. The analysis for dDVH showed that the dose distribution in the target volume calculated in the single isocenter technique is more homogeneous than the SSD technique. Wilcoxon test showed that the two treatment plans had the same quality (p > 0.05). The difference between calculated and measured dose was within 2.4 ± 3.3% for absolute point dose and the percentage of points passing gamma criteria was on average 99.8 ± 0.2%. Conclusion:This method provides a quantitative evaluation and comparison of the two irradiation techniques for breast cancer and the consequences of the technical change on dose calculation.

Reçu le 18 avril 2017 / Accepté le 12 décembre 2017Résumé -Cette étude vise à optimiser le protocole des rochers d'un scanner Siemens Somatom Sensation 40. Ainsi, plusieurs acquisitions du fantôme Catphan 600 sont réalisées pour évaluer quantitativement la qualité d'image. Pour cela, l'écart type du nombre CT et la fonction de transfert de modulation (FTM) sont mesurés à partir des images acquises. La qualité d'image est également évaluée visuellement par le radiologue, à partir d'images issues de patients. Trois critères sont considérés : la visualisation du tympan, la visualisation des osselets et la texture du bruit. L'utilisation de la table plutôt que de la têtière comme support conduit à une augmentation moyenne du bruit de 10 %. L'évaluation qualitative des images issues de patients a permis quant à elle de réduire les mAs effectifs de 28 % et donc l'IDSV de 150,8 mGy à 109,1 mGy. De plus, l'utilisation d'un filtre plus dur a conduit à une amélioration de la visualisation des plus petites structures de l'oreille interne. Ainsi, cette étude a permis d'assurer une qualité d'image satisfaisante pour le diagnostic tout en réduisant la dose délivrée au patient de 28 %.Abstract -Optimization of a computed tomography (CT) scan protocol of the temporal bone. This study aims to optimize the CT scan protocol for temporal bone for a Somatom Sensation 40CT scanner by SIEMENS. Several acquisitions of Catphan 600 phantom are realized in order to quantitatively assess the image quality. The standard deviation of the CT Hounsfield Unit and the Modulation Transfer Function (MTF) are measured from the images acquired. The image quality is visually evaluated by the radiologist thanks to the patient images considering three criteria: visualization of the eardrum, visualization of the ossicles and noise texture. Using the cradle as the main support, in place of the headrest, a noise increment of around 10% was observed. Qualitative assessments of the patient images allowed a reduction of the effective mAs of 28% and therefore the volumetric computed tomography dose index (CTDIvol) from 150.8 mGy to 109.1 mGy. In addition, the use of a harder filter led to improved visualization of the smallest structures of the inner ear. The study results allowed to find the best compromise between the highest image quality required to realize a confident diagnostic and a considerable reduction of the dose delivered to the patient (of 28%).

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