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Résumé: Objectifs de l’étude: La dosimétrie in vivo de transit utilisant l’imageur portal est une approche prometteuse pour l’assurance qualité en radiothérapie. Une évaluation comparative a été menée entre une solution commerciale, EPIgray® et un contrôle de dosimétrie in vivo de référence utilisant des diodes à semi-conducteurs. Matériel et méthodes: Les performances des deux méthodes de dosimétrie in vivo ont été évaluées. Le critère principal était l’écart de dose au point de prescription par rapport aux mesures de la chambre d’ionisation sur fantôme ou par rapport à la dose prévisionnelle calculée par le système de planification de traitement avec les patients. Le seuil de tolérance de l’écart de dose était défini à±5 %. Au total, 107 patients traités par irradiation conformationnelle tridimensionnelle avec ou sans modulation d’intensité pour des tumeurs de l’encéphale, du thorax, de la tête et du cou ont été inclus prospectivement. Résultats: La précision dosimétrique d’EPIgray® sur fantôme était comparable à celle des mesures par diodes en termes de répétabilité (0,11 %) et de reproductibilité (0,29–0,51 %), avec un écart de dose moyen de 0,17 % (écart-type : 1,11). Les taux de séances hors tolérances pour l’encéphale (irradiation conformationnelle tridimensionnelle avec ou sans modulation d’intensité), le thorax (irradiation conformationnelle tridimensionnelle) et la tête et le cou (irradiation conformationnelle avec modulation d’intensité) étaient respectivement de 0 % ; 9,6 % et 5,3 % avec un écart de dose moyen entre 0,49 % et 1,53 %. La moyenne de l’écart de dose sur trois séances consécutives avec EPIgray® permettait de valider 99,1 % des traitements. Conclusion: Les performances d’EPIgray® en dosimétrie in vivo sont compatibles avec les recommandations de l’European Society for Radiotherapy and Oncology (ESTRO) et équivalentes à celles des diodes à semi-conducteur pour l’irradiation conformationnelle tridimensionnelle. Elle permet un contrôle satisfaisant des irradiations conformationnelles avec modulation d’intensité, techniquement difficiles à réaliser avec les diodes. [Copyright &y& Elsevier]

Résumé: Objectifs de l’étude: La dosimétrie in vivo a pour objectif de détecter et de corriger les écarts de traitements significatifs en radiothérapie. L’utilisation des imageurs portals permet de rendre ces contrôles rapides et riches en informations. De nombreux travaux ont été publiés sur ce sujet, mais aucune recommandation n’a été établie pour définir les niveaux de tolérance pour ces contrôles. Nous proposons une méthodologie simple pour évaluer le niveau de tolérance global des résultats in vivo obtenus avec ces détecteurs plans. Matériels et méthodes: Dans un premier temps, les incertitudes liées à la méthode de reconstruction ont été évaluées. Elles ont été déterminées sur un ensemble de géométries, représentative des situations cliniques, pour lesquelles 45 points de mesure de dose ont été définis. Dans un deuxième temps, il a été réalisé une étude des variations induites par le cadre de la mesure in vivo en présence du patient afin de définir des niveaux d’actions réalistes et adaptés à la routine clinique. Pour finir, une méthodologie est proposée pour déterminer si la méthode, associée à ces niveaux de tolérance, permet de détecter des écarts significatifs de traitements, supérieures à 5 % à la dose prescrite. Résultats – conclusion: En appliquant cette méthodologie, nous avons conclu qu’un niveau de tolérance de 6,5 % (k =2) pouvait être associé à notre méthode de dosimétrie in vivo. Il a été montré que cette valeur permettait dans de nombreux cas de détecter des différences de 5 % de la dose prescrite. La dosimétrie in vivo de transmission ne peut pas prétendre détecter tous les écarts de traitements, mais elle permet d’augmenter la sécurité des patients. [ABSTRACT FROM AUTHOR]

Abstract: Purpose: In vivo dosimetry measurements are accepted when the difference between measured and calculated dose is under 5%. A statistical analysis has been conducted to determine whether this tolerance matched the clinical practice for the studied localizations: pelvis, thorax, head and neck, breast. Materials and methods: The technical characteristics of the detectors were checked before being used in clinical practice. Then an automatic statistical analysis was implemented using the 2450 in vivo dosimetry measurements obtained during 1year. Main results: The global average is 1.10%, the standard deviation 2.46% and the percentage of out of level measurements 4.09%. By distinguishing the localizations, the 5% tolerance appeared to be too narrow for the breast localization. Discussion/Conclusion: Several investigations were initiated to justify the modification of the tolerance for the breast localization. They highlighted an underestimation of the calculated dose when high beam angles are set: a new correction factor was defined to take account this error. A specific tolerance was also specified for the breast localization. [Copyright &y& Elsevier]

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Abstract: Purpose: Measurement of absorbed dose in target volume is widely considered to be an important tool for quality assurance in external radiotherapy. The aims of this work were to measure the entrance dose for patient treated for head and neck tumors and to compare this measured dose with the dose calculated. Patients and methods: Twenty patients were evaluated. Initially, the measurements were performed on a polystyrene phantom in order to calibrate diodes in terms of entrance dose and to determinate appropriate correction factors. In vivo entrance dosimetry check was performed for these patients treated for head and neck tumors in 60Co gamma-rays. Results: For the entrance dose evaluation over 100 field measurements, the mean deviation between the measured dose and the calculated dose was equal to 0.12% and the standard deviation was 1.84%. The deviation was less than 3% in 95% of measurements. Large deviation (more than 5%) was observed in one case. Conclusion: Simple in vivo dose measurements are an additional safeguard against major set-up errors and calculation or transcription errors that were missed during pretreatment chart check. [Copyright &y& Elsevier]

Abstract: Purpose: The objective of this work was the study of in vivo dosimetry performed in a series of 54 patients receiving total body irradiation (TBI) at the Salah-Azaiz Institute of Tunis since 2004. In vivo dosimetry measurements were compared to analytically calculated doses from monitor units delivered. Patients and method: The irradiation was conducted by a linear accelerator (Clinac® 1800, Varian, Palo Alto, USA) using nominal X-rays energies of 6 MV and 18 MV, depending on the thickness of the patient at the abdomen. The dose was measured by semi-conductors p-type EPD-20. These diodes were calibrated in advance with an ionization chamber “PTW Farmer” type of 0.6cm3 and were placed on the surface of plexiglas phantom in the same TBI conditions. A study of dosimetric characteristics of semi-conductors EPD-20 was carried out as a function of beam direction and temperature. Afterwards, we conducted a comparative analysis of doses measured using these detectors during irradiation to those calculated retrospectively from monitor units delivered to each patient conditioned by TBI. Results: Experience showed that semi-conductors are sensitive to the angle of beam radiation (0–90°) and the temperature (22–40°C). The maximum variation is respectively 5 and 7%, but in our irradiation conditions these correction factors are less than 1%. The analysis of the results of the in vivo dosimetry had shown that the ratio of the average measured doses and analytically calculated doses at the abdomen, mediastina, right lung and head are 1.005, 1.007, 1.0135 and 1.008 with a standard deviation “type A” respectively of 3.04, 2.37, 7.09 et 4.15%. Conclusion: In vivo dosimetry by semi-conductors is in perfect agreement with dosimetry by calculation. However, in vivo dosimetry using semiconductors is the only technique that can reflect the dose actually received instantly by the patient during TBI given the many factors that calculation can not take into account: patient and organs motions and the heterogeneity of the targets. [Copyright &y& Elsevier]

Aims. – The thyroid dysfunction after radiotherapy has led to evaluate the dose received by thyroid during nasopharynx and breast carcinoma irradiation. This evaluation was facilitated by in vivo dosimetry. The aims of this work were to evaluate the thyroid dose and to compare released dose at the reference point in the two localizations.Patients and methods. – A total of 30 patients were evaluated: 18 patients with nasopharynx carcinoma and 12 patients with breast carcinoma were included in the final analysis. In the first group, the total thyroid but the isthmus was irradiated. On the other hand, in the second group, only one thyroid lobe was included into the target volume. All patients have been treated by gamma rays of cobalt 60 and the thyroid absorbed dose was measured by semiconductor dosimeters (Scanditronix DPD6). These dosimeters were calibrated in the same geometric conditions of the irradiation.Results. – The measured absorbed dose of the thyroid parenchyma was equal to the calculated absorbed dose at the target that has been specified at a depth of –3 cm. Under the block, the isthmus received 5.1 ± 0.9 Gy (9.9 ± 1.8%) in nasopharynx carcinoma irradiation. However, in breast carcinoma irradiation, the distal thyroid lobe and the isthmus received 2.9 ± 0.7 Gy (6.55 ± 1.56%) and 3.69 ± 0.77 (8.39 ± 1.76%), respectively.Conclusion. – This study shows that the thyroid received a dose equal to the prescribed dose in both nasopharynx and breast cancer patients. It is recommended to follow the function of the thyroid gland in these patients. [Copyright &y& Elsevier]

The influence of thermoplastic masks used in clinical routine for patient immobilization in head and neck radiotherapy treatment on the absorbed skin dose has been investigated at Gustave-Roussy Institute. The measurements were performed in 60Co γ–rays, 4 and 6MV X-rays and in 8 and 10MeV electron beams. Initially, the measurements were performed with thermoluminescent dosimeters (LiF) and a NACP chamber on a polystyrene phantom in order to study the influence of physical parameters (distance, field size, energy…) on first millimeters depth variation dose. The study was completed with in vivo measurements on 14 patients using various dosimeters (thermoluminescent detectors, diodes) in order to assess the increase of dose on first millimeters depth and to verify the delivered dose during treatment sessions (quality control). In treatment conditions, masks lead to an important increase of dose on the first millimeter in 60Co γ–rays beams (dose value normalized to maximum of dose increase from 57.1% to 77.7% for 0.5 mm-water depth and from 78.5% to 88% for 1 mm-water depth) ; its contribution is less important in 4 and 6 MV X-rays beams (dose value normalized to maximum of dose increase from 49.5% to 63.2% for 0.5 mm-water depth and from 59% to 70.1% for 1 mm-water depth). Concerning 8 and 10 MeV electron beams, the normalized dose value increase respectively from 78.4% to 81.7% and from 82.2% to 86.1% for 0.5 mm-water depth. In vivo dosimetry enabled the quality control of delivered dose during treatment. Measured dose is in agreement within ± 5% with the prescribed dose for 92.3% of cases. In routine, in vivo dosimetry allowed to quantify the increase of skin dose induced by thermoplastic masks for various energies of photon and electron beams as well as quality control. [Copyright &y& Elsevier]

Introduction – Verification of absorbed dose in target volume is a key factor for quality assurance in radiotherapy. In vivo measurements allow evaluation of the variations in dose with time and variations between measured doses and calculated doses by TPS. The aim of this work were to evaluate reproducibility of patient positioning and to compare calculated doses by 2 different TPS.Patients and methods – Twenty patients were divided in 2 groups according to the thickness of their breast (mean SSD = 92,9 cm). In vivo measurement was performed within the first two sessions.Results – Reproducibility of SSD evaluation was made on 12 beams between 2 fractions. With a tolerance margin of 0,5 cm, positioning errors were present in 33% (4/12). The 2 TPS were in agreement in 75% (30/40).Conclusion – In vivo dosimetry can be a very interesting tool to assess patients positioning variations and TPS dose calculation. [Copyright &y& Elsevier]