Determination of the kfclin,fmsr Qclin,Qmsr correction factors for detectors used with an 800 MU/min CyberKnife® system equipped with fixed collimators and a study of detector response to small photon beams using a Monte Carlo method

International audiencePurpose In a previous work, output ratio (ORdet) measurements were performed for the 800 MU/min CyberKnife® at the Oscar Lambret Center (COL, France) using several commercially available detectors as well as using two passive dosimeters (EBT2 radiochromic film and micro-LiF TLD-700). The primary aim of the present work was to determine by Monte Carlo calculations the output factor in water (OFMC,w) and the kf clin,fmsr Qclin,Qmsr correction factors. The secondary aim was to study the detector response in small beams using Monte Carlo simulation. Methods The LINAC head of the CyberKnife® was modeled using the PENELOPE Monte Carlo code system. The primary electron beam was modeled using a monoenergetic source with a radial gaussian distribution. The model was adjusted by comparisons between calculated and measured lateral profiles and tissue-phantom ratios obtained with the largest field. In addition, the PTW 60016 and 60017 diodes, PTW 60003 diamond, and micro-LiF were modeled. Output ratios with modeled detectors (OR MC,det) and OFMC,w were calculated and compared to measurements, in order to validate the model for smallest fields and to calculate kfclin,fmsr Q clin,Qmsr correction factors, respectively. For the study of the influence of detector characteristics on their response in small beams; first, the impact of the atomic composition and the mass density of silicon, LiF, and diamond materials were investigated; second, the material, the volume averaging, and the coating effects of detecting material on the detector responses were estimated. Finally, the influence of the size of silicon chip on diode response was investigated. Results Looking at measurement ratios (uncorrected output factors) compared to the OFMC,w, the PTW 60016, 60017 and Sun Nuclear EDGE diodes systematically over-responded (about +6% for the 5 mm field), whereas the PTW 31014 Pinpoint chamber systematically under-responded (about -12% for the 5 mm field). ORdet measured with the SFD diode and PTW 60003 diamond detectors were in good agreement with OFMC,w except for the 5 mm field size (about -7.5% for the diamond and +3% for the SFD). A good agreement with OFMC,w was obtained with the EBT2 film and micro-LiF dosimeters (deviation less than 1.4% for all fields investigated). kfclin,fmsr Q clin,Qmsr correction factors for several detectors used in this work have been calculated. The impact of atomic composition on the dosimetric response of detectors was found to be insignificant, unlike the mass density and size of the detecting material. Conclusions The results obtained with the passive dosimeters showed that they can be used for small beam OF measurements without correction factors. The study of detector response showed that ORdet is depending on the mass density, the volume averaging, and the coating effects of the detecting material. Each effect was quantified for the PTW 60016 and 60017 diodes, the micro-LiF, and the PTW 60003 diamond detectors. None of the active detectors used in this work can be recommended as a reference for small field dosimetry, but an improved diode detector with a smaller silicon chip coated with tissue-equivalent material is anticipated (by simulation) to be a reliable small field dosimetric detector in a nonequilibrium field. © 2014 American Association of Physicists in Medicine

Characterization of the gafchromic EBT3 films for dose distribution measurements in stereotactic radiotherapy

International audienceRadiochromic film dosimetry is a promising technique, but at this time there are some artefacts, including non-uniformity, energy dependence and scanner artefacts. Accurate dosimetry with radiochromic films requires characterizing the film as well as the scanning and analysis procedures. In this work, the performance of the EBT3 films in combination with the EPSON Dual Lens Perfection V700 scanner for dose distribution measurements in stereotactic radiotherapy has been evaluated. It has been shown that it was necessary to perform 20 blank scans to obtain the stability of the scanner. In order to reduce the uncertainties due to the non-uniformity of the scan field, it was then decided to use the 12 × 12 cm2 central part of the scanner bed. Regarding EBT3 films, intra-sheet and inter-sheet uniformity of unexposed EBT3 films in terms of pixel value has been found to be respectively 0.27% (1 SD) and 0.15% (1 SD). No significant energy dependence has been observed above 30 keV and no angular dependence has been found. © 2014 Elsevier Ltd. All rights reserved

Study of commercial detector responses in non-equilibrium small photon fields of a 1000 MU/min CyberKnife system

International audiencePurpose The purpose of this study was to analyze the detector responses in non-equilibrium small photon fields. Methods Five detectors (PTW 31014 ionization chamber, PTW 60016, PTW 60017 and Sun Nuclear EDGE diodes and PTW 60003 diamond detector) and one passive dosimeter (Harshaw micro-LiF) as well as a 1000 MU/min CyberKnife were modeled with the PENELOPE Monte Carlo code. Field factors, ΩQclin,Qmsrfclin,fmsr were calculated and perturbations due to volume averaging effect, active material effect and coating effect were quantified for the five detectors and passive dosimeter. Results The PTW 31014 ionization chamber under-response is mainly due to the fluence perturbation caused by the presence of air as detecting material. Regarding diodes, the high density materials used in their active volume and in their coating is responsible for their over-response. Regarding the PTW 60003 diamond, its under-response for the 5 mm field size is due to a large volume averaging effect whereas for largest field sizes a nearly perfect compensation between the volume averaging effect and the material effect due to the diamond density occurs. Despite its small size, a volume averaging effect was observed for the micro-LiF for the 5 mm field size. Conclusion Perturbations due to volume averaging effect, active material effect and coating effect were investigated and quantified for five active detectors. Since these perturbations can cause opposite effects, wrong conclusions may be drawn regarding the radiological water-equivalence of detectors. Thus, we recommend performing such a study for each novel detector available on the market. © 2016 Associazione Italiana di Fisica Medica

Development of a protocol for small beam bi-dimensional dose distribution measurements with radiochromic films

International audienceGafchromic™ films have become popular due to their ease of use and their near water equivalence. This last property is crucial for stereotactic small beam dosimetry as demonstrated in recent papers. An accurate bi-dimensional dose measurement with Gafchromic™ films is very challenging mainly because of the non-uniformity response of flatbed scanners (used for films digitalization) and their own non-uniformity. The first proposal of this work is to develop bi-dimensional protocol for small beams and evaluate the associated uncertainty. The second proposal is to validate this protocol for the bi-dimensional measurements of treatment plans performed with the CyberKnife® system. First, the uniformity of an Epson V700 flatbed scanner and a batch of EBT3 Gafchromic™ films has been investigated. A "four films" dosimeter was designed to reduce the errors (statistic and systematic) due to their non-uniformity. Then, the "four films" dosimeter protocol in both a homogeneous (RW3 material) and heterogeneous (RW3, lung-like and bone-like materials) phantoms has been used to measure the bi-dimensional dose distributions of three simple CyberKnife® treatment plans. Two tumor locations (middle of the lung and near lung/bone interface) were considered for the heterogeneous phantom. These plans were achieved with the 10 mm fixed collimator and modeled with the PENELOPE Monte Carlo code in order to calculate accurate dose distributions. Finally, the "four films" bi-dimensional dose distributions were compared to the PENELOPE Monte Carlo simulations. Regarding the uncertainty associated to the bi-dimensional dose measurement protocol, the relative standard deviation σD on the dose was 1.2% in the range from 0.5 to 4.0 Gy. Regarding the protocol validation on CyberKnife® treatment plans, a very good agreement was found with all measurement points passing the {3% - 3 mm} Gamma Index criteria. © 2016 Elsevier Ltd. All rights reserved

Benefits of the dual-energy CT imaging for proton treatment of breast cancer with silicone implant

CERVOXYInternational audienc

Determination of small field output factors and correction factors using a Monte Carlo method for a 1000 MU/min CyberKnife® system equipped with fixed collimators

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Experimental characterisation of a proton kernel model for pencil beam scanning techniques

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Dosimetric characteristics of four PTW microDiamond detectors in high-energy proton beams

International audienceSmall diamond detectors are useful for the dosimetry of high-energy proton beams. However, linear energy transfer (LET) dependence has been observed in the literature with such solid state detectors. A novel synthetic diamond detector has recently become commercially available from the manufacturer PTW-Freiburg (PTW microDiamond type 60019). This study was designed to thoroughly characterize four microDiamond detectors in clinical proton beams, in order to investigate their response and their reproducibility in high LET regions. Very good dosimetric characteristics were observed for two of them, with good stability of their response (deviation less than 0.4% after a pre-irradiation dose of approximately 12 Gy), good repeatability (coefficient of variation of 0.06%) and a sensitivity of approximately 0.85 nC Gy(-1). A negligible dose rate dependence was also observed for these two microDiamonds with a deviation of the sensitivity less than 0.7% with respect to the one measured at the reference dose rate of 2.17 Gy min(-1), in the investigated dose rate range from 1.01 Gy min(-1) to 5.52 Gy min(-1). Lateral dose profile measurements showed the high spatial resolution of the microDiamond oriented with its stem perpendicular to the beam axis and with its small sensitive thickness of about 1 mu m in the scanning profile direction. Finally, no significant LET dependence was found with these two diamond dosimeters in comparison to a reference ionization chamber (model IBA PPC05). These good results were in accordance to the literature. However, this study showed also a non reproducibility between the devices in terms of stability, sensitivity and LET dependence, since the two other microDiamonds characterized in this work showed different dosimetric characteristics making them not suitable for proton beam dosimetry with a maximum difference of the peak-to-plateau ratio of 6.7% relative to the reference ionization chamber in a clinical 138 MeV proton beam

Dependence of coronary 3-dimensional dose maps on coronary topologies and beam set in breast radiation therapy A study based on ct angiographies

International audiencePurpose In left-side breast radiation therapy (RT), doses to the left main (LM) and left anterior descending (LAD) coronary arteries are usually assessed after delineation by prior anatomic knowledge on the treatment planning computed tomography (CT) scan. In this study, dose sensitivity due to interindividual coronary topology variation was assessed, and hot spots were located. Methods and Materials Twenty-two detailed heart models, created from heart computed tomography angiographies, were fitted into a single representative female thorax. Two breast RT protocols were then simulated into a treatment planning system the first protocol comprised tangential and tumoral bed beams (TGs-TB) at 50 + 16 Gy, the second protocol added internal mammary chain beams at 50 Gy to TGs-TB (TGs-TB-IMC). For the heart, the LAD, and the LM, several dose indicators were calculated dose-volume histograms, mean dose (D mean), minimal dose received by the most irradiated 2% of the volume (D2%), and 3-dimensional (3D) dose maps. Variations of these indicators with anatomies were studied. Results For the LM, the intermodel dispersion of Dmean and D2% was 10% and 11%, respectively, with TGs-TB and 40% and 80%, respectively, with TGs-TB-IMC. For the LAD, these dispersions were 19% (Dmean) and 49% (D2%) with TGs-TB and 35% (Dmean) and 76% (D2%) with TGs-TB-IMC. The 3D dose maps revealed that the internal mammary chain beams induced hot spots between 20 and 30 Gy on the LM and the proximal LAD for some coronary topologies. Without IMC beams, hot spots between 5 and 26 Gy are located on the middle and distal LAD. Conclusions Coronary dose distributions with hot spot location and dose level can change significantly depending on coronary topology, as highlighted by 3D coronary dose maps. In clinical practice, coronary imaging may be required for a relevant coronary dose assessment, especially in cases of internal mammary chain irradiation. © 2014 Elsevier Inc. All rights reserved

Clinical research in radiation oncology: how to move from the laboratory to the patient?

International audienceTranslational research in radiation oncology is undergoing intense development. An increasingly rapid transfer is taking place from the laboratory to the patients, both in the selection of patients who can benefit from radiotherapy and in the development of innovative irradiation strategies or the development of combinations with drugs. Accelerating the passage of discoveries from the laboratory to the clinic represents the ideal of any translational research program but requires taking into account the multiple obstacles that can slow this progress. The ambition of the RadioTransNet network, a project to structure preclinical research in radiation oncology in France, is precisely to promote scientific and clinical interactions at the interface of radiotherapy and radiobiology, in its preclinical positioning, in order to identify priorities for strategic research dedicated to innovation in radiotherapy. The multidisciplinary radiotherapy teams with experts in biology, medicine, medical physics, mathematics and engineering sciences are able to meet these new challenges which will allow these advances to be made available to patients as quickly as possible