Depth Of Maximum Dose Research Articles

Measurement of 6 MV small field beam profiles – comparison of micro ionization chamber and linear diode array with monte carlo code

The objective of this study is to analyze small field photon beams acquired with commonly available detectors. Beam profiles of 6 MV photons from the Siemens Primus Linear Accelerator were measured with a micro ion chamber (IC CC01, IBA) and linear diode array (LDA-99SC, IBA). Data was acquired using a water phantom for small fields (0.5×0.5 cm2 to 4×4 cm2) at depth of maximum dose, 5 cm and 10 cm. Profiles were also generated with EGSnrc Monte Carlo code. Measured and simulated profiles were compared in terms of percentage difference of the area under the simulated and measured profiles (PD), ratio of the measured to simulated dose at the point of maximum deviation within the central region of profile (R), full width half maximum (FWHM) and penumbra. For field sizes ≥1×1 cm2, the maximum PD is 3.17 % and 2.87 % for IC and LDA respectively, whereas R is in the range of 0.95-1.05 for IC and 0.99-1.05 for LDA. LDA measured FWHM and penumbra are also in better agreement with the simulated results. This study demonstrated that LDA can be used for acquisition of beam profiles for field size as low as 1×1 cm2.

Effects of Ionizing Radiation Detector Characteristics on the Results of Measurements of Percent Depth Doses in Small Photon Fields

Studies of the dosimetric parameters of small fields of ionizing radiation are extremely important in current clinical dosimetry. This article considers analysis of plots of percent depth dose in small photon fields obtained using an Elekta Synergy medical linear electron accelerator with energies of 6 and 10 MeV using ionization chambers with different active volumes and a semiconductor detector. Comparative data for the following parameters are presented: depth of maximum dose, surface dose, dose at a depth of 10 cm, and various other aspects of the plots. These data were found to depend on the type of detector used in identical measurement conditions. The main features of the detection of dose distributions by different detectors are noted and conclusions are drawn regarding the advantages of using detectors with the smallest available active volumes in clinical dosimetry.

Feasibility of linear diode array based small field data acquisition for 6 MV & 15 MV photon beams – An intercomparison with micro ion chamber

This study aims at evaluating the response of different radiation detectors for measurement of small field radiotherapy beam profiles for high energy photon beams. Micro ion chamber (IC, CC01, IBA) and linear diode array (LDA-99SC, IBA) were used to measure beam data on Siemens ONCOR Impression linear accelerator for 6 MV and 15 MV photon beams. For both detectors the beam profiles were acquired in water phantom for nominal small field sizes ranging from 1 × 1 cm2 to 5 × 5 cm2 at depths of maximum dose (dmax), 5 cm and 10 cm. Measured profiles were inter compared using gamma analysis at pass criteria of 1.5%/1.5 mm and 1%/1 mm and characterized with full width half maximum (FWHM) and penumbrae width. The gamma pass rate for all profiles of both energies has been found to be 100% (using 1.5%/1.5 mm) and 90% (using 1%/1 mm) except for 15 MV - 1 × 1 cm2 field size. Though, the effect of measurement depth has been found to be insignificant on beam matching with two detectors. For both energies, no considerable difference has been observed in FWHM of profiles as measured with the two detectors, with a slight increase in penumbrae size for LDA. It has been found that 6 MV beam profiles of IC and LDA show a reasonable match whereas a noticeable difference has been observed for 15 MV beam in penumbrae region. In addition, a substantial dose overestimation can be seen for LDA in the out of field region for both energies.

Measurements of Photon Beam Flattening Filter Using an Anisotropic Analytical Algorithm and Electron Beam Employing Electron Monte Carlo

Introduction: This study aimed to report the measurement of photon and electron beams to configure the Analytical Anisotropic Algorithm and Electron Monte Carlo used in clinical treatment. Material and Methods: All measurements were performed in a large water phantom using a 3-dimensional scanning system (PTW, Germany). For photon beams, the data were measured with a 0.125cc cylindrical chamber. For electron, the data were performed with a Roos chamber. Results: In photon beams, flatness and symmetry for reference field size 10×10cm2 were within the tolerance intervals. Flatness were 0.79% and 1.55% for X6MV and X18MV, respectively. Symmetry were 0.57 and 0.25 for X6MV and X18MV, respectively. The output factor vary between 0.83 and 1.11 for X6MV. Moreover, it varies between 0.74 and 1.09 for X18MV. The leaf transmission factors were 0.97% for X6MV and1.14% for X18MV. The DLG were 1.31 and 1.34 for X6MV and X18MV, respectively. For electron beams, the quality index R50 for applicator 15×15cm2 were in the tolerance. Maximum depth dose for 6, 9, 12, 16 and 20MeV were 1.2, 1.9, 2.7, 2.99 and 2.4cm, respectively. Bremsstrahlung tail were 6MeV–2.86cm, 9MeV–4.32cm, 12MeV–5.96cm, 16MeV–7.93cm, and 20MeV–10.08cm per energy levels. Conclusion: The obtained results and international recommendations were in a good agrement

Perturbation effect of parallel-plate ionization chambers on buildup dose measurements in transverse magnetic fields

The aim of this study is to investigate the perturbation effect of parallel-plate ionization chambers on the buildup dose measurement in transverse magnetic fields, using Monte Carlo (MC) simulation. The NACP-02 and ROOS parallel-plate chambers and a PTW31010 cylindrical chamber were modeled for buildup dose measurement in magnetic fields, using the EGSnrc/cavity code. The irradiation condition was set to a 10 × 10 cm2 field in a water phantom at a source-to-surface distance (SSD) of 100 cm, using 6-MV photon spectrum. Magnetic fields of 0 0.35, 1.0, 1.5, and 3.0 T were applied perpendicularly to the direction of the photon beam. The overall perturbation factor PQ,B for the ionization chambers in the magnetic fields was also calculated. The dose to water was enhanced with increasing the magnetic field strength at a depth of less than 1 cm. Over a depth of 1.5 cm, there was no significant difference in the depth doses with and without magnetic field in water. The maximum depth dose (%) for the NACP-02 and ROOS chambers at 1.5 T was higher up to 12% and 14% than the maximum depth dose at 0 T, respectively. The depth dose curves of a PTW31010 chamber have a similar tendency to those of water. The PQ,B values for each chamber were the largest at the phantom surface. The transverse magnetic field has a greater effect on the dose response of the NACP and ROOS chambers than that of the PTW31010 chamber in the buildup region.

Performance Evaluation of a Transmission Reference Detector for Commissioning Beam Data in a Wedge Field

The purpose of this study is to evaluate the performance of a “stealth chamber” as a novel reference detector for measuring the commissioning beam data for a wedge field. The stealth chamber is a transmission ionization chamber intended to be used as a reference chamber for relative dosimetry. The percentage depth doses (PDDs) and the dose profiles of 6-, 8-, and 10-MV photon beams of four wedge filters were measured using the stealth and the CC13 chambers as reference detectors. The PDD and the profile measurements were performed under various field sizes ranging from 4 × 4 cm2 to 20 × 20 cm2. For the evaluate of the performance of the stealth chamber in fields with a physical wedge, all measurement data obtained using the stealth chamber as a reference detector were compared with those measured using two CC13 chambers as both field and reference detectors. For the PDDs, the differences in dosimetric parameters such as the depth of maximum dose (dmax) and the PDD at 10 cm (D10) and 5 cm (D5) were analyzed for both reference detectors. In addition, the root-mean-square error (RMSE) was calculated to measure the agreement of the profiles at dmax and a 10 cm depth. The percentage differences in D10 and D5 between both detectors were less than 1% for all energies and fields with four wedges. Furthermore, the RMSEs of the profiles at dmax and 10-cm depth in both chambers were within 1% for all energies and fields except for the 60° wedge angle. This study found that the use of a stealth chamber to measure the commissioning beam data for a wedge field is feasible, although a relatively large difference in the PDD was observed in the build-up region. However, we recommend careful verification of the PDDs and dose profiles for large field size and wedge angle before clinical use.

Research of effect of cavity between bolus and skin on dose deposition in shallow tissues

Objective To evaluate the effect of the thickness and area of the cavity between bolus and skin upon the dose deposition in the shallow tissues. Methods The linear accelerator head model of 6 MV X ray was constructed using Geant4, which simulated the accelerator working condition of 10 cm×10 cm field and recorded the information of emergent particles as phase space file. A water phantom of 30 × 30 × 30 cm3 was designed at the SAD level. A 30 cm × 30 cm × 1 cm water film was constructed adjacent to or with different cavities to the upper surface of the phantom. The phase space file was utilized as particle source to simulate the particle transport process. The axis depth dose distribution and lateral dose profiles in the center area of the field at a depth of 1 mm, 5 mm, 9 mm, 15 mm and 21 mm of the phantom were obtained, respectively. The simulated data obtained from water film with different cavities were compared with those of adjacent to the water phantom. Results When the cavity was ≤ 5 mm, the cavity exerted slight effect. When the thickness was increased, the maximum dose depth (Dmax) was increased, the PDD at Dmax(PDDmax) became less, the depth of influenced lateral dose profiles was increased and the dose in the center area was decreased. Along with the increase of cavity area, the Dmax was initially increased and then decreased, the PDDmax was at first decreased and subsequently increased, the depth of influenced lateral dose profile was initially increased and then decreased, the dose in the center area was first decreased and subsequently increased. The lateral dose profile distant from the cavity or located at a depth ≥15 mm was almost not affected by the cavity. Conclusion The thickness of the cavity between bolus and skin should be less than 5 mm and the cavity area should be small as possible. Key words: Monte Carlo; Bolus; Cavity; Dose deposition; Profile

Monte Carlo simulation of whole body dose distribution in patients with total body X (γ) ray irradiation

Objective To explore the feasibility of application of the Monte Carlo method to simulate the whole body dose distribution in patients with total body X (γ) ray irradiation by comparing the actual measurement results. Methods A Monte Carlo model of a 6 MV Elekta Synergy Clinical linear accelerator was established by MCNPX. According to the relationship between the CT value and the density of the material, the CT of the ATOM physical phantom was converted into a voxel phantom for MCNPX calculation. The dose distribution of the whole body was simulated in the total body X (γ) ray irradiation. The simulated results were compared with the measurement values of the thermoluminescence dosimetry at different positions in the ATOM physical phantom to analyze the differences. Results The difference between the depth dose curve and the off-axis dose curve and the actual measurement values calculated by the 6 MV accelerator treatment head model in the water tank was less than 2%, with the maximum dose depth of approximately 1.5 cm and field size of 10 cm×10 cm, which were consistent with the actual measurement values. The maximum difference between the simulated results at different locations in the body and the thermoluminescence dosimeter was approximately 4%, and the simulated results of MCNPX were almost in good agreement with the results of thermoluminescence. Conclusions The whole body dose distribution in patient with total body X (γ) ray irradiation can be accurately simulated by MCNPX. Monte Carlo simulation makes it possible to optimize the uniformity of the total body dose during the total body irradiation process. Key words: Total body irradiation; Monte Carlo simulation; Dose uniformity; Thermoluminescence dosimetry

Analysis of Effect of Change Source to Surface Distance (SSD) and the Field Size to Distribution Dose Using Monte Carlo Method-EGSnrc

Research was conducted to analyze the effect of changes in the distance radiation source to the surface it is called the medium Source to Surface Distance (SSD) and wide exposure field (Field Size) on the distribution of the dose in linear air Accelerator (LINAC) using Monte Carlo - EGSnrc. Monte Carlo simulation is used for modeling and simulation head linac at BEAMnrc. Virtual model of the linac is made based on the data characteristics of the aircraft Linac Varian Clinac iX then its output obtained information characteristic photon beam using BEAMDP, while modeling and simulation phantom done on DOSXYZnrc with the size of the phantom is (40x40x40) cm3 , with the material in the form of water, using a beam of photons 6 MV, testing variation SSD at 80 cm, 90 cm, 100.1 cm, 110 cm, 120 cm and variation field size is (6x6) cm2, (10x10) cm2, (20x20) cm2 to obtain disribution of dosage form of curves Percentage Depth dose (PDD) and Profile dose. The results showed that the smaller distance radiation source to the surface of the medium (SSD) and the greater the broad field (field size), then increasing the dose distribution is obtained. In the SSD and Field Size variation, the radiation dose will continue to rise significantly from the surface of the medium 0 cm to a depth of maximum dose (Dmax), then the dose began to decline after passing the depth Dmax.

Comparing experimental assessment of the peripheral dose outside the applicator in electron beams of ELEKTA with Treatment planning system

Introduction: The use of electrons in the electron therapy to destroy tumoral tissue is dedicated significant contribution of different methods of radiation therapy. Scattered radiation due to exited electrons of the applicator affect the dose out of the field in the patient's normal tissue. The aim of this study is to determine the peripheral dose outside the applicator in the electron beams of Elekta accelerator. Materials and Methods: In this study the peripheral dose outside the applicator in electron beams was measured using linear accelerator Elekta Synergy Plateform and it was compared with the obtained results from the treatment planning system (TPS) Isogray model. The peripheral dose profiles were measured and compared Using the solid water phantom set and the film dosimetry system of EBT3 at energy levels 6, 10 and 18 MeV in the applicators with the different dimensions and at different angles of 0, 10 and 20 at the depth 0 and 0.5 and 1cm and for each energy level at depth of maximum dose (Dmax). The peripheral dose Profiles has been normalized to the edge of the field. Results: The highest Peak dose was observed in 18 MeV beam outside the applicator. Peak dose was reduced with increasing the electron beam energy. Using the applicator 20 × 20 cm2 for energy 18 MeV, a dose peak of 1.6% was observed on the surface at a distance of 2 cm from the outer edge of applicator and also for energy 6 MeV, 1.15% at the same distance from the edge of the applicator. It was found that the dose peak decreases with increasing depth and increases with increasing field size. Also the dose peak increased with increasing gantry angle from zero to 20 degrees. The result of measured peripheral dose outside the applicator by using TPS of Clarkson algorithm showed that the dose Calculation can be evaluated only up to distance of 3cm from the edge of the field. Conclusion: Totally, anticipation of dose distribution of the electron beams in collision with matter is different and difficult in various accelerators because of the complexity of the behavior of electrons. However, according to the results of this study in Elekta accelerator, noticeably dose peak was observed in use of high-energy electron beams because of more incidence of bremsstrahlung radiation outside the treatment field. The radiotherapy team can have prevented from damage to organs at risk outside the applicator by awareness of the value and location of peripheral dose outside the treatment field with applying the principles of radiation protection like shields. The evaluated TPS is not a good way to measure the peripheral dose outside the field.

Total skin electron therapy (TSET): Monte Carlo Simulation and implementation

Introduction: Total skin electron irradiation technique is used in treatment of the mycosis fungoid. The implementation of this technique requires non-standard measurements and complex dosimetry methods. Operating procedures for total skin electron irradiation and its dosimetry vary in different radiation therapy centers in the world. In this article, validation of TSET technique dosimetry data by Monte Carlo simulation is done. Materials and Methods: The electron beam characteristic of RTSEI for the only electron accelerator, located at the radiation center of the Seyed Alshohada Hospital of Isfahan (NEPTUN 10PC) determined by performing Monte Carlo simulations and using EGSnrc-based codes (BEAMnrc and DOSXYZnrc). For the best uniformity of the vertical profile, the optimal angle of gantry was defined at SSD=300 cm. The effect of the degrader plane that is located at a distance of 20 cm from the patient surface, was evaluated on the amount of energy reduction of the beam, the opening of the electron beam field and the homogeneity of the dose distribution. The transversal dose distribution from the whole treatment with Rotational technique was simulated in a CT- based anthropomorphic phantom. Also, the percentage depth dose in the head, neck, thorax, abdomen and legs was obtained for RTSET technique. TSET dosimetry requires measurements in nonreference conditions. Because of the complexity of the required measurements for the commissioning process of this technique, different dosimetric systems were employed such as radiochromic films (EBT3) and an ionization chamber. In particular, for dual-field beams irradiation, the optimal tilt angle was investigated and the dose distribution in the treatment plane was measured by radiochromic films. Dose distributions and percentage depth dose measurements for a total skin electron therapy were measured in an anthropomorphic phantom. Results: The optimal angle of 20o would give the most uniform total profile and the use of a 0.8 cm PMMA degrader in front of the patient leads to a homogeneous distribution of the dose in all directions (the mean relative dose value was 97%±5%, normalizing to 100% at the calibration point level). The percentage depth dose curves in different organs of anthropomorphic phantom for RTSEI indicates that the depth of maximum dose is on the surface of the phantom, Isodose curve of 80% is formed at a depth less than 4 mm and at the depth less than 1.5 mm, the dose decreases to 20% of the maximum dose. Conclusion: The main purpose of this study was to commission and optimize a TSET technique for the treatment of mycosis fungiodes with the NEPTUN 10PC linear accelerator. This was done through an extensive set of measurements and a large number of MC simulations. The results of Monte Carlo calculations were found to be in general agreement with the measurements, providing a promising tool for further studies of dose distribution calculations in TSEI.

33 Commissioning and dosimetric characteristics of new halcyon system

Introduction A new Halcyon linear accelerator had been installed at clinique Charcot in Sainte-Foy-Les-Lyon. It’s the first one in France. The accelerator head is mounted on a ring gantry. There is no jaws and, for each bank, two MLC (proximal and distal) of 29 and 28 leaves of 1 cm width at isocenter with an offset of 5 mm to minimize the transmission. The maximum speed of the leaves is 5 cm/s. The maximum field size is 28 × 28 cm2. Halcyon system has one unique energy of 6XFFF and a maximum dose rate of 800 UM/min. The MV imaging panel size is 43 × 43 cm2, fxed at a distance source to imager of 154 cm and its resolution is 0.22 mm at isocenter. Currently, Halcyon system has only MV imaging (MV/MV or MV CBCT). Imaging dose is calculated by TPS Eclipse (Varian) and taken into account during optimization. Beam model of Halcyon is already preinstalled and fully modelled in Eclipse. Methods All the preinstalled data in Eclipse were measured in a water tank Blue Phantom2 (IBA) using CC13 (IBA), EDGE (SunNuclear) et Farmer (PTW) detectors in accordance with Varian measurements. PDD, profiles, FOC, transmission factor and DLG measurements were compared with preinstalled beam model. Calculated dose distribution with AAA algorithm for PDD and profiles were compared with our measurements using tolerances from SFPM report n°27. End-to-end and RapidArc commissioning were performed. Imaging dose from clinical radiotherapy plans were calculated from several anatomical regions and measured in an anthropomorphic phantom CIRS with a semiflex (PTW) ionisation chamber. Results A very good superposition of preinstalled data with our measurements was observed for every data. Largest variations were observed for small field profiles 2 × 2 cm2. Mean and maximum variations for FOC measurements were −0.03% and 0.71%. Leaf transmission measured through both banks is 0.005%. Beam quality index (PDD(10) with SSD = 100 cm) is 63.5% (Varian specification 63% ± 1). Maximum depth dose for a 10x10cm2 field size at SSD = 100 cm is 1.28 cm (Varian specification 1.3 cm ± 0.2). For every field sizes 100% of the points for PDD and profiles pass the tolerances from SFPM report n°27. Gantry isocentricity measured with Drum phantom (Varian) is 0.65 mm. Absolute dose difference at isocenter for a VMAT plan measured during an End-to-End test performed in the IMRT Thorax Phantom (CIRS) is 1%. Conclusions An excellent agreement between Varian preinstalled beam model and our measures were observed and therefore no changes to the preinstalled beam model have been made.

The IAEA/WHO TLD Audit Program. The results of the TLD postal dosimetry audits in the National Centre for Radiotherapy and Nuclear Medicine Department, Korle-Bu, Accra (1998 to 2012)

During therapeutic procedures, the patient is always protected by the quality assurance program legitimately required for medical exposures. This comprehensive quality assurance programs, or quality management systems, include all components of radiation therapy practice, and has proven to be necessary in reducing the likelihood of dose misadministration. The aim of this paper is to gives a summary of various studies performed during the period of 15 years (1998-2012), and provides the results of a survey conducted by the IAEA. The methodology involve checking the beam output with its dosimeters, and to irradiate two TLD capsules in sequence in such a way that the irradiation time or monitor units calculation would deliver 200 cGy to a ‘tumour’ whose centre (the TLD capsule) is at 5 cm depth. Here, the irradiations were done in reference conditions using the agency’s Perspex holder, and the delivered dose is not a ‘given dose’ at the depth of maximum dose, but a dose at depth. The results of this study demonstrate that the QUATRO audit programme is a very useful tool for reviewing practices at our Centre, and shown beyond doubt to be effective in assuring the quality of dose determination at the Centre. Our study further indicates that it is possible to have the dose determination within acceptance limits by implementing correct procedures and cautiously performing dose calculations and measurements

Precise EBT3 Gafchromic film dosimetry for Grid therapy

ObjectivesGrid therapy is a radiation therapy technique for the treatment of bulky tumours. The high dose gradients and non-uniformity of dose distributions within the target lead to a challenge in the dosimetry of the Grid radiation fields. The aim of this study is to perform a precise EBT3 Gafchromic film dosimetry for Grid therapy fields using a commercially available flatbed colour scanner. MethodsIn this project, samples of the EBT3 Gafchromic films are exposed to Grid radiation fields. The irradiated EBT3 films were read using a flatbed Microtek scanner. The responses of these films (i.e. films from the same batch) as functions of the absorbed dose values are calibrated by irradiation under a fixed standard technique (i.e. 10 × 10 cm2 filed, 100 cm SSD, and depth of the maximum dose). These films are also read with the same scanner using the red, green, and blue channels. Four different approaches were used to evaluate film dosimetry for the Grid therapy applications: 1) single channel film dosimetry method (SCM), 2) dual channel film dosimetry method (DCM), 3) linearized dose-response curve method (LRCM), and 4) triple channel film dosimetry method (TCM). A dose of 20 Gy was delivered to the point along the central axis of the grid hole at the depth of maximum dose (dmax) for a 20 × 20 cm2 Grid field size. Beam profiles and percentage depth dose distributions of the Grid radiation have been measured in water-equivalent phantom material, using EBT3 films. The accuracy of the relative and the absolute dosimetry of the films were examined by comparison of the TLD measured data with the Monte Carlo simulated values. ResultsThe results of these investigations show that for a gamma index criterion of 5%/3 mm, the agreements between the MC calculations dose profiles and the SCM, DCM, LRCM, and TCM film dosimetry approaches the passing rates of 91%, 92%, 95%, and 95%, respectively. A much closer agreement was observed for using a linearized dose-response curve and triple-channel methods. ConclusionsSelection of an appropriate methodology in Gafchromic film dosimetry may lead to an accurate dose-response in a high dose gradient radiation field such as Grid therapy.

Technical Note: Experimental verification of EGSnrc calculated depth dose within a parallel magnetic field in a lung phantom.

The calculation of depth doses from a 6 MV photon beam in polystyrene using EGSnrc Monte Carlo, within a parallel magnetic field, has been previously verified against measured data. The current work experimentally investigates the accuracy of EGSnrc calculated depth doses in lung within the same parallel magnetic field. Two cylindrical bore electromagnets produced a magnetic field parallel to the central axis of a Varian Silhouette beam. A Gammex lung phantom was used, along with a parallel plate ion chamber, for the depth dose measurements. Two experimental setups were investigated: top of phantom coinciding with the top of the magnet's bore, and top of phantom coinciding with the center of the bore. EGSnrc was modified to read the 3D magnetic field distribution and then used to simulate the depth dose in lung. The parallel magnetic field caused measurable increases in dose at the surface and in the buildup region for both setups. For the setup where the top of the lung phantom coincides with the top of the magnet, the surface dose increased by ~11% compared to the no magnetic field case but the depth of maximum dose remained unchanged. When the phantom's top surface coincided with the center of the magnet, the surface dose increased by 32% and dose maximum occurred at a shallower depth. EGSnrc was able to calculate these dose increases due to the magnetic field accurately for both setups. All the simulated depth dose values were within 2% (with respect to Dmax ) of the measured ones, and most of the investigated points were within 1.5%. Surface and dose increases due to a parallel magnetic field have been measured in a lung phantom at two separate locations within the magnetic field. EGSnrc has been shown to match these measurements to within 2%.

P233] Investigation of the response of cylindrical ionization chamber in non-reference conditions when using high-energy photon beams

Purpose To investigate the response of CC13 cylindrical ionization chamber in non-reference conditions with focus on the penumbra region and the depth of maximum dose when using high-energy photon beams. This chamber is used for beam data measurements and also for pre-treatment quality assurance procedures for dynamic treatments. Methods Measurements and Monte Carlo calculations are performed, employing a Varian iX Linac (6 MV photon beam), at the depths of 15 mm and 100 mm for the field sizes of 20 × 20 mm 2 ; 20×100 mm 2 and 100×100 mm 2 . The Monte Carlo model is developed using the egs-chamber in the EGSnrc package. Conversion factors are calculated relating the absorbed dose to air in the air cavity of the chamber to the absorbed dose to water through using: (a) water volume equal to the air cavity of the chamber, (b) 1×1×1 mm 3 water voxel representing point dose. Corresponding correction factors, C w/chamber and C w,point/chamber , are determined to quantify the variations in the conversion factors. Results Agreement within 3.5%/0.5 mm is obtained between the measurements and calculations. The correction factors for a chamber position at the central axis of the beam and at the center of the field penumbra are between 0.98 and 1.02 for all field sizes and depths examined. The largest corrections are seen for off-axis positions 3 mm beyond the penumbra center where C w,point/chamber varies between 0.54 and 0.71 for the depth of 100 mm and 0.45 and 0.58 for the depth of 15 mm indicating an overestimation of the absorbed dose to water when using the CC13 chamber. C w,point/chamber factors up to 1.27 are obtained in regions close to the penumbra inside the 20×20 mm 2 and 20×100 mm 2 fields indicating an underestimation of the absorbed dose to water. The maximum variation of the C w/chamber factors is between 0.70 and 1.06 for the 20×20 mm 2 field at the depth of 15 mm. Conclusions Uncorrected chamber response of the CC13 chamber, or similar, results in under or overestimation of the absorbed dose to water at positions inside or outside of the beam close to the penumbra for small as well as large field sizes.

OA237] The output factor measurements of cyberknife system using various detectors

Purpose In Stereotactic Radiosurgery, the extremely high radiation dose is delivered to the target in a single or few fractions using narrow radiation fields. The radiation fields are collimated with 12 fixed circular collimators ranging from 5 to 60 mm in diameter in CyberKnife which is a robotic radiosurgery system. The accuracy of the output factors directly affects the accuracy of treatment delivery to the patients. The output factor (OF) measurement of small fields is very problematic and the small field measurements are more sensitive to physical properties of detectors. The aim of the present study is to perform the relative OF measurements of 12 fixed collimators in CyberKnife system using different detectors. Methods In this study, the OF measurements of 12 fixed collimators were made with CyberKnife system at maximum dose depth in water equivalent RW3 slab phantom using PTW 60019 microDiamond, PTW 31014 0.015 cc PinPoint, PTW 31010 0.125 cc Semiflex, EBT3 Gafchromic film and GR-200A TLD. All detectors were positioned at the center of the radiation field with the point laser from treatment head. The measured data were compared with Monte Carlo (MC) factors. Results The OF values measured at 30 mm and greater collimator sizes were found similar for all detectors. The OF values decrease rapidly as the collimator size becomes smaller. The measured data with TLD and microDiamond detector at the smallest collimator sizes such as 7.5, 10, 12.5 and 15 mm were found very similar with MC factors. Conclusions 0.125 cc Semiflex ion chamber may lead to large uncertainties in the OF measurements for the smallest collimator sizes. TLD and microDiamond detector show similar results for all collimator sizes except 5 mm. TLD is an utilizable dosimetric tool in the small field OF measurements by means of its small active volume.

OA236] Patient-specific quality assurance based on electronic portal imaging devices via artificial neural networks

Purpose In this work, Artificial Neural Networks (ANN) are used in External Beam Radiotherapy, especially for dosimetry purposes using Electronic Portal Imaging Device (EPID), which needs frequent calibration and complex setting of dedicated software. This work is preliminarily oriented towards quality assurance of intensity-modulated radiotherapy (IMRT). This first step will allow adjusting ANN before their implementation in more complex treatments and in vivodosimetry. Methods Like humans, artificial neural networks work via two phases – learning and recognition. The supervised learning phase entails the creation of “neurons” by linking input and output data. The recognition phase consists of using input data to predict output data. During the learning phase, EPID signals and desired absorbed dose distributions (taken from treatment planning system – TPS) are used as the neural network’s input and output, respectively. Once the learning is complete, new EPID signal can be used to predict the delivered absorbed dose distribution, allowing comparison with the TPS. EPID images were taken during 6MV treatment delivery, with a dose rate of 600 MU/min, from aSi1000 EPID (Varian). Mounted with Exact-arm on a Clinac 23iX equipped with a multi-leaf collimator (120 leaves). The EPID were acquired using the half-resolution mode. 2D plane images were calculated in EclipseTM at the maximum depth dose in a water phantom. Results Learning was performed using 11 input/output datasets from IMRT treatments. All of the used datasets (both EPID inputs and absorbed dose distribution outputs) consisted of 384 × 512 pixels. Learning can be time consuming but once the ANN has been fixed, its use during the recognition phase will be instantaneous. The gamma index, γ, was used to evaluate the difference between the ANN calculated and planned distributions. γ gives the number of pixels (as a percentage) that respect a given objective. γ(2% , 2 mm) for Head and Neck cancers was found to be 99.7%, highlighting the ANN capability to predict the absorbed dose distribution based on EPIDs. Conclusions It was shown that patient-specific quality assurance of IMRT based on EPID can be performed with neural networks algorithms. Next work would be extending algorithms for in vivo dosimetry purpose.

P296] Use of statistical interference for tolerance determination in dose measurements

Purpose The most important dosimetry quantity that is determined at radiotherapy centres is absorbed dose to water. Fixed tolerances for absorbed doses under reference conditions measured with ionisation chamber for photon resp. electron beams are usually 2% resp. 3%. The aim was to develop a new approach to the evaluation of agreement between measured and reported values based on statistical interference rather than to use fixed tolerance levels. Methods The approach takes partial uncertainties of compared doses into consideration. Three statistical models were proposed describing three different situations to assess the critical values for rejecting the zero hypothesis on agreement: 1) Two measurements performed in time with the same dosimetry chain, 2) Two measurements performed with two different dosimetry chains which were calibrated at the same calibration laboratory, 3) Two measurements performed with two different dosimetry chains at two different calibration laboratories (dose calculations are completely independent). For electron dosimetry, the uncertainty of PDD was comprehensively evaluated and its influence on recalculated dose values from reference depth to the depth of maximum dose was taken into account. In the contribution, uncertainties from TRS 398 were taken as an example. Results Critical values for photon and electron beams case 1 resp. 2 were 2.5% resp. 3.1% (k = 2). When uncertainty of 0.5% (k = 1) for electron PDD at zref was taken into account, critical values for electron beams case 1 resp. 2 were 2.9% resp. 3.4% (k = 2). The sensitivity of critical values to actual partial uncertainties will be shown. Conclusions The models can be adopted to implement statistical interference into the decision on agreement for all quantities, where partial uncertainties are known. This work was supported by the Ministry of Industry and Trade of the Czech Republic, project No. FV20411.

Monte Carlo study and design of system for implementation of Rotational Total Skin Electron Irradiation technique

Total skin electron irradiation technique is used in treatment of the mycosis fungoid. The implementation of this technique requires non-standard measurements and complex dosimetry methods. Depending on the linear accelerator (Linac) type, bunker size, room dimensions and dosimetry equipment, the design of instruments for appropriate set up and implementation of TSEI in different radiation therapy centers varies. The studies which have been done in this article provide an introduction to the implementing of this method for the first time in Iran and its results can be used for the centers with similar specifications in the world. This article determined the electron beam characteristic of TSEI for the only electron accelerator, located at the radiation center of the Seyed Alshohada Hospital of Isfahan (NEPTUN 10PC), by performing Monte Carlo simulations and using EGSnrc-based codes (BEAMnrc and DOSXYZnrc). For the best uniformity of the vertical profile, the optimal angle of gantry was defined at SSD=350 cm. The effect of the degrader plane that is located at a distance of 20 cm from the patient surface, was evaluated on the amount of energy reduction of the beam, the opening of the electron beam field and the homogeneity of the dose distribution. The transversal dose distribution from the whole treatment with Stanford technique (six dual fields) and Rotational technique was simulated in a CT-based anthropomorphic phantom. Also, the percentage depth dose in the head, neck, thorax, abdomen and legs was obtained for both methods. The simulation results show that the 20̂ angle between the horizontal and the beam central axis is optimal in order to provide the best vertical dose uniformity. The mean energy decreases from 6.1 MeV (the exit window) to 3 MeV (the treatment surface) by placing a degrader with 0.8 cm thickness in front of the treatment plane. FWHM of the angular distribution of the electron beam increased from 15̂ at SSD=100 cm to more than 30̂ on the treatment surface by traversing the PMMA degrader. The MC calculated percentage depth dose curves in different organs of anthropomorphic phantom for RTSEI indicates that the depth of maximum dose is on the surface of the phantom and Isodose curve of 80% is formed at a depth less than 4 mm. the results also show, with the degrader plane in front of the patient, the degree of homogeneity of the dose distribution for both Stanford and rotational techniques is the same.