Lateral Electronic Equilibrium Research Articles

Dosimetric Evaluation of Semiflex Three-dimensional Chamber under Unflatten Beam in Comparison among Different Detectors.

The goal of this study is to investigate the dosimetric properties of a Semiflex three-dimensional (3D) chamber in an unflatten beam and compare its data from a small to a large field flattening filter-free (FFF) beam with different radiation detectors. The sensitivity, linearity, reproducibility, dose rate dependency, and energy dependence of a Semiflex 3D detector in flattening filter and filter-free beam were fully investigated. The minimum radiation observed field widths for all detectors were calculated using lateral electronic charged particle equilibrium to investigate dosimetric characteristics such as percentage depth doses (PDDs), profiles, and output factors (OPFs) for Semiflex 3D detector under 6FFF Beam. The Semiflex 3D measured data were compared to that of other detectors employed in this study. The ion chamber has a dosage linearity deviation of +1.2% for <10 MU, a dose-rate dependency deviation of +0.5%, and significantly poorer sensitivity due to its small volume. There is a difference in field sizes between manufacturer specs and derived field sizes. The measured PDD, profiles, and OPFs of the Semiflex 3D chamber were within 1% of each other for all square field sizes set under linac for the 6FFF beam. It was discovered to be an appropriate detector for relative dose measurements for 6 FFF beams with higher dose rates for field sizes more than or equal to 3 cm × 3 cm.

Comparison of the Characteristics of Two Types of Parallel-plate Ionization Chamber Under Small-field Electron Irradiation.

This study compared two types of parallel-plate ionization chamber to clarify the pitfalls of dosimetry in electron radiation therapy. The ion recombination correction factor and polarity effect correction factor, sensitivity, and percentage depth doses (PDDs) of PPC05 and PPC40 parallel-plate ionization chambers were compared in a small-field electron beam. The output ratios were measured for 4-20 MeV electron beams with field sizes of 10 cm × 10 cm, 6 cm × 6 cm, and 4 cm × 4 cm. Furthermore, the films were placed in water and positioned in the beam with their surface perpendicular to the beam axis, and lateral profiles were obtained for each beam energy and each field. Regarding PDDs, at depths greater than the peak dose, the percentage depth dose for PPC40 was smaller than that for PPC05 in small fields and at beam energies greater than 12 MeV, which could be attributed to the lack of lateral electron equilibrium at small depths and multiple scattering events at large depths. The output ratio of PPC40 was approximately 0.025-0.038, which was lower than that of PPC05 in a 4 cm × 4 cm field. For large fields, the lateral profiles were similar, regardless of the beam energy, however, for small fields, the flatness of the lateral profile was beam energy dependent. The PPC05 chamber, which has a smaller ionization volume, is therefore more suitable than the PPC40 chamber for small-field electron dosimetry, in particular at high beam energies.

Small field output factor measurement and verification for CyberKnife robotic radiotherapy and radiosurgery system using 3D polymer gel, ionization chamber, diode, diamond and scintillator detectors, Gafchromic film and Monte Carlo simulation

The dosimetry of small fields has become tremendously important with the advent of intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery, where small field segments or very small fields are used to treat tumors. With high dose gradients in the stereotactic radiosurgery or radiotherapy treatment, small field dosimetry becomes challenging due to the lack of lateral electronic equilibrium in the field, x-ray source occlusion, and detector volume averaging. Small volume and tissue-equivalent detectors are recommended to overcome the challenges. With the lack of a perfect radiation detector, studies on available detectors are ongoing with reasonable disagreement and uncertainties. The joint IAEA and AAPM international code of practice (CoP) for small field dosimetry, TRS 483 (Alfonso et al., 2017) provides guidelines and recommendations for the dosimetry of small static fields in external beam radiotherapy. The CoP provides a methodology for field output factor (FOF) measurements and use of field output correction factors for a series of small field detectors and strongly recommends additional measurements, data collection and verification for CyberKnife (CK) robotic stereotactic radiotherapy/radiosurgery system using the listed detectors and more new detectors so that the FOFs can be implemented clinically. The present investigation is focused on using 3D gel along with some other commercially available detectors for the measurement and verification of field output factors (FOFs) for the small fields available in the CK system. The FOF verification was performed through a comparison with published data and Monte Carlo simulation. The results of this study have proved the suitability of an in-house developed 3D polymer gel dosimeter, several commercially available detectors, and Gafchromic films as a part of small field dosimetric measurements for the CK system.

Characterization and implementation of the L-alanine detector for quality control of lung SBRT treatments with the VMAT technique

New treatments in radiotherapy have some difficulties, among them, the geometric and dosimetric characterization of the radiation beam and the use of small radiation fields. Determination of the prescribed dose in the target volume in cases of small fields is difficult due to the absence of lateral electronic equilibrium and the sharp dose gradient at the edges of the fields. In this way, the choice of the radiation detector becomes relevant when performing the dosimetry of small fields. Alanine dosimeters have proven to be a good option for measurements of high radiation doses in these field sizes. This work aims to characterize the alanine detector through dosimetric tests for the VMAT (Volumetric Modulated Arc Therapy) technique in cases of SBRT (Stereotactic Body Radiation Therapy) to demonstrate the feasibility of using the alanine dosimetric system for use in quality control. End-to-End test was performed in a phantom, simulating two situations: homogeneous and heterogeneous regions. The response of L-alanine showed a strong linear correlation with dose (R2 = 0.99997), and insignificant dependence to dose rate. In the End-to-End test for a planned dose of 18 Gy, the doses obtained in alanine presented expanded uncertainty of 4.60% at 95% confidence level. Therefore, this work demonstrated that alanine dosimeter is suitable for quality control of SBRT with the VMAT technique in routine applications.

Comparison of Monaco treatment planning system algorithms and Monte Carlo simulation for small fields in anthropomorphic RANDO phantom: The esophagus case.

In this study, the dose distributions obtained by the algorithms used in Monaco treatment planning system (TPS) and Monte Carlo (MC) simulation were compared for small fields in the anthropomorphic RANDO phantom, and then, the results were analyzed using the gamma analysis method. In the study, dose distributions obtained from the collapse cone algorithm, MC algorithm, and MC simulation were examined. The EGSnrc was utilized for MC simulation. In radiation fields smaller than 3 cm × 3 cm, the doses calculated by the CC algorithm are particularly high in the region of lung/soft-tissue interfaces. In the region of soft-tissue/vertebral interfaces, the doses calculated by the CC algorithm and the MC algorithm are compatible with the MC simulation. For each algorithm, the main reason for the non-overlapping dose curves in small fields compared to MC simulation is that the lateral electronic equilibrium loss is not taken into account by the algorithms. The doses calculated by the algorithms used in TPS may differ, especially in environments where density changes are sharp. Even if the radiation dose from different angles is calculated similarly in the target area by the algorithms, the calculated doses in the tissues in each radiation field path may be different. Therefore, to increase the quality of radiotherapy and to protect critical organs more accurately, the accuracy of the algorithms in TPS should be checked before treatment, especially in multi-field treatments such as stereotactic body radiation therapy and intensity-modulated radiotherapy for tumors in the abdominal region.

The effect of SSD, Field size, Energy and Detector type for Relative Output Factor measurement in small photon beams as compared with Monte Carlo simulation

Abstract Introduction: Small fields photon dosimetry is associated with many problems. Using the right detector for measurement plays a fundamental role. This study investigated the measurement of relative output for small photon fields with different detectors. It was investigated for three-photon beam energies at SSDs of 90, 95, 100 and 110 cm. As a benchmark, the Monte Carlo simulation was done to calculate the relative output of these small photon beams for the dose in water. Materials and Methods: 6, 10 and 15 MV beams were delivered from a Synergy LINAC equipped with an Agility 160 multileaf collimator (MLC). A CC01 ion chamber, EFD-3G diode, PTW60019 microdiamond, EBT2 radiochromic film, and EDR2 radiographic film were used to measure the relative output of the linac. Measurements were taken in water for the CC01 ion chamber, EFD-3G diode, and the PTW60019. Films were measured in water equivalent RW3 phantom slabs. Measurements were made for 1 × 1, 2 × 2, 3 × 3, 4 × 4, 5 × 5 and a reference field of 10 × 10 cm2. Field sizes were defined at 100cm SSD. Relative output factors were also compared with Monte Carlo (MC) simulation of the LINAC and a water phantom model. The influence of voxel size was also investigated for relative output measurement. Results and Discussion: The relative output factor (ROF) increased with energy for all fields large enough to have lateral electronic equilibrium (LEE). This relation broke down as the field sizes decreased due to the onset of lateral electronic disequilibrium (LED). The high-density detector, PTW60019 gave the highest ROF for the different energies, with the less dense CC01 giving the lowest ROFs. Conclusion: These are results compared to MC simulation, higher density detectors give higher ROF values. Relative to water, the ROF measured with the air-chamber remained virtually unchanged. The ROFs, as measured in this study showed little variation due to increased SSDs. The effect of voxel size for the Monte Carlo calculations in water does not lead to significant ROF variation over the small fields studied.

A new reference-type ionization chamber with direction-independent response for use in small-field photon-beam dosimetry – An experimental and Monte Carlo study

The frequently applied narrow and non-standard transverse dose profiles of intensity modulated photon-beam radiotherapy, lacking lateral secondary electron equilibrium, require the use of high-resolution dosimetry detectors, and small air-filled detectors are recommended as the reference detectors for cross-calibration of the high-resolution detectors. The present study focuses on the dosimetric properties of a novel cylindrical ionization chamber, the PTW Semiflex 3D 31021. The chamber's effective point of measurement was found to lie at (0.41±0.04) r downstream the tip of the inner surface of the spherical front wall in the axial orientation and (0.46±0.04) r upstream the chamber axis in the radial orientation. Due to its symmetrical design, the sigma values of its lateral dose response functions for all chamber's orientations are the same (2.10±0.05mm). The polarity correction factors obtained in this work do not exceed 0.1% and the saturation correction factor was below 1% up to a dose-per-pulse value of 0.956mGy. The radiation quality correction factor kQ of the chamber as a function of the tissue-phantom-ratio, TPR20,10, has been calculated by Monte Carlo simulation and has been determined experimentally at the German Metrology Institute (Physikalisch-Technische Bundesanstalt, PTB). The values of the non-reference condition correction factor kNR have been Monte-Carlo-calculated for use of the chamber at various depths and field sizes.

Verification of relative output factor measurement using Gafchromic films for small-field radiosurgery photon beams.

Accurate dosimetry of small-field photon beams has always been difficult to achieve, due to the steep dose gradient and absence of lateral electronic equilibrium. The purpose of this study was to verify the measurement of relative output factor (ROF), which is one of the dosimetric parameter required for stereotactic radiosurgery (SRS) treatment planning. The ROFs for Radionics circular cone collimators with diameter in the range of 10.0 to 45.0 mm were measured using Gafchromic EBT2 and EBT3 films. The measurements were then compared with the ROFs obtained using a PinPoint ionisation chamber and Monte Carlo (MC) simulation. From the results, the ROFs measured by the ionisation chamber, EBT2, and EBT3 films were good agreement with the MC simulation, with deviations of less than 1.5, 2.6, and 5.0 % respectively. Based on the film dosimetry, the EBT2 film showed in a more reliable measurement for field size ranging from 15.0 to 45.0 mm, compared with EBT3. As a conclusion, based on the special characteristic of the small-field photon beams, ROF measurement using PinPoint ionisation chamber are being favoured, due to its accuracy. However, the EBT2 film can be used as an alternative, when high spatial resolution is required.

The ‘cutting away’ of potential secondary electron tracks explains the effects of beam size and detector wall density in small-field photon dosimetry

The well-known field-size dependent overresponse in small-field photon-beam dosimetry of solid-state detectors equipped with very thin sensitive volumes, such as the PTW microDiamond, cannot be caused by the photon and electron interactions within these sensitive layers because they are only a few micrometers thick. The alternative explanation is that their overresponse is caused by the combination of two effects, the modification of the secondary electron fluence profile (i) by a field size too small to warrant lateral secondary electron equilibrium and (ii) by the density-dependent electron ranges in the structural detector materials placed in front of or backing the sensitive layer. The present study aims at the numerical demonstration and visualization of this combined mechanism.The lateral fluence profiles of the secondary electrons hitting a 1 µm thick scoring layer were Monte-Carlo simulated by modelling their generation and transport in the upstream or downstream adjacent layers of thickness 0.6 mm and densities from 0.0012 to 3 g cm−3, whose atomic composition was constantly kept water-like. The scoring layer/adjacent layer sandwich was placed in an infinite water phantom irradiated by circular 60Co, 6 MV and 15 MV photon beams with diameters from 3 to 40 mm.The interpretation starts from the ideal case of lateral secondary electron equilibrium, where the Fano theorem excludes any density effect. If the field size is then reduced, electron tracks potentially originating from source points outside the field border will then be numerically ‘cut away’. This geometrical effect reduces the secondary electron fluence at the field center, but the magnitude of this reduction also varies with the density-dependent electron ranges in the adjacent layers. This combined mechanism, which strongly depends on the photon spectrum, explains the field size and material density effect on the response of detectors with very thin sensitive layers used in small-field photon-beam dosimetry.

Experimental verification of Acuros XB in the presence of lung-equivalent heterogeneities

Acuros XB (AXB) is a deterministic algorithm that directly accounts for the effects of heterogeneities and improves the accuracy of photon dose calculations in radiotherapy. Nevertheless, the accuracy of AXB inside lung has not been yet fully examined by means of experimental measurements. The aim of this study was to evaluate the accuracy of AXB in the presence of lung using 7Li-based thermoluminescent dosimeters (TLD-700). Percentage depth-dose distributions (PDD) obtained by AXB on a slab water-equivalent phantom with a lung-equivalent heterogeneity were compared with the TLD-700 measurements for 6 MV and 18 MV photon beams and a set of field sizes ranging from 2×2cm2to20×20cm2. Dose distributions obtained by the Anisotropic Analytical Algorithm (AAA) were also included in the comparison as a reference to a non-deterministic dose calculation algorithm. The agreement between AXB and the TLD results was kept within clinical tolerance levels (3%) for all beam configurations. On the contrary, AAA failed at correctly predicting the absorbed dose when the lateral electronic equilibrium was lost. AXB is capable of providing reliable dose estimations in the presence of lung and clearly outperforms AAA.

Small fields dosimetry: Output factors and correction factors determination for an elketa axesse medical linac equipped with circular cones

Introduction The peculiarities of small beams (high dose gradient, source occlusion, lack of lateral electronic equilibrium) and the features of the detector (active volume dimension, components with high-Z materials) make the dosimetry very challenging. Purpose The aim of this work is to determine small fields output factors (OF) for several detectors and correction factors for active detectors for comparison with a passive dosimeter. Materials and methods Small fields beams, ranging from 5 mm to 30 mm in diameter, were defined using circular cones. OF measurements were performed with six active detectors (ionizing microchambers air-filled: Exradin A26, Exradin A16; ionizing microchamber isooctane-filled: PTW microLion; plastic scintillator: Exradin W1; diode: Razor IBA) and one passive detector (Gafchromic EBT3 films). Results Exradin W1 and A26 shown excellent agreement with EBT3 films (better than 2%). A significant underestimation was observed for Exradin A16, particularly for the smallest field, up to 12%. The results obtained with the PTW microLion and the IBA RAZOR indicate a dose overestimation for the smaller radiation fields, up to 4% and 7% for the 5 mm-diameter field for microLion and RAZOR respectively. Conclusion The present study points out that it is crucial to apply the appropriate correction factors in order to provide accurate measurements in small beam geometry. The results show that the Exradin W1 and Exradin A26 can be used for small fields dosimetry without correction factors. The correction factors should be employed for the other detectors, in particular for field diameter smaller than 10 mm. Disclosure Nothing to declare.

Evaluation of dose perturbation at the interface of two different density medium using GAFCHROMIC film EBT2 and Monte Carlo code EGSnrc for Co-60 beam

Accurate dosimetry at the interface of two different density medium (e.g., air cavity in the head and neck cancers and lungs in thoracic region) is a major cause of concern in external beam radiation therapy. It has been observed that there is dose variation in and around air cavities, which occur as a result of the loss of both longitudinal and lateral electronic equilibrium. Heterogeneous structures with spatial differences in functionality and sensitivity for radiation pose challenge to radiation dosimetry. This study is an attempt to evaluate the dose perturbations produced at the interface of two medium for C0-60 gamma radiation. Low density polyethene foam has been used to mimic air cavity. GAFCHROMIC EBT2 dosimetry film was used for the measurement of dose at different locations. Simulation studies were performed using DOSRZnrc user code that comes with EGSnrc V4 2.4.0. Cylindrical geometry is used for all the simulations. We observed significant variation in dose for smaller fields. There is a dose build down in the backward region and a dose build up in the forward direction. In the region of electronic disequilibrium, dose reduction near interface (proximal end) will have negative impact if target region is embedded there, on the contrary, it would be beneficial if there is normal tissue/critical organ adjacent to it.

Prediction method of sensors response positioned at the surface with and without lateral electronic equilibrium in conformal arctherapy

Prediction method of sensors response positioned at the surface with and without lateral electronic equilibrium in conformal arctherapy

Accuracy and limits of accuray® fast Monte-Carlo algorithm in the presence of heterogeneities

s / Physica Medica 30 (2014) e123ee145 e137 Material and methods: This study was performed on a Novalis 600N (mMLC m3, BrainLab) with MOSFET sensors (TN-502-RD,BestMedical) calibrated with a local method (SFPM-2010). Sensor equipped with a RW3 cap was positioned at the spherical phantom surface of Lucy©3D-QA (Standard Imaging). Then the position of the detector was changed by stages away from the axis using the arm of the accelerator to entry angles a from 0 to 25 . This operation was repeated for the detector placed at the exit for a values of 0 to 40 . For each angle position a the MOSFET was irradiated at 100MU, keeping the angle of the arm fixed at 0 for fields sizes ranging from 18 18 to 42 42 mm. Sensitivity profiles(Sa)A A were interpolated for each size of field in 1 steps from -25 to +25 in respect of the entry beam, and from -40 to +40 in respect of the exit beam. A programwritten inMatlab® enables the profiles of (Sa)A A interpolated to be obtained. For a given angle position of the detector, the value of the dose at the point identified on the TPS at the intersection of the axis of theMOSFET and of RW3 thickness of depth in the spherical phantom, was calculated with the Point Kernel algorithm of TPS Isogray (DosiSoft®) and extracted in 1 steps. The correlation coefficients between the dose profiles thus obtained and the sensitivity angles (Sa,i)A Awere determined for each field size. Results: For all field sizes studied a good correlation was observed between the measured angular profiles interpolated at entry and exit and the dose profiles calculated by TPS: the risk value a remained low, i.e. p<0.05. Conclusion: These results validated the method of experimental determination of angular profiles with and without lateral electronic equilibrium. The next stage of the investigation will be the Monte-Carlo simulation of MOSFET sensors and to extend the method to other sensors.

PRESAGE 3D dosimetry accurately measures Gamma Knife output factors

Small-field output factor measurements are traditionally very difficult because of steep dose gradients, loss of lateral electronic equilibrium, and dose volume averaging in finitely sized detectors. Three-dimensional (3D) dosimetry is ideal for measuring small output factors and avoids many of these potential challenges of point and 2D detectors. PRESAGE 3D polymer dosimeters were used to measure the output factors for the 4 mm and 8 mm collimators of the Leksell Perfexion Gamma Knife radiosurgery treatment system. Discrepancies between the planned and measured distance between shot centers were also investigated. A Gamma Knife head frame was mounted onto an anthropomorphic head phantom. Special inserts were machined to hold 60 mm diameter, 70 mm tall cylindrical PRESAGE dosimeters. The phantom was irradiated with one 16 mm shot and either one 4 mm or one 8 mm shot, to a prescribed dose of either 3 Gy or 4 Gy to the 50% isodose line. The two shots were spaced between 30 mm and 60 mm apart and aligned along the central axis of the cylinder. The Presage dosimeters were measured using the DMOS-RPC optical CT scanning system. Five independent 4 mm output factor measurements fell within 2% of the manufacturer’s Monte Carlo simulation-derived nominal value, as did two independent 8 mm output factor measurements. The measured distances between shot centers varied by ±0.8 mm with respect to the planned shot displacements. On the basis of these results, we conclude that PRESAGE dosimetry is excellently suited to quantify the difficult-to-measure Gamma Knife output factors.

Dosimetry of cone-defined stereotactic radiosurgery fields with a commercial synthetic diamond detector.

Small field x-ray beam dosimetry is difficult due to lack of lateral electronic equilibrium, source occlusion, high dose gradients, and detector volume averaging. Currently, there is no single definitive detector recommended for small field dosimetry. The objective of this work was to evaluate the performance of a new commercial synthetic diamond detector, namely, the PTW 60019 microDiamond, for the dosimetry of small x-ray fields as used in stereotactic radiosurgery (SRS). Small field sizes were defined by BrainLAB circular cones (4-30 mm diameter) on a Novalis Trilogy linear accelerator and using the 6 MV SRS x-ray beam mode for all measurements. Percentage depth doses (PDDs) were measured and compared to an IBA SFD and a PTW 60012 E diode. Cross profiles were measured and compared to an IBA SFD diode. Field factors, ΩQclin,Qmsr (fclin,fmsr) , were calculated by Monte Carlo methods using BEAMnrc and correction factors, kQclin,Qmsr (fclin,fmsr) , were derived for the PTW 60019 microDiamond detector. For the small fields of 4-30 mm diameter, there were dose differences in the PDDs of up to 1.5% when compared to an IBA SFD and PTW 60012 E diode detector. For the cross profile measurements the penumbra values varied, depending upon the orientation of the detector. The field factors, ΩQclin,Qmsr (fclin,fmsr) , were calculated for these field diameters at a depth of 1.4 cm in water and they were within 2.7% of published values for a similar linear accelerator. The corrections factors, kQclin,Qmsr (fclin,fmsr) , were derived for the PTW 60019 microDiamond detector. The authors conclude that the new PTW 60019 microDiamond detector is generally suitable for relative dosimetry in small 6 MV SRS beams for a Novalis Trilogy linear equipped with circular cones.

Correction of measured Gamma-Knife output factors for angular dependence of diode detectors and PinPoint ionization chamber

Dosimetry for Gamma-Knife requires detectors with high spatial resolution and minimal angular dependence of response. Angular dependence and end effect time for p-type silicon detectors (PTW Diode P and Diode E) and PTW PinPoint ionization chamber were measured with Gamma-Knife beams. Weighted angular dependence correction factors were calculated for each detector. The Gamma-Knife output factors were corrected for angular dependence and end effect time. For Gamma-Knife beams angle range of 84°–54°. Diode P shows considerable angular dependence of 9% and 8% for the 18 mm and 14, 8, 4 mm collimator, respectively. For Diode E this dependence is about 4% for all collimators. PinPoint ionization chamber shows angular dependence of less than 3% for 18, 14 and 8 mm helmet and 10% for 4 mm collimator due to volumetric averaging effect in a small photon beam. Corrected output factors for 14 mm helmet are in very good agreement (within ±0.3%) with published data and values recommended by vendor (Elekta AB, Stockholm, Sweden). For the 8 mm collimator diodes are still in good agreement with recommended values (within ±0.6%), while PinPoint gives 3% less value. For the 4 mm helmet Diodes P and E show over-response of 2.8% and 1.8%, respectively. For PinPoint chamber output factor of 4 mm collimator is 25% lower than Elekta value which is generally not consequence of angular dependence, but of volumetric averaging effect and lack of lateral electronic equilibrium. Diodes P and E represent good choice for Gamma-Knife dosimetry.

Dose response of selected solid state detectors in applied homogeneous transverse and longitudinal magnetic fields.

MR-Linac devices under development worldwide will require standard calibration, commissioning, and quality assurance. Solid state radiation detectors are often used for dose profiles and percent depth dose measurements. The dose response of selected solid state detectors is therefore evaluated in varying transverse and longitudinal magnetic fields for this purpose. The Monte Carlo code PENELOPE was used to model irradiation of a PTW 60003 diamond detector and IBA PFD diode detector in the presence of a magnetic field. The field itself was varied in strength, and oriented both transversely and longitudinally with respect to the incident photon beam. The long axis of the detectors was oriented either parallel or perpendicular to the photon beam. The dose to the active volume of each detector in air was scored, and its ratio to dose with zero magnetic field strength was determined as the "dose response" in magnetic field. Measurements at low fields for both detectors in transverse magnetic fields were taken to evaluate the accuracy of the simulations. Additional simulations were performed in a water phantom to obtain few representative points for beam profile and percent depth dose measurements. Simulations show significant dose response as a function of magnetic field in transverse field geometries. This response can be near 20% at 1.5 T, and it is highly dependent on the detectors' relative orientation to the magnetic field, the energy of the photon beam, and detector composition. Measurements at low transverse magnetic fields verify the simulations for both detectors in their relative orientations to radiation beam. Longitudinal magnetic fields, in contrast, show little dose response, rising slowly with magnetic field, and reaching 0.5%-1% at 1.5 T regardless of detector orientation. Water tank and in air simulation results were the same within simulation uncertainty where lateral electronic equilibrium is present and expectedly differed at the beam edge in transverse field orientations only. Due to the difference in design, the two detectors behaved differently. When transverse magnetic fields are present, great care must be taken when using diamond or diode detectors. Dose response varies with relative detector orientation, magnetic field strength, and between detectors. This response can be considerable (∼20% for both detectors). Both detectors in longitudinal fields exhibit little to no dose response as a function of magnetic field. Water tank simulations seem to suggest that the diode detector is better suited to general beam commissioning, and each detector must be investigated separately.

SU-E-T-84: Comparison of Three Different Systems for Patient-Specific Quality Assurance: Cranial Stereotactic Radiosurgery Using VMAT with Multiple Non Coplanar Arcs

Purpose: Patient-specific quality assurance in volumetric modulated arc therapy (VMAT) brain stereotactic radiosurgery raises specific issues on dosimetric procedures, mainly represented by the small radiation fields associated with the lack of lateral electronic equilibrium, the need of small detectors and the high dose delivered. The purpose of the study is to compare three different dosimeters for pre-treatment QA. Methods: Nineteen patients (affected by neurinomas, brain metastases, and by meningiomas) were treated with VMAT plans computed on a Monte Carlo based TPS. Gafchromic films inside a slab phantom (GF), 3-D cylindrical phantom with two orthogonal diodes array (DA), and 3-D cylindrical phantom with a single rotating ionization chambers array (ICA), have been evaluated. The dosimeters are, respectively, characterized by a spatial resolution of: 0.4 (in our method), 5 and 2.5 mm. For GF we used a double channel method for calibration and reading protocol; for DA and ICA we used the 3-D dose distributions reconstructed by the two software sold with the dosimeters. With the need of a common system for analyze different measuring approaches, we used an in-house software that analyze a single coronal plane in the middle of the phantoms and Gamma values (2% / 2 mm and 3% / 3 mm) were computed for all patients and dosimeters. Results: The percentage of points with gamma values less than one was: 95.7% for GF, 96.8% for DA and 95% for ICA, using 3%/3mm criteria, and 90.1% for GF, 92.4% for DA and 92% for ICA, using 2% / 2mm gamma criteria. Tstudent test p-values obtained by comparing the three datasets were not statistically significant for both gamma criteria. Conclusion: Gamma index analysis is not affected by different spatial resolution of the three dosimeters.

The use of normoxic polymer gel for measuring dose distributions of 1, 4 and 30mm cones

Abstract This study demonstrates the use of normoxic polymer gel for measuring dose distributions of small fields that lack lateral electronic equilibrium. Two different types of normoxic polymer gel, MAGAT and PAGAT, are studied in a larger field (10 cm×10 cm) and 1, 4 and 30 mm cones to obtain cone factors, dose profiles and percentage depth doses. These results were then compared to KODAK XV film measurements and BEAMnrc Monte Carlo simulations. The results show that the sensitivity of PAGAT gel is 0.090±0.074 s −1 Gy −1 , which may not be suitable for small-field dosimetry with a 0.3 mm resolution scanned using a 3 T MR imager in a dose range lower than 2.5 Gy. There are good agreements between cone factors estimated using KODAK XV film and MAGAT gel. In a dose profile comparison, good dose agreement among MAGAT gel, XV film and MC simulation can be seen in the central area for a 30 mm cone. In penumbra, the distance to agreement is at most 1.2 mm (4 pixel), and less than 0.3 mm (1 pixel) for 4 and 1 mm cones. In a percentage depth dose comparison, there were good agreements between MAGAT and MC up to a depth of 8 cm. Possible factors for gel uncertainty such as MRI magnetic field inhomogeneity and temperature were also investigated.