Field Size Dependence Research Articles

On the Possibility to Use the Charge Imbalance in Patients Undergoing Radiotherapy: A New Online, In Vivo, Noninvasive Dose Monitoring System

This paper describes a new real-time, in vivo, noninvasive, biasless detector system acting as a beam monitoring and relative dose measurement system. The detector is based on the idea that when a beam current is injected into the body of a patient undergoing a charged particle therapy, the current itself can be collected using a conductive electrode in contact with the patient’s skin. This new approach was studied in vitro using an electrically isolated water tank irradiated with monoenergetic proton beams. The conductive electrode was immersed in water and positioned outside the irradiation field. The detection system performance was evaluated by comparing its response against a SEM (Secondary Emission Monitor) detector, used as a reference beam current monitor, and an Advanced Markus ionization chamber. Short-, mid- and long-term reproducibility, current monitoring capability, field size dependence, electrode position and environment temperature dependence, linearity with dose, and dose rate dependence were investigated. Few preliminary in vivo tests were also performed that demonstrated the possibility to apply the system in clinical practice. The potential of the proposed method is considerable, representing a simple and economical system for online, in vivo, and noninvasive monitoring of the beam current and relative released dose into the patient during treatment, without perturbing the irradiation field. The system presented in this work is protected with both a National Italian (N. 102017000087851) and an International N. WO 2019/025933 patent.

Dosimetry with a clinical linac adapted to FLASH electron beams.

PurposeTo assess dosimetric properties and identify required updates to commonly used protocols (including use of film and ionization chamber) pertaining to a clinical linac configured into FLASH (ultra‐high dose rate) electron mode.MethodsAn 18MV photon beam of a Varian iX linac was converted to FLASH electron beam by replacing the target and the flattening filter with an electron scattering foil. The dose was prescribed by entering the MUs through the console. Fundamental beam properties, including energy, dose rate, dose reproducibility, field size, and dose rate dependence on the SAD, were examined in preparation for radiobiological experiments. Gafchromic EBT‐XD film was evaluated for usability in measurements at ultra‐high dose rates by comparing the measured dose to the inverse square model. Selected previously reported models of chamber efficiencies were fitted to measurements in a broad range of dose rates.ResultsThe performance of the modified linac was found adequate for FLASH radiobiological experiments. With exception of the increase in the dose per MU on increase in the repetition rate, all fundamental beam properties proved to be in line with expectations developed with conventional linacs. The field size followed the theorem of similar triangles. The highest average dose rate (2 × 104 Gy/s) was found next to the internal monitor chamber, with the field size of FWHM = 1.5 cm. Independence of the dose readings on the dose rate (up to 2 × 104 Gy/s) was demonstrated for the EBT‐XD film. A model of recombination in an ionization chamber was identified that provided good agreement with the measured chamber efficiencies for the average dose rates up to at least 2 × 103 Gy/s.ConclusionDosimetric measurements were performed to characterize a linac converted to FLASH dose rates. Gafchromic EBT‐XD film and dose rate‐corrected cc13 ionization chamber were demonstrated usable at FLASH dose rates.

Characteristics of very high-energy electron beams for the irradiation of deep-seated targets.

Driven by advances in accelerator technology and the potential of exploiting the FLASH effect for the treatment of deep-seated targets (>5cm), there is an active interest in the construction of devices to deliver very high-energy electron (VHEE) beams for radiation therapy. The application of novel VHEE devices, however, requires an assessment of the tradeoffs between the different beam parameter choices including beam energies, beam divergences, and maximal field sizes. This study systematically examines the dosimetric beam properties of VHEE beams, determining their clinical usefulness while marking their limits of applications for different beam configurations. We performed Monte Carlo simulations of the dose distributions of electron beams for different energies (25-250MeV), source-to-surface distances (SSD) (50cm, 100cm, parallel), and field sizes (2cm2 ×2cm2 to 15cm2 ×15cm2 ) in water using a research version of the RayStation treatment planning system (RaySearch Labs 9A IONPG). The beam was simulated using a monoenergetic point source and perfect collimation. Central axis percentage depth dose (PDD) and transverse dose profiles at multiple depths were evaluated and compared to those of MV photon beams. Profile characteristics including therapeutic range (TR) at 90%, proximal fall-off (PFO) at 90%, lateral penumbra (LP) at 90%-10%, and field width (FW) at 90% were obtained. Very high-energy electrons beams with SSD 100cm and parallel beams (infinite SSD) exhibit a linear to near-linear increase of TR as a function of energy in the simulated energy range and reach values well beyond the typical depths of lesions encountered in clinics (<20cm). Their TR show a marked field size dependence only for field sizes <10cm2 ×10cm2 . For VHEE beams with SSD 50cm, TR are largely reduced (4-8cm). For beam energies >150MeV with large SSD (>100cm), for many configurations, there is no substantial difference in PDD when adding an opposed beam. This may potentially reduce the number of VHEE beams needed for treatment by a factor of two compared to a treatment using lower energies and lower SSD. In order to cover deep-seated targets homogeneously, VHEE devices with a parallel beam must provide a maximum field size up to several centimeters larger than the tumor size. For the investigated diverging beams, there is not such a significant field width reduction with depth for larger fields as it is compensated by divergence. Penumbrae of VHEE beams are smaller than those of clinical MV photon beams for lower depths (<5cm) but increase quickly for larger depths. There is only a relatively small dependence of penumbra on the SSD of the beam. The findings presented in this study assess the performance of VHEE beams and offer a first estimate of treatment indications and tradeoffs for a given design of a VHEE device. SSD >100cm results in clinically more favorable PDD. Beam energies of 100MeV and above are needed to cover common tumors (5-15cm in-depth) conformally. Higher energies provide an additional benefit specifically for small and deep-seated lesions due to their reduced lateral penumbrae.

Characterization of Gas Electron Multiplier‐based detector for external beam radiation therapy dosimetry

Electronic portal imaging devices (EPIDs) are commonly installed on modern linear accelerators (LINACs) and are convenient for imaging and, potentially, dosimetry. However, owing to their construction with metal and scintillating layers of high atomic number, they exhibit nonwater-equivalent response and oversensitivity to low-energy photons. Therefore, EPIDs are not ideal for dosimetry purposes. Additionally, nonlinearities due to the combined use of scintillators and photodiodes have been reported. Here, an EPID which employs a variable gain Gas Electron Multiplier (GEM) and direct detection of electrons is introduced. To investigate its dosimetric performance, measurements characterizing the novel EPID are performed and compared with measurements from ionization chambers and conventional EPIDs. Linearity, dose rate dependence, field size dependence, off-axis response, and transmission response were measured for all available energy settings (6, 10, 6MV Flattening Filter Free (FFF) and 10MVFFF) using three different detector gain settings. Additionally, an evaluation of the ghosting and image lag of the panel was completed. Reference ionization chamber measurements were performed for the off-axis and transmission response and existing data for conventional EPIDs and ionization chambers from equivalent measurements were used for comparison of the field size dependence. Elsewhere, values from the linac monitoring chambers were used. In the range from 10 to 1000 Monitor Units (MU), the detector was linear within 1% for all combinations of gain settings and energies. The dose rate dependence was also within 1% for all energies and for two out of three gain settings. Regarding field size dependency, the ratio of ionization chamber and panel values was 0.94 and 0.98 for the conventional EPID and GEMini respectively, at 20×20cm2 and 10MV. For 6MV, 6MVFFF, and 10MVFFF these ratios were 0.97, 0.98, and 0.99 for the GEMini, and 0.95, 0.97, and 0.97 for the conventional EPID. Similar performance between the GEMini and conventional EPID is observed for field sizes smaller than 10×10cm2 . The transmission response was within 5% for all energies for thicknesses up to 30cm, compared to 10-20% for a conventional EPID. The off-axis response for shifts up to 16cm was within 1% and 3% for 6MV and 10MV, with and without phantom. The rise and fall of the signal from the detector correspond well to monitor chamber measurements indicating little ghosting and image lag, regardless of gain setting. The GEM EPID exhibits dose rate dependence and linearity within 1%, and negligible ghosting and image lag. In this regard, it performs particularly well using 50 and 250V of gain, and either could be chosen. For higher sensitivity, 250V is the recommended base gain setting, although other applications may warrant different gains. For most tests performed in this study, the GEM EPID demonstrates a more water-equivalent response than conventional EPIDs making GEMs a viable technology for dosimetry in radiation therapy.

Evaluation of MgB4O7:Ce, Li and Ce-doped 80MgB2O4–20MgB4O7 as alternative OSL materials for use in quality assurance of 6 MV photon beams

We investigated the potential use of two new borate matrices for quality assurance (QA) tests in radiotherapy. Both matrices, MgB4O7:Ce, Li (MBO) and Ce-doped 80MgB2O4–20MgB4O7 (80-20MBO), were doped with 0.5% cerium oxide. The following dosimetric properties of these materials prepared as pellets were tested: optically stimulated luminescence (OSL) sensitivity, dose response, fading, reproducibility, and the supralinearity index. Subsequently, the pellets were exposed to a 6 MV photon beam, and the percentage depth dose (PDD) curve, OSL response to different dose rates, and radiation field size dependencies were compared with those obtained with a reference detector (ionization chamber, I.C.) used in QA. The results indicated a higher OSL sensitivity of 80–20MBO as compared with MBO, on the order of 6- to 8-fold. The dose response of MBO presented a linear behavior within the range evaluated (20 mGy–2 Gy), unlike 80-20MBO, which had a supralinear behavior. The OSL signal of 80–20MBO was also marked by 60% fading within 5 h post irradiation; afterwards, the OSL signal stabilized. A better reproducibility of 80–20MBO compared with MBO was also observed. No dose rate dependence of the OSL response was observed; moreover, a maximum 5% standard deviation was seen in this test. For the field size tests, the pellets over-responded by 3.1% (80-20MBO) and 3.4% (MBO), in a 3 × 3 cm2 field, compared with the I.C. For the PDD, a high deviation was observed if compared with the I.C. data at the build-up region (at a depth d ≤ 2 cm), maximum 4.3% and 7.8% for 80-20MBO and MBO, respectively, and beyond this value, good agreement among detectors was seen. Our preliminary results indicate the potential utility of these pellets in QA for radiotherapy.

Verification system for intensity-modulated radiation therapy with scintillator.

In the preparation of intensity-modulated radiation therapy (IMRT), patient-specific verification is widely employed to optimize the treatment. To accurately estimate the accumulated dose and obtain the field-by-field or segment-by-segment verification, an original IMRT verification tool using scintillator light and an analysis workflow was developed in this study. The raw light distribution was calibrated with respect to the irradiated field size dependency and light diffusion in the water. The calibrated distribution was converted to dose quantity and subsequently compared with the results of the clinically employed plan. A criterion of 2-mm dose-to-agreement and 3% dose difference was specified in the gamma analysis with a 10% dose threshold. By applying the light diffusion calibration, the maximum dose difference was corrected from 7.7cGy to 3.9cGy around the field edge for a 60cGy dose, 7 × 7 cm2 irradiation field, and 10 MV beam energy. Equivalent performance was confirmed in the chromodynamic film. The average dose difference and gamma pass rate of the accumulated dose distributions in six patients were 0.8 ± 4.5cGy and 97.4%, respectively. In the field-by-field analysis, the average dose difference and gamma pass rate in seven fields of Patient 1 were 0.2 ± 1.2cGy and 93.9%, respectively. In the segment-by-segment analysis, the average dose difference and gamma pass rate in nine segments of Patient 1 and a 305° gantry angle were - 0.03 ± 0.2cGy and 93.9%, respectively. This system allowed the simultaneous and independent analysis of each field or segment in the accumulated dose analysis.

Dosimetric performance of continuous EPID imaging in stereotactic treatment conditions.

This study aims at investigating the dosimetric characteristics of a Varian aS1000 EPID, focusing on its continuous acquisition mode under the challenging conditions that can be met in stereotactic radiotherapy verification. An aS1000 EPID installed on a Varian TrueBeamSTx was irradiated with 6 and 10 MV unflattened and flattened photon beams. In order to avoid detector saturation, the source-to-detector distance (SDD) was set to 150 or 180cm depending on the dose rate. EPID image sets were acquired in continuous mode (CM) and also in the commonly used integrated mode (IM) for comparison, to evaluate dose linearity (including dose rate dependence), repeatability, reproducibility, stability, ghosting effect and field size dependence. CM response linearity was found to be within 0.8% of IM and independent of dose rate. Response repeatability was slightly better for IM and FF beams, being in all cases within 0.9%. Reproducibility was within 0.6% for both modes and all beam qualities. Response stability between continuous frames varied within 1% for dynamic and static irradiations and for all the beam qualities, showing its independence from these parameters. Ghosting effect was not significant, being comparable to signal variations between continuous frames (±1%). Field size dependence in both modes agreed within 1%. The dosimetric response of the aS1000 EPID in CM with FFF beams and high dose rates is comparable to that in IM and for flattened beams provided that the appropriate SDD is used. aS1000 EPID in continuous acquisition mode is therefore suitable for stereotactic applications.

Thermoluminescence Dosimetric Characteristics of Fabricated Germanium (Ge) Doped Optical Fibres for Electron Beams Dosimetry: A Preliminary Study

The basic dosimetric favourable responses of tailored fabricated germanium (Ge) doped cylindrical optical fibres, make use of 2.3% mol have been extensively studied with clinical electron beams irradiation in terms of dose linearity, reproducibility, fading signals, minimum detectable dose (MDD), energy- and field size dependence. The irradiation was performed at the Radiotherapy Unit, Advanced Medical and Dental Institute (AMDI), which utilizes Elekta Synergy® linear accelerator (LINAC) at 6-, 9-, 12- and 15 MeV electron beam energies at specific doses ranging from 1 to 5 Gy. Thermoluminescence (TL) signals exhibited a linear dose-response overdose ranges, mean reproducibility with a coefficient of variation (CV) of better than 10% and no dependency with different field sizes at p > 0.05. The MDD values were typically 3.51 to 4.13 mGy. The minimum TL fading of the fabricated Ge-doped cylindrical optical fibres was reported favourably for 9 MeV electron beam at day of 74.

Characterization of an inorganic scintillator for small-field dosimetry in MR-guided radiotherapy.

IntroductionAim of this study is to dosimetrically characterize a new inorganic scintillator designed for magnetic resonance‐guided radiotherapy (MRgRT) in the presence of 0.35 tesla magnetic field (B).MethodsThe detector was characterized in terms of signal to noise ratio (SNR), reproducibility, dose linearity, angular response, and dependence by energy, field size, and B orientation using a 6 MV magnetic resonance (MR)‐Linac and a water tank.Field size dependence was investigated by measuring the output factor (OF) at 1.5 cm. The results were compared with those measured using other detectors (ion chamber and synthetic diamond) and those calculated using a Monte Carlo (MC) algorithm.Energy dependence was investigated by acquiring a percentage depth dose (PDD) curve at two field sizes (3.32 × 3.32 and 9.96 × 9.96 cm2) and repeating the OF measurements at 5 and 10 cm depths.ResultsThe mean SNR was 116.3 ± 0.6. Detector repeatability was within 1%, angular dependence was <2% and its response variation based on the orientation with respect to the B lines was <1%.The detector has a temporal resolution of 10 Hz and it showed a linear response (R2 = 1) in the dose range investigated. All the OF values measured at 1.5 cm depth using the scintillator are in accordance within 1% with those measured with other detectors and are calculated using the MC algorithm. PDD values are in accordance with MC algorithm only for 3.32 × 3.32 cm2 field. Numerical models can be applied to compensate for energy dependence in case of larger fields.ConclusionThe inorganic scintillator in the present form can represent a valuable detector for small‐field dosimetry and periodic quality controls at MR‐Linacs such as dose stability, OFs, and dose linearity.In particular, the detector can be effectively used for small‐field dosimetry at 1.5 cm depth and for PDD measurements if the field dimension of 3.32 × 3.32 cm2 is not exceeded.

Towards customizable thin-panel low-Z detector arrays: electrode design for increased spatial resolution ion chamber arrays

The purpose of the present development is to employ 3D printing to prototype an ion chamber array with a scalable design potentially allowing increased spatial resolution and a larger active area. An additional goal is to design and fabricate a custom size thin-panel detector array with low-Z components. As a proof of principle demonstration, a medium size detector array with 30 × 30 air-vented ion chambers was 3D-printed using PLA as frame for the electrodes. The active-area is 122 mm × 120 mm with 4 × 4 mm2 spatial resolution. External electrodes are cylindrical and made from conductive PLA. Internal electrodes are made from microwire. The array is symmetric with respect to the central plane and its thickness is 10 mm including build-up/-down plates of 2.5 mm thickness. Data acquisition is realized by biasing only selected chamber rows and reading only 30 chambers at a time. To test the device for potential clinical applications, 1D dose profiles and 2D dose maps with various square and irregular fields were measured. The overall agreement with the reference doses (film and treatment planning system) was satisfactory, but the measured dose differs in the penumbra region and in the field size dependence. Both of these features are related to the thin walls between neighboring ion chambers and different lateral phantom scatter in the detector panel vs homogeneous material. We demonstrated feasibility of radiation detector arrays with minimal number of readout channels and low-cost electronics. The acquisition scheme based on selected row or column ‘activation’ by bias voltage is not practical for 2D dosimetry but it allows for rapid turn-around when testing of custom arrays with the aid of multiple 1D dose profiles. Future progress in this area includes overcoming the limitations due high chamber packing ratio, which leads to the lateral scattering effects.

Dosimetric characteristics of a 2D silicon diode array for stereotactic radiotherapy end-to-end patient-specific QA

The measurement accuracy of stereotactic radiotherapy (SRT) patient-specific quality assurance (QA) is challenging as small-field dosimetry contains lateral charge disequilibrium, partial blocking of the beam source, and limitations based on the detector size. The aim of this study was to investigate the dosimetric characteristics of a two-dimensional high-resolution silicon diode array for SRT end-to-end patient-specific QA.The dosimetric characteristics of the diode array, delivered to an end-to-end QA phantom, were evaluated for 6 MV, with a flattening filter (FF) and flattening filter-free (FFF), and 10 MV FFF photon beams. Dose linearity, dose rate dependence, dose-per-pulse dependence, field size dependence, angular dependence, and beam directivity were investigated for all photon beams. A total of 27 clinical plans were selected to evaluate the SRT end-to-end patient-specific QA for treatment delivery using volumetric modulated arc therapy (VMAT).For all beam qualities, the differences of dose and dose rate linearity of the diode array were below 0.7% (10–1400 MU range) and 0.6% (100–2400 MU/min range), respectively. The diodes had a decrease in sensitivity of about 4% for the 21-fold decrease in dose-per-pulse. Output factors were in good agreement for field size down to 2 × 2 cm2 (<0.6% for both 6 MV FF and FFF beams, <1.6% for the 10 MV FFF beam), while the field size down to 0.6 × 0.6 cm2 had an under-response (<4% for all beam qualities). Angular dependence was within ±2% and ±4%, for coplanar and non-coplanar beams, respectively. However, this was not the case for the beam direction nearly parallel to the diode array plane. Beam directivity response on both sides of the array was in excellent agreement. For the end-to-end patient-specific QA, the average gamma passing rate (GPR) with a 2%/1 mm criterion was 96.8% and the minimum value was 93.0%. There was no significant difference in the GPRs between coplanar and non-coplanar VMAT plans.The diode array, with several corrections, demonstrated good dosimetric characteristics for SRT QA. The GPRs of end-to-end patient-specific QA for non-coplanar and multiple metastases were as high as that for coplanar and single-target, and a gamma criterion of 2%/1 mm would be suitable for SRT QA when using the array.

Investigation of LINAC Structural Effects on Photoneutron Specified Parameters Using FLUKA code

Introduction: The utilization of high-energy photons in the medical linear accelerator can lead to photoneutron production. This study aimed to evaluate the effect of the physical components of the head, including flattening filter (FF) andmultileaf collimator (MLC), as well as the dependence of therapeutic field size on the photoneutron spectrum, dose, and flux. Material and Methods: The present study reported the simulation of the fundamental linac head components of the Varian Clinac 2100 performing in X-ray mode with 18 MV energy by the FLUKA code. The percentage depth dose and lateral dose profile were measured using a PTW thimble chamber to ensure the simulation reliability. Results: Photoneutron spectrum analysis indicated that neutrons with highest relative biological effectiveness were delivered to the phantom surface, and opening the field from 0×0 to 40×40 shifted the spectrum by 24.545% to the higher energies. The target and the vicinity parts played the most prominent roles in neutron contamination. The relationship between the field size and the photoneutron dose was non-linear, and it reached a peak of 20×20 . Although using small fields formed by the MLC contribute to a lower dose compared to those shaped by the jaws, MLC-equipped machines result in 21.98% higher dose. Moreover, the flattening filter removal unexpectedly increased the isocenter photoneutron dose by 11.63%. This undesirable dose can be up to 2.54 mSv/Gy for the reference field at the isocenter while the out-of-field dose is about 0.5 mSv/Gy for most of the field dimensions. Conclusion: As a result, it is critical to consider this unwanted absorbed dose, which is seriously influenced by the implemented therapeutic conditions.

MOSFET dosimeter characterization in MR‐guided radiation therapy (MRgRT) Linac

PurposeWith the increasing use of MR‐guided radiation therapy (MRgRT), it becomes important to understand and explore accuracy of medical dosimeters in the presence of magnetic field. The purpose of this work is to characterize metal‐oxide‐semiconductor field‐effect transistors (MOSFETs) in MRgRT systems at 0.345 T magnetic field strength.MethodsA MOSFET dosimetry system, developed by Best Medical Canada for in‐vivo patient dosimetry, was used to study various commissioning tests performed on a MRgRT system, MRIdian® Linac. We characterized the MOSFET dosimeter with different cable lengths by determining its calibration factor, monitor unit linearity, angular dependence, field size dependence, percentage depth dose (PDD) variation, output factor change, and intensity modulated radiation therapy quality assurance (IMRT QA) verification for several plans. MOSFET results were analyzed and compared with commissioning data and Monte Carlo calculations.ResultsMOSFET measurements were not found to be affected by the presence of 0.345 T magnetic field. Calibration factors were similar for different cable length dosimeters either placed at the parallel or perpendicular direction to the magnetic field, with variations of less than 2%. The detector showed good linearity (R2 = 0.999) for 100–600 MUs range. Output factor measurements were consistent with ionization chamber data within 2.2%. MOSFET PDD measurements were found to be within 1% for 1–15 cm depth range in comparison to ionization chamber. MOSFET normalized angular response matched thermoluminescent detector (TLD) response within 5.5%. The IMRT QA verification data for the MRgRT linac showed that the percentage difference between ionization chamber and MOSFET was 0.91%, 2.05%, and 2.63%, respectively for liver, spine, and mediastinum.ConclusionMOSFET dosimeters are not affected by the 0.345 T magnetic field in MRgRT system. They showed physics parameters and performance comparable to TLD and ionization chamber; thus, they constitute an alternative to TLD for real‐time in‐vivo dosimetry in MRgRT procedures.

Characteristics of the Exradin W1 scintillator in the magnetic field.

To investigate the angular dependency of the W1 scintillator with and without a magnetic field, the beam incidence angles to the detector varied from 0° to 360° at intervals of 30° when the detector was pointed in both the craniocaudal and right‐to‐left directions. The beam incidence angles also varied from 0° to 360° at intervals of 45° when the W1 scintillator was in the anterior‐to‐posterior direction. To investigate the field size dependency of the W1 scintillator with and without a magnetic field, the doses by an identical beam‐on time were measured at various square field sizes and the measured doses were normalized to the dose at the field of 10.5 cm × 10.5 cm (FS10.5). With and without a magnetic field, the deviations of the doses to the dose at the beam incident angle of 0° were always less than 1% regardless of the dosimeter positioning relative to the magnetic field direction. When the field sizes were equal to or less than FS10.5, the differences in the output factors with and without a magnetic field were less than 0.7%. However, those were larger than 1% at fields larger than FS10.5, and up to 3.1%. The W1 scintillator showed no angular dependency to the magnetic field. Differences larger than 1% in the output factors with and without a magnetic field were observed at field sizes larger than 10.5 cm × 10.5 cm.

Physical and biological impacts of collimator-scattered protons in spot-scanning proton therapy.

To improve the penumbra of low‐energy beams used in spot‐scanning proton therapy, various collimation systems have been proposed and used in clinics. In this paper, focused on patient‐specific brass collimators, the collimator‐scattered protons' physical and biological effects were investigated. The Geant4 Monte Carlo code was used to model the collimators mounted on the scanning nozzle of the Hokkaido University Hospital. A systematic survey was performed in water phantom with various‐sized rectangular targets; range (5–20 cm), spread‐out Bragg peak (SOBP) (5–10 cm), and field size (2 × 2–16 × 16 cm2). It revealed that both the range and SOBP dependences of the physical dose increase had similar trends to passive scattering methods, that is, it increased largely with the range and slightly with the SOBP. The physical impact was maximized at the surface (3%–22% for the tested geometries) and decreased with depth. In contrast, the field size (FS) dependence differed from that observed in passive scattering: the increase was high for both small and large FSs. This may be attributed to the different phase‐space shapes at the target boundary between the two dose delivery methods. Next, the biological impact was estimated based on the increase in dose‐averaged linear energy transfer (LET d) and relative biological effectiveness (RBE). The LET d of the collimator‐scattered protons were several keV/μm higher than that of unscattered ones; however, since this large increase was observed only at the positions receiving a small scattered dose, the overall LET d increase was negligible. As a consequence, the RBE increase did not exceed 0.05. Finally, the effects on patient geometries were estimated by testing two patient plans, and a negligible RBE increase (0.9% at most in the critical organs at surface) was observed in both cases. Therefore, the impact of collimator‐scattered protons is almost entirely attributed to the physical dose increase, while the RBE increase is negligible.

Transit and non-transit 3D EPID dosimetry versus detector arrays for patient specific QA.

PurposeDespite their availability and simplicity of use, Electronic Portal Imaging Devices (EPIDs) have not yet replaced detector arrays for patient specific QA in 3D. The purpose of this study is to perform a large scale dosimetric evaluation of transit and non‐transit EPID dosimetry against absolute dose measurements in 3D.MethodsAfter evaluating basic dosimetric characteristics of the EPID and two detector arrays (Octavius 1500 and Octavius 1000SRS), 3D dose distributions for 68 VMAT arcs, and 10 IMRT plans were reconstructed within the same phantom geometry using transit EPID dosimetry, non‐transit EPID dosimetry, and the Octavius 4D system. The reconstructed 3D dose distributions were directly compared by γ‐analysis (2L2 = 2% local/2 mm and 3G2 = 3% global/2 mm, 50% isodose) and by the percentage difference in median dose to the high dose volume (%∆HDVD 50).ResultsRegarding dose rate dependency, dose linearity, and field size dependence, the agreement between EPID dosimetry and the two detector arrays was found to be within 1.0%. In the 2L2 γ‐comparison with Octavius 4D dose distributions, the average γ‐pass rate value was 92.2 ± 5.2%(1SD) and 94.1 ± 4.3%(1SD) for transit and non‐transit EPID dosimetry, respectively. 3G2 γ‐pass rate values were higher than 95% in 150/156 cases. %∆HDVD 50 values were within 2% in 134/156 cases and within 3% in 155/156 cases. With regard to the clinical classification of alerts, 97.5% of the treatments were equally classified by EPID dosimetry and Octavius 4D.ConclusionTransit and non‐transit EPID dosimetry are equivalent in dosimetric terms to conventional detector arrays for patient specific QA. Non‐transit 3D EPID dosimetry can be readily used for pre‐treatment patient specific QA of IMRT and VMAT, eliminating the need of phantom positioning.

Experimental and Monte-Carlo characterization of the novel compact ionization chamber PTW 31023 for reference and relative dosimetry in high energy photon beams

IntroductionThe aim of the present work is to perform dosimetric characterization of a novel vented PinPoint ionization chamber (PTW 31023, PTW-Freiburg, Germany). This chamber replaces the previous model (PTW 31014), where the diameter of the central electrode has been increased from 0.3 to 0.6mm and the guard ring has been redesigned. Correction factors for reference and non-reference measurement conditions were examined. Materials and methodsMeasurements and calculations of the correction factors were performed according to the DIN 6800-2. The shifts of the effective point of measurement (EPOM) from the chamber's reference point were determined by comparison of the measured PDD with the reference curve obtained with a Roos chamber. Its lateral dose response functions, which act according to a mathematical convolution model as the convolution kernel transforming the dose profile D(x) to the measured signal M(x), have been approximated by Gaussian functions with standard deviation σ. Additionally, the saturation correction factors kS have been determined using different dose-per-pulse (DPP) values. The polarity effect correction factors kP were measured for field sizes from 5cm×5cm to 40cm×40cm. The influence of the diameter of the central electrode and the new guard ring on the beam quality correction factors kQ was studied by Monte-Carlo simulations. The non-reference condition correction factors kNR have been computed for 6MV photo beam by varying the field size and measurement depth. Comparisons on these aspects have been made to the previous model. ResultsThe shifts of the EPOM from the reference point, Δz, are found to be −0.55 (6MV) and −0.56 (10MV) in the radial orientation and −0.97mm (6MV) and −0.91mm (10MV) in the axial orientation. All values of Δz have an uncertainty of 0.1mm. The σ values are 0.80mm (axial), 0.75mm (radial lateral) and 1.76mm (radial longitudinal) for 6MV photon beam and are 0.85mm (axial), 0.75mm (radial lateral) and 1.82mm (radial longitudinal) for 15MV photon beam. All σ values have an uncertainty of 0.05mm. The correction factor kS was found to be 1.0034±0.0009 for the PTW 31014 chamber and 1.0024±0.0007 for the PTW 31023 chamber at the highest DPP (0.827mGy) investigated in this study. Under reference conditions, the polarity effect correction factor kP of the PTW 31014 chamber is 1.0094 and 1.0116 for 6 and 10MV respectively, while the kP of the PTW 31023 chamber is 1.0005 and 1.0013 for 6 and 10MV respectively, all values have an uncertainty of 0.002. The kP of the new chamber also exhibits a weaker field size dependence. The kQ values of the PTW 31023 chamber are closer to unity than those of the PTW 31014 chamber due to the thicker central electrode and the new guard ring design. The kNR values of the PTW 31023 chamber for 6MV photon beam deviate by not more than 1% from unity for the conditions investigated. DiscussionsCorrection factors associated with the new chamber required to perform reference and relative dose measurements have been determined according to the DIN-protocol. The correction factor kS of the new chamber is 0.1% smaller than that of the PTW 31014 at the highest DPP investigated. Under reference conditions, the correction factor kP of the PTW 31023 chamber is approximately 1% smaller than that of the PTW 31014 chamber for both energies used. The dosimetric characteristics of the new chamber investigated in this work have been demonstrated to fulfil the requirements of the TG-51 addendum for reference-class dosimeters at reference conditions.

Characteristics and clinical application of the TomoDose diode array for quality control of Tomotherapy

Objective To evaluate the dosimetric characteristics of the TomoDose diode and its application in detecting the couch velocity and assessing the beam Profile stability for convenient and efficient quality control of Tomotherapy. Methods Fundamental properties of the TomoDose detector, such as dose linearity, dose per pulse dependence, directional dependence and field size dependence were tested by the water-equivalent phantom and the results were statistically compared with those measured by the ion-chamber (0.057 cm3, A1SL). Five different couch velocities with a time interval of 0.0005 s were designed to test the sensitivity of TomoDose for detecting the couch velocity, and then the clinical data of 7 patients with different couch velocities were selected to verify the test accuracy. The beam profile of three different jaws of 1.0, 2.5 and 5.0 cm were measured using TomoDose with water-equivalent phantom in the x and y directions under the same conditions as water tank with an underwater depth of 15, 50, 100, 150 and 200 mm, respectively. The Profile data obtained by water tank and TomoDose were evaluated using the Gamma (γ) evaluation method with the 2%/1 mm criterion. Results The dose linearity of the TomoDose and ion chamber was in a linear pattern within the beam-on-time≤30 s. The difference between the TomoDose and ion chamber was less than 2% for the dose per pulse dependence, and the response trends of them were consistent. TomoDose showed angular response dependence with a maximum difference of 2.53% for the gantry angle of ±60°.For the field size dependence, the response difference between the TomoDose and ion chamber was increased with the decreasing field size when the Jaw was 5.0 cm, and the maximum deviation was 0.78% when the field size was 5.0 cm×2.5 cm. TomoDose could detect the couch distance error of 0.5 mm, and determine the couch velocity deviation of 200 mm), the value of γ was larger and even with γ>1.In the y direction, all comparison results of Profile under three field width demonstrated that γ index was larger on the edge of beam, whereas γ<1 was found in all cases. Conclusions TomoDose is suitable f or the quality control of Tomotherapy, which can accurately measure the couch velocity, precisely monitor the stability of beam Profile of Tomotherapy and complete the quality control process in a convenient and efficient manner. Key words: Semiconductor detector; Couch velocity; Profile; Tomotherapy

Spatial scaling of illusory motion perceived in a static figure.

In a phenomenon known as the Rotating Snakes illusion (Kitaoka & Ashida, 2003), illusory motion is perceived in a static figure with a specially designed luminance profile. It is known that the strength of this illusion increases with eccentricity, suggesting that the underlying mechanism of the illusion has a spatial property that changes with eccentricity. If a change in receptive-field size of responsible neurons causes the eccentricity dependence of the illusion, its strength should be spatially scalable using a scaling factor that increases with eccentricity, because the receptive field size of neurons in visual areas with retinotopy generally obeys quantitative dependence on eccentricity. For the luminance micropatterns comprising the figure for the Rotating Snakes illusion, we varied eccentricity from 9 to 15 deg and spatial frequency from 0.25 to 1.6 cycles/deg, and measured illusion strength. Illusion strength was found to increase with decreasing spatial frequency and with increasing eccentricity. Furthermore, the profiles of illusion strength at different eccentricities were spatially scalable into a single parabola as a function of the spatially scaled visual angle. The estimated scaling factors linearly increased with eccentricity with a slope similar to the eccentricity dependence of the receptive field size of V1 neurons, suggesting the involvement of early visual areas in the generation of the illusion.

15 Characterization of an aSi-1000 EPID response in integrated and continuous acquisition modes for SBRT dosimetry applications

Introduction The amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) mounted on linacs have been extensively studied and have shown dose-response characteristics suitable for dose verification. Their use for dosimetric purposes is already in clinical routine. However, the response of EPID in continuous acquisition mode was not so studied in literature, and in less extent when considering it for dosimetric applications in stereotactic body radiotherapy (SBRT) conditions (i.e. small fields, and high dose rates using flattening filter free (FFF) beams). The purpose of this work was therefore to characterize the EPID response in terms of relevant dosimetric properties, mainly for the potential use of continuous mode for further SBRT dosimetry with dynamic techniques. Methods The EPID used in this study was the aSi-1000 model installed on a Varian TrueBeamSTx. Its response was characterized in FFF photon beams with nominal energies of 6 and 10 MV and maximum available dose rates (1400 and 2400 MU/min, for 6 and 10 MV respectively). Different sets of images were acquired in both integrated and continuous modes, with EPID at 150 cm from the source, to study dose linearity, repeatability and reproducibility of response, field-size dependence, ghosting effect and response to energy spectrum variations. Before data acquisition, dark field and flood filed images were acquired for offset and gain corrections. As fluctuations in dose rate can potentially influence the response of EPID when using dynamic techniques with continuous acquisition mode, the linearity of EPID response was further evaluated by irradiating a 10 × 10 cm2 field in 20° and 90° arcs focusing on low MU range (3–100 MU), and was also compared to static irradiations. Results Conclusions The characterization of the aSi-1000 EPID response to relevant dosimetric properties was investigated, showing the feasibility of its use for dosimetry with FFF beams and high dose rates. Results for continuous mode, which differed from dosimetric non-linearity reported in previous published studies [ [1] McCurdy B.M.C. Greer P.B. Dosimetric properties of an amorphous-silicon EPID used in continuous acquisition mode for application to dynamic and arc IMRT. Med Phys. 2009; 36: 3028-3039 Crossref PubMed Scopus (73) Google Scholar ], also encourage the use of EPID for SBRT with dynamic techniques in future works for 3D dosimetry applications (e.g. quality assurance including in vivo dosimetry). The amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) mounted on linacs have been extensively studied and have shown dose-response characteristics suitable for dose verification. Their use for dosimetric purposes is already in clinical routine. However, the response of EPID in continuous acquisition mode was not so studied in literature, and in less extent when considering it for dosimetric applications in stereotactic body radiotherapy (SBRT) conditions (i.e. small fields, and high dose rates using flattening filter free (FFF) beams). The purpose of this work was therefore to characterize the EPID response in terms of relevant dosimetric properties, mainly for the potential use of continuous mode for further SBRT dosimetry with dynamic techniques. The EPID used in this study was the aSi-1000 model installed on a Varian TrueBeamSTx. Its response was characterized in FFF photon beams with nominal energies of 6 and 10 MV and maximum available dose rates (1400 and 2400 MU/min, for 6 and 10 MV respectively). Different sets of images were acquired in both integrated and continuous modes, with EPID at 150 cm from the source, to study dose linearity, repeatability and reproducibility of response, field-size dependence, ghosting effect and response to energy spectrum variations. Before data acquisition, dark field and flood filed images were acquired for offset and gain corrections. As fluctuations in dose rate can potentially influence the response of EPID when using dynamic techniques with continuous acquisition mode, the linearity of EPID response was further evaluated by irradiating a 10 × 10 cm2 field in 20° and 90° arcs focusing on low MU range (3–100 MU), and was also compared to static irradiations. The characterization of the aSi-1000 EPID response to relevant dosimetric properties was investigated, showing the feasibility of its use for dosimetry with FFF beams and high dose rates. Results for continuous mode, which differed from dosimetric non-linearity reported in previous published studies [ [1] McCurdy B.M.C. Greer P.B. Dosimetric properties of an amorphous-silicon EPID used in continuous acquisition mode for application to dynamic and arc IMRT. Med Phys. 2009; 36: 3028-3039 Crossref PubMed Scopus (73) Google Scholar ], also encourage the use of EPID for SBRT with dynamic techniques in future works for 3D dosimetry applications (e.g. quality assurance including in vivo dosimetry).