Characterization of a novel micro-irradiator using Monte Carlo radiation transport simulations

Small animals are highly valuable resources for radiobiology research. While rodents have been widely used for decades, zebrafish embryos have recently become a very popular research model. However, unlike rodents, zebrafish embryos lack appropriate irradiation tools and methodologies. Therefore, the main purpose of this work is to use Monte Carlo radiation transport simulations to characterize dosimetric parameters, determine dosimetric sensitivity and help with the design of a new micro-irradiator capable of delivering irradiation fields as small as 1.0 mm in diameter. The system is based on a miniature x-ray source enclosed in a brass collimator with 3 cm diameter and 3 cm length. A pinhole of 1.0 mm diameter along the central axis of the collimator is used to produce a narrow photon beam. The MCNP5, Monte Carlo code, is used to study the beam energy spectrum, percentage depth dose curves, penumbra and effective field size, dose rate and radiation levels at 50 cm from the source. The results obtained from Monte Carlo simulations show that a beam produced by the miniature x-ray and the collimator system is adequate to totally or partially irradiate zebrafish embryos, cell cultures and other small specimens used in radiobiology research.     

A dosimetric comparison of various multileaf collimators

The dosimetric characteristics of three multileaf collimator (MLC) systems (Elekta, Siemens and Varian) having 10 mm leaf width are compared. A 6 MV photon beam was used from each unit for measurements. Film dosimetry was performed for the measurements and the analysis techniques were exactly duplicated in each system. Two of the collimators have rounded leaf ends (Elekta and Varian) and the third (Siemens) has a flat end that follows beam divergence. A scanning densitometer (Wellh?fer with 0.45 mm spot and 0.5 mm step size) was used for film analysis. The dosimetric characteristics studied include: penumbra width (80-20%) as a function of position of the leaf end in the field, inter- and intra-leaf radiation leakage, dose distribution of the tongue and groove, and isodose curves for stepped leaves forming 45 degrees angle beam edge. Results show that MLC designs with divergent and non-divergent leaves produce penumbra (80-20%) widths that are within 2.0 mm of each other. However, the distance of the collimator from the x-ray target plays an important role, and the smallest penumbra width was noted for the Varian MLC despite its rounded leaf-end design. Compared to the other systems, this collimator is positioned about 15 cm closer to the patient which affects the skin dose. The MLC with flat leaf end, although closer to the target, showed slightly poorer penumbra width. Inter-leaf leakage through the leaves is 1.3% for two of the collimators (Elekta and Varian) with the backup jaws and is nearly 1% for the third system (Siemens). The Siemens MLC produces reduced tongue-and-groove effect compared to the other two collimators (Elekta and Varian). The isodose undulation for a stepped edge is found to be significant for the collimator closest to the patient (Varian) and does not depend on the leaf-end shape. There is no perfect MLC system that can be recommended, rather each one has unique advantages and disadvantages that should be weighed with comfort, ease and cost effectiveness for clinical use.     

Dosimetric characteristics of a double-focused miniature multileaf collimator.

The dosimetric characteristics of a double-focused miniature multileaf collimator (mMLC) attached to a Philips SL75/5 linear accelerator (linac) have been investigated. Output factors, percentage depth-dose, penumbra, leaf transmission, and leakage between the leaves were measured for the 6 MV x-ray beam on this accelerator. Because leakage both through and between the leaves is minimal, the linac jaws can be kept fixed while the mMLC leaf configuration is modified for different aperture shapes. This allows for accurate output prediction using the equivalent square formalism. Percent depth-dose measured for fields defined by the mMLC show little deviation from the percent depth-dose measured for fields defined by the machine jaws or Lipowitz metal blocks. Because the mMLC matches beam divergence in both directions, allows minimal beam transmission, and has a large source-to-collimator distance, the penumbra is sharper for fields defined by the mMLC than for fields defined by the linac jaws or Lipowitz metal blocks. Based on these data, dose calculations for mMLC-defined fields can be applied with no change in procedures from those used for fields defined using conventional methods.     

Dosimetric characteristics of Novalis shaped beam surgery unit

The dosimetric characteristics of a new dedicated radiosurgical treatment unit are systematically measured in terms of its percent depth dose, beam profile, and relative scatter factor. High-resolution diode detector, mini-ion-chamber detector, and conventional Kodak XV films are used to measure dosimetric data for a range of field sizes from 6x6 mm to 100x100 mm. The effects of collimator size, micro-multileaf collimator shape, and detector type on the dosimetric data are investigated. Results indicate that, with careful design, accurate dosimetric data could be acquired using either a dedicated diode detector or a mini-ion-chamber detector, and film detector. Special attention is required when measuring dosimetric data for small field sizes such as 6x6 mm.     

Using a photon phase-space source for convolution/superposition dose calculations in radiation therapy

For a given linac design, the dosimetric characteristics of a photon beam are determined uniquely by the energy and radial distributions of the electron beam striking the x-ray target. However, in the usual commissioning of a beam from measured data, a large number of variables can be independently tuned, making it difficult to derive a unique and self-consistent beam model. For example, the measured dosimetric penumbra in water may be attributed in various proportions to the lateral secondary electron range, the focal spot size and the transmission through the tips of a non-divergent collimator; the head-scatter component in the tails of the transverse profiles may not be easy to resolve from phantom scatter and head leakage; and the head-scatter tails corresponding to a certain extra-focal source model may not agree self-consistently with in-air output factors measured on the central axis. To reduce the number of adjustable variables in beam modelling, we replace the focal and extra-focal sources with a single phase-space plane scored just above the highest adjustable collimator in a EGS/BEAM simulation of the linac. The phase-space plane is then used as photon source in a stochastic convolution/superposition dose engine. A photon sampled from the uncollimated phase-space plane is first propagated through an arbitrary collimator arrangement and then interacted in the simulation phantom. Energy deposition kernel rays are then randomly issued from the interaction points and dose is deposited along these rays. The electrons in the phase-space file are used to account for electron contamination. 6 MV and 18 MV photon beams from an Elekta SL linac are used as representative examples. Except for small corrections for monitor backscatter and collimator forward scatter for large field sizes (<0.5% with <20 x 20 cm2 field size), we found that the use of a single phase-space photon source provides accurate and self-consistent results for both relative and absolute dose calculations.     

Assessment of a new multileaf collimator concept using GEANT4 Monte Carlo simulations

The aim of the work was to investigate in advance the dosimetric properties of a new multileaf collimator (MLC) concept with the help of Monte Carlo (MC) simulations prior to the production of a prototype. The geometrical design of the MLC was implemented in the MC code GEANT4. For the simulation of a 6 MV treatment beam, an experimentally validated phase space and a virtual spatial Gaussian-shaped model placed in the origin were used. For the simulation of the geometry in GEANT4, the jaws and the two leaf packages were implemented with the help of computer-aided design data. First, transmission values for different tungsten alloys were extracted using the simulation codes GEANT4 and BEAMnrc and compared to experimental measurements. In a second step, high-resolution simulations were performed to detect the leakage at depth of maximum dose. The 20%-80% penumbra along the travel direction of the leaves was determined using 10 x 10 cm2 fields shifted along the x- and y-axis. The simulated results were compared with measured data. The simulation of the transmission values for different tungsten alloys showed a good agreement with the experimental measurements (within 2.0%). This enabled an accurate estimation of the attenuation coefficient for the various leaf materials. Simulations with varying width of the spatial Gaussian distribution showed that the leakage and the penumbra depend very much on this parameter: for instance, for widths of 2 and 4 mm, the interleaf leakage is below 0.3% and 0.75%, respectively. The results for the leakage and the penumbra (4.7+/-0.5 mm) are in good agreement with the measurements. This study showed that GEANT4 is appropriate for the investigation of the dosimetric properties of a multileaf collimator. In particular, a quantification of the leakage, the penumbra, and the tongue-and-groove effect and an evaluation of the influence of the beam parameters such as the width of the Gaussian distribution was possible.     

Quality assurance of a helical tomotherapy machine

Helical tomotherapy has been developed at the University of Wisconsin, and 'Hi-Art II' clinical machines are now commercially manufactured. At the core of each machine lies a ring-gantry-mounted short linear accelerator which generates x-rays that are collimated into a fan beam of intensity-modulated radiation by a binary multileaf, the modulation being variable with gantry angle. Patients are treated lying on a couch which is translated continuously through the bore of the machine as the gantry rotates. Highly conformal dose-distributions can be delivered using this technique, which is the therapy equivalent of spiral computed tomography. The approach requires synchrony of gantry rotation, couch translation, accelerator pulsing and the opening and closing of the leaves of the binary multileaf collimator used to modulate the radiation beam. In the course of clinically implementing helical tomotherapy, we have developed a quality assurance (QA) system for our machine. The system is analogous to that recommended for conventional clinical linear accelerator QA by AAPM Task Group 40 but contains some novel components, reflecting differences between the Hi-Art devices and conventional clinical accelerators. Here the design and dosimetric characteristics of Hi-Art machines are summarized and the QA system is set out along with experimental details of its implementation. Connections between this machine-based QA work, pre-treatment patient-specific delivery QA and fraction-by-fraction dose verification are discussed.     

The dosimetric properties of an intraoperative radiation therapy applicator system for a Mevatron-80

An applicator system for intraoperative radiation therapy has been fabricated which does not require physical docking with the accelerator. A dosimetric study has been completed which documents the properties of this system for a variety of electron beam energies, applicator sizes, collimator settings, both primary and secondary, and source-surface distance (SSD) settings. Sensitivity of the system to common misalignment errors was also determined. Results indicate (a) applicator leakage of less than 5%, (b) beam flatness to within plus or minus 5% at the dMAX with a single primary collimator setting, (c) smooth changes in output with cone size, beam energy and SSD, and (d) negligible changes in dose distributions within alignment errors permitted by the system.     

Dosimetric characteristics of novalis Tx system with high definition multileaf collimator

A new Novalis Tx system equipped with a high definition multileaf collimator (HDMLC) recently became available to perform both image-guided radiosurgery and conventional radiotherapy. It is capable of delivering a highly conformal radiation dose with three energy modes: 6 MV photon energy, 15 MV photon energy, and 6 MV photon energy in a stereotactic radiosurgery mode with 1000 MU/min dose rate. Dosimetric characteristics of the new Novalis Tx treatment unit with the HDMLC are systematically measured for commissioning. A high resolution diode detector and miniion-chamber detector are used to measure dosimetric data for a range of field sizes from 4 x 4 mm to 400 x 400 mm. The commissioned Novalis Tx system has passed the RPC stereotactic radiosurgery head phantom irradiation test. The Novalis Tx system not only expands its capabilities with three energy modes, but also achieves better beam conformity and sharer beam penumbra with HDMLC. Since there is little beam data information available for the new Novalis Tx system, we present in this work the dosimetric data of the new modality for reference and comparison.     

Dosimetric impact of geometric errors due to respiratory motion prediction on dynamic multileaf collimator-based four-dimensional radiation delivery

The synchronization of dynamic multileaf collimator (DMLC) response with respiratory motion is critical to ensure the accuracy of DMLC-based four dimensional (4D) radiation delivery. In practice, however, a finite time delay (response time) between the acquisition of tumor position and multileaf collimator response necessitates predictive models of respiratory tumor motion to synchronize radiation delivery. Predicting a complex process such as respiratory motion introduces geometric errors, which have been reported in several publications. However, the dosimetric effect of such errors on 4D radiation delivery has not yet been investigated. Thus, our aim in this work was to quantify the dosimetric effects of geometric error due to prediction under several different conditions. Conformal and intensity modulated radiation therapy (IMRT) plans for a lung patient were generated for anterior-posterior/posterior-anterior (AP/PA) beam arrangements at 6 and 18 MV energies to provide planned dose distributions. Respiratory motion data was obtained from 60 diaphragm-motion fluoroscopy recordings from five patients. A linear adaptive filter was employed to predict the tumor position. The geometric error of prediction was defined as the absolute difference between predicted and actual positions at each diaphragm position. Distributions of geometric error of prediction were obtained for all of the respiratory motion data. Planned dose distributions were then convolved with distributions for the geometric error of prediction to obtain convolved dose distributions. The dosimetric effect of such geometric errors was determined as a function of several variables: response time (0-0.6 s), beam energy (6/18 MV), treatment delivery (3D/4D), treatment type (conformal/IMRT), beam direction (AP/PA), and breathing training type (free breathing/audio instruction/visual feedback). Dose difference and distance-to-agreement analysis was employed to quantify results. Based on our data, the dosimetric impact of prediction (a) increased with response time, (b) was larger for 3D radiation therapy as compared with 4D radiation therapy, (c) was relatively insensitive to change in beam energy and beam direction, (d) was greater for IMRT distributions as compared with conformal distributions, (e) was smaller than the dosimetric impact of latency, and (f) was greatest for respiration motion with audio instructions, followed by visual feedback and free breathing. Geometric errors of prediction that occur during 4D radiation delivery introduce dosimetric errors that are dependent on several factors, such as response time, treatment-delivery type, and beam energy. Even for relatively small response times of 0.6 s into the future, dosimetric errors due to prediction could approach delivery errors when respiratory motion is not accounted for at all. To reduce the dosimetric impact, better predictive models and/or shorter response times are required.     

Dosimetric characterization of the 18-MV photon beam from the Siemens Mevatron 77 linear accelerator

A comprehensive set of dosimetric measurements has been made on the Mevatron 77.80.67 18-MV photon beam. Percentage depth dose, dose in the buildup region, field size dependence of output, transmission through lead, tray attenuation, and isodose curves for the open and wedged fields were measured using an ionization chamber in water and polystyrene phantoms. These dosimetric measurements sufficiently characterized the beam to permit clinical use. The depth dose at 10-cm depth for a 10 X 10 cm2 field at 100-cm source-to-skin distance (SSD) is 80.9%, which meets design specifications. Central axis depth-dose data were fitted to within 0.5% by a set of polynomial equations utilizing a two-dimensional linear regression analysis. Tissue-maximum ratios calculated from depth-dose data agree with measured data to within 2%. Output differences as large as 2.5% were measured for rectangular fields depending on which collimator jaws defined the long dimension of the field. The field size dependence of output was fit to within +/- 0.1% by a linear regression. The half-value thickness of the beam was measured to be 13 mm of lead.     

Hardware-sensitive optimization for intensity modulated radiotherapy

The multileaf collimator (MLC) hardware constraints are usually neglected in the process of intensity-modulated beam optimization. Consequently, it is not always possible to deliver planned beam modulation using dynamic MLC. Beam optimization is significantly diminished if the results must be approximated due to limitations imposed by the delivery device. To overcome this problem, an inverse beam optimization method which incorporates the hardware constraints has been developed. The hardware constraints, including the leaf velocity, the dose rate and the minimum required gap between opposing and adjacent leaves, were considered. An iterative search for feasible modulation was conducted alternately in the dosimetric space and the MLC position-time space. The optimization algorithm was designed for a unidirectional leaf trajectory and a constant dose rate. A scheme to reduce tongue-and-groove underdosage during optimization was also implemented. Comparisons were made between the solutions produced by this method and conventional optimization disregarding the hardware restrictions. The beam profiles generated by the conventional method were modified to satisfy the hardware specifications. The results indicate that inclusion of MLC constraints during optimization can improve the degree of conformity that is deliverable.     

Characterization of an add-on multileaf collimator for electron beam therapy

An add-on multileaf collimator for electrons (eMLC) has been developed that provides computer-controlled beam collimation and isocentric dose delivery. The design parameters result from the design study by Gauer et al (2006 Phys. Med. Biol. 51 5987-6003) and were configured such that a compact and light-weight eMLC with motorized leaves can be industrially manufactured and stably mounted on a conventional linear accelerator. In the present study, the efficiency of an initial computer-controlled prototype was examined according to the design goals and the performance of energy- and intensity-modulated treatment techniques. This study concentrates on the attachment and gantry stability as well as the dosimetric characteristics of central-axis and off-axis dose, field size dependence, collimator scatter, field abutment, radiation leakage and the setting of the accelerator jaws. To provide isocentric irradiation, the eMLC can be placed either 16 or 28 cm above the isocentre through interchangeable holders. The mechanical implementation of this feature results in a maximum field displacement of less than 0.6 mm at 90 degrees and 270 degrees gantry angles. Compared to a 10 x 10 cm applicator at 6-14 MeV, the beam penumbra of the eMLC at a 16 cm collimator-to-isocentre distance is 0.8-0.4 cm greater and the depth-dose curves show a larger build-up effect. Due to the loss in energy dependence of the therapeutic range and the much lower dose output at small beam sizes, a minimum beam size of 3 x 3 cm is necessary to avoid suboptimal dose delivery. Dose output and beam symmetry are not affected by collimator scatter when the central axis is blocked. As a consequence of the broader beam penumbra, uniform dose distributions were measured in the junction region of adjacent beams at perpendicular and oblique beam incidence. However, adjacent beams with a high difference in a beam energy of 6 to 14 MeV generate cold and hot spots of approximately 15% in the abutting region. In order to improve uniformity, the energy of adjacent beams must be limited to 6 to 10 MeV and 10 to 14 MeV respectively. At the maximum available beam energy of 14 MeV, radiation leakage results mainly from the intraleaf leakage of approximately 2.5% relative dose which could be effectively eliminated at off-axis distances remote from the field edge by adjusting the jaw field size to the respective opening of the eMLC. Additionally, the interleaf and leaf-end leakage could be reduced by using a tongue-and-groove leaf shape and adjoining the leaf-ends off-axis respectively.     

Dosimetry and mechanical accuracy of the first rotating gamma system installed in North America

The purpose of this paper is to present the dosimetry and mechanical accuracy of the first rotating gamma system (RGS) installed in North America for stereotactic radiosurgery. The data were obtained during the installation, acceptance test procedure, and commissioning of the unit. The RGS unit installed at UC Davis Cancer Center (RGSu) has modifications on the source and collimator bodies from the earlier version of the Chinese RGS (RGSc). The differences between these two RGSs are presented. The absolute dose at the focal point was measured in a 16-cm-diam acrylic phantom using a small volume chamber, which was calibrated at the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UW-ADCL). The dose in acrylic was then converted to a dose in water. A collimator output factor from each of the four different collimator sizes ranging from 4, 8, 14, and 18 mm was measured with (1) a smaller volume chamber and (2) approximately 3.0 mm x 3.0 mm x 1.0 mm TLD chips in the same acrylic phantom. The Gafchromic films were used for the dose profile, collimator output factor, and mechanical/radiation field isocentricity measurements. The TLD chips were processed in-house whereas Gafchromic films were processed both at the UW-ADCL and in-house. The timer error, timer accuracy, and timer linearity were also determined. The dose profiles were found to be similar between RGSc and RGSu. The 4 mm collimator output factor of the RGSu was approximately 0.6, similar to that from RGSc, in comparison to 0.8 in the report for a Leksell Model U Gamma-Knife. The mechanical/radiation field isocentricity for RGSc and RGSu is found to be similar and is within 0.3 mm in both X and Y directions. In the Z direction, the beam center of the RGSu is shifted toward the sources by 0.75 mm from the mechanical isocenter whereas no data are available for RGSc. Little dosimetric difference is found between RGSu and RGSc. It is reported that RGSc has the same dosimetric and mechanical characteristics as the Model U Gamma-Knife. Therefore, RGSu should be capable of achieving stereotactic radiosurgery with the same degree of dosimetric and mechanical accuracy as with the Gamma-Knife.     

Design considerations for a computer controlled multileaf collimator for the Harper Hospital fast neutron therapy facility

The d(48.5) + Be neutron beam from the Harper Hospital superconducting cyclotron is collimated using a unique multirod collimator (MRC). A computer controlled multileaf collimator (MLC) is being designed to improve efficiency and allow for the future development of intensity modulated radiation therapy with neutrons. For the current study the use of focused or unfocused collimator leaves has been studied. Since the engineering effort associated with the leaf design and materials choice impacts significantly on cost, it was desirable to determine the clinical impact of using unfocused leaves in the MLC design. The MRC is a useful tool for studying the effects of using focused versus unfocused beams on beam penumbra. The effects of the penumbra for the different leaf designs on tumor and normal tissue DVHs in two selected sites (prostate and head and neck) was investigated. The increase in the penumbra resulting from using unfocused beams was small (approximately 1.5 mm for a 5 x 5 cm2 field and approximately 7.6 mm for a 25 x 25 cm2 field at 10 cm depth) compared to the contribution of phantom scatter to the penumbra width (5.4 and 20 mm for the small and large fields at 10 cm depth, respectively). Comparison of DVHs for tumor and critical normal tissue in a prostate and head and neck case showed that the dosimetric disadvantages of using an unfocused rather than focused beam were minimal and only significant at shallow depths. For the rare cases, where optimum penumbra conditions are required, a MLC incorporating tapered leaves and, thus, providing focused collimation in one plane is necessary.     

Dosimetric effect of respiration-gated beam on IMRT delivery

Intensity modulated radiation therapy (IMRT) with a dynamic multileaf collimator (DMLC) requires synchronization of DMLC leaf motion with dose delivery. A delay in DMLC communication is known to cause leaf lag and lead to dosimetric errors. The errors may be exacerbated by gated operation. The purpose of this study was to investigate the effect of leaf lag on the accuracy of doses delivered in gated IMRT. We first determined the effective leaf delay time by measuring the dose in a stationary phantom delivered by wedge-shaped fields. The wedge fields were generated by a DMLC at various dose rates. The so determined delay varied from 88.3 to 90.5 ms. The dosimetric effect of this delay on gated IMRT was studied by delivering wedge-shaped and clinical IMRT fields to moving and stationary phantoms at dose rates ranging from 100 to 600 MU/min, with and without gating. Respiratory motion was simulated by a linear sinusoidal motion of the phantom. An ionization chamber and films were employed for absolute dose and 2-D dose distribution measurements. Discrepancies between gated and nongated delivery to the stationary phantom were observed in both absolute dose and 2-D dose distribution measurements. These discrepancies increased monotonically with dose rate and frequency of beam interruptions, and could reach 3.7% of the total dose delivered to a 0.6 cm3 ion chamber. Isodose lines could be shifted by as much as 3 mm. The results are consistent with the explanation that beam hold-offs in gated delivery allowed the lagging leaves to catch up with the delivered monitor units each time that the beam was interrupted. Low dose rates, slow leaf speeds and low frequencies of beam interruptions reduce the effect of this delay-and-catch-up cycle. For gated IMRT it is therefore important to find a good balance between the conflicting requirements of rapid dose delivery and delivery accuracy.     

The influence of cooling rate on the accuracy of normoxic polymer gel dosimeters

Polymer gel dosimeters offer a wide range of applications in the three-dimensional verification of complex radiation dose distributions such as in intensity-modulated radiotherapy (IMRT). With the release of polymer gel dosimeters that can be fabricated in normal atmospheric ('normoxic') conditions, the gel manufacturing process has been significantly simplified. Gel dosimeters are calibrated by use of a series of calibration vials irradiated with known doses or by use of a calibration phantom with a known dose distribution. The overall accuracy of the polymer gel dosimeters is determined by different dosimetric properties. In this study, we show the influence of the temperature history during storage of the gel dosimeter on the dose response curve for two gel dosimeters using the monomers acrylamide/N,N'-methylene-bis-acrylamide (nPAG) and methacrylic acid (nMAG) respectively and bis[tetrakis(hydroxymethyl)phosphonium]sulphate (THP) as antioxidant in both gel dosimeters. This study reveals that differences in temperature history after fabrication of normoxic polymer gel dosimeters may compromise the dosimetric accuracy. It was found that the acrylamide based gel dosimeter (nPAG) is less dependent on the post-manufacture temperature history than the methacrylic acid based gel dosimeter (nMAG). The importance of an equal temperature history for the gel dosimeter and calibration vials is emphasized by this study. A reproducibility study has also been performed on the nPAG gel dosimeter when additional efforts are made to control the temperature changes upon cooling.     

An experimental investigation into the effect of periodic motion on proton dosimetry using polymer gel dosimeters and a programmable motion platform

Organ motion in proton therapy affects treatment dose distribution during both double-scattering (DS) and uniform-scanning (US) deliveries. We investigated the dosimetric impact of target motion using three-dimensional polymer gel dosimeters and a programmable motion platform. A simple one-beam treatment plan with 16 cm range and 6 cm modulation was generated from the treatment planning system (TPS) in both the DS and US modes. One gel dosimeter was irradiated with a stationary DS beam. Two other gel dosimeters were irradiated with the DS and US beams while they moved in the same sinusoidal motion profile using a programmable motion platform. The dose distribution of the stationary DS delivery agreed with the TPS plan. Dosimetric comparisons between DS motion delivery and the MATLAB-based motion model showed insignificant differences. Dose-volume histograms of a cylindrical target volume inside the gel dosimeters showed target coverage degradation caused by motion. A three-dimensional gamma index calculation (3% and 3 mm) confirmed different dosimetric impacts from DS and US with the same target motion. This polymer-gel-dosimeter-based study confirmed the dosimetric impact of intrafraction target motion and its interplay with temporal delivery of different energy layers in US proton treatments.     

Leaf position verification during dynamic beam delivery: a comparison of three applications using electronic portal imaging

The use of a dynamic multileaf collimator (MLC) to deliver intensity-modulated beams presents a problem for conventional verification techniques. The use of electronic portal imaging to track MLC leaves during beam delivery has been shown to provide a solution to this problem. An experimental comparison of three different verification systems, each using a different electronic portal imaging technology, is presented. Two of the systems presented are commercially available imagers with in-house modifications, with the third system being an in-house built experimental system. The random and systematic errors present in each of the verifications systems are measured and presented, together with the study of the effects of varying dose rate and leaf speed on verification system performance. The performance of the three systems is demonstrated to be very similar, with an overall accuracy in comparing measured and prescribed collimator trajectories of approximately +/-1.0 mm. Systematic errors in the percentage delivered dose signal provided by the accelerator are significant and must be corrected for good performance of the current systems. It is demonstrated that, with suitable modifications, commercially available portal imaging systems can be used to verify dynamic MLC beam delivery.