Diameter Circular Field Research Articles

Electron FLASH Delivery at Treatment Room Isocenter for Efficient Reversible Conversion of a Clinical LINAC

In this study, procedures were developed to achieve efficient reversible conversion of a clinical linear accelerator (LINAC) and deliver ultrahigh-dose-rate (UHDR) electron or conventional beams to the treatment room isocenter for FLASH radiation therapy. The LINAC was converted to deliver UHDR beam within 20 minutes by retracting the x-ray target from the beam's path, positioning the carousel on an empty port, and selecting 10 MV photon beam energy in the treatment console. Dose rate surface and depth dose profiles were measured in solid water phantom at different field sizes with Gafchromic film and an optically stimulated luminescent dosimeter (OSLD). A pulse controller counted the pulses via scattered radiation signal and gated the delivery for a preset pulse count. A fast photomultiplier tube-based Cherenkov detector measured the per pulse beam output at a 2-ns sampling rate. After conversion back to clinical mode, conventional beam output, flatness, symmetry, field size, and energy were measured for all clinically commissioned energies. The surface average dose rates at the isocenter for 1-cm diameter and 1.5-in diameter circular fields and for a jaws-wide-open field were 238 ± 5 Gy/s, 262 ± 5 Gy/s, and 290 ± 5 Gy/s, respectively. The radial symmetry of the beams was within 2.4%, 0.5%, and 0.2%, respectively. The doses from simultaneous irradiation of film and OSLD were within 1%. The photomultiplier tube showed the LINAC required ramp up time in the first 4 to 6 pulses before the output stabilized, after which its stability was within 3%. At the isocenter of the treatment room, 10 MeV UHDR beams were achieved. The beam output was reproducible but requires further investigation of the ramp up time, equivalent to ∼1 Gy, requiring dose monitoring. The UHDR beam can irradiate both small and large subjects to investigate potential FLASH radiobiological effects in minimally modified clinical settings, and the dose rate can be further increased by reducing the source-to-surface distance.

CyberKnife reference dosimetry: An assessment of the impact of evolving recommendations on correction factors and measured dose.

Specialized treatment machines such as the CyberKnife, TomoTherapy, or the GammaKnife, utilize flattening filter free (FFF) photon beams and may not be able to generate a 10cm x 10cm reference field. A new Code of Practice has recently been published (IAEA TRS483) to give recommendations for these machines. Additionally, some standard laboratories provide measured beam quality correction factors for the user's reference chamber, which can be used instead of the published tabulated beam quality correction factors. The purpose of this study was first to assess how different recommendations, as outlined below, affect the reference dosimetry at the CyberKnife, and second, to assess the impact of using measured rather than tabulated beam quality correction factors on measured dose. Following recommendations in TRS398, three field chambers (IBA CC04, Exradin A19, and Exradin A12S) were cross-calibrated with a user's reference chamber (IBA FC65-G), which was calibrated in a Cobalt-60 (Co-60) beam by a primary standards laboratory. The chamber response was corrected for influence quantities such as temperature, pressure, ion recombination, polarity, and beam quality. Additionally, correction factors for volume averaging and differences due the FFF beam spectrum were determined for the CyberKnife beam. Three different methods were utilized - TRS398; Intermediate (i.e. TRS398 with additional published recommendations); and TRS483. The measurements were undertaken in a 10cm×10cm field defined by jaws for a uniform flattened (WFF) 6MV photon beam of a Varian TrueBeam linear accelerator (linac) with a source to detector distance (SDD) of 100cm, and in a 60mm diameter circular field for a 6MV flattening filter free (FFF) Accuray CyberKnife beam with SDD of 80cm. All measurement was performed at 10cm deep in a full scatter phantom as defined in TRS398. Differences between the three methods in volume averaging correction factors ranged from 0.01% to 0.45% depending on the chamber assessed. As expected, an increased chamber length leads to a larger correction factor. The differences in beam spectrum correction factors range from 0.09% to 0.3%. Negligible differences in beam quality correction factors were observed; however, differences up to 1% were found between measured and tabulated values. Differences in cross-calibrated chamber calibration coefficients range from 0.05% to 0.51% depending on the chamber assessed. Differences in measured dose are up to 0.87% between Method TRS398 and Intermediate, again chamber dependent, and 0.28% between Method Intermediate and TRS483. Using chambers cross-calibrated in the linac beam can lead to differences in measured dose per Monitor Unit (MU) in the CyberKnife beam of approximately 0.5% between chambers. Using Method Intermediate vs using recommendations given in TRS483 led to a difference of 0.28% in measured dose per MU, which is due to differences in volume averaging and beam spectrum correction factors. Using TRS483 is recommended as the cross-calibration is done in the CyberKnife beam and accounts for its specific reference conditions. It will also ensure consistency between different centers. The measured beam quality correction factors agree within the uncertainties with the tabulated values.

Characteristics of Megavoltage Electron Beams Directed through Silicone for Bolus Electron Therapy

In high energy electron beam therapy, a bolus can be used for the conformal dose to target and achieve the appropriate treatment quality. To evaluate the use of a customized silicone bolus, we analyzed the dosimetric properties of the electron beam according to the silicone bolus thickness. Eight silicone slits were fabricated using an acrylic mold. The size of each slit was 150 × 150 × 5 m3 (width × length × height). Degassing was performed to eliminate bubbles in the slits during the silicone casting process. Depth dose measurements were performed for a 6 MeV and 9 MeV electron beams with 10 × 10 cm2, 6 × 6 cm2 and 5 cm diameter circular fields with various silicone thickness and with the conventional bolus. For the depth dose measurement, the film was placed between two 6 cm solid water phantoms, and the electron beam was delivered parallel to the film. The Rando phantom dose with customized bolus were evaluated with the 10 × 10 cm2 6 MeV electron beam. As the slab thickness increased, the depth dose in the solid water phantom decreased after reaching the maximum dose depth (dmax). Depending on the silicone thickness, the surface dose of the phantom was increased until reaching the electron beam dmax and was decreased after dmax. The coefficient to convert the thickness of the silicone to water were 1.06 in 6 MeV and 1.07 in 9 MeV, respectively. We measured the electron beam dose properties for making patient-specific silicone boluses with optimal thickness. The results of this study can be used to predict the silicone bolus thickness required in order to deliver the appropriate dose to the target.

Experimental pencil beam kernels derivation for 3D dose calculation in flattening filter free modulated fields

This paper presents a method to obtain the pencil-beam kernels that characterize a megavoltage photon beam generated in a flattening filter free (FFF) linear accelerator (linac) by deconvolution from experimental measurements at different depths. The formalism is applied to perform independent dose calculations in modulated fields.In our previous work a formalism was developed for ideal flat fluences exiting the linac’s head. That framework could not deal with spatially varying energy fluences, so any deviation from the ideal flat fluence was treated as a perturbation. The present work addresses the necessity of implementing an exact analysis where any spatially varying fluence can be used such as those encountered in FFF beams. A major improvement introduced here is to handle the actual fluence in the deconvolution procedure. We studied the uncertainties associated to the kernel derivation with this method. Several Kodak EDR2 radiographic films were irradiated with a 10 MV FFF photon beam from two linacs from different vendors, at the depths of 5, 10, 15, and 20cm in polystyrene (RW3 water-equivalent phantom, PTW Freiburg, Germany). The irradiation field was a 50mm diameter circular field, collimated with a lead block.The 3D kernel for a FFF beam was obtained by deconvolution using the Hankel transform. A correction on the low dose part of the kernel was performed to reproduce accurately the experimental output factors. Error uncertainty in the kernel derivation procedure was estimated to be within 0.2%. Eighteen modulated fields used clinically in different treatment localizations were irradiated at four measurement depths (total of fifty-four film measurements). Comparison through the gamma-index to their corresponding calculated absolute dose distributions showed a number of passing points (3%, 3mm) mostly above 99%. This new procedure is more reliable and robust than the previous one. Its ability to perform accurate independent dose calculations was demonstrated.

SU-E-T-209: Independent Dose Calculation in FFF Modulated Fields with Pencil Beam Kernels Obtained by Deconvolution

Purpose: To obtain the pencil beam kernels that characterize a megavoltage photon beam generated in a FFF linac by experimental measurements, and to apply them for dose calculation in modulated fields. Methods: Several Kodak EDR2 radiographic films were irradiated with a 10 MV FFF photon beam from a Varian True Beam (Varian Medical Systems, Palo Alto, CA) linac, at the depths of 5, 10, 15, and 20cm in polystyrene (RW3 water equivalent phantom, PTW Freiburg, Germany). The irradiation field was a 50 mm diameter circular field, collimated with a lead block. Measured dose leads to the kernel characterization, assuming that the energy fluence exiting the linac head and further collimated is originated on a point source. The three-dimensional kernel was obtained by deconvolution at each depth using the Hankel transform. A correction on the low dose part of the kernel was performed to reproduce accurately the experimental output factors. The kernels were used to calculate modulated dose distributions in six modulated fields and compared through the gamma index to their absolute dose measured by film in the RW3 phantom. Results: The resulting kernels properly characterize the global beam penumbra. The output factor-based correction was carried out adding the amount of signal necessary to reproduce the experimental output factor in steps of 2mm, starting at a radius of 4mm. There the kernel signal was in all cases below 10% of its maximum value. With this correction, the number of points that pass the gamma index criteria (3%, 3mm) in the modulated fields for all cases are at least 99.6% of the total number of points. Conclusion: A system for independent dose calculations in modulated fields from FFF beams has been developed. Pencil beam kernels were obtained and their ability to accurately calculate dose in homogeneous media was demonstrated.

An empirical formula to obtain tissue-phantom ratios from percentage depth–dose curves for small fields

For small photon fields, accurate values of a tissue-phantom ratio (TPR) are difficult to obtain either by direct measurement or by the conventional method of converting from measured percentage depth doses (%dd). This study aims to develop an empirical method to accurately obtain TPRs from %dd curves for small radiosurgery beams. The Monte Carlo simulation codes BEAMnrc/DOSXYZnrc were used to simulate the accelerator head and small, collimated fields including the circular cone accessory. The Monte Carlo directly calculated TPR values as a function of depth were compared with TPRs converted from %dd curves in a water phantom for field sizes ranging from 4 mm diameter to 10 × 10 cm2 fields. Direct measurements of TPRs were performed with the detector remaining fixed at a SAD of 100 cm and increasing the detector depth by adding water. The %dd curves were measured at 100 cm SSD in a 50 × 50 × 50 cm3 water tank. Using the Monte Carlo values, we developed an empirical formula to obtain TPRs from %dd and validated its accuracy. The conventional method of obtaining TPRs from %dd underestimate TPR by 3.4% and 0.6% at a depth 1.5 cm and overestimate TPR by 6.4% and 1.7% at a depth of 25 cm for 4 mm and 30 mm diameter circular fields, respectively. The empirical formula is derived from realistic Monte Carlo simulations using field sizes ranging from 4 to 30 mm and depth ranging from 1.5 to 25 cm. TPRs calculated using this function deviate from TPRs directly calculated from Monte Carlo by less than 0.5%. The accuracy of this empirical formula is validated against the directly measured TPRs in water. The developed empirical method has the potential to greatly simply the work in obtaining TPRs from measured %dd curves for small fields. By using this developed empirical formula the uncertainties between directly measured TPRs and converted TPRs from measured %dd curves are within 1%.

SU-E-T-544: An Empirical Formula to Obtain Tissue-Phantom Ratios From Percentage Depth Dose Curves for Small Fields

Purpose: For small photon fields, accurate values of tissue-phantom ratios (TPR) are difficult to obtain either by direct measurement or by the conventional method of converting from measured percentage-depth doses (%dd). This study aims to develop an empirical method to accurately obtain TPRs from %dd curves for small radiosurgery beams. Methods: The Monte Carlo simulation codes BEAMnrc/DOSXYZnrc were used to simulate the accelerator head and accessory. The Monte Carlo directly calculated TPR values as a function of depth were compared with TPRs converted from %dd curves in a water phantom for field sizes ranging from 4 mm diameter to 10×10 cm2 fields. Direct measurements of TPRs were performed with the detector remaining fixed at an SAD of 100 cm and increasing the detector depth by adding water. Results: Using the Monte Carlo values, we developed an empirical formula to obtain TPRs from %dd. The conventional method of obtaining TPRs from %dd underestimate TPR by 3.4% and 0.6% at a depth 1.5 cm and overestimate TPR by 6.2% and 1.7% at a depth of 25 cm for 4 mm and 30 mm diameter circular fields, respectively. The empirical formula is derived from realistic Monte Carlo simulations using field sizes ranging from 4 mm to 30 mm and depth ranging from 1.5 cm to 25 cm. TPRs calculated using this function deviate from TPRs directly calculated from Monte Carlo by less than 0.5%. The accuracy of this empirical formula is validated against the directly measured TPRs in water. Conclusion: The developed empirical method has the potential to greatly simply the work in obtaining TPRs from measured %dd curves for small fields. By using this developed empirical formula the uncertainties between directly measured TPRs and converted TPRs from measured %dd curves are within 1%.

WE-E-108-06: Demonstration of a CBCT Based Monte Carlo Model for Small Animal Treatment Planning

Purpose: To demonstrate feasibility of planning small animal irradiation using cone beam computed tomography (CBCT) data as input to a Monte Carlo (MC) based treatment planning system (TPS). Methods: The BEAMnrc/EGSnrc code was used to model 225 kVp photon beams produced by a small animal irradiator (XRAD 225Cx, Precision X-ray). Homogenous phantom plugs (Gammex RMI 467, Middleton, WI) of known material and physical density were scanned using the XRAD 225Cx CBCT system to associate unique CBCT units to the corresponding material density. Additionally, the known physical densities were then correlated to appropriate atomic compositions to create a material data set library. This enabled correlating phantom CBCT units to a material/density matrix used as input to DOSXYZnrc MC computations. A 4×4×4 cm3 multi slab phantom in homogeneous (all solid water) and heterogeneous (lung block between water slabs) configurations were CBCT scanned using 80 kVp and 0.3 mAs setting. MC dose calculation was performed for 10 mm diameter circular field. Ultimately, 10 mm circular fields in homogenous and heterogeneous phantom configuration were used to compare absolute dose computed by MCTPS and film measurement. The CBCT based MC model was finally demonstrated using an animal CBCT scan. Results: The gamma map of CBCT based MC calculation and film measurement at 1cm depth plane in solid water medium shows excellent agreement of 98.7%within selected region of interest using 3%/0.5 mm gamma criteria. The absolute dose comparison between measurement and simulation in homogenous and heterogeneous phantom is within 1%. In a small animal the CBCT based MC model qualitatively demonstrated higher dose to bone relative to dose deposited to soft tissues. Conclusion: The CBCT based MC model is a valuable tool for improved dose calculation accuracy for small animal treatment planning in orthovoltage energy range.

WE‐E‐108‐09: An Investigation of the Feasibility of Rodentmorphic 3D Dosimeters for Verification of Precision Micro‐Irradiator Treatment

Purpose: To evaluate the feasibility of novel rodentmorphic 3D dosimeters for comprehensive high‐resolution verification of the treatment accuracy of a state‐of‐the‐art micro‐irradiator equipped with on‐line cone‐beam‐CT guidance. Methods: Anatomically accurate 3D dosimeter molds were created in a two step procedure. First, rodents were CT scanned and the structures of interest contoured (e.g. body and spine). These contours were then exported for input to a 3D printer, to generate positive dosimeter molds. The rat body dosimeter was made of regular water‐equivalent PRESAGE, while the spine was made from high‐Z PRESAGE with an effective atomic‐number close to bone. Evaluation of the dosimeters involved (i) establishing the feasibility of manufacture and accurate positioning of heterogenous inserts, (ii) verification of accurate dose‐readout by optical‐CT, and (iii) verification of sufficient bony/soft‐tissue contrast for representative CBCT IGRT positioning. Simulated rat prostate treatments were delivered with dose of 16Gy given by 4 2.5cm diameter circular fields. Results: High resolution (0.5mm isotropic) 3D dosimetry data was acquired in rodentmorphic dosimeters both with and without the high‐Z spinal insert. The spinal insert was visible under kV radiographs and CBCT, demonstrating the feasibility of IGRT positioning. Pronounced edge artifacts were observed near the flat undersurface of the rat, and near regions of sharp curvature. Further artifacts were observed in some regions near the spinal insert caused by bubbles trapped during manufacture. New manufacturing procedures utilizing the flexibility of 3D printing to precisely customize contours in non‐critical regions have improved on both of these limitations. Conclusion: This work demonstrates promising feasibility for anatomically accurate 3D rodentmorphic dosimeters compatible with very high resolution 3D dosimetry. The ability to create such dosimeters is an important step forward in enabling accurate verification of complex micro‐irradiator treatments in the pre‐clinical setting. NIH Grant No. R01 CA 100835

WE-E-108-05: Evaluation of the XRAD 225Cx MC Source Model in Heterogeneous Mediums

Purpose: This study was performed to benchmark a Monte Carlo source model of the XRAD 225Cx irradiator under heterogeneous conditions. Methods: The BEAM/EGS was used to model 225 kV photon beams from a small animal irradiator (Precision XRAD225Cx). Benchmarking the model in heterogeneous media was performed in a heterogeneous phantom (4×4×4 cm3) composed of solid water and cortical bone. The 10 mm field size virtual source from BEAMnrc was used to compute the 3D dose distribution in DOSXYZnrc. Benchmarking the simulation against measurements was performed using EBT2 film. Heterogeneous conditions were further validated using the phase space file for the 20 mm field size. Gamma analysis was performed to further validate the beam model for the heterogeneous configuration phantom. Results: The MC simulation indicates that there is a 2.4 times increased dose deposition in cortical bone, in agreement with published data. This increased dose deposition, however, was not observed in film measurements and it is investigated further. Gamma analysis was performed for 20 mm field size applicator and the Result of the analysis for a confined region. The single beam with 20 mm diameter circular field irradiation demonstrates a good agreement within the region of interest, with 94% of the calculated data meeting the 5%/0.5 mm criterion. Conclusion: Our kV Monte Carlo source model demonstrates excellent agreement with measurement in heterogeneous conditions.

Beam characteristics and stopping-power ratios of small radiosurgery photon beams

Small megavoltage (MV) photon fields of dimensions less than 3 × 3 cm2 are increasingly being used in modern radiation therapy. To our knowledge, small beam characteristics and dosimetric parameters, such as the energy spectra, particle fluence, and water-to-air stopping-power ratios (SPRs) directly affect the accuracy of small field dosimetry. This study presents the characteristics of small photon beams and investigates the variations of energy spectra of photons and electrons as a function of field size and their effects on the water-to-air SPRs for field sizes ranging from a small 4 mm diameter circular field to a 10 × 10 cm2 field. It sheds light on the differences between small fields collimated by the cone accessory and X- and Y-jaws and on beam characteristics outside the primary radiation fields. In addition, we also investigated the use of an ‘intermediate machine-specific-reference field’ (Alfonso et al 2008 Med. Phys. 35 5179–86) to determine if the variations between a small and a reference field can be eased by introducing an intermediate 4 × 4 cm2 field instead of a standard 10 × 10 cm2 reference field. The Monte Carlo simulation codes BEAMnrc, DOSXYZnrc and SPRRZnrc were used in this study. The accelerator head and circular cone accessory were simulated in detail including two designs of flattening filters: one for a standard-dose rate (100–600 MU min−1) and the other for a high-dose rate (1000 MU min−1) 6 MV beam. The mean energy of photons at depths (1.5–30 cm) in water are 1.72–2.36 MeV, 1.55–1.97 MeV, and 1.44–1.74 MeV for field sizes of 4 mm diameter, 4 × 4 cm2, and 10 × 10 cm2, respectively. The mean energy also varies significantly for electrons at depths (1.5–30 cm): 0.99–1.25 MeV, 0.94–1.09 MeV, and 0.93–1.04 MeV for field sizes of 4 mm, 4 × 4 cm2, and 10 × 10 cm2, respectively. The calculated water-to-air SPRs at depths (1.5–30 cm) are 1.120–1.113, 1.121–1.117, and 1.122–1.119 for field sizes of 4 mm, 4 × 4 cm2 and 10 × 10 cm2, respectively. Although the differences in mean energy are > 20% for photons and > 5% for electrons between 4 mm field and 10 × 10 cm2, the effects on the water-to-air SPRs are small (<0.5%). For detectors with responses to energy changes that are not negligible, significant energy variations between small fields and the reference 10 × 10 cm2 field may have a significant impact on the dosimetry accuracy. However, the use of an intermediate field is capable of greatly reducing these energy variations. This study also found negligible differences in dosimetric parameters between beams with different flattening filters and different incident electron energies on the target when each has the same beam quality kQ values specified by %dd(10)x.

The evaluation of 6 and 18 MeV electron beams for small animal irradiation

A small animal irradiator is critical for providing optimal radiation dose distributions for pre-clinical animal studies. This paper focuses on the evaluation of using 6 or 18 MeV electron beams as small animal irradiators. Compared with all other prototypes which use photons to irradiate small animals, an electron irradiator has many advantages in its shallow dose distribution. Two major approaches including simulation and measurement were used to evaluate the feasibility of applying electron beams in animal irradiation. These simulations and measurements were taken in three different fields (a 6 cm × 6 cm square field, and 4 mm and 30 mm diameter circular fields) and with two different energies (6 MeV and 18 MeV). A PTW Semiflex chamber in a PTW-MP3 water tank, a PTW Markus chamber type 23343, a PTW diamond detector type 60003 and KODAK XV films were used to measure PDDs, lateral beam profiles and output factors for either optimizing parameters of Monte Carlo simulation or to verify Monte Carlo simulation in small fields. Results show good agreement for comparisons of percentage depth doses (⩽2.5% for 6 MeV e; ⩽1.8% for 18 MeV e) and profiles (FWHM ⩽ 0.5 mm) between simulations and measurements on the 6 cm field. Greater deviation can be observed in the 4 mm field, which is mainly caused by the partial volume effects of the detectors. The FWHM of the profiles for the 18 MeV electron beam is 32.6 mm in the 30 mm field, and 4.7 mm in the 4 mm field at d90. It will take 1–13 min to complete one irradiation of 5–10 Gy. In addition, two different digital phantoms were also constructed, including a homogeneous cylindrical water phantom and a CT-based heterogeneous mouse phantom, and were implemented into Monte Carlo to simulate dose distribution with different electron irradiations.

SU-FF-J-166: The Development of a Precise Electron Irradiator for Small Animal Studies

Purpose: This study focuses on the development, validation, and evaluation of a precise MV-electron irradiator. Compared with all other prototypes which use photons to irradiate small animals, an electron irradiator has many advantages in its shallow dose distribution. Method and Materials: Two major approaches including simulation and measurement were used to evaluate the feasibility of an electron animal irradiator. These simulations/measurements were taken in three different fields (a 6 cm square field, and 30 and 4 mm diameter circular fields) and with two different energies (6 and 18 MeV). A PTW Semiflex chamber i n a PTW-MP3 water tank, a PTW Markus chamber type 23343, a PTW diamond detector type 60003, and KODAK XV films were used to measure PDDs, lateral beam profiles, and output factors for either optimizing parameters of Monte Carlo simulation or to verify Monte Carlo simulation in small fields. Results: Results show good agreement for the comparisons of PDD (⩽ 2.5% for 6 MeV e; ⩽ 1.8% for 18 MeV e) and profiles (FWHM ⩽ 0.5 mm) between simulations and measurements on the 6 cm field. Greater deviation can be observed in the 4 mm field due to the partial volume effects of detectors. The output factor for the 18 MeV electron beam is 0.970 in the 30 mm field, and 0.610 in the 4 mm field; the FWHM of profiles for the 18 MeV electron beam is 32.6 mm in the 30 mm field, and 4.7 mm in the 4 mm field at the d90. Two different digital phantoms were also constructed, including a homogeneous cylindrical water phantom and a CT-based heterogeneous mouse phantom, and were implemented into Monte Carlo to simulate the dose distribution with different electron irradiations. Conclusion: These results ensure that our proposed electron irradiator is feasible for precise small animal irradiation.

SU‐FF‐T‐365: Dosimetry QA of Scanned Proton Beam with An Ion Chamber Array

Purpose: To investigate an IC Profiler for clinical proton dosimetry QA. Methods and Materials: An IC Profiler (model 1122) (Sun Nuclear Corporation, Melbourne, FL) consisting of four linear arrays (X, Y, and two diagonals) with 251 vented ion chambers was investigated. The vented ion chamber has 2.6 mm effective collection width and 0.9 g/cm2 inherent buildup. It was irradiated with a proton beam with 16 cm penetration depth at a depth of 11 cm corresponding to the middle of a 10 cm spread‐out Bragg peak (SOBP) for a 10‐cm diameter circular field using uniform scanning proton beam delivery system. Lateral dose profiles measured with ICP was compared with results obtained from a Markus chamber (MC) positioned in water tank, a large multi‐pad ionization chamber (MPIC) transverse detector, and a MatriXX detector (Scanditronix / Wellhofer). Field width (50%–50%), lateral penumbra (20%–80%), field flatness and symmetry were analyzed. Results: The measured field widths were 10.92 cm, 10.70 cm, 10.21 cm and 10.51 cm from ICP, MC, MPIC and, Matrixx detector, respectively. The measured lateral penumbras were 5.6 mm, 6.0 mm, 8.2 mm and 8.1 mm for ICP, MC, MPIC, and Matrixx, respectively. The flatness values obtained with ICP, MC, MPIC and Matrixx were 107.40%, 104.03%, 108.05%, and 104.55%. The symmetries obtained with ICP, MC, MPIC and Matrixx are 0.62%, 0.16%, 0.15%, and 0.12%, respectively. Conclusions: Results obtained from the IC Profiler are similar to those obtained from the other three detectors. From these investigations it can be concluded that the newly available IC PROFILER is a suitable device for quality assurance and dose verifications in proton therapy either in double scattering or scanned beam.

An investigation of central axis depth dose distribution perturbation due to an air gap between patient and bolus for electron beams

Electron beams can be used for the radiotherapy treatment of superficial cancers. In many cases of electron beam radiotherapy, tissue equivalent bolus material placed on the skin is to be used to enhance skin dose. An air gap might be present between the bolus and the skin due to variation in the patient contour. The impact of semi-infinite air gaps under bolus material on central axis depth dose distributions for electron beams was investigated in this study. Semi-infinite air gaps were introduced between bolus and the surface of a water phantom for air gap sizes up to 20.0 mm and for bolus thicknesses of 5, 10 and 15 mm. The electron beams studied had nominal energies of 6, 10 and 14 MeV and circular fields of 3, 5, 7 and 9 cm diameter. Depth dose measurements were carried out in the water phantom with a Scanditronix p-Si electron diode. It was found that the impact of an air gap is dependent on beam energy, field size, air gap size and bolus thickness used. The impact of the air gap on central axis depth dose distribution increased with decreasing field size, increasing air gap size, decreasing electron beam energy and increasing bolus thickness. For 15 mm bolus, 3 cm diameter circular field, 6 MeV beam and the 20 mm air gap, the maximum dose and the surface dose was reduced by approximately 60% and the depth of dose maximum shifted 3.5 mm. An air gap between bolus and a patient should be avoided to ensure that there is no impact on the treatment. The measured data in this study can be used to determine the likely degree of impact on the treatment, of unavoidable air gaps between bolus and the patient.

Dosimetric parameters for small field sizes using Fricke xylenol gel, thermoluminescent and film dosimeters, and an ionization chamber

Dosimetric measurements in small therapeutic x-ray beam field sizes, such as those used in radiosurgery, that have dimensions comparable to or smaller than the build-up depth, require special care to avoid incorrect interpretation of measurements in regions of high gradients and electronic disequilibrium. These regions occur at the edges of any collimated field, and can extend to the centre of small fields. An inappropriate dosimeter can result in an underestimation, which would lead to an overdose to the patient. We have performed a study of square and circular small field sizes of 6 MV photons using a thermoluminescent dosimeter (TLD), Fricke xylenol gel (FXG) and film dosimeters. PMMA phantoms were employed to measure lateral beam profiles (1 × 1, 3 × 3 and 5 × 5 cm2 for square fields and 1, 2 and 4 cm diameter circular fields), the percentage depth dose, the tissue maximum ratio and the output factor. An ionization chamber (IC) was used for calibration and comparison. Our results demonstrate that high resolution FXG, TLD and film dosimeters agree with each other, and that an ionization chamber, with low lateral resolution, underestimates the absorbed dose. Our results show that, when planning small field radiotherapy, dosimeters with adequate lateral spatial resolution and tissue equivalence are required to provide an accurate basic beam data set to correctly calculate the absorbed dose in regions of electronic disequilibrium.

SU-FF-T-328: Dosimetry of (&lt; 3 Cm Diameter) 6 MeV Circular Field Electron Beams for a Varian 21EX Linac

Purpose: Small (≥ 3 cm diameter-φ) 6 MeV electron beam circular fields present complex dosimetry. This work provides absorbed dose rates in water at dmax, percentage depth doses (PDD), profile coverages, penumbra widths, uniformity and homogeneity indices for these fields for clinical treatments. Method and Materials: Circular (2 cm to 3 cm in 0.2 cm diameter increment) cerrobend cutouts were mounted in 6-cm × 6-cm cone. Central-axis depth-ionizations were measured with NACP parallel-plate ionization chamber in a solid water phantom. PDD were obtained using the TG-51 protocol. Cross-beam profiles at dmax of each cutout were measured with XV film in solid water phantom. Absorbed dose rates in water were determined as the ratio of maximum central-axis absorbed doses for each cutout, corrected for TG-51 parameters, to that of the 10-cm × 10-cm reference field. Results: As cutout φ decreases, the build-up portions of the PDD curves shift toward the phantom surface, and the dmax decrease from 1.2 cm to 0.6 cm. The dose gradient, Go, an ICRU 35 measure of the steepness of the descending portion of the PDD, decreases from 2 to 1.5, but the practical ranges, Rp, remain constant. The absorbed dose rate decreases linearly with decreasing φ, but surface absorbed dose remains constant. 90% profile coverages (= width of 90% over cutout φ at dmax, 55%, and 80% coverages, 72%; homogeneity coefficients, 0.70, and the uniformity indices, 0.53 remain reasonably constant. The 80–20% penumbra to diameter ratios increase from 0.52 to 0.62 as diameter decreases. Conclusion: These data for ≤ 3 cm diameter circular fields facilitate routine clinical treatments. The absorbed dose rates and other data for small cutouts can be quickly obtained for patient treatments rather than performing measurements for individual patients.

Clinical experience with routine diode dosimetry for electron beam radiotherapy

Purpose: Electron beam radiotherapy is frequently administered based on clinical setups without formal treatment planning. We felt, therefore, that it was important to monitor electron beam treatments by in vivo dosimetry to prevent errors in treatment delivery. In this study, we present our clinical experience with patient dose verification using electron diodes and quantitatively assess the dose perturbations caused by the diodes during electron beam radiotherapy. Methods and Materials: A commercial diode dosimeter was used for the in vivo dose measurements. During patient dosimetry, the patients were set up as usual by the therapists. Before treatment, a diode was placed on the patient’s skin surface and secured with hypoallergenic tape. The patient was then treated and the diode response registered and stored in the patient radiotherapy system database via our in-house software. A customized patient in vivo dosimetry report showing patient details, expected and measured dose, and percent difference was then generated and printed for analysis and record keeping. We studied the perturbation of electron beams by diodes using film dosimetry. Beam profiles at the 90% prescription isodose depths were obtained with and without the diode on the beam central axis, for 6–20 MeV electron beams and applicator/insert sizes ranging from a 3-cm diameter circular field to a 25 × 25 cm open field. Results: In vivo dose measurements on 360 patients resulted in the following ranges of deviations from the expected dose at the various anatomic sites: Breast (222 patients) −20.3 to +23.5% (median deviation 0%); Head and Neck (63 patients) −21.5 to +14.8% (median −0.7%); Other sites (75 patients) −17.6 to +18.8% (median +0.5%). Routine diode dosimetry during the first treatment on 360 patients (460 treatment sites) resulted in 11.5% of the measurements outside our acceptable ±6% dose deviation window. Only 3.7% of the total measurements were outside ±10% dose deviation. Detailed investigations revealed that the dose discrepancies, overwhelmingly, were due to inaccurate diode orientation and positioning, especially in the regions with rapidly changing contours and/or sloping surfaces. The presence of a diode in the treatment field was found, in some cases, to cause significant dose reduction, most noticeable with smaller fields and lower energy beams. The reduction in dose ranged from 16% (for a 6 MeV beam and a 3 cm diameter circular field) to 4% (for a 12 MeV beam and a 10× 10 cm field). Conclusions: Diode dosimetry was found to be convenient and valuable for verifying in real time the dose delivery accuracy of electron beam treatments, but with some caveats. When treating a small field by low energy electrons, frequent use of diodes is undesirable, because it might result in appreciable reduction of dose to the target volume.

ROSAT observations of the unusual supernova remnant CTB 80 containing the pulsar PSR 1951 + 32

view Abstract Citations (68) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS ROSAT Observations of the Unusual Supernova Remnant CTB 80 Containing the Pulsar PSR 1951+32 Safi-Harb, Samar ; Ogelman, Hakki ; Finley, John P. Abstract The unusal supernova remnant CTB, 80 containing the 39.5 ms pulsar PSR 1951 + 32, has been observed with the Position Sensitive Proportional Counter (PSPC) and the High Resolution Imager (HRI) aboard ROSAT. The HRI image, centered on the pulsar, is composed of a bright compact core of approximately 1 arcminute radius containing the pulsar and a compact nebula, as well as a diffuse nebula extending approximately 5 arcminutes eastward of the pulsar. The PSPC allowed us to model the spectra of the point source, the compact nebula and the 5 arcminute diffuse nebula. For a power-law spectrum with photon index Gamma approximately 2 and an interstellar column density of NH approximately 3 x 1021/cm21 the derived luminosities are approximately 2.3 x 1033 d2.52 ergs/s from the pointlike source, approximately 3.9 x 1033 d2.52 ergs/s from the compact nebula, and approximately 1.8 x 1033 d2.52 ergs/s from the 5 arcminutes diffuse nebula. In addition, the 2 deg diameter circular field of view of the PSPC reveals a hard emission feature southeast of the pulsar with a conical geometry extending out to the edge of the detector. The spectrum from this region is well described by a two-temperature Raymond-Smith thermal plasma with an average temperature of approximately 107 K and a luminosity of approximately 1034d2.52 ergs/s. Pulsations from the 39.5 ms pulsar, PSR 1951 + 32, are detected at the 99% confidence level. The implied pulsed fration is approximately 35% with a complicated energy-dependent behavior. The compact core and the extended diffuse nebula can be explained as synchrotron radiation from the relativistic pulsar wind confined by the ram pressure of the surrounding inhomgeneous medium. The conelike feature detected southeast of PSR 1951 + 32 is consistent with emission from an optically thin SNR in the radiative cooling phase of its evolution. Publication: The Astrophysical Journal Pub Date: February 1995 DOI: 10.1086/175212 Bibcode: 1995ApJ...439..722S Keywords: Interstellar Matter; Pulsars; Supernova Remnants; X Ray Astronomy; Brightness; Data Reduction; Flux Density; Radio Observation; Rosat Mission; X Ray Spectra; Astrophysics; ISM: INDIVIDUAL ALPHANUMERIC: CTB 80; ISM: SUPERNOVA REMNANTS; STARS: PULSARS: INDIVIDUAL ALPHANUMERIC: PSR 1951; 32; X-RAYS: ISM full text sources ADS | data products SIMBAD (2) HEASARC (1)