Water-equivalent Plastic Phantom Research Articles

Low-cost dual-energy CBCT by spectral filtration of a dual focal spot X-ray source

Dual-energy cone beam computed tomography (DE-CBCT) has been shown to provide more information and improve performance compared to a conventional single energy spectrum CBCT. Here we report a low-cost DE-CBCT by spectral filtration of a carbon nanotube x-ray source array. The x-ray photons from two focal spots were filtered respectively by a low and a high energy filter. Projection images were collected by alternatively activating the two beams while the source array and detector rotated around the object, and were processed by a one-step materials decomposition and reconstruction method. The performance of the DE-CBCT scanner was evaluated by imaging a water-equivalent plastic phantom with inserts containing known densities of calcium or iodine and an anthropomorphic head phantom with dental implants. A mean energy separation of 15.5 keV was achieved at acceptable dose rates and imaging time. Accurate materials quantification was obtained by materials decomposition. Metal artifacts were reduced in the virtual monoenergetic images synthesized at high energies. The results demonstrated the feasibility of high quality DE-CBCT imaging by spectral filtration without using either an energy sensitive detector or rapid high voltage switching.

Simultaneous Measurements of Dose and Microdosimetric Spectra in a Clinical Proton Beam Using a scCVD Diamond Membrane Microdosimeter.

A single crystal chemical vapor deposition (scCVD) diamond membrane-based microdosimetric system was used to perform simultaneous measurements of dose profile and microdosimetric spectra with the Y1 proton passive scattering beamline of the Center of Proton Therapy, Institute Curie in Orsay, France. To qualify the performance of the set-up in clinical conditions of hadrontherapy, the dose, dose rate and energy loss pulse-height spectra in a diamond microdosimeter were recorded at multiple points along depth of a water-equivalent plastic phantom. The dose-mean lineal energy () values were computed from experimental data and compared to silicon on insulator (SOI) microdosimeter literature results. In addition, the measured dose profile, pulse height spectra and values were benchmarked with a numerical simulation using TOPAS and Geant4 toolkits. These first clinical tests of a novel system confirm that diamond is a promising candidate for a tissue equivalent, radiation hard, high spatial resolution microdosimeter in beam quality assurance of proton therapy.

Use of a plastic scintillator detector for patient-specific quality assurance of VMAT SRS.

PurposeTo evaluate a scintillator detector for patient‐specific quality assurance of VMAT radiosurgery plans.MethodsThe detector was comprised of a 1 mm diameter, 1 mm high scintillator coupled to an acrylic optical fiber. Sixty VMAT SRS plans for treatment of single targets having sizes ranging from 3 mm to 30.2 mm equivalent diameter (median 16.3 mm) were selected. The plans were delivered to a 20 cm × 20 cm x 15 cm water equivalent plastic phantom having either the scintillator detector or radiochromic film at the center. Calibration films were obtained for each measurement session. The films were scanned and converted to dose using a 3‐channel technique.ResultsThe mean difference between scintillator and film was ‒0.45% (95% confidence interval ‒0.1% to 0.8%). For target equivalent diameter smaller than the median, the mean difference was 1.1% (95% confidence interval 0.5% to 1.7%). For targets larger than the median, the mean difference was ‒0.2% (95% confidence interval ‒0.7% to 0.1%). ConclusionsThe scintillator detector response is independent of target size for targets as small as 3 mm and is well‐suited for patient‐specific quality assurance of VMAT SRS plans. Further work is needed to evaluate the accuracy for VMAT plans that treat multiple targets using a single isocenter.

A multicentre audit of HDR/PDR brachytherapy absolute dosimetry in association with the INTERLACE trial (NCT015662405)

A UK multicentre audit to evaluate HDR and PDR brachytherapy has been performed using alanine absolute dosimetry. This is the first national UK audit performing an absolute dose measurement at a clinically relevant distance (20 mm) from the source. It was performed in both INTERLACE (a phase III multicentre trial in cervical cancer) and non-INTERLACE brachytherapy centres treating gynaecological tumours. Forty-seven UK centres (including the National Physical Laboratory) were visited. A simulated line source was generated within each centre’s treatment planning system and dwell times calculated to deliver 10 Gy at 20 mm from the midpoint of the central dwell (representative of Point A of the Manchester system). The line source was delivered in a water-equivalent plastic phantom (Barts Solid Water) encased in blocks of PMMA (polymethyl methacrylate) and charge measured with an ion chamber at 3 positions (120° apart, 20 mm from the source). Absorbed dose was then measured with alanine at the same positions and averaged to reduce source positional uncertainties. Charge was also measured at 50 mm from the source (representative of Point B of the Manchester system). Source types included 46 HDR and PDR 192Ir sources, (7 Flexisource, 24 mHDR-v2, 12 GammaMed HDR Plus, 2 GammaMed PDR Plus, 1 VS2000) and 1 HDR 60Co source, (Co0.A86). Alanine measurements when compared to the centres’ calculated dose showed a mean difference (±SD) of +1.1% (±1.4%) at 20 mm. Differences were also observed between source types and dose calculation algorithm. Ion chamber measurements demonstrated significant discrepancies between the three holes mainly due to positional variation of the source within the catheter (0.4%–4.9% maximum difference between two holes). This comprehensive audit of absolute dose to water from a simulated line source showed all centres could deliver the prescribed dose to within 5% maximum difference between measurement and calculation.

TH-CD-201-11: Optimizing the Response and Cost of a DNA Double-Strand Break Dosimeter

Purpose: A DNA double-strand break (DSB) dosimeter was developed to measure the biological effect of radiation. The goal here is to refine the fabrication method of this dosimeter to reproducibly create a low coefficient of variation (CoV) and reduce the cost for the dosimeter. Methods: Our dosimeter consists of 4 kilo-base pair DNA strands (labeled on one end with biotin and on the other with fluorescein) attached to streptavidin magnetic beads. The final step of the DNA dosimeter fabrication is to suspend these attached beads in phosphate-buffered saline (PBS). The amount of PBS used to suspend the attached beads and the relative volume of the DNA strands to the beads both affect the CoV and dosimeter cost. We diluted the beads attached with DNA in different volumes of PBS (100, 200, and 400 µL) to create different concentrations of the DNA dosimeter. Then we irradiated these dosimeters (50 µL samples) in a water-equivalent plastic phantom at 25 and 50 Gy (three samples per dose) and calculated the CoV for each dosimeter concentration. Also, we used different masses of DNA strands (1, 2, 8, 16, 24, and 32 µg) to attach to the same volume of magnetic beads (100 µL) to explore how this affects the cost of the dosimeter. Results: The lowest CoV was produced for the highest concentration of dosimeter (100 µL of PBS), which created CoV of 2.0 and 1.0% for 25 and 50 Gy, respectively. We found that the lowest production cost for the dosimeter occurs by attaching 16 µg of DNA strands with 100 µL of beads. Conclusion: : We optimized the fabrication of the DNA dosimeter to produce low CoV and cost, but we still need to explore ways to further improve the dosimeter for use at lower doses. This work was supported in part by Yarmouk University (Irbid, Jordan) and CPRIT (RP140105)

Evaluation of alanine as a reference dosimeter for therapy level dose comparisons in megavoltage electron beams

When comparing absorbed dose standards from different laboratories (e.g. National Measurement Institutes, NMIs, for Key or Supplementary comparisons) it is rarely possible to carry out a direct comparison of primary standard instruments, and therefore some form of transfer detector is required. Historically, air-filled, unsealed ionization chambers have been used because of the long history of using these instruments, very good stability over many years, and ease of transport. However, the use of ion chambers for therapy-level comparisons is not without its problems. Findings from recent investigations suggest that ion chambers are prone to non-random variations, they are not completely robust to standard courier practices, and failure at any step in a comparison can render all measurements potentially useless.An alternative approach is to identify a transfer system that is insensitive to some of these concerns—effectively a dosimeter that is inexpensive, simple to use, robust, but with sufficient precision and of a size relevant to the disseminated quantity in question. The alanine dosimetry system has been successfully used in a number of situations as an audit dosimeter and therefore the purpose of this investigation was to determine whether alanine could also be used as the transfer detector for dosimetric comparisons, which require a lower value for the measurement uncertainty.A measurement protocol was developed for comparing primary standards of absorbed dose to water in high-energy electron beams using alanine pellets irradiated in a water-equivalent plastic phantom. A trial comparison has been carried out between three NMIs and has indicated that alanine is a suitable alternative to ion chambers, with the system used achieving a precision of 0.1%. Although the focus of the evaluation was on the performance of the dosimeter, the comparison results are encouraging, showing agreement at the level of the combined uncertainties (~0.6%). Based on this investigation, a large-scale comparison of primary standards for high-energy electron beams is currently being developed under the auspices of the BIPM.

Comparison of Gafchromic EBT2 and EBT3 for patient-specific quality assurance: Cranial stereotactic radiosurgery using volumetric modulated arc therapy with multiple noncoplanar arcs

Patient-specific quality assurance in volumetric modulated arc therapy (VMAT) brain stereotactic radiosurgery raises specific issues on dosimetric procedures, mainly represented by the small radiation fields associated with the lack of lateral electronic equilibrium, the need of small detectors and the high dose delivered (up to 30 Gy). GafchromicTM EBT2 and EBT3 films may be considered the dosimeter of choice, and the authors here provide some additional data about uniformity correction for this new generation of radiochromic films. A new analysis method using blue channel for marker dye correction was proposed for uniformity correction both for EBT2 and EBT3 films. Symmetry, flatness, and field-width of a reference field were analyzed to provide an evaluation in a high-spatial resolution of the film uniformity for EBT3. Absolute doses were compared with thermoluminescent dosimeters (TLD) as baseline. VMAT plans with multiple noncoplanar arcs were generated with a treatment planning system on a selected pool of eleven patients with cranial lesions and then recalculated on a water-equivalent plastic phantom by Monte Carlo algorithm for patient-specific QA. 2D quantitative dose comparison parameters were calculated, for the computed and measured dose distributions, and tested for statistically significant differences. Sensitometric curves showed a different behavior above dose of 5 Gy for EBT2 and EBT3 films; with the use of inhouse marker-dye correction method, the authors obtained values of 2.5% for flatness, 1.5% of symmetry, and a field width of 4.8 cm for a 5×5 cm2 reference field. Compared with TLD and selecting a 5% dose tolerance, the percentage of points with ICRU index below 1 was 100% for EBT2 and 83% for EBT3. Patients analysis revealed statistically significant differences (p<0.05) between EBT2 and EBT3 in the percentage of points with gamma values<1 (p=0.009 and p=0.016); the percent difference as well as the mean difference between calculated and measured isodoses (20% and 80%) were found not to be significant (p=0.074, p=0.185, and p=0.57). Excellent performances in terms of dose homogeneity were obtained using a new blue channel method for marker-dye correction on both EBT2 and EBT3 GafchromicTM films. In comparison with TLD, the passing rates for the EBT2 film were higher than for EBT3; a good agreement with estimated data by Monte Carlo algorithm was found for both films, with some statistically significant differences again in favor of EBT2. These results suggest that the use of GafchromicTM EBT2 and EBT3 films is appropriate for dose verification measurements in VMAT stereotactic radiosurgery; taking into account the uncertainty associated with Gafchromic film dosimetry, the use of adequate action levels is strongly advised, in particular, for EBT3.

Effects on the photon beam from an electromagnetic array used for patient localization and tumor tracking

One of the main components in a Calypso 4D localization and tracking system is an electromagnetic array placed above patients that is used for target monitoring during radiation treatment. The beam attenuation and beam spoiling properties of the Calypso electromagnetic array at various beam angles were investigated. Measurements were performed on a Varian Clinac iX linear accelerator with 6 MV and 15 MV photon beams. The narrow beam attenuation properties were measured under a field size of 1 cm×1 cm, with a photon diode placed in a cylindrical graphite buildup cap. The broad beam attenuation properties were measured under a field size of 10 cm×10 cm, with a 0.6 cc cylindrical Farmer chamber placed in a polystyrene buildup cap. Beam spoiling properties of the array were studied by measuring depth‐dose change from the array under a field size of 10 cm×10 cm cm in a water‐equivalent plastic phantom with an embedded Markus parallel plate chamber. Change in depth doses were measured with the array placed at distances of 2, 5, and 10 cm from the phantom surface. Narrow beam attenuation and broad beam attenuation from the array were found to be less than 2%–3% for both 6 MV and 15 MV beams at angles less than 40°, and were more pronounced at more oblique angles. Spoiling effects are appreciable at beam buildup region, but are insignificant at depths beyond dmax. Dose measurements in a QA phantom using patient IMRT and VMAT treatment plans were shown to have less than 2.5% dose difference with the Calypso array. The results indicate that the dose difference due to the placement of Calypso array is clinically insignificant.PACS number: 87.56.‐v

Evaluation of Portal Dosimetry for VMAT Treatments

Evaluation of Portal Dosimetry for VMAT Treatments

SU-E-T-153: Proton Linearity and Energy Dependence Studies of Optically Stimulated Luminescent Detectors for Remote Audits of Proton Beam Calibrations by the Radiological Physics Center

Purpose: Implement Optically Stimulated Luminescent Detectors (OSLD) into Radiological Physics Centerˈs (RPC) remote audit quality assurance (QA) program for protons. Methods: The OSLDs were aluminum oxide (Al2O3:C) nanoDots™ from Landauer, Inc. (Glenwood, IL.). A standard RPC remote audit electron phantom with detector inserts at two different locations with three scatter rings and slabs of water equivalent plastic phantom, added on top of the phantom to place both dosimeter locations within the SOBP, were used. Two nanoDots™ were placed in each detector insert of the phantom. For the linearity study, a 250 MeV proton beam with reference setup (FS = 10 × 10 cm2, 10cm SOBP) was used with the beam isocenter located between the two detector depths. Doses of 25, 50, 100, 200, 300, 350 cGy at isocenter were delivered. For the energy dependence study, doses of 200 cGy each were delivered with the reference setup for 250, 200, and 160 MeV proton beams. Co-60 measurement was performed as a standardization process. The OSLDs were read on a MicroStar reader from Landauer between 5 to 7 days after irradiation. Results: The OSLD irradiated had a linear dose response of y=−8.3513E−5x+1.0084 with an R2 = 1.000 over the dose range where y=Kl (dose linearity factor) and x=dose. The energy dependence of the OSLD for the three different energies was less than 3% for the irradiation conditions and fluctuation for different OSLDs less than 1% for the same energy used for the RPC remote audit program. Conclusions: The OSLD showed a linear dose response and consistent energy dependence for three proton energies at the MD Anderson Cancer Center Proton Center. OSLD can be used to replace the TLD for RPCˈs remote audit QA program for proton beam output. Work supported by PHS CA0 10953 and CA081647 awarded by NCI, DHHS

SU‐E‐I‐13: A Model for CT Contrast Agent Evaluation

Purpose: A model is presented that has been designed to evaluate and optimize the use of CT based contrast agents. Initial validation of the model was undertaken and presented here. Methods: The model employs computed spectra from the x‐ray tube at a specified kilovoltage, attenuation correct ion applied to the spectra for filtration and projection of the spectra through a cylindrically symmetric phantom which may also contain a virtual contrast agent filled target at its center. Noise was applied to computed projection data based on photon counting and standard filtered back projection applied to provide a simulated image from which signal‐to‐noise (SNR) can be computed. In the case of a virtual contrast agent filled target, the contrast‐to‐noise ratio (CNR) can be determined as well. Dose at the center of the phantom is computed from the calculated photon fluence. In this model, the CNR per unit radiation dose and/or per unit of contrast agent may be computed. The impact of kilovoltage and filtration on this parameter may be assessed allowing for optimized imaging conditions. In this initial validation of the model a uniform water equivalent plastic phantom was assumed and the CT number and radiation dose were computed for a CT system operating at 120 kVp and 100 mAs. The results were compared to direct measurement for a GE CT system under these conditions.Results: The CT number and dose provided by the model was ‐ 0.04 and 0.440 cGy respectively. This agreed within 3.5 H.U. and 11% of measured value when scatter is added to the model Conclusions: Computed tomography dose, and with further refinement, SNR and CNR may be accurately computed using this model. Placing a virtual contrast agent containing target at the phantom center will allow for prototype contrast agent evaluation and allow for the optimization of technical parameters.

Watch glasses exposed to 6 MV photons and 10 MeV electrons analysed by means of ESR technique: A preliminary study

Abstract In this work we report a case study of the ESR response of watch glasses exposed to 6 MV photons and to 10 MeV electrons. The choice of watch glasses is justified by the fact that watch glasses are very close to the exposed individual. For both types of radiation beams, the absorbed doses belong to the range between 1 and 20 Gy. The samples have been irradiated in water-equivalent plastic phantom with a linear accelerator used for radiotherapy. After exposure watch glass samples have been cut in small strip-shaped pieces with suitable size to be put into the quartz tube for ESR measurements. The signal induced by radiation (RIS) lies in the g ∼ 2 region and must be discriminated from the background signal (BS). An analysis of both signals has been carried out in order to isolate the signal sensitive to ionizing radiation from the background one. We have studied the ESR response of irradiated samples as a function of the absorbed dose. Furthermore estimates of the detection limits have been achieved.

SU-GG-T-117: Gated RapidArc Treatment Delivery Using the New Varian Trilogy MX Linear Accelerator

Purpose: The new Trilogy MX linear accelerator has been designed by Varian Medical Systems to offer new treatment delivery capabilities such as the ability to perform respiratory gated RapidArc treatments. A preliminary evaluation of gated RapidArc treatment deliveries on the Trilogy MX has been performed. Method and Materials: A dosimetric comparison between the gated and non‐gated RapidArc field deliveries was performed using film and an ion chamber. Film was placed within a water equivalent plastic phantom at three planes; isocenter and +/− 3 cm from isocenter, and all were irradiated simultaneously. The absolute dose was also measured at the isocenter using an ion chamber. For all of the gated RapidArc measurements the gating threshold was arbitrarily set to 50% of the waveform amplitude. Three treatment delivery conditions were tested; standard delivery, Jaw Tracking mode, and Flattening Filter Free (FFF) mode. Results: The mean percent difference between the gated and non‐gated absolute dose measurements at the isocenter was 0.5% and 1.5% for the ion chamber and film, respectively. A best agreement analysis shows an average discrepancy of 3%/1.0mm between the gated and non‐gated dose distributions (10% dose threshold, 95%+ pass rate). Gated and non‐gated dose distribution profiles agreed well. Compared to calculated dose distributions, the accuracy of the gated and non‐gated dose distributions deliveries was similar, approximately 4%/2.0mm for all three treatment delivery conditions. Conclusion: Overall some discrepancies between the gated and non‐gated field deliveries are detectable but not clinically significant. The Trilogy MX offers a useful tool for radiotherapy treatment in the thoracic region given that RapidArc provides lower peripheral lungdose and gating combined with the high dose rate FFF beam minimizes the effects of target movement. Conflict of Interest: Research sponsored by Varian.

Characterization of a novel phantom for three-dimensional in vitro cell experiments

A novel intensity-modulated radiation therapy (IMRT) phantom for use in three-dimensional in vitro cell experiments, based on a commercially available system (CIRS Inc., Norfolk, VA), was designed and fabricated. The water-equivalent plastic phantom can, with a set of water-equivalent plastic inserts, enclose 1–3 multi-well tissue culture plates. Dosimetry within the phantom was assessed using thermoluminescence dosimeters (TLDs) and film. The phantom was loaded with three tissue culture plates, and an array of TLDs or a set of three films was placed underneath each plate within the phantom, and then irradiated using an IMRT plan created for it. Measured doses from each dosimeter were compared to those acquired from the treatment planning system. The percent differences between TLD measurements and the corresponding points in the treatment plan ranged from 1.3% to 2.9%, differences which did not show statistical significance. Average point-by-point percent dose differences for each film plane ranged from 1.6% to 3.1%. The percentage dose difference for which 95% of the points in the film matched those corresponding to the calculated dose plane to within 3.0% ranged from 2.8% to 4.2%. The good agreement between predicted and measured dose shows that the phantom is a useful and efficient tool for three-dimensional in vitro cell experiments.

Thermoluminescence dosimetry measurements of brachytherapy sources in liquid water

Radiation therapy dose measurements are customarily performed in liquid water. The characterization of brachytherapy sources is, however, generally based on measurements made with thermoluminescence dosimeters (TLDs), for which contact with water may lead to erroneous readings. Consequently, most dosimetry parameters reported in the literature have been based on measurements in water-equivalent plastics, such as Solid Water. These previous reports employed a correction factor to transfer the dose measurements from a plastic phantom to liquid water. The correction factor most often was based on Monte Carlo calculations. The process of measuring in a water-equivalent plastic phantom whose exact composition may be different from published specifications, then correcting the results to a water medium leads to increased uncertainty in the results. A system has been designed to enable measurements with TLDs in liquid water. This system, which includes jigs to support water-tight capsules of lithium fluoride in configurations suitable for measuring several dosimetric parameters, was used to determine the correction factor from water-equivalent plastic to water. Measurements of several 125I and 131Cs prostate brachytherapy sources in liquid water and in a Solid Water phantom demonstrated a correction factor of 1.039 +/- 0.005 at 1 cm distance. These measurements are in good agreement with a published value of this correction factor for an 125I source.

SU-FF-T-212: Energy Response of a CR Plate Exposed to Megavoltage X-Ray and Electron Beams

Purpose: To investigate the mechanism underlying the energy dependence when a CR plate is exposed to therapeutic beams. Method and Materials: Small circular disks were cut from one CR plate and placed in a water‐equivalent plastic (WEP) phantom and exposed. The photostimulated luminescence (PSL) signal was recorded until the signal dropped to the background level to obtain the bleaching curve (PSL vs. time). The area under the bleaching curve (AUC) gives a measure of stored information in the CR plate. Monte Carlo simulations were used to obtain the ratio, DCR/DWater of the energy absorbed in the active layer of the CR plate in a WEP phantom to the energy absorbed in water when the entire phantom (including the CR plate) is replaced by water. Results: For electron beams, the AUC was independent of energy for any given dose to water, and the work function, W, i.e. the energy required to produce one PSL photon was also energy independent. In contrast, for photon beams, the AUC was 15% and 30% higher for 18 MV than for 6MV and Co‐60, respectively where the ratio DCR/DWater was 0.81, 1.08 and 1.11, respectively. Taking AUC data into account, the W for 18 MV had to be lower than for 6MV and Co‐60, our data showing 37% and 45%, respectively, in order to give a higher AUC despite a lower energy absorption in the active layer. While there is no obvious reason for this energy dependence, the differences in the secondary electron spectra produced by the different photon beams are probably not responsible. Conclusion: The method presented here will help researchers to both understand the response to ionizing radiation and develop new applications such as megavoltage dosimetry for IMRT verification. The energy dependence of W on beam modalities requires caution regarding CR dosimetry.

Novel methods of measuring single scan dose profiles and cumulative dose in CT

Computed tomography dose index (CTDI) is a conventional indicator of the patient dose in CT studies. It is measured as the integration of the longitudinal single scan dose profile (SSDP) by using a 100-mm-long pencil ionization chamber and a single axial scan. However, the assumption that most of the SSDP is contained within the chamber length may not be valid even for thin slices. We have measured the SSDPs for several slice widths on two CT scanners using a PTW diamond detector placed in a 300 mm x 200 mm x 300 mm water-equivalent plastic phantom. One SSDP was also measured using lithium fluoride (LiF) TLDs and an IC-10 small volume ion chamber, verifying the general shape of the SSDP measured using the diamond detector. Standard cylindrical PMMA CT phantoms (140 mm length) were also used to qualitatively study the effects of phantom shape, length, and composition on the measured SSDP. The SSDPs measured with the diamond detector in the water-equivalent phantom were numerically integrated to calculate the relative accumulated dose D(L)(0)calc at the center of various scan lengths L. D(L)(0)calc reached an equilibrium value for L > 300 mm, suggesting the need for phantoms longer than standard CT dose phantoms. We have also measured the absolute accumulated dose using an IC-10 small volume ion chamber, D(L)(0)SV, at three points in the phantom cross section for several beamwidths and scan lengths. For one CT system, these measurements were made in both axial and helical scanning modes. The absolute CTDI100, measured with a 102 mm active length pencil chamber, were within 4% of D(L)(0)SV measured with the small volume ion chamber for L approximately 100 mm suggesting that nonpencil chambers can be successfully used for CT dosimetry. For nominal beam widths ranging from 3 to 20 mm and for L approximately 250 mm, D(L)(0)SV values at the center of the water-equivalent phantom's elliptic cross section were approximately 25%-30% higher than the measured CTDI100. For small beamwidths, the difference in D(L)(0)SV for L approximately 250 mm and L approximately 14 x beamwidth (CTDI14nT) reached up to 50%. Peripheral point doses at 70 mm depth along the major axis of the phantom for L approximately 250 mm were up to 22% higher than for L approximately 100 mm. The differences between CTDI100 and D(L)(0)SV for L approximately 250 mm were in good agreement with the predictions made from the numerical integration of the measured SSDPs. Due to the considerable dose measured beyond the length of standard CT phantoms, CT dosimetry for longer body scan series should be performed in longer phantoms. Measurements could be made as we have shown, using a small volume chamber translating through the beam using multiple scans.

Peripheral dose distributions for a linear accelerator equipped with a secondary multileaf collimator and universal wedge

The American Association of Physicists in Medicine Task Group 36 (AAPM TG-36) data can be used to estimate peripheral dose (PD) distributions outside the primary radiation field. However, the report data does not apply to linear accelerators equipped with a multileaf collimator (MLC) and universal wedge (UW). Tertiary multileaf collimators have been shown to significantly affect PD distributions and TG-36 reported data. Measurements were performed to evaluate PD distributions for a linear accelerator equipped with a secondary MLC, backup diaphragms, and UW. This data can be used to compliment the TG-36 report for estimation of doses to critical structures outside primary radiation fields. For the evaluated linear accelerator, an MLC is incorporated in the upper secondary collimator jaws. Backup shielding diaphragms are located underneath the MLC. At the nominal collimator position, the MLC and the backup diaphragm provide collimation primarily in the transverse direction. Conventional, solid tungsten-alloy jaws, located underneath the backup diaphragms, provide secondary collimation in the longitudinal direction. The universal wedge provides dose modulation in the direction of the conventional jaws. Measurements were made with an ionization chamber inserted into a water-equivalent plastic phantom with the secondary collimator and MLC settings of , and with and without UW. Data was acquired along the machine's longitudinal axis for 6, 10, and 18 MV photons. Peripheral dose distributions were measured with the collimator rotated to 0° and 270° for open field measurements and to 0°, 180°, and 270° for wedged fields (IEC 1217). This allowed evaluation of peripheral dose distributions as a function of collimator rotation. Wedged fields were normalized to deliver the same dose at the depth of maximum dose on the central axis as open fields. The measured PD distributions were generally comparable to data reported by TG-36. At distances close to the field edge (less than 30 or 40 cm), the measured PD distributions were lower when the measurement point was shielded by solid jaws than with MLC and backup diaphragm. At longer distances, this trend reversed for all energies and evaluated field sizes. However, the difference in PD distribution with collimator rotation was not large enough to warrant strategic positioning of the collimator to reduce dose to critical structures outside the primary radiation field. Because internal scatter dominates close to the field edge, wedged PD distributions were comparable to open field doses at distances closer than 30 cm. However, at distances larger than 30 cm from the field edge, wedged PD distributions were significantly grater than those for open fields due to increased contribution of leakage radiation. Increased leakage radiation is due to the increase in wedged field monitor units, which is related to a small wedge factor (0.27 to 0.29). PACS number(s): 87.53.–j, 87.66.–a

Attenuation of 4-20 MV x rays by a new compensator material of cement.

The properties of a new cement based material for production of compensators are presented. Broad beam attenuation of 4-20 MV x rays by slabs of the material have been measured at various field sizes and depths in a large water phantom. For comparison the attenuation of aluminum was determined at some of the photon energies and it was found that the attenuation properties of the cement based material are very close to those of aluminum. At 6 and 18 MV, a comparison of different phantoms for attenuation measurements was carried out. For this investigation the ionization chamber was placed in a 50x50x50 cm3 water phantom, a 20x20x20 cm3 water equivalent plastic phantom, and a cylindrical mini-phantom. Agreement was obtained between the measurements in the large water phantom and in the water equivalent plastic phantom. The measurements carried out with the mini-phantom in a 6 MV x-ray beam gave a higher transmission versus absorber thickness than the transmission found with the water phantom resulting in a lower value of the effective attenuation coefficient. At 18 MV x rays the difference between the measurements in the water phantom and in the mini-phantom was less. This shows that a water equivalent plastic phantom with an area comparable with the largest field size applied can be used for measurements of effective attenuation coefficients, whereas a mini-phantom cannot be used directly, especially at low photon energies.

Peripheral dose distributions for a linear accelerator equipped with a secondary multileaf collimator and universal wedge.

The American Association of Physicists in Medicine Task Group 36 (AAPM TG‐36) data can be used to estimate peripheral dose (PD) distributions outside the primary radiation field. However, the report data does not apply to linear accelerators equipped with a multileaf collimator (MLC) and universal wedge (UW). Tertiary multileaf collimators have been shown to significantly affect PD distributions and TG‐36 reported data. Measurements were performed to evaluate PD distributions for a linear accelerator equipped with a secondary MLC, backup diaphragms, and UW. This data can be used to compliment the TG‐36 report for estimation of doses to critical structures outside primary radiation fields. For the evaluated linear accelerator, an MLC is incorporated in the upper secondary collimator jaws. Backup shielding diaphragms are located underneath the MLC. At the nominal collimator position, the MLC and the backup diaphragm provide collimation primarily in the transverse direction. Conventional, solid tungsten‐alloy jaws, located underneath the backup diaphragms, provide secondary collimation in the longitudinal direction. The universal wedge provides dose modulation in the direction of the conventional jaws. Measurements were made with an ionization chamber inserted into a 20×40×120cm3 water‐equivalent plastic phantom with the secondary collimator and MLC settings of 5×5,10×10,15×15, and 25×25cm2 with and without UW. Data was acquired along the machine's longitudinal axis for 6, 10, and 18 MV photons. Peripheral dose distributions were measured with the collimator rotated to 0° and 270° for open field measurements and to 0°, 180°, and 270° for wedged fields (IEC 1217). This allowed evaluation of peripheral dose distributions as a function of collimator rotation. Wedged fields were normalized to deliver the same dose at the depth of maximum dose on the central axis as open fields. The measured PD distributions were generally comparable to data reported by TG‐36. At distances close to the field edge (less than 30 or 40 cm), the measured PD distributions were lower when the measurement point was shielded by solid jaws than with MLC and backup diaphragm. At longer distances, this trend reversed for all energies and evaluated field sizes. However, the difference in PD distribution with collimator rotation was not large enough to warrant strategic positioning of the collimator to reduce dose to critical structures outside the primary radiation field. Because internal scatter dominates close to the field edge, wedged PD distributions were comparable to open field doses at distances closer than 30 cm. However, at distances larger than 30 cm from the field edge, wedged PD distributions were significantly grater than those for open fields due to increased contribution of leakage radiation. Increased leakage radiation is due to the increase in wedged field monitor units, which is related to a small wedge factor (0.27 to 0.29).PACS number(s): 87.53.–j, 87.66.–a