Linac Machine Research Articles

TU‐F‐CAMPUS‐J‐05: Fast Volumetric MRI On An MRI‐Linac Enables On‐Line QA On Dose Deposition in the Patient

Purpose:The introduction of the MRI‐linac in radiotherapy brings MRI‐guided treatment with daily plan adaptions within reach. This paradigm demands on‐line QA. With its ability to perform continuous volumetric imaging in an outstanding soft‐tissue contrast, the MRI‐ linac promises to elucidate the dose deposition process during a treatment session. Here we study for a prostate case how dynamic MRI combined with linac machine parameters and a fast dose‐engine can be used for on‐line dose accumulation.Methods:Prostate imaging was performed in healthy volunteer on a 1.5T MR‐scanner (Philips, Best, NL) according to a clinical MR‐sim protocol, followed by 10min of dynamic imaging (FLASH, 4s/volume, FOV 40×40×12cm3, voxels 3×3×3mm3, TR/TE/α=3.5ms/1.7ms/5°). An experienced radiation oncologist made delineations, considering the prostate CTV. Planning was performed on a two‐compartment pseudoCT (air/water density) according to clinical constraints (77Gy in PTV) using a Monte‐Carlo (MC) based TPS that accounts for magnetic fields. Delivery of one fraction (2.2Gy) was simulated on an emulator for the Axesse linac (Elekta, Stockholm, SE). Machine parameters (MLC settings, gantry angle, dose rate, etc.) were recorded at 25Hz. These were re‐grouped per dynamic volume and fed into the MC‐engine to calculate a dose delivered for each of the dynamics. Deformations derived from non‐rigid registration of each dynamic against the first allowed dose accumulation on a common reference grid.Results:The DVH parameters on the PTV compared to the optimized plan showed little changes. Local deformations however resulted in local deviations, primarily around the air/rectum interface. This clearly indicates the potential of intra‐fraction adaptations based on the accumulated dose. Application in each fraction helps to track the influence of plan adaptations to the eventual dose distribution. Calculation times were about twice the delivery time.Conclusion:The current Result paves the way to perform on‐line treatment delivery QA on the MRI‐linac in the near future.

Accuracy of positioning and irradiation targeting for multiple targets in intracranial image-guided radiation therapy: a phantom study

This study investigated the accuracy of positioning and irradiation targeting for multiple off-isocenter targets in intracranial image-guided radiation therapy (IGRT). A phantom with nine circular targets was created to evaluate both accuracies. First, the central point of the isocenter target was positioned with a combination of an ExacTrac x-ray (ETX) and a 6D couch. The positioning accuracy was determined from the deviations of coordinates of the central point in each target obtained from the kV-cone beam computed tomography (kV-CBCT) for IGRT and the planning CT. Similarly, the irradiation targeting accuracy was evaluated from the deviations of the coordinates between the central point of each target and the central point of each multi-leaf collimator (MLC) field for multiple targets. Secondly, the 6D couch was intentionally rotated together with both roll and pitch angles of 0.5° and 1° at the isocenter and similarly the deviations were evaluated. The positioning accuracy for all targets was less than 1 mm after 6D positioning corrections. The irradiation targeting accuracy was up to 1.3 mm in the anteroposterior (AP) direction for a target 87 mm away from isocenter. For the 6D couch rotations with both roll and pitch angles of 0.5° and 1°, the positioning accuracy was up to 1.0 mm and 2.3 mm in the AP direction for the target 87 mm away from the isocenter, respectively. The irradiation targeting accuracy was up to 2.1 mm and 2.6 mm in the AP direction for the target 87 mm away from the isocenter, respectively. The off-isocenter irradiation targeting accuracy became worse than the positioning accuracy. Both off-isocenter accuracies worsened in proportion to rotation angles and the distance from the isocenter to the targets. It is necessary to examine the set-up margin for off-isocenter multiple targets at each institution because irradiation targeting accuracy is peculiar to the linac machine.

SU-E-T-554: Comparison of Electron Disequilibrium Factor in External Photon Beams for Different Models of Linear Accelerators

Purpose: The dose in the buildup region of a photon beam is usually determined by the transport of the primary secondary electrons and the contaminating electrons from accelerator head. This can be quantified by the electron disequilibrium factor, E, defined as the ratio between total dose and equilibrium dose (proportional to total kerma), E = 1 in regions beyond buildup region. Ecan be different among accelerators of different models and/or manufactures of the same machine. This study compares E in photon beams from different machine models/ Methods: Photon beam data such as fractional depth dose curve (FDD) and phantom scatter factors as a function of field size and phantom depth were measured for different Linac machines. E was extrapolated from these fractional depth dose data while taking into account inverse-square law. The ranges of secondary electron were chosen as 3 and 6 cm for 6 and 15 MV photon beams, respectively. The field sizes range from 2x2 to 40x40 cm2. Results: The comparison indicates the standard deviations of electron contamination among different machines are about 2.4 - 3.3% at 5 mm depth for 6 MV and 1.2 - 3.9% at 1 cm depth for 15 MV for the same field size. The corresponding maximum deviations are 3.0 - 4.6% and 2 - 4% for 6 and 15 MV, respectively. Both standard and maximum deviations are independent of field sizes in the buildup region for 6 MV photons, and slightly decreasing with increasing field size at depths up to 1 cm for 15 MV photons. Conclusion: The deviations of electron disequilibrium factor for all studied Linacs are less than 3% beyond the depth of 0.5 cm for the photon beams for the full range of field sizes (2-40 cm) so long as they are from the same manufacturer.

SU‐E‐J‐22: Implementation and Validation of a 2D‐3D Rigid Registration Algorithm for Proton Gantry and Stereotactic Radiosurgery Systems

Purpose: To implement and validate the accuracy of an intensity‐based 2D to 3D rigid registration algorithm for proton treatment systems. Methods: A 2D‐3D rigid registration algorithm (REG23) was previously validated for linac machines. We adapted REG23 for use in both gantry‐based and Stereotactic Alignment in Radiosurgery (STAR) proton treatment systems. REG23 registration was run on NVIDIA Quadra600 GPU card using NCC metrics and AMOEBA optimizer and validated using dual orthogonal kV images acquired during cranial target treatments: 66 fractions to 7 patients in gantry system and 135 fractions to 9 patients in STAR. Rectangle ROIs covering the whole region superior to the base‐of‐skull were used for REG23 registration and the accuracy was evaluated using clinical utilized fiducial‐based 2D‐3D ray back‐projection rigid registration as the baseline. Results: For the gantry system, the differences between REG23 and baseline were ‐ 0.01±0.63 mm, 0.43±0.61 mm, 0.12±0.60 mm in the left‐right, superior‐inferior, and anterior‐posterior directions; 0.15±0.44°, 0.11±0.23°, and 0.19±0.37° in pitch, roll, and yaw, respectively. The vector difference was 1.04±0.48 mm. For STAR system, the REG23 results were − 0.34±0.45 mm, −0.10±0.36 mm, −0.38±0.75 mm, −0.14±0.54°, 0.04±0.18°, and 0.03±0.35° different than the baseline in the left‐right, superior‐inferior, and anterior‐posterior directions, pitch, roll, and yaw. The vector difference was 0.93±0.53 mm. The time for REG23 automatic registration was 29.7±9.5 seconds. Conclusion: We demonstrated that the intensity based 2D‐3D rigid registration algorithm REG23 provided sub‐millimeter accuracy and better than 0.5° for both gantry‐based and STAR proton treatment systems for cranial patients. This accuracy is within patient setup tolerance for current fractionated proton treatment systems. Given its accuracy and efficiency, we believe REG23 has great potential for clinical utilization in proton radiation therapy by replacing the implanted fiducial or anatomic feature based patient setup method.The project was supported by the Federal Share of program income earned by Massachusetts General Hospital on C06 CA059267, Proton Therapy Research and Treatment Center.

SU‐C‐500‐05: CT‐Based Treatment Planning for 4Pi Non‐Coplanar Converging (4PiNC) Beams in An External‐Beam Treatment Planning System

Purpose: A 4‐Pi non‐coplanar converging (4PiNC) modality has been actively developed in our institution, in which the radiation beams are delivered along conic trajectories and rotate around the midline of a patient. The proposed design eliminates the potential gantry‐couch collision, a major concern of non‐coplanar beam delivery. This study introduces a method to implement the 4PiNC planning in the public‐domain treatment planning system (TPS) PLUNC. Methods: The cone angle of a 4PiNC delivery is defined as the angle between the cone edge and the transverse iso‐plane. The beam angle defines its relative position on the cone. We have mathematically proved that there is a one‐to‐one correspondence between a 4PiNC beam specification (cone and beam angle) and a LINAC beam specification (gantry and couch angle). Therefore, a graphic interface was written in PLUNC to design a 4PiNC RT plan using existing commissioned LINAC machines. The method was validated visually on a spherical phantom of 15cm radius with 12 beams evenly distributed on a cone, and with cone angle 40, ‐ 30, 20, ‐ 10 and 0 degree respectively. A 4PiNC RT plan was made for a prostate SBRT case with the afore‐mentioned 5 cones (20 beams on each cone) and was compared with the conventional five‐beam IMRT plan. Results: In the phantom study, the transverse planes at −3, 0 and 3 cm from the iso‐plane as well as the sagittal iso‐plane were examined; the beam separations and cone angles were consistent with those specified. The prostate study suggested that the OAR mean dose can be reduced by up to 44.5% by the 4PiNC RT plan with comparable PTV coverage. Conclusion: The RT planning for the proposed 4PiNC delivery system can be implemented in a TPS for conventional LINAC beams. This allows dosimetric investigation of the proposed system even before finalizing the hardware design. This work was partially supported by a DoD grant DoD W81XWH1010862

WE-G-217BCD-07: Implementation and Evaluation of Helical On-Board CBCT and Exact Image Reconstruction.

Longitudinal coverage of CBCT, which is 17 cm for head scan and 15.5 cm for body scan, is not enough to cover the entire PTV for over 90% of head/neck and gastrointestinal/genitourinary/gynecologic patients if lymph nodes are involved. Helical CBCT, which was accomplished using external beam LINAC in its research mode, is one promising way to extend the CBCT longitudinal coverage. Aim of this study is to compare Katsevich's exact algorithm with traditional FDK algorithm for helical CBCT image reconstruction. CBCT projection raw data were acquired on a TrueBeam LINAC machine (Varian Medical Systems) in the research mode in helical trajectories that encompass a 360 degree rotation, 25 cm pitch, 100 kVp, 80 mA, and 25 ms, with a Catphan 600. Reconstruction was done with Katsevich's exact and FDK approximate algorithms. Scatter correction, beam-hardening correction, and non-uniform gantry angle correction, are performed on projection data to reduce artifacts and noise. Image qualities (CT number accuracy, uniformity, SNR) were evaluated and compared between the reconstruction algorithms. Images reconstructed by Katsevich's algorithm show better qualities, compared to ones by FDK algorithm and HU numbers have higher uniformity and accuracy. The HU-density calibration curve closely conforms to the manufacturer recommended values. The level of noise computed as the standard deviation in the phantom uniform region is 28.07 for the Katsevich algorithm, compared to 44.64 for the FDK algorithm. Katsevich's exact reconstruction algorithm provided better image qualities than FDK for helical CBCT scans. This result will very useful for our ongoing investigation of helical CBCT, which would lead to improvement of CBCT longitudinal coverage of PTV and would be essential for future image-guided adaptive radiation therapy applications. Varian Research Agreement with Washington University in St. Louis.

TU-E-BRB-04: Adaptive Stereotactic-Body Radiation Therapy (SBRT) Planning for Lung Cancer.

Tumor shrinkage during lung SBRT has been reported. Adaptive SBRT planning can potentially escalate dose to the target and mitigate dose to surrounding tissues. In this study, we investigated dosimetric effects of adaptive lung SBRT planning using cone-bean CT (CBCT) images. 48 patients who had SBRT treatments for lung cancer in our institution were included in this study. All patients underwent a 3D helical CT scan and 4DCT scan for treatment planning. Treatments were delivered on Linac machines equipped with kV-OBI, CBCT, and MV EPID. CBCT images of all patients were retrospectively analyzed to determine tumor size changes. 10 patients with the largest percentages of tumor shrinkage were selected for adaptive planning. Plans were created on CBCT images of each fraction using the same planning parameters as the original CT-based plans. ITV was contoured on CBCT. PTV was generated by adding 5 mm margin to ITV. Two accumulative dose plans, non-adaptive (P-NON) and adaptive (P-ADP), were determined for each patient using deformable image registration via Velocity AI. Dosimetric comparisons between PNON and PADP were performed for all OARs using RTOG0915 metrics. Statistical significances were tested using paired t-test with a significance level of 0.05. Comformality ranged from 1.03 to 1.41, with small intra-subject variations (ranged from 0.01 to 0.06). Compared to PNON, PADP reduced all dosimetric metrics and significantly in the following (p-value range: 0.02-0.04): V20Gy of lungs; D0.35cc, D1.2cc and maximum dose of cord, D5cc and maximum dose of esophagus, D30cc of chest wall, and D15cc of heart. Coverage to original GTV is comparable in two plans. Adaptive SBRT planning for selected patients resulted in significant dose reduction for adjacent OARs. It has the potential to further escalate dose to the target and mitigate dose to surrounding tissues.

SU‐E‐T‐09: Investigate the Impact of Prolonged Treatment Delivery Time on Tumor Control Probability

Purpose: Stereotactic body radiotherapy (SBRT) can be delivered by a linac machine using multiple beams or a Cyberknife machine. The Cyberknife‐ based SBRT usually allows greater dose conformity to the tumor target and reduce normal tissue toxicity. However, it requires much longer treatment time. This work explores the potential impact of such prolonged delivery times on treatment outcome. Methods: The tumor‐control probability (TCP) utilizing an extended linear‐quadratic (LQ) model, which accounts for sublethal damage repair and clonogen proliferation, was used to calculate the cell‐killing efficiency of clinical SBRT plans for 30 patients. LQ parameters derived from compiled clinical data for lung cancer (α=0.31, =0.062, α/β=10 Gy, Tk = 21d, Td = 3d and a 0.025 s‐1 repair constant) were used to compute changes in the TCP due to prolonged delivery time associated with Cyberknife‐ delivery. The dose‐volume histograms were obtained from actual SBRT plans using 12 Gy × 4 or 10Gy ×5 fractions. The impact of delivery time at 10, 50, and 70 min on TCP were studied for all 30 patients. For a few typical patients with small, intermediate, and larger tumors, the TCP was calculated for a range of delivery time from 5 to 100 min. Results: Our calculations showed that an average 99.8% of TCP was reached for most of the patients, if 10 minute delivery time was assumed, except for a few patients with large or multiple targets. By increasing treatment delivery time from 10 to 50 and 70 minutes, the TCP decreased by an average of 3% for most of the patients. Significant decrease was found for those same patients with larger tumors or multiple targets, with the TCP vales in the 67% range. Conclusions: The delivery time may have a significant impact on Cyberknife‐based SBRT treatment outcomes for patients with larger or multiple tumors.

SU‐E‐T‐208: Optimizing Parameters in Imaging for Accurate EPID Exit Dosimetry Application

Purpose: Besides being used for patient treatment alignment, EPIDs treatment quality control is advancing to application of exit dosimetry analysis. The absolute dosimetry is determined by the accuracy of the calibration between portal image pixel value and dose. In this study, a generic IMRT field was selected as an evaluation reference, the dosimetric effect of pixel value and dose calibration was investigated by varying energy, dose rate and total delivered dose. Method and Materials:Varian EPID and Imaging acquisition system (IAS3‐aS1000) were used to create images from dose delivered from a Varian Trilogy Linac machine. The doses were delivered with a 60 degree enhance dynamic wedges (EDW) field varied with different energies, total doses and dose rates. And the images were exported to the RIT system (RIT113V5.3) for analysis. The dosimetric calibration curves were used to analyze the effect of each parameter. The slopes between the calibration curves were also calculated. then a prostate IMRT field was employed for absolute dosimetry verification. Results: At dose rate of 400MU per minute, the magnitudes of slope range from 10 pixel value per cGy to 27 pixel value per cGy for calibration from 50MU to 5000MU. When dose rates vary from 100 to 600 MU per minute, the slopes change from 29 pixel value per cGy to 1 pixel value per cGy. A selected point dose from an IMRT field was compared with EPID measurement and was within 5% using calibrations at 200MU delivered at 600 MU per minute, or 100MU delivered at 400MU per minute. Conclusions: Accurate dosimetric application for EPID requires meticulous selection of imaging parameters. For high precision exit dosimetry based on EPID imaging, further investigation is needed for total Monitor Unit and delivery temporal effect from IMRT Fields.

SU‐FF‐T‐316: 3 Years of Data Collection and Analysis of Film Flatness and Symmetry Using Vidar/ RIT Software From Different Centers Nation Wide

Purpose: To analyze accuracy and dependability of RIT software / Vidar Scanner in film flatness and symmetry measurement nationwide. Method and Materials: EDR and V films were exposed to 200 MU 6X & 15X radiations at depth of 10cm in solid water phantom, 100cmSSD with filed size of 20×20cm. Measurements were repeated with different phantom orientation, phantom of different vendors, and different packs of films, different processor, and different linac machines. Water beam scanning was done with PTW water tank at the lowest speed, central axis and at steps of 3cm off axis. Profiles were analyzed using Memphysto software. Exposed films were scanned at different 90' intervals and flipped in each orientation for comparison. Measurements were repeated at different centers with different linac and different RIT software and Vidar Scanner Results: RIT's film flatness scanning shows an area of increased intensity on the left hand side of the scan regardless of using different Linac machine, Vidar scanner or the orientation of the phantom being used. No mirror images were obtained in RIT when the flatness film was scanned flipped or if rotated at different angles. Results of ‘Scan info’ test performed weekly on Vidar scanner for three months showed no failure and passed the overall performance and water beam scanning profile collected by PTW showed persistent uniform symmetry and flatness using TG‐45 guidelines. Conclusions: None of the mentioned effects were seen while scanning a patient's QA film. Only in scanning of flatness film, one would notice the above effects. RIT believes that a systemic scanner bias (within Vidar production specifications) may be causing this effect and is working on methods to compensate for any systemic scanner bias.

Inverse Compton backscattering source driven by the multi-10 TW laser installed at Daresbury

AbstractInverse Compton scattering is a promising method to implement a high brightness, ultra-short, energy tunable X-ray source at accelerator facilities. We have developed an inverse Compton backscattering X-ray source driven by the multi-10 TW laser installed at Daresbury. Hard X-rays, with spectral peaks ranging from 15 to 30 keV, depending on the scattering geometry, will be generated through the interaction of laser pulses with electron bunches delivered by the energy recovery linac machine, initially known as energy recovery linac prototype and subsequently renamed accelerators and lasers in combined experiments. X-ray pulses containing 9 × 107 photons per pulse will be created from head-on collisions, with a pulse duration comparable to the incoming electron bunch length. For transverse collisions 8 × 106 photons per pulse will be generated, where the laser pulse transit time defines the X-ray pulse duration. The peak spectral brightness is predicted to be ~1021 photons/(s mm2 mrad2 0.1% Δλ/λ).

TH-D-352-08: Radiation Treatment Machine Mechanical Quality Assurance Check with Megavoltage Portal Imaging Device

Purpose:Treatment machine mechanical check is an important procedure in quality assurance (QA). Linac machines are commonly equipped with electronic portal imaging device(EPID).EPID can be used to perform mechanical QA more accurately and easily. We developed a method to use EPID to measure gantry, collimator and table isocenter runout. Method and Materials: Measurements were performed on an Elekta's ASSEX machine that equips with an a‐Si EPID and kilo‐voltage cone beam CT. A ball‐bearing phantom was position in the radiation isocenter following KV‐MV isocenter calibration procedure. Portal images were taken at various gantry, collimator and table angles. Matlab algorithms were developed to analyze the gantry, collimator and table isocenter positions. Results: Gantry, collimator and table isocenter positions were measured quantitatively with accuracy better than 0.1mm. Results clearly show the discrepancies of the three isocenters. Conclusion:EPID is a useful tool in LINAC mechanic QA. EPID isocenter measurements are more accurate and convenient than traditional approaches.

SU-FF-T-281: Leakage Radiation of Elekta Synergy-S LINAC Machine

Purpose: To evaluate leakage radiation through treatment head of Elekta Synergy-S linear accelerator, which has a new “beam modulator” MLC design. Method and Materials: As part of the acceptance test of a newly installed Elekta Synergy-S, Kodak XV films were used to encompass the entire treatment head to locate potential radiation hot spots. After evaluating film results, an ion chamber (with appropriate buildup caps) was used to measure the leakage radiation for 6MV and 15MV x-ray beams with MLC leaves fully closed. The chamber was placed in a plane perpendicular to the beam axis and which contained the machine isocenter, at distances ranging from 13–100 cm away from the machine isocenter in three directions (inline, crossline and diagonal). A 4.5 cm lead block placed on the blocking tray was used to evaluate the scattered radiation contribution from closed MLC leaves to ion chamber measurements at selected locations near the secondary collimator edges. Results: Leakage radiation was greater than 0.1% of the dose at isocenter for the original beam modulator treatment head design. A Beam Modulator Head Shielding Kit was installed and measurements repeated. Leakage radiation was only slightly reduced. A second version of Beam Modulator Head Shielding Kit was installed, and subsequent measurements showed that head leakage radiation was finally reduced below 0.1%. Contributions to ion chamber readings from scattered radiation generated from closed MLC leaves can be as high as 60% at close proximity to the secondary collimator edge. Conclusions: Thorough measurement of head leakage is necessary during acceptance testing of a LINAC machine, especially for new type machines such as the Synergy-S, to ensure patient and staff safety. It is recommended leakage radiation measurements be made by blocking the scattered radiation contribution from the closed MLC leaves.

A MONTE CARLO SIMULATION OF PHOTON BEAM GENERATED BY A LINEAR ACCELERATOR

ntroduction: Monte Carlo simulation is the most accurate method of simulating radiation transport and predicting doses at different points of interest in radiotherapy. A great advantage of the Monte Carlo method compared to the deterministic methods is the ability to deal accurately with any complex geometry. Its disadvantage is the extremely long computing time required to obtain a dose distribution with good statistical accuracy. Materials and Methods: The MCNP-4C Monte Carlo code was used to simulate a 9 MV photon beam from a Neptun 10PC linear accelerator. The accelerator was modeled as a complete unit consisting of a target, exit window, initial collimator, primary collimator, flattening filter, monitor chamber and secondary collimator. The geometrical details and the composition of each component was either obtained from the manufacturer or was directly measured. The simulation of the source was performed in a two step process. Initially, the electron source was defined. Secondly, the bremsstrahlung energy spectra and the fluence distribution at the scoring planes were used to define the photon source. The simulated electron beam energy followed a Gaussian distribution, with FWHM equal to 12% in nominal energy. The used intensity distribution of the electron beam also followed a Gaussian distribution with a FWHM equal to 0.34 cm. To compute the photon beam data a 50 × 50 × 40 cm 3 water phantom located at SSD = 100 cm was simulated. The depth dose and the dose profile curves were calculated for four different field sizes (5×5, 10×10, 20×20 and 30×30 cm 2 ) and compared against the measured values. The low-energy cut-off for the photons and electrons was 10 and 500 KeV, respectively. The measurements were carried out by using a Scanditronix dose scanning system and a 0.12 cm 3 RK ionization chamber. Results: To verify the simulated model, the calculated Monte Carlo dose data were compared against the corresponding measured values. The energy spectra and the angular distribution of the x-ray beam generated by the Neptun 10PC linac was examined. The result showed an efficiency of about 73% for the production of bermsstrahlung photon by the target. The agreement between the calculated and the measured depth dose and the dose profile was generally better than 2% for all the fields. Discussion and Conclusion: The simulation of the Neptun 10PC linac performed in this work is capable of computing the depth dose data and the beam profiles in water phantom for all the predefined fields including 5×5, 10×10, 20×20 and 30×30 cm 2 . Therefore, it can be concluded that MCNP-4C is a suitable tool for the dose calculation in radiotherapy. The simulated linac machine and the resulting data can be used to predict the dose distribution in all complex fields.

Towards optimization of megavoltage cone-beam CT for image-guided radiotherapy

Towards optimization of megavoltage cone-beam CT for image-guided radiotherapy

Towards optimization of megavoltage cone-beam CT for image-guided radiotherapy

Soft tissue imaging in the treatment room is one of the main challenges faced today in high-precision radiation therapy. Megavoltage cone beam CT (MV-CBCT) is a promising imaging technique for image-guided radiotherapy due to its simplicity and its potentially higher accuracy than kV-CBCT (i.e., cone beam CT with a kilovoltage x-ray source mounted on a Linac machine). MV-CBCT uses an electronic portal imaging device (EPID) attached to a Linac to acquire CT images by rotating the MV x-ray source (emitting a cone beam) and the EPID around the patient.

Evaluation of factors to convert absorbed dose calibrations from graphite to water for the NPL high-energy photon calibration service

The National Physical Laboratory (NPL) provides a high-energy photon calibration service using 4–19 MV x-rays and 60Co γ-radiation for secondary standard dosemeters in terms of absorbed dose to water. The primary standard used for this service is a graphite calorimeter and so absorbed dose calibrations must be converted from graphite to water. The conversion factors currently in use were determined prior to the launch of this service in 1988. Since then, it has been found that the differences in inherent filtration between the NPL LINAC and typical clinical machines are large enough to affect absorbed dose calibrations and, since 1992, calibrations have been performed in heavily filtered qualities. The conversion factors for heavily filtered qualities were determined by interpolation and extrapolation of lightly filtered results as a function of tissue phantom ratio 20,10 (TPR20,10). This paper aims to evaluate these factors for all mega-voltage photon energies provided by the NPL LINAC for both lightly and heavily filtered qualities and for 60Co γ-radiation in two ways. The first method involves the use of the photon fluence-scaling theorem. This states that if two blocks of different material are irradiated by the same photon beam, and if all dimensions are scaled in the inverse ratio of the electron densities of the two media, then, assuming that all photon interactions occur by Compton scatter the photon attenuation and scatter factors at corresponding scaled points of measurement in the phantom will be identical. The second method involves making in-phantom measurements of chamber response at a constant target–chamber distance. Monte Carlo techniques are then used to determine the corresponding dose to the medium in order to determine the chamber calibration factor directly. Values of the ratio of absorbed dose calibration factors in water and in graphite determined in these two ways agree with each other to within 0.2% (1σ uncertainty). The best fit to both sets of results agrees with values determined in previous work to within 0.3% (1σ uncertainty). It is found that the conversion factor is not sensitive to beam filtration.

Computer Controlled Energy Spread Measurement at the CERN PS Linac

A new method of energy spread measurement is described which permits a quick evaluation of the percentage of the PS Linac beam inside the acceptance range of energy spread for the PS Synchrotron. The execution of the measurement, for which a minimum of two beam pulses is lost, is under control of the PS Control Computer, IBM 1800, and makes use of two beam slits, a triplet lens and a beam deflecting magnet. The result of the measurement is simply obtained as the ratio of two beam current transformer readings. These are read near the beginning and near the end of the beam pulse and the computed results together with other Linac machine parameter readings of interest, such as RF tank levels, are displayed on TV-screens at the Linac and in the PS Main Control Room. On request, a print-out of the results can also be obtained. Before executing the measurement, the computer program checks the status and levels of important parameters and in case of malfunctioning warns the operator with a diagnostic message on the TV-screen.