Treatment Head Research Articles

TU-A-108-01: Four-Dimensional Monte Carlo Using the TOPAS TOol for PArticle Simulation

Purpose: As the number and diversity of 4D radiation therapy treatment modalities increases, with techniques from IMRT, VMAT, SBRT, IGRT and IMPT to proton and ion beam scanning, the value of a comprehensive 4D Monte Carlo dose calculation tool has become evident. In silico experiments provide a powerful platform to study sensitivity of dose calculation techniques to motion errors, beam instabilities and interplay effects. We present a comprehensive system to meet this growing need. Methods: We have provided rich 4D capabilities within the TOPAS TOol for PArticle Simulation. TOPAS allows any one or more simulation parameter to vary over time, such as a beam current, the geometrical position of a treatment head component, a magnetic field, a 4D CT time slice or a gating signal for scoring. In use at 25 research institutions, TOPAS has been brought to bear on a wide range of 4D problems. We present 4D capabilities, including modulator wheel rotation during beam current modulation, beam scanning, MLC leaf motion, gantry rotation and 4D CT. Results: It is imperative that 4D simulation tools provide 4D-scoring capabilities, such as scorers that can be gated by time, or phase space output that can associate time to each saved particle. Only with such tools can one confirm that the simulation is accurate in not just three dimensions but four. We show such results that confirm TOPAS 4D performance, both as time-sliced histograms and through simulated 4D radiography (simulated films darkening as dose accumulates over time). Conclusion: TOPAS has proven to be a comprehensive 4D simulation tool. While current users are mainly in proton therapy, TOPAS is being extended for use in ion, x-ray, electron and brachytherapy. While there are many excellent MC tools already for some of these modalities, TOPAS is unique in the 4D domain.

Geometrical splitting technique to improve the computational efficiency in Monte Carlo calculations for proton therapy

To present the implementation and validation of a geometrical based variance reduction technique for the calculation of phase space data for proton therapy dose calculation. The treatment heads at the Francis H Burr Proton Therapy Center were modeled with a new Monte Carlo tool (TOPAS based on Geant4). For variance reduction purposes, two particle-splitting planes were implemented. First, the particles were split upstream of the second scatterer or at the second ionization chamber. Then, particles reaching another plane immediately upstream of the field specific aperture were split again. In each case, particles were split by a factor of 8. At the second ionization chamber and at the latter plane, the cylindrical symmetry of the proton beam was exploited to position the split particles at randomly spaced locations rotated around the beam axis. Phase space data in IAEA format were recorded at the treatment head exit and the computational efficiency was calculated. Depth-dose curves and beam profiles were analyzed. Dose distributions were compared for a voxelized water phantom for different treatment fields for both the reference and optimized simulations. In addition, dose in two patients was simulated with and without particle splitting to compare the efficiency and accuracy of the technique. A normalized computational efficiency gain of a factor of 10-20.3 was reached for phase space calculations for the different treatment head options simulated. Depth-dose curves and beam profiles were in reasonable agreement with the simulation done without splitting: within 1% for depth-dose with an average difference of (0.2 ± 0.4)%, 1 standard deviation, and a 0.3% statistical uncertainty of the simulations in the high dose region; 1.6% for planar fluence with an average difference of (0.4 ± 0.5)% and a statistical uncertainty of 0.3% in the high fluence region. The percentage differences between dose distributions in water for simulations done with and without particle splitting were within the accepted clinical tolerance of 2%, with a 0.4% statistical uncertainty. For the two patient geometries considered, head and prostate, the efficiency gain was 20.9 and 14.7, respectively, with the percentages of voxels with gamma indices lower than unity 98.9% and 99.7%, respectively, using 2% and 2 mm criteria. The authors have implemented an efficient variance reduction technique with significant speed improvements for proton Monte Carlo simulations. The method can be transferred to other codes and other treatment heads.

Finding an improved amorphous-silicon x-ray flat-panel detector configuration for the in-line geometry

We have previously investigated the use of a conventional amorphous-silicon flat-panel detector (FPD) for intrafractional image guidance in the in-line geometry. In this configuration, the FPD is mounted between the patient and the treatment head, with the front of the FPD facing towards the patient. By geometrically separating signals from the diagnostic (kV) and treatment (MV) beams, it is possible to monitor the patient and treatment beam at the same time. In this study, we propose an FPD design based on existing technology with a 70% reduced up-stream areal density that is more suited to this new application. We have investigated our FPD model by means of a validated Monte Carlo simulation. Experimentally, simple rectangular fields were used to irradiate through the detector and observe the impact of removing detector components such as the support structure or the phosphor screen on the measured signal. The proposed FPD performs better than the conventional FPD: (i) attenuation of the MV beam is decreased by 60%; (ii) the MV signal is reduced by 20% for the primary MV field region which can avoid saturation of the FPD; and (iii) long range scatter from the MV into the kV region of the detector is greatly reduced.

Clinical Application of In-Room Positron Emission Tomography for In Vivo Treatment Monitoring in Proton Radiation Therapy

The purpose of this study is to evaluate the potential of using in-room positron emission tomography (PET) for treatment verification in proton therapy and for deriving suitable PET scan times. Nine patients undergoing passive scattering proton therapy underwent scanning immediately after treatment with an in-room PET scanner. The scanner was positioned next to the treatment head after treatment. The Monte Carlo (MC) method was used to reproduce PET activities for each patient. To assess the proton beam range uncertainty, we designed a novel concept in which the measured PET activity surface distal to the target at the end of range was compared with MC predictions. The repositioning of patients for the PET scan took, on average, approximately 2 minutes. The PET images were reconstructed considering varying scan times to test the scan time dependency of the method. The measured PET images show overall good spatial correlations with MC predictions. Some discrepancies could be attributed to uncertainties in the local elemental composition and biological washout. For 8 patients treated with a single field, the average range differences between PET measurements and computed tomography (CT) image-based MC results were <5 mm (<3 mm for 6 of 8 patients) and root-mean-square deviations were 4 to 11 mm with PET-CT image co-registration errors of approximately 2 mm. Our results also show that a short-length PET scan of 5 minutes can yield results similar to those of a 20-minute PET scan. Our first clinical trials in 9 patients using an in-room PET system demonstrated its potential for invivo treatment monitoring in proton therapy. For a quantitative range prediction with arbitrary shape of target volume, we suggest using the distal PET activity surface.

Quantitative analysis of in-air output ratio

Output factor (Scp) is one of the important factors required to calculate monitor unit (MU), and is divided into two components: phantom scatter factor (Sp) and in-air output ratio (Sc). Generally, Sc for arbitrary fields are calculated using several methods based on Sc determined by the absorbed dose measurement for several square fields. However, there are calculation errors when the treatment field has a large aspect ratio and the opening of upper and lower collimator are exchanged. To determine Sc accurately, scattered photons from the treatment head and backscattered particles into the monitor chamber must be analyzed individually. In this report, a simulation model that agreed well with measured Sc was constructed and dose variation by scattered photons from the treatment head and by backscattered particles into the monitor chamber was analyzed quantitatively. The results showed that the contribution of scattered photons from the primary collimator was larger than that of the flattening filter, and backscattered particles were affected by not only the upper jaw but also the lower jaw. In future work, a new Sc determination algorism based on the result of this report will be proposed.

TOPAS: An innovative proton Monte Carlo platform for research and clinical applications

While Monte Carlo particle transport has proven useful in many areas (treatment head design, dose calculation, shielding design, and imaging studies) and has been particularly important for proton therapy (due to the conformal dose distributions and a finite beam range in the patient), the available general purpose Monte Carlo codes in proton therapy have been overly complex for most clinical medical physicists. The learning process has large costs not only in time but also in reliability. To address this issue, we developed an innovative proton Monte Carlo platform and tested the tool in a variety of proton therapy applications. Our approach was to take one of the already-established general purpose Monte Carlo codes and wrap and extend it to create a specialized user-friendly tool for proton therapy. The resulting tool, TOol for PArticle Simulation (TOPAS), should make Monte Carlo simulation more readily available for research and clinical physicists. TOPAS can model a passive scattering or scanning beam treatment head, model a patient geometry based on computed tomography (CT) images, score dose, fluence, etc., save and restart a phase space, provides advanced graphics, and is fully four-dimensional (4D) to handle variations in beam delivery and patient geometry during treatment. A custom-designed TOPAS parameter control system was placed at the heart of the code to meet requirements for ease of use, reliability, and repeatability without sacrificing flexibility. We built and tested the TOPAS code. We have shown that the TOPAS parameter system provides easy yet flexible control over all key simulation areas such as geometry setup, particle source setup, scoring setup, etc. Through design consistency, we have insured that user experience gained in configuring one component, scorer or filter applies equally well to configuring any other component, scorer or filter. We have incorporated key lessons from safety management, proactively removing possible sources of user error such as line-ordering mistakes. We have modeled proton therapy treatment examples including the UCSF eye treatment head, the MGH stereotactic alignment in radiosurgery treatment head and the MGH gantry treatment heads in passive scattering and scanning modes, and we have demonstrated dose calculation based on patient-specific CT data. Initial validation results show agreement with measured data and demonstrate the capabilities of TOPAS in simulating beam delivery in 3D and 4D. We have demonstrated TOPAS accuracy and usability in a variety of proton therapy setups. As we are preparing to make this tool freely available for researchers in medical physics, we anticipate widespread use of this tool in the growing proton therapy community.

Tbx3 represses PTEN and is over-expressed in head and neck squamous cell carcinoma

BackgroundDespite advances in diagnostic and treatment strategies, head and neck squamous cell cancer (HNSCC) constitutes one of the worst cancer types in terms of prognosis. PTEN is one of the tumour suppressors whose expression and/or activity have been found to be reduced in HNSCC, with rather low rates of mutations within the PTEN gene (6-8%). We reasoned that low expression levels of PTEN might be due to a transcriptional repression governed by an oncogene. Tbx2 and Tbx3, both of which are transcriptional repressors, have been found to be amplified or over-expressed in various cancer types. Thus, we hypothesize that Tbx3 may be over expressed in HNSCC and may repress PTEN, thus leading to cancer formation and/or progression.MethodsUsing immunohistochemistry and quantitative PCR (qPCR), protein and mRNA levels of PTEN and Tbx3 were identified in samples excised from cancerous and adjacent normal tissues from 33 patients who were diagnosed with HNSCC. In addition, HeLa and HEK cell lines were transfected with a Tbx3 expressing plasmid and endogenous PTEN mRNA and protein levels were determined via qPCR and flow cytometry. Transcription assays were performed to demonstrate effects of Tbx3 on PTEN promoter activity. Mann–Whitney, Spearman’s Correlation and Wilcoxon signed-rank tests were used to analyze the data.ResultsWe demonstrate that in HNSCC samples, Tbx3 mRNA levels are increased with respect to their normal tissue counterparts (p<0.001), whereas PTEN mRNA levels are significantly reduced in cancer tissues. Moreover, Tbx3 protein is also increased in HNSCC tissue sections. Over-expression of Tbx3 in HeLa and HEK cell lines causes reduction in endogenous PTEN mRNA and protein levels. In addition, transcription activity assays reveal that Tbx3 is capable of repressing both the basal and induced promoter activity of PTEN.ConclusionsWe show that Tbx3 is up-regulated in tissue samples of HNSCC patients and that Tbx3 represses PTEN transcription. Thus, our data not only reveals a new mechanism that may be important in cancer formation, but also suggests that Tbx3 can be used as a potential biomarker in cancer.

Development of a Geant4 based Monte Carlo Algorithm to evaluate the MONACO VMAT treatment accuracy

A method to evaluate the dosimetric accuracy of volumetric modulated arc therapy (VMAT) treatment plans, generated with the MONACOTM (version 3.0) treatment planning system in realistic CT-data with an independent Geant4 based dose calculation algorithm is presented.Therefore a model of an Elekta Synergy linear accelerator treatment head with an MLCi2 multileaf collimator was implemented in Geant4. The time dependent linear accelerator components were modeled by importing either logfiles of an actual plan delivery or a DICOM-RT plan sequence. Absolute dose calibration, depending on a reference measurement, was applied. The MONACO as well as the Geant4 treatment head model was commissioned with lateral profiles and depth dose curves of square fields in water and with film measurements in inhomogeneous phantoms. A VMAT treatment plan for a patient with a thoracic tumor and a VMAT treatment plan of a patient, who received treatment in the thoracic spine region including metallic implants, were used for evaluation.MONACO, as well as Geant4, depth dose curves and lateral profiles of square fields had a mean local gamma (2%, 2mm) tolerance criteria agreement of more than 95% for all fields. Film measurements in inhomogeneous phantoms with a global gamma of (3%, 3mm) showed a pass rate above 95% in all voxels receiving more than 25% of the maximum dose. A dose-volume-histogram comparison of the VMAT patient treatment plans showed mean deviations between Geant4 and MONACO of -0.2% (first patient) and 2.0% (second patient) for the PTVs and (0.5±1.0)% and (1.4±1.1)% for the organs at risk in relation to the prescription dose.The presented method can be used to validate VMAT dose distributions generated by a large number of small segments in regions with high electron density gradients. The MONACO dose distributions showed good agreement with Geant4 and film measurements within the simulation and measurement errors.

Monte Carlo computed machine-specific correction factors for reference dosimetry of TomoTherapy static beam for several ion chambers

To determine k(Q(msr),Q(o) ) (f(msr),f(o) ) correction factors for machine-specific reference (msr) conditions by Monte Carlo (MC) simulations for reference dosimetry of TomoTherapy static beams for ion chambers Exradin A1SL, A12; PTW 30006, 31010 Semiflex, 31014 PinPoint, 31018 microLion; NE 2571. For the calibration of TomoTherapy units, reference conditions specified in current codes of practice like IAEA∕TRS-398 and AAPM∕TG-51 cannot be realized. To cope with this issue, Alfonso et al. [Med. Phys. 35, 5179-5186 (2008)] described a new formalism introducing msr factors k(Q(msr),Q(o) ) (f(msr),f(o) ) for reference dosimetry, applicable to static TomoTherapy beams. In this study, those factors were computed directly using MC simulations for Q(0) corresponding to a simplified (60)Co beam in TRS-398 reference conditions (at 10 cm depth). The msr conditions were a 10 × 5 cm(2) TomoTherapy beam, source-surface distance of 85 cm and 10 cm depth. The chambers were modeled according to technical drawings using the egs++ package and the MC simulations were run with the egs_chamber user code. Phase-space files used as the source input were produced using PENELOPE after simulation of a simplified (60)Co beam and the TomoTherapy treatment head modeled according to technical drawings. Correlated sampling, intermediate phase-space storage, and photon cross-section enhancement variance reduction techniques were used. The simulations were stopped when the combined standard uncertainty was below 0.2%. Computed k(Q(msr),Q(o) ) (f(msr),f(o) ) values were all close to one, in a range from 0.991 for the PinPoint chamber to 1.000 for the Exradin A12 with a statistical uncertainty below 0.2%. Considering a beam quality Q defined as the TPR(20,10) for a 6 MV Elekta photon beam (0.661), the additional correction k(Q(msr,)Q) (f(msr,)f(ref) ) to k(Q,Q(o) ) defined in Alfonso et al. [Med. Phys. 35, 5179-5186 (2008)] formalism was in a range from 0.997 to 1.004. The MC computed factors in this study are in agreement with measured factors for chamber types already studied in literature. This work provides msr correction factors for additional chambers used in reference dosimetry. All of them were close to one (within 1%).

The use of non-standard CT conversion ramps for Monte Carlo verification of 6 MV prostate IMRT plans

Monte Carlo (MC) dose calculation algorithms have been widely used to verify the accuracy of intensity-modulated radiotherapy (IMRT) dose distributions computed by conventional algorithms due to the ability to precisely account for the effects of tissue inhomogeneities and multileaf collimator characteristics. Both algorithms present, however, a particular difference in terms of dose calculation and report. Whereas dose from conventional methods is traditionally computed and reported as the water-equivalent dose (Dw), MC dose algorithms calculate and report dose to medium (Dm). In order to compare consistently both methods, the conversion of MC Dm into Dw is therefore necessary.This study aims to assess the effect of applying the conversion of MC-based Dm distributions to Dw for prostate IMRT plans generated for 6 MV photon beams. MC phantoms were created from the patient CT images using three different ramps to convert CT numbers into material and mass density: a conventional four material ramp (CTCREATE) and two simplified CT conversion ramps: (1) air and water with variable densities and (2) air and water with unit density. MC simulations were performed using the BEAMnrc code for the treatment head simulation and the DOSXYZnrc code for the patient dose calculation. The conversion of Dm to Dw by scaling with the stopping power ratios of water to medium was also performed in a post-MC calculation process.The comparison of MC dose distributions calculated in conventional and simplified (water with variable densities) phantoms showed that the effect of material composition on dose-volume histograms (DVH) was less than 1% for soft tissue and about 2.5% near and inside bone structures. The effect of material density on DVH was less than 1% for all tissues through the comparison of MC distributions performed in the two simplified phantoms considering water. Additionally, MC dose distributions were compared with the predictions from an Eclipse treatment planning system (TPS), which employed a pencil beam convolution (PBC) algorithm with Modified Batho Power Law heterogeneity correction. Eclipse PBC and MC calculations (conventional and simplified phantoms) agreed well (<1%) for soft tissues. For femoral heads, differences up to 3% were observed between the DVH for Eclipse PBC and MC calculated in conventional phantoms. The use of the CT conversion ramp of water with variable densities for MC simulations showed no dose discrepancies (0.5%) with the PBC algorithm. Moreover, converting Dm to Dw using mass stopping power ratios resulted in a significant shift (up to 6%) in the DVH for the femoral heads compared to the Eclipse PBC one.Our results show that, for prostate IMRT plans delivered with 6 MV photon beams, no conversion of MC dose from medium to water using stopping power ratio is needed. In contrast, MC dose calculations using water with variable density may be a simple way to solve the problem found using the dose conversion method based on the stopping power ratio.

MO-F-213AB-03: Potential Reduction in Out-Of-Field Dose in Pencil Beam Scanning Proton Therapy Through Use of a Patient-Specific Aperture.

Patient specific apertures are commonly employed in passive double scattering (DS) proton therapy (PT). This study was aimed at identifying the potential benefits of using such an aperture in pencil beam scanning (PBS). An accurate Geant4 Monte Carlo model of the PBS PT treatment head at Massachusetts General Hospital (MGH) was developed based on an existing model of the passive double-scattering (DS) system. The Monte Carlo code specifies the treatment head at MGH with sub-millimeter accuracy and was configured based on the results of experimental measurements performed at MGH. This model was then used to compare out-of-field doses in simulated DS treatments and PBS treatments. The PBS treatments were simulated both with and without the patient-specific aperture used in the DS treatment. For the conditions explored, a typical prostate field, the lateral penumbra in PBS is wider than in DS, leading to higher absorbed doses and equivalent doses adjacent to the primary field edge. For lateral distances greater than 10cm from the field edge, the doses in PBS appear to be lower than those observed for DS. Including an aperture at nozzle exit reduces the penumbral width by preventing wide-angle scatter from reaching the patient. This can reduce the dose in PBS for lateral distances of less than 10cm from the field edge by over an order of magnitude and allow better dose conformity. Placing a patient-specific aperture at nozzle exit during PBS treatments can potentially reduce doses lateral to the primary radiation field by over an order of magnitude. This has the potential to further improve the normal tissue sparing capabilities of PBS. The magnitude of this effect depends on the beam spot size of the scanning system and is thus facility dependent.

SU‐C‐213CD‐01: Automated Marker Tracking for Intra‐Fractional Image Guidance in Radiotherapy

Purpose: We have developed two automated methods for real‐time detection of intra‐fractional target motion during treatment with the aid of metallic markers based on ‘in‐line’ acquired x‐ray images. Fast detection algorithms are desirable to facilitate tumor tracking applications. Methods: A Siemens linac was equipped with a 16×16″ a‐Si flat panel detector (FPD) underneath the treatment head. A diagnostic x‐ray tube was positioned opposite to the treatment head; geometrically separating MV treatment and kV imaging fields on the FPD. We analyzed x‐ray images of a lung phantom equipped with a marker for better contrast. For the automated marker detection we developed two algorithms as part of our image acquisition application. Both algorithms are based on gray‐value template matching with normalized cross‐correlation (NCC). One of the algorithms pre‐ calculates suitable candidate regions by employing a canny edge detection filter. In this study, we wanted to a) study the quality of template matching under the influence of MV induced image artifacts (e.g. stripe artifacts), and b) investigate if pre‐selection of candidate regions leads to reliable marker detection. Results: Our lung‐phantom study has demonstrated that matching a 21×21 pixel template over an 81×81 image with NCC takes about 9.9+−4.5 ms. This method is very reliable and works for a wide dose range. Using canny pre‐filtering reduces the latency to 1.1+−0.7 ms, but marker detection becomes less reliable at our lowest dose level (0.6 mAs). In the presence of an 8×8 cm̂2 MV field, the latencies remain unchanged but the correlation factors decrease sharply by up to 22%. We observed that MV induced stripe artifacts overlapping with the marker are the greatest hurdle to successful detection. Conclusions: While template‐matching over the entire image reliably finds the correct marker position, it is possible to accelerate the search for high contrast images with the help of canny filtering. Supported by Siemens Healthcare/CR.

SU-E-T-473: Performance Assessment of the TOPAS Tool for Particle Simulation for Proton Therapy Applications.

The TOPAS Tool for Particle Simulation was developed to make Geant4 Monte Carlo simulation more readily available for research and clinical physicists. Before releasing this new tool to the proton therapy community, several test have been performed to ensure accurate simulations in a variety of proton therapy setups. TOPAS can model a passive scattering or scanning beam treatment head, model a patient geometry based on CT images, score dose, fluence, etc., save and replay a phase space, provides advanced graphics, and is fully four-dimensional (4D) to handle variations in beam delivery and patient geometry during treatment. An innovative control system meets requirements for ease of use, reliability and repeatability without sacrificing flexibility. To test the TOPAS code, we modeled proton therapy treatment examples including the UCSF eye treatment beamline (UCSFETB), the MGH STAR radiosurgery beamline and the MGH gantry treatment head in passive scattering and scanning modes. The simulations included time-dependent geometry and time- dependent beam current delivery. At the UCSFETB, time- dependent depth dose distributions were accurately simulated with time- varying energy modulation from a rotating propeller. At the MGH STAR beamline, distal and proximal ranges agreed within measurement uncertainty and the shape of the simulated SOBP followed measured data. For the MGH gantry treatment head in passive scattering mode, SOBPs were simulated for the full set of range modulator wheel and second scatterer combinations. TOPAS simulation was within clinical required accuracy. For the MGH nozzle in scanning mode, a variety of scan patterns were simulated with fluence maps generated for cases including beam current modulation, energy modulation and target tracking. Our results demonstrate the functionality of TOPAS. They show agreement with measured data and demonstrate the capabilities of TOPAS in simulating beam delivery in 3D and 4D. This work was supported by IH/NCI under R01 CA 140735-01.

SU-E-T-478: Geometrical Splitting Technique to Improve the Computational Efficiency in Monte Carlo Calculations for Proton Therapy.

To implement a geometry based particle splitting technique in order to reduce the computation time when generating treatment head phase space files for proton therapy dose calculations using Monte Carlo (MC) calculations and to validate the doses generated from these phase spaces with respect to reference simulations. The treatment nozzles at the Francis H Burr Proton Therapy Center (FHBPTC) were modeled with a new MC tool ('TOPAS' based on Geant4). For variance reduction purposes, two particle-splitting planes were implemented, one downstream of the second ionization chamber the other upstream of the aperture of the nozzle and phase spaces in IAEA format were recovered. The symmetry of the proton beam was considered to split the particles by a factor of 4 per plane. Split particles were randomly positioned at different locations rotated around the beam axis. The computational efficiency was calculated and dose profiles compared for a voxelized water phantom for different treatment fields for both the reference and optimized simulations. Depth-dose curves and beam profiles were analyzed. Dose calculation in patients was simulated to compare the performance. Normalized computational efficiency between 10 and 14.5 were reached. Percentage difference between dose profiles in water for simulations done with and without particle splitting is within the statistical precision of 2%, 1 standard deviation. Dose distributions for the realistic patient treatment show differences up to 4% in the regions of interest, within 2 standard deviations. By considering the cylindrically symmetric region of the nozzle and the splitting planes separated at strategic distance, considerable time reduction can be achieved without compromising the precision. This approach will reduce the time for phase space simulations for clinical MC dose calculation at FHBPTC by more than a factor of 10.

TH‐C‐BRA‐12: Tomographic Measurement of X‐Ray Beam Spot Profiles Using a Rotating Edge

Purpose: X‐ray beam spot size and shape are critical performance determinants of an imaging or treatment system. However, quantitative assessment of such spot profiles can prove difficult, particularly at MV energies. We have developed a novel and convenient tomographic spot measurement technique that uses a rotating edge phantom. The method can be applied to x‐ray systems equipped with flat panel imagers. Methods: Data were acquired at 10MV and 6MV on a Varian TrueBeam system. A 0.5 mm thick tantalum sheet (attenuation ∼5%) was placed on the system's rotatable treatment head with an edge abutting the axis of rotation. A total of 144 projections, each 10MU, were acquired at 2.5 degree steps. For each projection, a line‐spread function (LSF) is generated by differentiating the measured edge spread function. For sufficiently high source magnifications, the LSF, taken at a given rotation angle, is a tomographic projection of the x‐ray beam spot at that angle. The LSF's were then assembled into a sinogram and the corresponding spot profile was reconstructed using a parallel‐beam CT algorithm. The reconstructed profiles were compared to those measured using a film‐based ‘spot camera’ made from a 15 cm thick tungsten cylinder pierced with an array of small holes. Monte Carlo simulations of the edge and the spot camera experiments were also performed. Results: The edge technique produced profiles similar to those from the spot camera yet with higher resolution (0.17mm vs. 0.25mm). These results were confirmed by Monte Carlo simulations. The measured FWHM of the 10 MV spot was 1.6 mm. The 6 MV spot was slightly asymmetric with an average FWHM of 1.5 mm. Conclusions: A thin rotating edge with low attenuation can be used to accurately and conveniently measure x‐ray beam spot profiles. NIH NIH 1R01CA138426 Employees of Varian Medical Systems

SU-E-T-289: On the Use of a Diode Array for the Commissioning of Dynamically-Wedged Asymmetric Fields Generated by Varian EDW's in the Pinnacle Treatment Planning System.

To present a validation study of enhanced dynamic wedges (EDWs) implemented into Pinnacle3 treatment planning system (TPS) using a diode array, Monte Carlo (MC) simulation, and measurements. Modeling of EDW dose distribution in the Pinnacle3 TPS is based on a combination of open-field beam data and the Varian 'Golden Segmented Treatment Table' (GSTT) unique to each photon beam. To validate the EDW models, dose profiles of 6- and 10-MV photon beams from a Clinac 2100C/D were measured in virtual water at depths from near- surface to 30 cm for a wide range of asymmetric field sizes and wedge angles using the Profiler-2 diode array system. The EDW output factors for asymmetric fields were measured in solid water using a small-volume cylindrical ionization chamber placed at a depth of 10 cm on the central axis. In addition, absolute doses on central axis at depths of 5, 10, 15 and 20 cm were measured in virtual water using a small-volume cylindrical chamber for comparison with the collapsed-cone convolution (CCC) calculations. The 6- and 10-MV photon beams emerging from the treatment head of the Clinac 2100C/D were fully simulated using the Monte Carlo code BEAMnrc and DOSXYZnrc. This code was used to calculate the central-axis percentage depth doses and dose profiles for the open and dynamically-wedged fields in a water phantom. The off-axis dose profiles of various EDWs computed with the CCC dose model and MC simulations agreed with measured EDW dose distributions to an accuracy of better than 2%/2 mm. Measured EDW output factors used for MU calculations in Pinnacle3 TPS agreed with MC predictions within 1-2%. The utility of Profiler-2 diode array system to measure dose profiles for commissioning of EDWs into the Pinnacle3 TPS has been demonstrated to have acceptable accuracy for clinical implementation.

MO-A-213AB-07: Evaluation of Distal Dose Surface with In-Room PET for Proton Therapy Monitoring.

The objective of this study is to evaluate the feasibility of proton beam treatment verification using in-room PET. As of February 2012, four patients have been studied in a clinical trial. In addition, we suggest a new method comparing the distal surface of the measured and simulated PET activities to verify the location of the distal dose surface. Patients were scanned for 20 minutes with an in-room PET positioned next to the proton treatment head in a gantry room for beam delivery using passive scattering. The time between end of treatment and the start of the scan was within about 2 minutes. The predicted distribution of the PET activities and the proton dose distributions in the patients were also calculated using Monte Carlo (MC). Along the beam direction, the 50% fall-off positions of the maximum PET activity at each line profile were compared with the MC simulated and the measured PET images, and then the differences were assessed with root-mean-square deviation (RMSD) and mapped in the beam's eye view. The measured PET images showed a good spatial correlation with the simulated PET images and the proton dose distributions even though the treated volumes and locations varied between patients. The RMSD values, representing the surface differences between the measured and simulated PET, were assessed to be 4.3-5.1 mm for four patients. Some region including the penumbra showed larger differences but was excluded. We have explored the potential of the in-room PET for proton therapy monitoring through a clinical trial. The PET image analysis method based on MC simulations showed that the distal dose surface could be determined within a few millimeters but not within the aimed accuracy of 2-3 mm. Improvements in PET-CT image registration and biological washout modeling will most likely increase the accuracy further. NIH/NCI P01 CA021239.

SU‐C‐213AB‐03: Monte Carlo Computed KQmsr Correction Factors for Reference Dosimetry of Tomotherapy Beams for 7 Ion Chambers

Purpose: To compute kQmsr correction factors for various types of ion chamber (Exradin A1SL, A12; PTW 30006, 30010, 31014, MicroLion and NE‐2571) using Monte Carlo (MC) simulations according to the new formalism of Alfonso et al (Med. Phys. 35 2008) Methods: TomoTherapy units are not able to realize reference conditions specified in current codes of practice like IAEA/TRS‐398 and AAPM/TG‐51. To cope with this issue, Alfonso et al described a new formalism introducing machine‐specific (msr) factors kQmsr for reference dosimetry, applicable to static TomoTherapy beams. In this study, those factors were computed directly using MC simulations for Q0 corresponding to a simplified 60Co beam in TRS‐398 reference conditions (at 10 cm depth). Msr conditions were a 10×5 cm2 TomoTherapy beam, source‐surface distance of 85 cm and 10 cm depth. The chambers were modeled according to technical drawings using the egs++ package and the MC simulations were run with the egschamber user code. Phase‐space files used as the source input were produced using Penelope after simulation of a simplified 60Co beam and the TomoTherapy treatment head modeled according to technical drawings. Correlated sampling, intermediate phase‐space storage and photon cross‐section enhancement variance reduction techniques were used. The simulations were stopped when the combined standard uncertainty was below 0.2%. Results: All correction factors were close to one, from 0.991 for the PTW 31014 to 1.000 for Exradin A12 (+− 0.002). The factor obtained for Exradin A1SL (0.997 +− 0.002) confirmed previous findings. This work provides msr correction factors for additional chambers used in reference dosimetry. Conclusions: Global msr correction factors obtained in this study can be used directly for reference dosimetry of static tomotherapy beams. In the Alfonso et al formalism, intermediate factors are introduced to link the new formalism to existing protocols. Those intermediate factors are not needed here. This work has been partially supported by Accuray. Thomas Rockwell Mackie is a consultant of Accuray and has therefore a potential conflict of interest.

Monte Carlo study of the potential reduction in out-of-field dose using a patient-specific aperture in pencil beam scanning proton therapy

This study is aimed at identifying the potential benefits of using a patient-specific aperture in proton beam scanning. For this purpose, an accurate Monte Carlo model of the pencil beam scanning (PBS) proton therapy (PT) treatment head at Massachusetts General Hospital (MGH) was developed based on an existing model of the passive double-scattering (DS) system. The Monte Carlo code specifies the treatment head at MGH with sub-millimeter accuracy. The code was configured based on the results of experimental measurements performed at MGH. This model was then used to compare out-of-field doses in simulated DS treatments and PBS treatments. For the conditions explored, the penumbra in PBS is wider than in DS, leading to higher absorbed doses and equivalent doses adjacent to the primary field edge. For lateral distances greater than 10 cm from the field edge, the doses in PBS appear to be lower than those observed for DS. We found that placing a patient-specific aperture at nozzle exit during PBS treatments can potentially reduce doses lateral to the primary radiation field by over an order of magnitude. In conclusion, using a patient-specific aperture has the potential to further improve the normal tissue sparing capabilities of PBS.

Generation of a novel phase‐space‐based cylindrical dose kernel for IMRT optimization

Improving dose calculation accuracy is crucial in intensity-modulated radiation therapy (IMRT). We have developed a method for generating a phase-space-based dose kernel for IMRT planning of lung cancer patients. Particle transport in the linear accelerator treatment head of a 21EX, 6 MV photon beam (Varian Medical Systems, Palo Alto, CA) was simulated using the EGSnrc/BEAMnrc code system. The phase space information was recorded under the secondary jaws. Each particle in the phase space file was associated with a beamlet whose index was calculated and saved in the particle's LATCH variable. The DOSXYZnrc code was modified to accumulate the energy deposited by each particle based on its beamlet index. Furthermore, the central axis of each beamlet was calculated from the orientation of all the particles in this beamlet. A cylinder was then defined around the central axis so that only the energy deposited within the cylinder was counted. A look-up table was established for each cylinder during the tallying process. The efficiency and accuracy of the cylindrical beamlet energy deposition approach was evaluated using a treatment plan developed on a simulated lung phantom. Profile and percentage depth doses computed in a water phantom for an open, square field size were within 1.5% of measurements. Dose optimized with the cylindrical dose kernel was found to be within 0.6% of that computed with the nontruncated 3D kernel. The cylindrical truncation reduced optimization time by approximately 80%. A method for generating a phase-space-based dose kernel, using a truncated cylinder for scoring dose, in beamlet-based optimization of lung treatment planning was developed and found to be in good agreement with the standard, nontruncated scoring approach. Compared to previous techniques, our method significantly reduces computational time and memory requirements, which may be useful for Monte-Carlo-based 4D IMRT or IMAT treatment planning.