Linac Photon Beam Research Articles

TU‐AB‐BRC‐11: Moving a GPU‐OpenCL‐Based Monte Carlo (MC) Dose Engine Towards Routine Clinical Use: Automatic Beam Commissioning and Efficient Source Sampling

Purpose:We have previously developed a GPU‐OpenCL‐based MC dose engine named goMC with built‐in analytical linac beam model. To move goMC towards routine clinical use, we have developed an automatic beam‐commissioning method, and an efficient source sampling strategy to facilitate dose calculations for real treatment plans.Methods:Our commissioning method is to automatically adjust the relative weights among the sub‐sources, through an optimization process minimizing the discrepancies between calculated dose and measurements. Six models built for Varian Truebeam linac photon beams (6MV, 10MV, 15MV, 18MV, 6MVFFF, 10MVFFF) were commissioned using measurement data acquired at our institution. To facilitate dose calculations for real treatment plans, we employed inverse sampling method to efficiently incorporate MLC leaf‐sequencing into source sampling. Specifically, instead of sampling source particles control‐point by control‐point and rejecting the particles blocked by MLC, we assigned a control‐point index to each sampled source particle, according to MLC leaf‐open duration of each control‐point at the pixel where the particle intersects the iso‐center plane.Results:Our auto‐commissioning method decreased distance‐to‐agreement (DTA) of depth dose at build‐up regions by 36.2% averagely, making it within 1mm. Lateral profiles were better matched for all beams, with biggest improvement found at 15MV for which root‐mean‐square difference was reduced from 1.44% to 0.50%. Maximum differences of output factors were reduced to less than 0.7% for all beams, with largest decrease being from1.70% to 0.37% found at 10FFF. Our new sampling strategy was tested on a Head&Neck VMAT patient case. Achieving clinically acceptable accuracy, the new strategy could reduce the required history number by a factor of ∼2.8 given a statistical uncertainty level and hence achieve a similar speed‐up factor.Conclusion:Our studies have demonstrated the feasibility and effectiveness of our auto‐commissioning approach and new efficient source sampling strategy, implying the potential of our GPU‐based MC dose engine goMC for routine clinical use.

Small Volume Ionization Chambers Angular Dependence and Its Influence on Point-Dose Measurements

Purpose: Cylindrical ionization chambers as the PinPoint and the CC01 have high-spatial resolution and cylindrical symmetry in relevance to small fields. Dosimetry of newly advanced technologies that include non-coplanar beams, such as in Gamma-Knife, VMAT and IMRT treatment plans, the angular dependence of the ion chamber is essential. Therefore in this study we simulated two common used ion chambers using a Monte Carlo method. Methods: The angular dependence for the CC01 and the PinPoint were calculated for a vast angular range in a cylindrical water phantom for two LINAC photon beams, 6 MV and 15 MV. The calculations were performed for fully structural description of each ion chamber using Monte Carlo code, in order to reveal unsymmetrical response dose higher than 2% in medical applications. The calculated dose response, for the CC01 ion chamber, was validated with measurements. Results: The CC01 ion chamber showed a very small angular dependence compared to the PinPoint ion chamber. The relative angular dependence amplitude of the PinPoint was about 12% in the range of 0° - 110°. Conclusions: The differences in the ion chambers structure led to the variations in the relative angular dependence amplitude. Therefore, when choosing a detector, not only the volume of the ionization chamber but also its structure must be taken into consideration.

Angular independent silicon detector for dosimetry in external beam radiotherapy.

In this work, the "edgeless" silicon detector technology is investigated, in combination with an innovative packaging solution, to manufacture silicon detectors with negligible angular response. The new diode is also characterized as a dosimeter for radiotherapy with the aim to verify its suitability as a single detector for in vivo dosimetry as well as large area 2D array that does not require angular correction to their response. For the characterisation of the "edgeless-drop-in" detector technology, a set of samples have been manufactured with different sensitive areas (1 × 1 and 0.5 × 0.5 mm(2)) and different thicknesses (0.1 and 0.5 mm) in four different combinations of top and peripheral p-n junction fabricated on p-type and n-type silicon substrates. The diode probes were tested in terms of percentage depth dose (PDD), dose rate, and linearity and compared to ion chambers. Measurements of the output factor have been compared to film. The angular response of the diodes probes has been tested in a cylindrical PMMA phantom, rotated with bidirectional accuracy of 0.25° under 10 × 10 cm(2) 6 MV Linac photon beam. The radiation hardness has been investigated as well as the effect of radiation damage on the angular and dose rate response of the diode probes when irradiated with photons from a Co-60 gamma source up to dose of 40 kGy. The PDDs measured by the edgeless detectors show an agreement with the data obtained using ion chambers within ±2%. The output factor measured with the smallest area edgeless diodes (0.5 × 0.5 mm(2)-0.1 and 0.5 mm thick) matches EBT3 film to within 2% for square field size from 10 to 0.5 cm side equivalent distance. The dose rate dependence in a dose per pulse range of 0.9 × 10(-5)-2.7 × 10(-4) Gy/pulse was less than -7% and +300% for diodes fabricated on p-type and n-type substrates, respectively. The edgeless diodes fabricated on the p-type substrate demonstrated degradation of the response as a function of the irradiation dose within 5%-15%, while diodes on the n-type substrate show a variation of approximately 30% after 40 kGy. The angular response of all probes is minimal (within 2%) but the N on N and P on P configurations show the best performances with an angular dependence of ±1.0% between 0° and 180° in the transversal direction. In this configuration, the space charge region of the passive diode extends from the behind and sidewall toward the anode on the top providing beneficial electric field distribution in the peripheral area of the diode. Such performance has also been tested after irradiation by Co-60 up to 40 kGy with no measurable change in angular response. A new edgeless-drop-in silicon diode fabrication and packaging technology has been used to develop detectors that show no significant angular dependence in their response for dosimetry in radiation therapy. From the characterisation of the diodes, proposed in a wide range of different geometries and configurations, the authors recommend the P-on-P detectors in conjunction with "drop in" packaging technology as the candidate for further development as single diode probe or 2D diode array for dosimetry in radiotherapy.

TH-CD-304-06: Amplitude and Frequency Characteristics of Acoustic Waves Induced Following Radiotherapy Linear Accelerator Irradiation

Purpose: To enhance understanding of the characteristics of acoustic waves induced following irradiation by a linac photon beam through experiments and simulations. Such waves have potential applications in in vivo dosimetry since their properties are related to the dose distribution following a single pulse of irradiation. Methods: The acoustic waves induced following linac photon beam irradiation of different metal blocks suspended in a water tank were investigated. Experimentally, these waves were detected with an ultrasound transducer and amplification electronics. Computationally, the acoustic waves were modeled by combining Monte Carlo dose calculations and acoustic wave transport techniques. The effect of varying metal block size, material, depth, and linac photon beam energy on the amplitude and frequency of the induced acoustic signal was investigated. Results: Experimental and simulated acoustic waveforms indicated that increasing photon beam energy and decreasing metal block depth along the central axis increases signal amplitude by an amount proportional to the increased dose deposited in the block. Decreasing in-plane lead block width increased the frequency of acoustic waves due to the smaller dimension of the lead-induced perturbation in the global dose distribution. Varying the metal block material from lead to aluminum resulted in smaller amplitude acoustic waves due to aluminum’s lower electron density, while aluminum’s higher speed of sound resulted in higher frequency waves. Both simulated and experimental waveforms showed similar trends in signal amplitude and frequency when set-up variables were altered. Conclusion: This study revealed factors that affect the amplitude and frequency of radiation-induced acoustic waves and their relation to the deposited dose characteristics. The agreement between simulated and experimental waveforms verifies the simulation workflow’s ability to model such acoustic waves, and indicates that it could be utilized to explore more complex irradiation scenarios and potentially aid in the development of experimental detection systems for in vivo dosimetry. This work was supported by NSERC and CIHR grants. S.H. acknowledges support by the NSERC CREATE Medical Physics Research Training Network grant 432290.

TH‐E‐304‐03: Absorbed Dose Standards for Brachytherapy

It is fair to say that the clinical determination of absorbed dose in a high‐energy photon beam is the only situation where there is currently a direct chain of traceability from the standards laboratory to the end‐user and the protocol requires only ‘simple’ conversion/ correction factors. For all other modalities the primary standard is in a different quantity (e.g., air kerma) or in the right quantity but a different radiation beam (e.g., for proton dosimetry) from what is required for the clinical dose delivery. Complex conversion factors are often then required in applying protocols to give the desired final quantity. Such conversion factors can be from kerma to dose (in the case of kilovoltage x‐rays or brachytherapy) or from one type of radiation beam to another (in the case of electron or proton beams). Even non‐standard MV photon beams (e.g. non‐standard geometry, presence of a magnetic field) can have a significant impact on the procedure that must be followed compared to what is found in TG‐51. While national and international bodies, such as the AAPM and the IAEA, primarily address the end‐user's needs with protocols and guidelines, calibration laboratories are developing new measurement standards and approaches that should simplify the calibration chain and therefore lead to simpler, more robust procedures to yield absorbed dose in the beam of interest in the radiation therapy clinic.This session will provide a broad overview of the worldwide developments currently in progress, and also planned for the near future, of absorbed dose standards for a range of beam modalities including: Proton beamsHDR and LDR brachytherapyNon‐standard photon fields (small fields, impact of magnetic fields, etc)Learning Objectives: Understand the present traceability chain for all beam modalities in regular use in the radiation therapy clinic. Understand the current and planned development of absorbed dose standards for modalities other than linac photon beams. Understand the physical basis of these standards and the potential impact on end‐user dosimetry. Understand the likely impact of these standards developments on dosimetry protocols in the coming 5–10 years.

TH‐E‐304‐04: Standards for Non‐Standard Photon Beams

It is fair to say that the clinical determination of absorbed dose in a high‐energy photon beam is the only situation where there is currently a direct chain of traceability from the standards laboratory to the end‐user and the protocol requires only ‘simple’ conversion/ correction factors. For all other modalities the primary standard is in a different quantity (e.g., air kerma) or in the right quantity but a different radiation beam (e.g., for proton dosimetry) from what is required for the clinical dose delivery. Complex conversion factors are often then required in applying protocols to give the desired final quantity. Such conversion factors can be from kerma to dose (in the case of kilovoltage x‐rays or brachytherapy) or from one type of radiation beam to another (in the case of electron or proton beams). Even non‐standard MV photon beams (e.g. non‐standard geometry, presence of a magnetic field) can have a significant impact on the procedure that must be followed compared to what is found in TG‐51. While national and international bodies, such as the AAPM and the IAEA, primarily address the end‐user's needs with protocols and guidelines, calibration laboratories are developing new measurement standards and approaches that should simplify the calibration chain and therefore lead to simpler, more robust procedures to yield absorbed dose in the beam of interest in the radiation therapy clinic.This session will provide a broad overview of the worldwide developments currently in progress, and also planned for the near future, of absorbed dose standards for a range of beam modalities including: Proton beamsHDR and LDR brachytherapyNon‐standard photon fields (small fields, impact of magnetic fields, etc)Learning Objectives: Understand the present traceability chain for all beam modalities in regular use in the radiation therapy clinic. Understand the current and planned development of absorbed dose standards for modalities other than linac photon beams. Understand the physical basis of these standards and the potential impact on end‐user dosimetry. Understand the likely impact of these standards developments on dosimetry protocols in the coming 5–10 years.

TH‐E‐304‐00: Reference Dosimetry for Beam Modalities Other Than MV Photons

It is fair to say that the clinical determination of absorbed dose in a high‐energy photon beam is the only situation where there is currently a direct chain of traceability from the standards laboratory to the end‐user and the protocol requires only ‘simple’ conversion/ correction factors. For all other modalities the primary standard is in a different quantity (e.g., air kerma) or in the right quantity but a different radiation beam (e.g., for proton dosimetry) from what is required for the clinical dose delivery. Complex conversion factors are often then required in applying protocols to give the desired final quantity. Such conversion factors can be from kerma to dose (in the case of kilovoltage x‐rays or brachytherapy) or from one type of radiation beam to another (in the case of electron or proton beams). Even non‐standard MV photon beams (e.g. non‐standard geometry, presence of a magnetic field) can have a significant impact on the procedure that must be followed compared to what is found in TG‐51. While national and international bodies, such as the AAPM and the IAEA, primarily address the end‐user's needs with protocols and guidelines, calibration laboratories are developing new measurement standards and approaches that should simplify the calibration chain and therefore lead to simpler, more robust procedures to yield absorbed dose in the beam of interest in the radiation therapy clinic.This session will provide a broad overview of the worldwide developments currently in progress, and also planned for the near future, of absorbed dose standards for a range of beam modalities including: Proton beamsHDR and LDR brachytherapyNon‐standard photon fields (small fields, impact of magnetic fields, etc)Learning Objectives: Understand the present traceability chain for all beam modalities in regular use in the radiation therapy clinic. Understand the current and planned development of absorbed dose standards for modalities other than linac photon beams. Understand the physical basis of these standards and the potential impact on end‐user dosimetry. Understand the likely impact of these standards developments on dosimetry protocols in the coming 5–10 years.

SU‐E‐T‐781: Using An Electronic Portal Imaging Device (EPID) for Correlating Linac Photon Beam Energies

Purpose:Electronic portal imaging devices (EPIDs) have proven to be useful for measuring several parameters of interest in linear accelerator (linac) quality assurance (QA). The purpose of this project was to evaluate the feasibility of using EPIDs for determining linac photon beam energies.Methods:Two non‐clinical Varian TrueBeam linacs (Varian Medical Systems, Palo Alto, CA) with 6MV and 10MV photon beams were used to perform the measurements. The linacs were equipped with an amorphous silicon based EPIDs (aSi1000) that were used for the measurements. We compared the use of flatness versus percent depth dose (PDD) for predicting changes in linac photon beam energy. PDD was measured in 1D water tank (Sun Nuclear Corporation, Melbourne FL) and the profiles were measured using 2D ion‐chamber array (IC‐Profiler, Sun Nuclear) and the EPID. Energy changes were accomplished by varying the bending magnet current (BMC). The evaluated energies conformed with the AAPM TG142 tolerance of ±1% change in PDD.Results:BMC changes correlating with a ±1% change in PDD corresponded with a change in flatness of ∼1% to 2% from baseline values on the EPID. IC Profiler flatness values had the same correlation. We observed a similar trend for the 10MV beam energy changes. Our measurements indicated a strong correlation between changes in linac photon beam energy and changes in flatness. For all machines and energies, beam energy changes produced change in the uniformity (AAPM TG‐142), varying from ∼1% to 2.5%.Conclusions:EPID image analysis of beam profiles can be used to determine linac photon beam energy changes. Flatness‐based metrics or uniformity as defined by AAPM TG‐142 were found to be more sensitive to linac photon beam energy changes than PDD.Research funding provided by Varian Medical Systems. Dr. Sasa Mutic receives compensation for providing patient safety training services from Varian Medical Systems, the sponsor of this study.

TH‐E‐304‐02: Standards for Proton and Heavy Ion Beams

It is fair to say that the clinical determination of absorbed dose in a high‐energy photon beam is the only situation where there is currently a direct chain of traceability from the standards laboratory to the end‐user and the protocol requires only ‘simple’ conversion/ correction factors. For all other modalities the primary standard is in a different quantity (e.g., air kerma) or in the right quantity but a different radiation beam (e.g., for proton dosimetry) from what is required for the clinical dose delivery. Complex conversion factors are often then required in applying protocols to give the desired final quantity. Such conversion factors can be from kerma to dose (in the case of kilovoltage x‐rays or brachytherapy) or from one type of radiation beam to another (in the case of electron or proton beams). Even non‐standard MV photon beams (e.g. non‐standard geometry, presence of a magnetic field) can have a significant impact on the procedure that must be followed compared to what is found in TG‐51. While national and international bodies, such as the AAPM and the IAEA, primarily address the end‐user's needs with protocols and guidelines, calibration laboratories are developing new measurement standards and approaches that should simplify the calibration chain and therefore lead to simpler, more robust procedures to yield absorbed dose in the beam of interest in the radiation therapy clinic.This session will provide a broad overview of the worldwide developments currently in progress, and also planned for the near future, of absorbed dose standards for a range of beam modalities including: Proton beamsHDR and LDR brachytherapyNon‐standard photon fields (small fields, impact of magnetic fields, etc)Learning Objectives: Understand the present traceability chain for all beam modalities in regular use in the radiation therapy clinic. Understand the current and planned development of absorbed dose standards for modalities other than linac photon beams. Understand the physical basis of these standards and the potential impact on end‐user dosimetry. Understand the likely impact of these standards developments on dosimetry protocols in the coming 5–10 years.

TH‐E‐304‐01: Overview of Primary Standards for Reference Dosimetry

It is fair to say that the clinical determination of absorbed dose in a high‐energy photon beam is the only situation where there is currently a direct chain of traceability from the standards laboratory to the end‐user and the protocol requires only ‘simple’ conversion/ correction factors. For all other modalities the primary standard is in a different quantity (e.g., air kerma) or in the right quantity but a different radiation beam (e.g., for proton dosimetry) from what is required for the clinical dose delivery. Complex conversion factors are often then required in applying protocols to give the desired final quantity. Such conversion factors can be from kerma to dose (in the case of kilovoltage x‐rays or brachytherapy) or from one type of radiation beam to another (in the case of electron or proton beams). Even non‐standard MV photon beams (e.g. non‐standard geometry, presence of a magnetic field) can have a significant impact on the procedure that must be followed compared to what is found in TG‐51. While national and international bodies, such as the AAPM and the IAEA, primarily address the end‐user's needs with protocols and guidelines, calibration laboratories are developing new measurement standards and approaches that should simplify the calibration chain and therefore lead to simpler, more robust procedures to yield absorbed dose in the beam of interest in the radiation therapy clinic.This session will provide a broad overview of the worldwide developments currently in progress, and also planned for the near future, of absorbed dose standards for a range of beam modalities including: Proton beamsHDR and LDR brachytherapyNon‐standard photon fields (small fields, impact of magnetic fields, etc)Learning Objectives: Understand the present traceability chain for all beam modalities in regular use in the radiation therapy clinic. Understand the current and planned development of absorbed dose standards for modalities other than linac photon beams. Understand the physical basis of these standards and the potential impact on end‐user dosimetry. Understand the likely impact of these standards developments on dosimetry protocols in the coming 5–10 years.

SU‐E‐T‐451: Accuracy and Application of the Standard Imaging W1 Scintillator Dosimeter

Purpose:To evaluate the Standard Imaging W1 scintillator dosimeter in a range of clinical radiation beams to determine its range of possible applications.Methods:The W1 scintillator is a small perturbation‐free dosimeter which is of interest in absolute and relative clinical dosimetry due to its small size and water equivalence. A single version of this detector was evaluated in Co‐60 and linac photon and electron beams to investigate the following: linearity, sensitivity, precision, and dependence on electrometer type. In addition, depth‐dose and cross‐plane profiles were obtained in both photon and electron beams and compared with data obtained with wellbehaved ionization chambers.Results:In linac beams the precision and linearity was very impressive, with typical values of 0.3% and 0.1% respectively. Performance in a Co‐60 beam was much poorer (approximately three times worse) and it is not clear whether this is due to the lower signal current or the effect of the continuous beam (rather than pulsed beam of the linac measurements). There was no significant difference in the detector reading when using either the recommended SI Supermax electrometer or two independent high‐quality electrometers, except for low signal levels, where the Supermax exhibited an apparent threshold effect, preventing the measurement of the bremsstrahlung background in electron depth‐dose curves. Comparisons with ion chamber measurements in linac beams were somewhat variable: good agreement was seen for cross‐profiles (photon and electron beams) and electron beam depth‐dose curves, generally within the 0.3% precision of the scintillator but systematic differences were observed as a function of measurement depth in photon beam depth‐dose curves.Conclusion:A first look would suggest that the W1 scintillator has applications beyond small field dosimetry but performance appears to be limited to higher doserate and/or pulsed radiation beams. Further work is required to resolve discrepancies compared to ion chambers.

SU-E-T-578: MCEBRT, A Monte Carlo Code for External Beam Treatment Plan Verifications

Purpose: Present a new Monte Carlo code (MCEBRT) for patient-specific dose calculations in external beam radiotherapy. The code MLC model is benchmarked and real patient plans are re-calculated using MCEBRT and compared with commercial TPS. Methods: MCEBRT is based on the GEPTS system (Med. Phys. 29 (2002) 835–846). Phase space data generated for Varian linac photon beams (6 – 15 MV) are used as source term. MCEBRT uses a realistic MLC model (tongue and groove, rounded ends). Patient CT and DICOM RT files are used to generate a 3D patient phantom and simulate the treatment configuration (gantry, collimator and couch angles; jaw positions; MLC sequences; MUs). MCEBRT dose distributions and DVHs are compared with those from TPS in absolute way (Gy). Results: Calculations based on the developed MLC model closely matches transmission measurements (pin-point ionization chamber at selected positions and film for lateral dose profile). See Fig.1. Dose calculations for two clinical cases (whole brain irradiation with opposed beams and lung case with eight fields) are carried out and outcomes are compared with the Eclipse AAA algorithm. Good agreement is observed for the brain case (Figs 2-3) except at the surface where MCEBRT dose can be higher by 20%. This is due to better modeling of electron contamination by MCEBRT. For the lung case an overall good agreement (91% gamma index passing rate with 3%/3mm DTA criterion) is observed (Fig.4) but dose in lung can be over-estimated by up to 10% by AAA (Fig.5). CTV and PTV DVHs from TPS and MCEBRT are nevertheless close (Fig.6). Conclusion: A new Monte Carlo code is developed for plan verification. Contrary to phantombased QA measurements, MCEBRT simulate the exact patient geometry and tissue composition. MCEBRT can be used as extra verification layer for plans where surface dose and tissue heterogeneity are an issue.

SU-E-T-233: Modeling Linac Couch Effects On Attenuation and Skin Dose

Purpose: Treatment couch tops in medical LINAC rooms lead to attenuation to beams penetrating them, plus higher skin dose which can become a significant concern with the high fraction doses associated with Stereotactic Body Radiation Therapy. This work measures the attenuation and shallow depth dose due to a BrainLab couch, and studies the modeling of the couch top in our treatment planning system (TPS) as a uniform solid material with homogeneous density. Methods: LINAC photon beams of size 10×10 cm and nominal energy 6 MV were irradiated from different gantry angles on a stack of solid water. Depth dose were measured with two types of parallel plate chambers, MPPK and Markus. In the Philips Pinnacle TPS, the couch was modeled as a slab with varying thickness and density. A digital phantom of size 30×30×10 cm with density 1 g/cc was created to simulate the measurement setup. Both the attenuation and skin dose effects due to the couch were studied. Results: An orthogonal attenuation rate of 3.2% was observed with both chamber measurements. The attenuation can be modeled by couch models of varying thicknesses. Once the orthogonal attenuation was modeled well, the oblique beam attenuation in TPS agreed with measurement withinmore » 1.5%. The depth dose at shallow depth (0.5 cm) was also shown to be modeled correctly within 1.5% of the measurement using a 12 mm thick couch model with density of 0.9 g/cc. Agreement between calculation and measurement diverges at very shallow depths (≤1 mm) but remains acceptable (<5%) with the aforementioned couch model parameters. Conclusion: Modeling the couch top as a uniform solid in a treatment planning system can predict both the attenuation and surface dose simultaneously well within clinical tolerance in the same model.« less

Measuring linac photon beam energy through EPID image analysis of physically wedged fields

Electronic portal imaging devices (EPIDs) have proven to be useful tools for measuring several parameters of interest in linac quality assurance (QA). However, a method for measuring linac photon beam energy using EPIDs has not previously been reported. In this report, such a method is devised and tested, based on fitting a second order polynomial to the profiles of physically wedged beams, where the metric of interest is the second order coefficient α. The relationship between α and the beam quality index [percentage depth dose at 10 cm depth (PDD10)] is examined to produce a suitable calibration curve between these two parameters. Measurements were taken in a water-tank for beams with a range of energies representative of the local QA tolerances about the nominal value 6 MV. In each case, the beam quality was found in terms of PDD10 for 100 × 100 mm(2) square fields. EPID images of 200 × 200 mm(2) wedged fields were then taken for each beam and the wedge profile was fitted in MATLAB 2010b (The MathWorks, Inc., Natick, MA). α was then plotted against PDD10 and fitted with a linear relation to produce the calibration curve. The uncertainty in α was evaluated by taking five repeat EPID images of the wedged field for a beam of 6 MV nominal energy. The consistency of measuring α was found by taking repeat measurements on a single linac over a three month period. The method was also tested at 10 MV by repeating the water-tank crosscalibration for a range of energies centered approximately about a 10 MV nominal value. Finally, the calibration curve from the test linac and that from a separate clinical machine were compared to test consistency of the method across machines in a matched fleet. The relationship between α and PDD10 was found to be strongly linear (R(2) = 0.979) while the uncertainty in α was found to be negligible compared to that associated with measuring PDD10 in the water-tank (± 0.3%). The repeat measurements over a three month period showed the method to be reasonably consistent (i.e., well within the limits defined by local QA tolerances). The measurements were repeated on a matched machine and the same linear relationship between α and PDD10 was observed. The results for both machines were found to be indistinguishable across the energy range of interest (i.e., across and close to the thresholds defined by local QA tolerances), hence a single relation could be established across a matched fleet. Finally, the experiment was repeated on both linacs at 10 MV, where the linear relationship between α and PDD10 was again observed. The authors conclude that EPID image analysis of physically wedged beam profiles can be used to measure linac photon beam energy. The uncertainty in such a measurement is dominated by that associated with measuring PDD10 in the water-tank; hence, the accuracies of these two methods are directly comparable. This method provides a useful technique for quickly performing energy constancy measurements while saving significant clinical downtime for QA.

Organ motion in children during image-guided high-precision radiotherapy

Background: For IMRT commissioning in our department, the AAPM's TG119 Report was implemented on the Delta4 (Scandidos, Uppsala, Sweden) to evaluate step-and-shoot IMRT plans and verify the dose distribution using a 6MV linac photon beam. Materials and methods: TG-119 recommends a set of test cases to assess the overall accuracy of planning and delivery of IMRT treatments. Delta4 is used for quality assurance of patient specific treatment delivery. It is a cylindrical phantom consisting of two detector planes with p-type diodes that canmeasure point doses. Seven TG-119 plans, including the respective structure sets, were transferred on the Delta4 geometry. The respective dose distributions for a 6MV photon beam from a Siemens Oncor Impression linac with a multileaf collimator (82 leaves, 1cm thick) were calculated in the Oncentra v4.3 (Nucletron, Elekta) treatment planning system. Two preliminary tests were performed with simple parallelopposed fields irradiating the cylindrical phantom. Additionally, five more tests with mock structures were planned and irradiated with IMRT fields. Dose distribution for all test cases was calculated using a collapsed cone convolution algorithm for optimization and final dose calculation. TG119’s optimization goals were intended for all cases. For the acceptance of the verification process, gamma criteria of 3% dose and 3mm distance to agreement were used. Results: The gamma index passing rate for all tests was measured higher than 90%. Preliminary tests were measured 99.3% for anterior-posterior and 99.6% for the bands test. For the IMRT cases, it was measured 97.5% for multitarget, 99.6% for prostate, 98.5% for head and neck, 95.2 for the easy C-shape and 90.3% for the harder C-shape test. Discussion: TG119 structures and plans were easily transferred and implemented on the Delta4 dosimetric phantom. For all test cases the gamma index passing ratewasmeasured higher than 90%. Delta4 phantom has proven to be fast applicable and reliable for the step-and-shoot IMRT commissioning following the TG119 recommended tests.

Neutron dose measurements of Varian and Elekta linacs by TLD600 and TLD700 dosimeters and comparison with MCNP calculations

High-energy linacs produce secondary particles such as neutrons (photoneutron production). The neutrons have the important role during treatment with high energy photons in terms of protection and dose escalation. In this work, neutron dose equivalents of 18 MV Varian and Elekta accelerators are measured by thermoluminescent dosimeter (TLD) 600 and TLD700 detectors and compared with the Monte Carlo calculations. For neutron and photon dose discrimination, first TLDs were calibrated separately by gamma and neutron doses. Gamma calibration was carried out in two procedures; by standard 60Co source and by 18 MV linac photon beam. For neutron calibration by 241Am-Be source, irradiations were performed in several different time intervals. The Varian and Elekta linac heads and the phantom were simulated by the MCNPX code (v. 2.5). Neutron dose equivalent was calculated in the central axis, on the phantom surface and depths of 1, 2, 3.3, 4, 5, and 6 cm. The maximum photoneutron dose equivalents which calculated by the MCNPX code were 7.06 and 2.37 mSv.Gy-1 for Varian and Elekta accelerators, respectively, in comparison with 50 and 44 mSv.Gy-1 achieved by TLDs. All the results showed more photoneutron production in Varian accelerator compared to Elekta. According to the results, it seems that TLD600 and TLD700 pairs are not suitable dosimeters for neutron dosimetry inside the linac field due to high photon flux, while MCNPX code is an appropriate alternative for studying photoneutron production.

WE-G-BRA-06: Calibrating an Ionisation Chamber: Gaining Experience Using a Dosimetry 'flight Simulator'.

Recently there has been great interest in the use of simulation training, with the view to enhance safety within radiotherapy practice. We have developed a Virtual Environment for Radiotherapy Training (VERT) which facilitates this, including the simulation of a number of 'Physics practices'. One such process is the calibration of an ionisation chamber for use in Linac photon beams. The VERT system was used to provide a life sized 3D virtual environment within which we were able to simulate the calibration of a departmental chamber for 6MV and 15 MV beams following the UK 1990 Code of Practice. The characteristics of the beams are fixed parameters in the simulation, whereas default (Absorbed dose to water) correction factors of the chambers are configurable thereby dictating their response in the virtual x-ray beam. When the simulation is started, a random, realistic temperature and pressure is assigned to the bunker. Measurement and chamber positional errors are assigned to the chambers. A virtual water phantom was placed on the Linac couch and irradiated through the side using a 10 × 10 field. With a chamber at the appropriate depths and irradiated iso-centrically, the Quality Indices (QI) of the beams were obtained. The two chambers were 'inter-compared', allowing the departmental chamber calibration factor to be calculated from that of the reference chamber. For the virtual 6/15 MV beams, the QI were found to be 0.668/ 0.761 and the inter-comparison ratios 0.4408/ 0.4402 respectively. The departmental chamber calibration factors were calculated; applying these and appropriate environmental corrections allowed the output of the Linac to be confirmed. We have shown how a virtual training environment can be used to demonstrate practical processes and reinforce learning. The UK CoP was used here, however any relevant protocol could be demonstrated. Two of the authors (Beavis and Ward) are Founders of Vertual Ltd, a spin-out company created to commercialise the research presented in this abstract.

Uncertainty and sensitivity analysis of the effect of the mean energy and FWHM of the initial electron fluence on the Bremsstrahlung photon spectra of linear accelerators

A calculation of the correct dose in radiation therapy requires an accurate description of the radiation source because uncertainties in characterization of the linac photon spectrum are propagated through the dose calculations. Unfortunately, detailed knowledge of the initial electron beam parameters is not readily available, and many researchers adjust the initial electron fluence values by trial-and-error methods. The main goal of this work was to develop a methodology to characterize the fluence of initial electrons before they hit the tungsten target of an Elekta Precise medical linear accelerator. To this end, we used a Monte Carlo technique to analyze the influence of the characteristics of the initial electron beam on the distribution of absorbed dose from a 6MV linac photon beam in a water phantom. The technique is based on calculations with Software for Uncertainty and Sensitivity Analysis (SUSA) and Monte Carlo simulations with the MCNP5 transport code. The free parameters used in the SUSA calculations were the mean energy and full-width-at-half-maximum (FWHM) of the initial electron distribution. A total of 93 combinations of these parameters gave initial electron fluence configurations. The electron spectra thus obtained were used in a simulation of the electron transport through the target of the linear accelerator, which produced different photon (Bremsstrahlung) spectra. The simulated photon spectra were compared with the 6-MV photon spectrum provided by the linac manufacturer (Elekta). This comparison revealed how the mean energy and FWHM of the initial electron fluence affect the spectrum of the generated photons. This study has made it possible to fine-tune the examined electron beam parameters to obtain the resulted absorbed doses with acceptable accuracy (error <1%).

Comparison of MCNP5 Dose Calculations inside the RANDO® Phantom Irradiated with a MLC LinAc Photon Beam against Treatment Planning System PLUNC

Comparison of MCNP5 Dose Calculations inside the RANDO® Phantom Irradiated with a MLC LinAc Photon Beam against Treatment Planning System PLUNC

1385 poster COMMISSIONING OF PENELOPE AND GATE MONTE CARLO MODELS FOR 6 AND 18 MV PHOTON BEAMS FROM THE SIEMENS ARTISTE LINAC

Purpose: Monte Carlo (MC) simulations currently allow accurate calculation of the dose delivered to patients in radiotherapy treatments, provided that the linear accelerator can be modelled accurately. The commissioning of the linac model is thus an important task to ensure the overall accuracy of the calculated dose. This work aims at commissioning MC models developed with PENELOPE and GATE for the 6 and 18 MV photon beams of the Siemens ARTISTE linac, by comparing simulation to measurement. Materials: Linac photon beam models at 6 and 18 MV were commissioned. Percentage depth doses (PDD) and lateral beam profiles at dmax, 5 and 10 cm were measured for open square fields from 5x5 to 30x30 cm at 100 SSD using a PTW large MP3 water tank with a PTW Semiflex 0.125 cm3 ionization chamber. MC simulations were performed using PENELOPE 2006/PENFAST and GATE v6.0 (based on GEANT4 9.3.p02) in two steps: phase space files storing particles exiting the linac head were first computed and then used as input data for dose computation. Both codes are optimized for radiotherapy purposes and include commonly used variance reduction techniques. The linac geometry was described in the simulation based on the manufacturer specifications. The main parameters defining the incident electron beam, i.e. its mean energy and its radial intensity distribution, were determined following the methodology suggested by Pena et al (Med. Phys. 2007), using PDDs and lateral profiles of the 5x5, 10x10 and 30x30 cm fields. The incident electron beam was assumed to be monoenergetic and monodirectional. Dose computations in the water phantom were performed using PENFAST and GATE 6.0.0, with a voxel size of 4 mm in all directions for all field sizes. Results: At depth greater than dmax, simulated PDDs with GATE and PENELOPE both agreed with measurement to better than 1% for 6 and 18 MV. At shallower depths, both MC codes underestimated the measured dose. Lateral profiles matched measured ones within 1%/1 mm at 6 MV and within 2%/2 mm at 18 MV, for both codes. Larger discrepancies are however observed for field sizes larger than 25x25 cm. Work is ongoing to identify the reasons for these discrepancies; the impact of the initial electron energy distribution is particularly investigated. Conclusions: Photon beam models of the Siemens ARTISTE linac at 6 and 18 MV were developed with PENELOPE and GATE and commissioned. Good agreement was obtained between measured and simulated dose distributions, confirming the validity of the models.