Clinical Linac Research Articles

WE‐E‐134‐02: High Resolution Acoustic Angiography for Early Assessment of Tumor Response to Radiation Treatment in Rat Tumor Model

Purpose: To test the hypothesis that acoustic angiography would enable early assessment of tumor response to radiation therapy at multiple timepoints after a single‐fraction radiotherapy treatment. Methods: 15 fibrosarcoma tumor‐bearing rats were irradiated (field‐size:1.8×1.8cm) by 6MV photon beam from a clinical linac. Tumors were implanted subcutaneously in the lower flank area and treated 14 days later (Mean‐diameter: 7.03+/−1.62mm). Animals were randomized into three groups (doses of 0, 5 or 20 Gy). Tumors were imaged prior to treatment (Day 0) using both acoustic angiography (microbubble contrast‐based microvessel imaging) and standard high frequency b‐mode, and then again at Days 3, 6, 9, 12, and 18. Tumor volume, and perfused volume ratio (PVR ‐ the ratio of tumor volume with perfused voxels, to total tumor volume) were quantified at each timepoint. Results: The tumors in the 0 or 5 Gy groups continued to grow exponentially after treatment. All animals in the 20 Gy group experienced an inflection point in their growth curve at Day 3, and tumor volume reduced exponentially until Day 12. Three of these tumors are non‐responders(20Gy‐NRs) and had second inflection point at Day 12, and proceeded to increase in volume, on average to their original Day 0 tumor size by Day 18. The other two tumors in the 20 Gy group are responders(20Gy‐R' s) and reduced to undetectable tumor volumes by Day 18. The PVR metric for vascularization revealed differences in the 20Gy‐NR and 20Gy‐R groups at Day 12, which correlated with tumor volumetric differences. Conclusion: Our preliminary data indicate that the acoustic angiography‐derived PVR metric can detect rat fibrosarcoma tumors responded (N=2) and those which did not (N=3) to 20 Gy radiation six days before there is a detectable difference in tumor volumes.

SU‐E‐J‐44: Imaging and Radiation Therapy: GATE Monte Carlo Simulation of a MV‐CBCT Flat Panel with Specific Application in Head and Neck Cancer

Purpose: In radiotherapy treatment, it is necessary to position the patient before each session to ensure that the beams target the tumor while sparing the surrounding healthy tissue. This step is made possible via different imaging techniques like the “electronic portal imaging device” (EPID) which is the most common approach. In this configuration, the imaging source is the same as the treatment source, while a flat panel detector is automatically placed under the patient via an articulated arm. In this context, our study was focused on the Monte Carlo modeling of such an EPID in order to simulate both planar and 3D CBCT images. Methods: The work presented here follows a preliminary study in which the treatment part of a clinical LINAC (Siemens Oncor) was modeled in GATE v6.2. The objective was to calculate the dose delivered by complex treatments like IMRT. The objective of the study is to extend this model by incorporating the imaging part of the LINAC, which consists of a flat panel detector (Perkin Elmer XRD1640). For this purpose, geometrical and physical data provided by the manufacturer were first accurately modeled in GATE. As a second step, 2D and 3D images simulations were performed according to clinical protocols: projections over a 200° arc, 1° increments, and 41cm square field of view. IMRT QA and, RANDO anthropomorphic phantom and head‐and‐neck patient CT were used as input objects for simulations. Results: From the obtained series of 2D projections a 3D reconstruction was performed in each case, and the dimensions of the reconstructed structures were compared with actual objects. Conclusion: We have developed within the GATE platform a complete and detailed model of a 160MLC LINAC with its MVCBCT flat panel. Future work will consist in simulating epid‐based transit dosimetry.

Imaging and radiation therapy: Gate Monte Carlo simulation of a MV-CBCT flat panel with specific application in head and neck cancer

Purpose In radiotherapy treatment, it is necessary to position the patient before each session to ensure that the beams target the tumor while sparing the surrounding healthy tissue. This step is made possible via different imaging techniques like the ‘‘electronic portal imaging device” (EPID) which is the most common approach. In this configuration, the imaging source is the same as the treatment source, while a flat panel detector is automatically placed under the patient via an articulated arm. In this context, our study was focused on the Monte Carlo modeling of such an EPID in order to simulate both planar and 3D CBCT images. Methods The work presented here follows a preliminary study in which the treatment part of a clinical LINAC (Siemens Oncor) was modeled in GATE v6.2. The objective was to calculate the dose delivered by complex treatments like IMRT. The objective of the study is to extend this model by incorporating the imaging part of the LINAC, which consists of a flat panel detector (Perkin Elmer XRD1640). For this purpose, geometrical and physical data provided by the manufacturer were first accurately modeled in GATE. Assessments of portal imaging were performed by performing dose calculations in a CQ IMRT phantom with GATE for two measurement points with protocols of 8, 15 and 60 UM and comparing the doses with clinical measurements. And with performing 2D CT projections over a 200° arc, 1° increments, and 41 cm × 41 cm field of view for homogeneous and heterogeneous structures (introduction of a patient head and neck CT patient). Results For the calculation of the deposit dose at the isocentre of the CQ IMRT phantom, respectively GATE gave doses of 5.734 ± 0.169, 11.008 ± 0.777 and 46.006 ± 0.711 cGy compared with measurements are: 6.821, 12,469 and 49.600 cGy protocols for 8, 15 and 60 UM. From the obtained series of 2D projections a 3D reconstruction was performed in each case, and the dimensions of the reconstructed structures were compared with actual objects. Conclusion We have successfully developed within the GATE platform a complete and detailed model of a 160 MLC LINAC with its MV-CBCT flat panel. Future work will consist in simulating epidbased transit dosimetry.

SU‐E‐T‐250: Measurements of Surface and Interface Dose for 6 MV Photons in a Parallel Magnetic Field

Purpose: The patients treated using Linac‐MR systems, being developed for real time MR guided radiotherapy, will be irradiated in the presence of a magnetic field. The present study presents the entrance surface, buildup and interface doses measured in a 6 MV beam in the presence of a parallel magnetic field. Methods: A set of phantoms with one angled surface, and a phantom with an adjustable air gap were constructed out of polystyrene. These phantoms were designed to measure both the percent depth dose and the surface dose at the same time using Gafchromic film. Two solenoid electromagnets, placed on top of each other, were used to generate a magnetic field along their vertical bore (26.5 cm deep x 17.5 cm diameter) and a decreasing fringe field outside. The magnetic field strength is variable from 0 T to 0.2 T at the bore's center. The phantoms were placed inside the bore and irradiated using field sizes that avoided irradiation of the magnetic coils. Results: The parallel magnetic field was found to focus the contaminating electrons produced in the linac head and in the air column above the phantom, within the beam area. The surface dose, for all the surface angles investigated, increased by ∼15% of the Dmax dose at 0.2 T magnetic field. For the air gap phantom the concentration of electrons inside the beam area by the parallel magnetic field is clearly visible inside the air gap on the depth dose film and in the percent depth dose curve. Conclusion: We have devised an experimental system to measure the radiation dose of a clinical linac in a parallel magnetic field. We are currently performing Monte Carlo simulations that include magnetic fields replicating the measurement system. The verified Monte Carlo system will make the dose calculation engine for future linac‐MR systems.

SU-E-T-52: Evaluation of EBT2 and EBT3 Films for Dosimetry in Laser-Driven Ion Accelerators

Purpose: Laser‐driven ion accelerators could be a future option for a compact medical ion accelerator for radiation therapy. Laser ion accelerators can deliver a typical dose of few Gy in a single ion pulse of ns duration. Their dose rate, thus, exceeds the typical dose rate of a medical accelerator by several orders of magnitude. Dosimetry at high repetition rate laser ion accelerators requires 2D dosimeters with dose‐rate independent response. Both requirements can be fulfilled by Gafchromic EBT2 or EBT3 films. Energies for application of laser accelerated ions (e.g. in cell irradiation experiments) are still limited to about 10 MeV. Dose response and energy dependence of Gafchromic EBT2 or EBT3 films are, therefore, evaluated in the low proton energy regime as delivered by a laser ion accelerator and compared to clinical photon and proton beams. Methods: The dose response of EBT2 and EBT3 film has been characterized for a wide proton energy range. 6 MV photon irradiations at a clinical Linac provide a reference curve. The energy dependence of both films, has been studied in particular for low proton energies as predominating in the vicinity of the Bragg peak. Mono‐energetic beams of 3 MeV, 11 MeV and 20 MeV but also up to a maximum energy of 200 MeV were used for these studies. Results: In general, EBT2 and EBT3 have a comparable performance and show similar response to photons and protons down to a proton energy of 10 MeV. A dose under‐estimation by up to 20 % in the Bragg peak region is visible in depth dose measurements of both types of film. Conclusion: EBT 2 and EBT3 films can be applied as 2D dosimeter in the low proton energy regime of a laser ion accelerator. Dose under‐response corrections are required for proton energies below 10 MeV. This work was funded by the DFG cluster of excellence Munich Centre for Advanced Photonics (MAP).

Development of a fast and feasible spectrum modeling technique for flattening filter free beams

To develop a fast and robust technique for the determination of optimized photon spectra for flattening filter free (FFF) beams to be applied in convolution/superposition dose calculations. A two-step optimization method was developed to derive optimal photon spectra for FFF beams. In the first step, a simple functional form of the photon spectra proposed by Ali ["Functional forms for photon spectra of clinical linacs," Phys. Med. Biol. 57, 31-50 (2011)] is used to determine generalized shapes of the photon spectra. In this method, the photon spectra were defined for the ranges of field sizes to consider the variations of the contributions of scattered photons with field size. Percent depth doses (PDDs) for each field size were measured and calculated to define a cost function, and a collapsed cone convolution (CCC) algorithm was used to calculate the PDDs. In the second step, the generalized functional form of the photon spectra was fine-tuned in a process whereby the weights of photon fluence became the optimizing free parameters. A line search method was used for the optimization and first order derivatives with respect to the optimizing parameters were derived from the CCC algorithm to enhance the speed of the optimization. The derived photon spectra were evaluated, and the dose distributions using the optimized spectra were validated. The optimal spectra demonstrate small variations with field size for the 6 MV FFF beam and relatively large variations for the 10 MV FFF beam. The mean energies of the optimized 6 MV FFF spectra were decreased from 1.31 MeV for a 3 × 3 cm(2) field to 1.21 MeV for a 40 × 40 cm(2) field, and from 2.33 MeV at 3 × 3 cm(2) to 2.18 MeV at 40 × 40 cm(2) for the 10 MV FFF beam. The developed method could significantly improve the agreement between the calculated and measured PDDs. Root mean square differences on the optimized PDDs were observed to be 0.41% (3 × 3 cm(2)) down to 0.21% (40 × 40 cm(2)) for the 6 MV FFF beam, and 0.35% (3 × 3 cm(2)) down to 0.29% (40 × 40 cm(2)) for the 10 MV FFF beam. The first order derivatives from the functional form were found to improve the speed of computational time up to 20 times compared to the other techniques. The derived photon spectra resulted in good agreements with measured PDDs over the range of field sizes investigated. The suggested method is easily applicable to commercial radiation treatment planning systems since it only requires measured PDDs as input.

Unfolding linac photon spectra and incident electron energies from experimental transmission data, with direct independent validation

In a recent computational study, an improved physics-based approach was proposed for unfolding linac photon spectra and incident electron energies from transmission data. In this approach, energy differentiation is improved by simultaneously using transmission data for multiple attenuators and detectors, and the unfolding robustness is improved by using a four-parameter functional form to describe the photon spectrum. The purpose of the current study is to validate this approach experimentally, and to demonstrate its application on a typical clinical linac. The validation makes use of the recent transmission measurements performed on the Vickers research linac of National Research Council Canada. For this linac, the photon spectra were previously measured using a NaI detector, and the incident electron parameters are independently known. The transmission data are for eight beams in the range 10-30 MV using thick Be, Al and Pb bremsstrahlung targets. To demonstrate the approach on a typical clinical linac, new measurements are performed on an Elekta Precise linac for 6, 10 and 25 MV beams. The different experimental setups are modeled using EGSnrc, with the newly added photonuclear attenuation included. For the validation on the research linac, the 95% confidence bounds of the unfolded spectra fall within the noise of the NaI data. The unfolded spectra agree with the EGSnrc spectra (calculated using independently known electron parameters) with RMS energy fluence deviations of 4.5%. The accuracy of unfolding the incident electron energy is shown to be ∼3%. A transmission cutoff of only 10% is suitable for accurate unfolding, provided that the other components of the proposed approach are implemented. For the demonstration on a clinical linac, the unfolded incident electron energies and their 68% confidence bounds for the 6, 10 and 25 MV beams are 6.1 ± 0.1, 9.3 ± 0.1, and 19.3 ± 0.2 MeV, respectively. The unfolded spectra for the clinical linac agree with the EGSnrc spectra (calculated using the unfolded electron energies) with RMS energy fluence deviations of 3.7%. The corresponding measured and EGSnrc-calculated transmission data agree within 1.5%, where the typical transmission measurement uncertainty on the clinical linac is 0.4% (not including the uncertainties on the incident electron parameters). The approach proposed in an earlier study for unfolding photon spectra and incident electron energies from transmission data is accurate and practical for clinical use.

Sci-Thur PM: YIS - 02: A validated approach for clinical linacs to accurately determine the photon spectra and the incident electron energy.

In clinical photon beams, independent determination of the photon spectra and the incident electron energy is useful for beam (re)commissioning and for detector response modelling. In this study, an approach is developed for that purpose, and validated on a research linac whose photon spectra and electron beams are directly and independently known. In this approach, an optimized combination of transmission curves is measured using multiple attenuators and detectors to maximize energy differentiation. For validation, transmission measurements are made for 8 beams from 10-30 MV, with bremsstrahlung targets from Be to Pb. A protocol is established to account for many influence quantities including linac drifts (2%), polarity (6%), ion recombination (0.2%), leakage (0.3%), room scatter (0.8%), non-ideal attenuation (1.5%), attenuator mass thickness (4%), and photonuclear effect (5.6%). The experimental accuracy on the smallest signals is 0.4%. EGSnrc is upgraded to model photonuclear attenuation (without tracking secondary particles), and then used to model the full experiment. For direct transmission comparisons, the agreement is 2%. This allows for an estimate of 0.5% on the upper limit of photon cross section uncertainties, which is much better than the current estimate of 1-2%. The unfolded spectra agree with the benchmark ones within 4.5%. The incident electron energy is accurate within 5%, with 95% confidence. The overall improvement over the commonly used methods is a factor of 3. This transmission study is the first to independently determine the incident electron energy, and to recognize the significant role of the photonuclear effect at higher energies.

Photon radiation testing of commercially available off-the-shelf microcontroller devices

The results of photon radiation testing of various microcontroller devices is described. This was testing was useful to select the microcontroller in a 6DOF MEMS-based INS. This systems is being developed for the in-vivo monitoring of tumour position during clinical radiotherapy treatments. This application requires a radiation-tolerant processor in order to perform appropriately in a radiotherapy environment. A phantom has been built to replicate the working conditions that the microcontroller devices are required to endure. Each time, a number of identical microcontroller devices have been exposed, in 5turn, to X-ray doses in excess of 50 Gy from a clinical radiotherapy LINAC.

Functional forms for photon spectra of clinical linacs

Specifying photon spectra of clinical linacs using a functional form is useful for many applications, including virtual source modelling and spectral unfolding from dosimetric measurements such as transmission data or depth-dose curves. In this study, 11 functional forms from the literature are compiled and quantitatively compared. A new function is proposed which offers improvements over existing ones. The proposed function is simple, physics-based and has four free parameters, one of which is the mean incident electron kinetic energy. A comprehensive benchmark set of validated, high-precision Monte Carlo spectra is generated and used to evaluate the strengths and limitations of different functions. The benchmark set has 65 spectra (3.5–30 MV) from Varian, Elekta, Siemens, Tomotherapy, Cyberknife and research linacs. The set includes spectra on- and off-axis from linacs with and without a flattening filter, and in treatment and imaging modes. The proposed function gives the lowest spectral deviations among all functions. It reproduces the energy fluence values in each bin for the benchmark set with a normalized root-mean-square deviation of 1.7%. The mean incident electron kinetic energy, maximum photon energy, most-probable energy and average energy are reproduced, on average, within 1.4%, 4.3%, 3.9% and 0.6% of their true values, respectively. The proposed function is well behaved when used for spectral unfolding from dosimetric data. The contribution of the 511 keV annihilation peak and the energy spread of the incident electron beam can be added as additional free parameters.

Transfer of the UK absorbed dose primary standard for photon beams from the research linac to the clinical linac at NPL

An Elekta Synergy clinical linac facility is now in routine use at the National Physical Laboratory (NPL). For the purpose of therapy-level dosimetry, this has replaced the NPL research linac, which is over 40 years old, and in which the NPL absorbed dose primary standard for high-energy photons was established. This standard has been disseminated to clinical beams by interpolation of the calibration factor as a function of tissue phantom ratio TPR20/10. In this work the absorbed dose standard has been commissioned in all the beams produced by the Elekta Synergy linac. Reference standard ionization chambers have been calibrated in terms of absorbed dose to graphite and this calibration has been converted to one in terms of absorbed dose to water. The results have been combined with the calibration in 60Co γ-rays to obtain measured values for the quality-dependent correction, kQ, for these reference standard chambers used in the Elekta beams. The resulting data are consistent with the interpolated kQ to within 0.4%, which is less than the combined standard uncertainty of kQ, 0.56%.

Ambient neutron dose equivalent outside concrete vault rooms for 15 and 18 MV radiotherapy accelerators

In this work, the ambient dose equivalent, H*(10), due to neutrons outside three bunkers that house a 15- and a 18-MV Varian Clinac 2100C/D and a 15-MV Elekta Inor clinical linacs, has been calculated. The Monte Carlo code MCNPX (v. 2.5) has been used to simulate the neutron production and transport. The complete geometries including linacs and full installations have been built up according to the specifications of the manufacturers and the planes provided by the corresponding medical physical services of the hospitals where the three linacs operate. Two of these installations, those lodging the Varian linacs, have an entrance door to the bunker while the other one does not, although it has a maze with two bends. Various treatment orientations were simulated in order to establish plausible annual equivalent doses. Specifically anterior-posterior, posterior-anterior, left lateral, right lateral orientations and an additional one with the gantry rotated 30° have been studied. Significant dose rates have been found only behind the walls and the door of the bunker, near the entrance and the console, with a maximum of 12 µSv h(-1). Dose rates per year have been calculated assuming a conservative workload for the three facilities. The higher dose rates in the corresponding control areas were 799 µSv y(-1), in the case of the facility which operates the 15-MV Clinac, 159 µSv y(-1), for that with the 15-MV Elekta, and 21 µSv y(-1) for the facility housing the 18-MV Varian. A comparison with measurements performed in similar installations has been carried out and a reasonable agreement has been found. The results obtained indicate that the neutron contamination does not increase the doses above the legal limits and does not produce a significant enhancement of the dose equivalent calculated. When doses are below the detection limits provided by the measuring devices available today, MCNPX simulation provides an useful method to evaluate neutron dose equivalents based on a detailed description of linac, patient and bunker.

SU-E-T-740: TG-51A: Part 2: Factors Affecting Alanine Response and Overall Uncertainty of Dose Measurements with Alanine under Conditions Suitable for Calibrations of Photon Beams of Medical Linacs

Purpose: Alanine dosimetry now appears suitable not only for measuring actual doses in QA, but also for calibrations of clinical linacs. Like with ionization chambers, alanine calibrations are traceable to calorimetry national standards, but photon energy dependence of alanine response is much weaker, which makes uncertainty related to differences in beam quality smaller. To compare performance of ionometry and alanine dosimetry objectively, it is necessary to carefully analyze factors affecting the alanine response and the overall uncertainty of dose measurements with alanine under conditions appropriate for linac calibrations. Method and Materials: Several Bruker EPR spectrometers equipped with standard reference samples were used to measure responses of alanine under various experimental conditions. Their calibrations were directly traceable to NIST. Alanine dosimeters from GammaService (Germany) and Harwell (UK) were used. Co-60, Varian Trilogy and Elekta Synergy were used for irradiations. Results: An analysis of all known factors that could possibly affect accuracy of alanine dosimetry will be presented using previously published and newly obtained data. Modality and energy of radiation, temperature and humidity at irradiation and post-irradiation storage, time between the irradiation and signal measurement, exposure to bright sunlight and some others will be characterized quantitatively in the ranges of variation typical for linac calibration conditions. Based on this analysis, a calculated overall uncertainty of calibration dose measurements with alanine will be presented. Conclusion: Alanine dosimetry now provides accuracy of linac calibration similar to accuracy typical for measurements with ionization chambers from secondary standard laboratories. The very weak energy dependence of the alanine response has been reliably characterized at several national laboratories, which makes this way of calibration free of the theoretical assumptions and uncertainties associated with kQ in ionometry. It appears that alanine dosimetry, based on entirely different physical phenomenon than ionometry, can successfully compliment ionometry in calibrations of clinical linacs.

SU‐E‐J‐74: A Dosimeter for In Vivo Real Time Verification of Intensity Modulated Radiation Therapy

Purpose: To study the feasibility of implementing a low cost, high speed in vivo real time dosimeter based on scintillating fiber arrays to verify intensity modulated radiation therapy (IMRT) to enhance treatment safety and delivered dose verification. Methods: Detector construction is performed by mounting linear scintillating fiber sensors in a water equivalent plastic support to form a homogenous water equivalent array. The fibers are aligned to match each MLC leaf pair and the detector is placed in the accessory tray of a clinical LINAC for beam output determination. Each fiber is coupled to a high speed photodetector and front end amplifier to process the detector output which is digitized and analyzed using in house developed software to compute the beam fluence, dose rate, and MLC opening. Results: The detector response correlates linearly with the LINAC beam intensity and MLC positions. The sensor and front end amplifier has a time response of 0.2 usec, thus every 2 usec LINAC pulse is recorded to determine the beam fluence with a precision better than 1%. The array offers a minimum beam attenuation of 2.4%, allowing the use of this technology as an in vivo transmission detector. We determined that the detector is capable of measuring the MLC opening with a precision of 2 mm per leaf. Conclusions: We have proven the feasibility of constructing a scintillating fiber sensor array for in vivo real time dosimetry to enhance patient safety during IMRT treatment and measure actual dose delivered to the patient. The technical highlight of this detector is based on the high speed of the linear sensors and front end amplifier which allows real time verification of the IMRT fluence output, dose rate, and leaf opening with a minimum of perturbation or absorption of the delivered beam.

TH-E-220-08: Flattening Filter Removal for Improved Megavoltage Imaging of Fiducial Markers

Purpose: Removal of the LINAC flattening filter enables a high rate of dose deposition with reduced treatment time. When used for megavoltage imaging, an unflat beam has reduced primary beam scatter resulting in sharper images. In fluoroscopic imaging mode, a higher photon count per image frame yields higher contrast-to-noise ratio. We investigated the effects of an unflat beam on the image quality of megavoltage portal and fluoroscopic images. The impact of flattening filter removal on imaging of moving fiducial markers also was investigated. Methods: 6MV projection images were acquired in fluoroscopic and portal modes using an electronic flat-panel imager. Portal images also were acquired using XV film. The effects of the flattening filter on the pre-sampling MTF, relative MTF and contrast-to-noise ratio were quantified using the QC3 phantom and tungsten wire. The ability of observers to visualize fiducial markers using flat versus unflat beams also was studied. Results: The unflat beam had improved contrast resolution, up to 40% increase in MTF contrast at the highest frequency measured (0.75 line pairs per mm). The contrast-to-noise ratio was increased as expected from the increased photon flux. The visualization of fiducial markers was markedly better using the unflat beam enabling visualization of thin gold fiducial markers, the thinnest of which was not visible using the unflat beam. Conclusions: The removal of the flattening filter from a clinical LINAC leads to quantifiable improvements in the image quality of megavoltage projection images. These gains enable observers to more easily visualize thin fiducial markers and track their motion on fluoroscopic images.

103 oral RE-ESTABLISHING THE PHOTON ABSORBED DOSE PRIMARY STANDARD ON THE NPL CLINICAL LINAC

A new state-of-the-art clinical linac facility was opened in November 2008 at the NPL in addition to the existing research linac facility. The new machine is an Elekta Synergy Digital Linac with iViewGT portal and XVi 3D x-ray volumetric imaging that can be confi gured to deliver seven x-ray beam energies (instead of the usual maximum of three in any one hospital machine). This feature, together with the ability to provide up to ten electron beam energies, will enable NPL to provide absorbed dose calibrations for the full range of energies currently in therapeutic use in the UK.

Neutron dose equivalent and neutron spectra in tissue for clinical linacs operating at 15, 18 and 20 MV

In this work, the dose equivalent due to photoneutrons and the neutron spectra in tissue was calculated for various linacs (Varian Clinac 2100C, Elekta Inor, Elekta SL25 and Siemens Mevatron KDS) operating at energies between 15 and 20 MV, using the Monte Carlo code MCNPX (v. 2.5). The dose equivalent in an ICRU tissue phantom has been calculated for anteroposterior treatments with a detailed simulation of the geometry of the linac head and the coupled electron-photon-neutron transport. Neutron spectra at the phantom entrance and at 1-cm depth in the phantom, depth distribution of the neutron fluence in the beam axis and dose distributions outside the beam axis at various depths have also been calculated and compared with previously published results. The differences between the neutron production of the various linacs considered has been analysed. Varian linacs show a larger neutron production than the Elekta and Siemens linacs at the same operating energy. The dose equivalent due to neutrons produced by medical linacs operating at energies >15 MeV is relevant and should not be neglected because of the additional doses that patients can receive.

MO‐E‐BRA‐01: Accounting for the Effect of a Magnetic Field from the Bending Magnet in Monte Carlo Accelerator Treatment Head Simulation

Purpose: To account for a fringe magnetic field from the bending magnet in Monte Carlo treatment head simulation. Method and Materials: A Tesla meter was used to map the magnetic field between the exit window and secondary scattering foil of a clinical linac for 6–21 MeV electron beams. The foil and monitor chamber were removed to access this region. Measurements extended 12.0 cm along the central axis and 1.0–3.0 cm off‐axis. The Monte Carlo code EGSnrc was adapted to incorporate the magnetic field with accuracy validated in simple geometries. Results: The magnetic field was orientated crossplane, with the deflection of the electron beam inplane. The highest intensity, measured at the exit window, increased with nominal beam energy from 0.006 T for 6 MeV to 0.020 T for 21 MeV. The field falloff, approximated by a cubic function, was 5% at the secondary scattering foil. The field deflected the electron beams by 0.82–1.24 cm at isocenter. The peak position in the calculated and measured profiles, with no scattering foils in the beam, matched to 1 mm inplane and crossplane. A preliminary comparison of calculated and measured dose profiles with the foils and chamber in the beam support our contention that the simulations can accurately account for the deflection of the electron beam by the fringe magnetic field from the bending magnet. It appears the lateral variation in the magnetic field can be neglected. Conclusion: The calculated fluence is expected to have an improved uncertainty of 1%/1 mm, without the non‐physical adjustment in the monitor chamber and secondary foil positions required when the field is left out of the simulation, having a better match to the undulations in the measured profiles with the bremsstrahlung peak in the correct position.

A new active method for the measurement of slow-neutron fluence in modern radiotherapy treatment rooms

This work focuses on neutron monitoring at clinical linac facilities during high-energy modality radiotherapy treatments. Active in-room measurement of neutron fluence is a complex problem due to the pulsed nature of the fluence and the presence of high photon background, and only passive methods have been considered reliable until now. In this paper we present a new active method to perform real-time measurement of neutron production around a medical linac. The device readout is being investigated as an estimate of patient neutron dose exposure on each radiotherapy session. The new instrument was developed based on neutron interaction effects in microelectronic memory devices, in particular using neutron-sensitive SRAM devices. This paper is devoted to the description of the instrument and measurement techniques, presenting the results obtained together with their comparison and discussion. Measurements were performed in several standard clinical linac facilities, showing high reliability, being insensitive to the photon fluence and EM pulse present inside the radiotherapy room, and having detector readout statistical relative uncertainties of about 2% on measurement of neutron fluence produced by 1000 monitor units irradiation runs.

Assessment of Pre-operational radiological conditions at the Linac Centre of the Lagos University Teaching Hospital, Nigeria

The Lagos University Teaching Hospital, Nigeria has acquired a 6 - 15 MeV ELEKTA linear accelerator for clinical teletherapy. The 15 MeV is a sufficient energy to activate photonuclear interactions of the type (h(nu), n) and (h(nu), e), in some materials found within the locality, thereby enhancing the background radiation exposure levels in the environments. The objective of this study is to conduct a pre-operational assessment of the radiological conditions of the new LINAC Centre and its environments which is required for future uses in the assessment of the environmental impacts due to the applications of the facility. The results of this study are to provide a baseline data needed for such assessments and the impact of eventual LINAC accidents. Radiation exposure levels due to natural background radioactivity in the controlled and supervised areas and offices in the LINAC localities have been measured. The environmental radiation survey around the Centre was conducted according to the requirements prescribed by the ICRP, IAEA Basic Safety Standards etc. The specific radioactivity contents of 100 building, furna and flora material samples have been determined using a Gamma Well Scintillation Counter. The mean exposure rates all over the LINAC Centre varied from 2.0 muSvh(-1) to 3.3 muSvh(-1). In the controlled areas, the mean value was 2.4 muSvh(-1) and it was 2.7 muSvh in the supervised areas, while in the adjacent offices, it was 2.5 muSvh(-1). The means of the measured specific activities in the cement concrete, soil, plant and grass samples were 42.7299 Bqkg(-1), 39.9592 Bqkg(-1), 23.9010 Bqkg(-1) and 20.8940 Bqkg(-1) respectively. A database for the assessment of the environmental impacts of the clinical LINAC and the impacts of its eventual accidents has been generated for future periodic uses.