Elekta Precise Linear Accelerator Research Articles

Calculation of Organ Dose Distribution (in-field and Out-of-field) in Breast Cancer Radiotherapy on RANDO Phantom Using GEANT4 Application for Tomographic Emission (Gate) Monte Carlo Simulation.

Organ dose distribution calculation in radiotherapy and knowledge about its side effects in cancer etiology is the most concern for medical physicists. Calculation of organ dose distribution for breast cancer treatment plans with Monte Carlo (MC) simulation is the main goal of this study. Elekta Precise linear accelerator (LINAC) photon mode was simulated and verified using the GEANT4 application for tomographic emission. Eight different radiotherapy treatment plans on RANDO's phantom left breast were produced with the ISOgray treatment planning system (TPS). The simulated plans verified photon dose distribution in clinical tumor volume (CTV) with TPS dose volume histogram (DVH) and gamma index tools. To verify photon dose distribution in out-of-field organs, the point dose measurement results were compared with the same point doses in the MC simulation. Eventually, the DVHs for out-of-field organs that were extracted from the TPS and MC simulation were compared. Based on the implementation of gamma index tools with 2%/2 mm criteria, the simulated LINAC output demonstrated high agreement with the experimental measurements. Plan simulation for in-field and out-of-field organs had an acceptable agreement with TPS and experimental measurement, respectively. There was a difference between DVHs extracted from the TPS and MC simulation for out-of-field organs in low-dose parts. This difference is due to the inability of the TPS to calculate dose distribution in out-of-field organs. Based on the results, it was concluded that the treatment plans with the MC simulation have a high accuracy for the calculation of out-of-field dose distribution and could play a significant role in evaluating the important role of dose distribution for second primary cancer estimation.

FLASH radiotherapy and the associated dosimetric challenges

At Lund University and Skåne University Hospital in Lund, Sweden, we have, as the first clinic, modified a clinical Elekta Precise linear accelerator for convertible delivery of ultra-high dose rate (FLASH) irradiation. Whereas recently published reviews highlighted the need for standardised protocols for ultra-high dose rate beam dosimetry to be able to determine the true potential of FLASH irradiation, several dosimetry studies as well as in-vitro and in-vivo experiments have been carried out at our unit. Dosimetric procedures for verification of accurate dose delivery of FLASH irradiation to cell cultures, zebrafish embryos and small animals have been established using radiochromic films and thermo-luminescent dosimeters. Also, recently the first experience of electron FLASH radiotherapy (FLASH-RT) in canine patients in our clinical setting was published. Our research facilities also include a laboratory for 3D polymer gel manufacturing. Recently, we started investigating the feasibility of a NIPAM polymer gel dosimeter for ultra-high dose rate dosimetry. Furthermore, in the bunker of the modified Elekta linear accelerator, a Surface Guided Radiotherapy (SGRT) system is accessible. The Catalyst™ system (C-Rad Positioning, Uppsala, Sweden) provides optical surface imaging for patient setup, real-time motion monitoring and breathing adapted treatment. Aiming at treating patients using ultra-high dose rates, a real-time validation of the alignment between the beam and the target is crucial as the dose is delivered in a fraction of a second. Our research group has during the last decade investigated and developed SGRT workflows which improved patient setup and breathing adapted treatment for several cancer patient groups. Recently, we also started investigating the feasibility of a real-time motion monitoring system for surface guided FLASH-RT. Both FLASH related studies; 3D polymer gel dosimetry and surface guided FLASH-RT are to our knowledge the first of their kind. Following an introduction to the field of FLASH and the associated dosimetric challenges, we here aim to present the two ongoing studies including some preliminary results.

Test cases for commissioning an add-on micro multi-leaf collimator Apex for stereotactic radiosurgery treatments

Two comprehensive test cases are presented for dosimetric commissioning of a radiosurgery system, in a hospital where dedicated phantoms are not available. The system consisted of an Elekta Precise linear accelerator, an Apex micro multi-leaf collimator, and a Monaco treatment planning system (TPS). The purpose of Test I was to assess the dose accuracy with coplanar arc beams. Test II was an end-to-end type test, a rigid Leksell stereotactic frame was fixed to a watermelon phantom and Ergo++ TPS was used for stereotactic coordinates definition. The purpose of Test II was to assess the dose accuracy with non-coplanar arc beams and the influence of geometrical accuracy in the whole process. Ionization chambers were used for dose measurements. Results of Test I showed that discrepancies below 1% are achievable, while results of Test II allowed detection of geometric shifts < 1 mm with dose discrepancies lower than 1%. To the best of our knowledge, there are not published works reporting test cases for commissioning a stereotactic radiosurgery system like the one tested in this work. The designed test cases showed adequacy for assessment of TPS accuracy in complex treatment configurations, like those used in stereotactic radiosurgery.

Determination of effective source to surface distance and cutout factor in small fields in electron beam radiotherapy: A comparison of different dosimeters

Abstract Objective: The main purpose of this study is to calculate the effective source to surface distance (SSDeff) of small and large electron fields in 10, 15, and 18 MeV energies, and to investigate the effect of SSD on the cutout factor for electron beams a linear accelerator. The accuracy of different dosimeters is also evaluated. Materials and methods: In the current study, Elekta Precise linear accelerator was used in electron beam energies of 10, 15, and 18 MeV. The measurements were performed in a PTW water phantom (model MP3-M). A Semiflex and Advanced Markus ionization chambers and a Diode E detector were used for dosimetry. SSDeff in 100, 105, 110, 115, and 120 cm SSDs for 1.5 × 1.5 cm2 to 5 × 5 cm2 (small fields) and 6 × 6 cm2 to 20 × 20 cm2 (large fields) field sizes were obtained. The cutout factor was measured for the small fields. Results: SSDeff in small fields is highly dependent on energy and field size and increases with increasing electron beam energy and field size. For large electron fields, with some exceptions for the 20 × 20 cm2 field, this quantity also increases with energy. The SSDeff was increased with increasing beam energy and field size for all three detectors. Conclusion: The SSDeff varies significantly for different field sizes or cutouts. It is recommended that SSDeff be determined for each electron beam size or cutout. Selecting an appropriate dosimetry system can have an effect in determining cutout factor.

Set-up error validation with EPID images: Measurements vs Egs_cbct simulation.

In this study, the egs_cbct code's ability to replicate an electronic portal imaging device (EPID) is explored. We have investigated head and neck (H&N) setup verification on an Elekta Precise linear accelerator. It is equipped with an electronic portal imaging device (EPID) that can capture a set of projection images over different gantry angles. Cone-beam computed tomography (CBCT) images were reconstructed from projection images of two different setup scenarios. Projections of an Anthropomorphic Rando head phantom were also simulated by using the egs_cbct Monte Carlo code for comparison with the measured projections.Afterwards, CBCT images were reconstructed from this data. Image quality was evaluated against a metric defined as the image acquisition interval (IAI). It determines the number of projection images to be used for CBCT image reconstruction. From this results it was established that phantom shifts could be determined within 2 mm and rotations within one degree accuracy using only 20 projection images (IAI = 10 degrees). Similar results were obtained with the simulated data. In this study it is demonstrated that a head and neck setup can be verified using substantially fewer projection images. Bony landmarks and air cavities could still be observed in the reconstructed Rando head phantom. The egs_cbct code can be used as a tool to investigate setup errors without tedious measurements with an EPID system.

Effective Source-Surface Distance in Various Field Sizes and Electron Beam Energies and its Effect on Cutout Factor in a Elekta Precise Linear Accelerator

Introduction: In electron beam treatment, because of the non-point electron beam source, inverse-square law cannot be applied for dosimetry in different treatment intervals. Therefore, providing source-surface distance (SSD) charts in all clinics is of paramount importance. This study aimed to determine the effective SSD for various electron beam energies and field sizes and to evaluate its effect on cutout factor in a linear accelerator. Materials and Methods: We used Elekta Precise linear accelerator in Ayatollah Khansari Hospital, Arak, Iran, for various energy levels (10, 15, and 18 MeV). The measurement environment was MP3-M water phantom (PTW Co., Canada), and diode detector was utilized for dosimetry. The effective SSD and cutout factor was estimated for 100, 105, 110, 115, and 120 cm SSDs and 1.5×1.5 and 20×20 cm2 square fields. Results: The effective SSD in the 1.5×1.5 to 20×20 cm2 fields altered from 29.95 to 93.95 cm, 50.40 to 96.50 cm, and 63.51 to 95.32 cm for different energy levels of 10 MeV, 15 MeV, and 18 MeV, respectively. The cutout factor increased along with the field size, but decreased by extending the SSD. These alterations were more significant for the energy level of 10 MeV. Conclusion: Since the effective SSD is dependent on energy level and field size, it is recommended to independently compute the effective SSD considering these variables. Furthermore, for designing accurate therapies, cutout factor variations should be considered for small-sized fields, especially at low energy levels.

Validation of treatment planning system using simulation PRIMO code.

Introduction: In radiation therapy, in order to double-check the dosimetric results of the main treatment planning system (TPS), a distinct TPS, with few capacitances in terms of contouring and a variety of dose calculation algorithms is used. This system has the capability to double check the planification and the accurate prediction of dose distribution in order to be ensured of the accuracy of the primary TPS results. The purpose of this study was to investigate the performance of Monte Carlo based algorithm, PRIMO, to double check the primary treatment planning software, and resolving the problem of limited budgets of radiotherapy centers in purchasing such systems. The present study evaluate the validation of the PRIMO code, in simulation of conformal treatments, which was performed with an Elekta Precise linear accelerator and an ISOgray treatment planning system. Materials and Methods: In this study, the PRIMO code version 0.3.1.1626 was used. This code contains the geometry of Eleckta, Varian, Siemens linear accelerators and also CyberKnife system. The CT images of the Alderson Rando phantom, and the slab water phantom, contouring and treatment planning including photon energy, treatment fields and dose prescription point which was taken from ISOgray system in Dicom format were inputs of the PRIMO software. DICOM Editor Tools was used to solve the possible problems in reading these dicom files. To accelerate the simulation, the phase space files of a 6 MV photon beam derived from the International Atomic Energy Agency (IAEA) database. Results: The gamma criteria of 3%/. 3 was used to compare the calculation results the simulation PRIMO code with ISOgray TPS system. Conclusion: The results of this study indicate the potential of the PRIMO code for treatment planning in radiation therapy.

136. Which is the optimum cylindrical phantom grid size calculation to evaluate the accuracy of gamma index for times optimizing in the pre-treatment quality assurance procedure of VMAT and IMRT tecnique?

PurposeIMRT and VMAT are the state-of-the-art irradiation techniques for the delivery of highly conformal radiation fields to PTV. The use of QA tools for the verification of the planned dose distribution prior to the treatment of the patient has become a standard procedure in clinical routine. Aim of this study is to evaluate the optimal grid size calculation on cylindrical phantoms to determine the gamma index (γ) levels for the routinely pre-treatment dosimetric QA. Methods and materialsThe measurements were made on a Elekta Precise linear accelerator and compared with the created plans by the Philips Pinnacle3 TPS. The pre-treatment evaluations were made with the PTW_OCTAVIUS-4D phantom with the 729 cubical ion chambers arranged in the OCTAVIUS_Detector 2D_array of 27 cm × 27 cm. The beam incidence is always perpendicular to the surface of the detector panel and the data were collected and evaluated by means of the PTW_VeriSoft software package. On about 100 patients who underwent to radiotherapy with VMAT, were evaluated the γ between the dose deliveries and the dose calculations on the cylindrical phantom with cubic step size of 4 mm, 3 mm and 2 mm. For each phantom grid size the γ value with [3 mm;3%], [2 mm;2%] and [1 mm;1%] as distance and dose difference criteria respectively were determined. ResultsFor each fixed γ[d(mm);D(%)] criteria the γ global ratios to the 3 mm voxel phantom calculation, followed a Gaussian distribution. That mean that there isn’t a correlation between the used phantom grid size and γ accordance. The agreement in γ differences were found within 2%(2SD), 3%(2SD) and 5%(2SD) for γ[3 mm;3%], γ[2 mm;2%] and γ[1 mm;1%] respectively. ConclusionsDue to the computer limits and the large amount of calculation, the TPS algorithms are often time consuming, strongly depending on the set grid size calculation. For a busy Center a voxel grid size of 3–4 mm is enough appropriate.

346. “Day after” high energy linear accelerator decomissioning: Characterization of the head of the accelerator

Purpose Italian laws are very restrictive in terms of release and exemption quantities in case of waste contaminated by radioactive isotopes. This study focuses on the characterization of a 18 MV Elekta Precise Linear Accelerator head that have been in clinical use since two days before its dismantling to make space to a new “state of the art” machine. Methods Dismantling have been carried out by an Italian specialized company, each part of the machine have been first measured with a high efficiency α, β, γ contamination monitor equipped with ZnS:Ag detector and a portable γ-ray spectrometer (NaI(Tl)). Activated parts have been subsequently measured with a low background, high reliability and high definition spectrometric chain (DSA-1000 and Genie 2000 software, Canberra), with an electric cooled High Purity Germanium detector (HPGe) with 20% relative efficiency (GX2018 detector and Cryo-Pulse5 plus cooler, Canberra). Energy, FWHM and efficiency calibration have been performed with a multigamma Marinelli source in the range 59 keV (241 Am) – 1836 keV (88Y). Efficiency curves for different geometries have been calculated with a 3D model of the detector-sample system and Montecarlo (MC) method by ISOCS/LabSOCS efficiency software (Canberra). Results 57Co, 58Co, 60Co, 54Mn have been among the more frequent radioisotopes found. However short lived nuclides (eg. 122Sb, 187W, 57Ni) were present as well. Conclusions Activation of long lived radionuclides was confirmed as previously reported in published paper [1] , [2] ; short lived radionuclides were detected as well. Waiting for full decay of short lived radioisotopes could allow to minimize the amount of radioactive waste. Download : Download high-res image (99KB) Download : Download full-size image

Early Clinical Outcome of the First Lung SBRT Program in Jordan: Importance of International Collaboration

Background: The stereotactic radiotherapy (SBRT) program was established at King Hussein Cancer Center (KHCC) in Jordan through cooperation with an internationally renowned institution MD Anderson Cancer Center (MDACC) in USA, and it went clinical in 2012. Until the present day, it stands as the only SBRT program in the entire country with patient population that has increased in the past few years due influx of refugees from regional conflicts. In this presentation, we will present the clinical outcome of our SBRT program. Methods: 17 patients treated to date in the SBRT service. All patients underwent 10-phase 4DCT and PET-CT scans. The internal target volume (ITV) was constructed from the minimum intensity projection (MIP) dataset and expanded, if needed, following PET findings. 5 mm margin added to create the PTV. A dose scheme of 48 Gy in 4 fractions or 50 Gy/4 fractions were used for all patients except for two patients who received 60 Gy in 8 fractions due to toxicity concerns. Lung heterogeneity correction was used during planning and treatment delivery was done on Elekta Precise Linear Accelerator and using CBCT imaging for positioning for every fraction. Follow- up done by performing CT chest 3 months after completion of SBRT, and CT/PET every 6 months in the two years, CT chest every 4-6 months thereafter, and keeping CT/PET whenever indicated. Results: All patients treated were males except one, with age ranging from 50-84 years old (mode of 79). All patients were unfit for surgery except for one who refused surgery. All patients except one are alive with only one patient recurred locally. One patient died not due to cancer. Conclusion: The newly established SBRT clinical service in our country, serves as the only such treatment of a population of 9.5 million including 2.5 million refugees. We have started recruiting inoperable lung patients to the service at a slow pace to gain more confidence and experience before admitting larger number of patients. Clinical results are encouraging with most showing tumor regression/complete response as of last follow-up. This model of collaboration between KHCC and MDACC represents a successful scientific collaboration between cancer centers in developed and developing countries that lead to effective and safe implementation of new techniques and procedures.

Assessment of the effects of radiation type and energy on the calibration of TLD-100

Introduction: In radiation therapy, knowing the dose rates to healthy organs and tumors is beneficial, and thermoluminescent dosimeter (TLD) allows for this possibility. This study was aimed at determining the dose-response differences of TLDs in various types of radiation, energy levels, and dose rate calibrated with other types of radiation beams and energy and dose levels. Materials and Methods: In this study, LiF:Mg,Ti (TLD-100) was used for dosimetry. Photon and electron irradiation was performed by Elekta Precise Linear Accelerator. First, TLDs were calibrated in three different groups of 6 MV photon, 6 MeV electron, and 60Co teletherapy photon beam with 50 cGy dose. Next, each group was irradiated with 6 MV photon, 6 MeV electron, and 60Co teletherapy photon beam separately at three different dose levels of 20, 60, and 100 cGy. Results: TLDs calibrated with electron were significantly different at all dose levels and with all types of radiation from TLDs calibrated with photon or 60Co teletherapy photon beam (P=0.000). P-value of the TLDs calibrated with 6 MV photon versus 60Co was less than 0.94. The maximum standard deviation belonged to 100 cGy irradiation, while the least pertained to 20 cGy irradiation. Conclusion: Calibration of TLDs depends on the type of radiation.

A Validation Study on Radiation properties of a Novel LiF: Mg, Ti Known as IAP-100

Introduction: LiF dosimeter has the most application in medicine. This study aimed to evaluate some dosimetric properties of a novel LiF: Mg, Ti. Materials and Methods: An ELEKTA Precise linear accelerator was used to calibrate dosimeters at 6 MV. In this survey, responses of dosimeters were evaluated up to 1000 cGy. Background effect was investigated in two different dosimeter states including irradiated and unirradiated.Thermoluminescence response dependence to dose rate was investigated, as well. Energy dependence was evaluated in diagnostic and therapeutic ranges. Furthermore, fading effect was evaluated by reading the dosimeters every 2 h up to 12 h post-irradiation. Results: The dosimeters had linear response up to 250 cGy. Readout values of dosimeters receiving 120 cGy at three dose rates of 21, 212, 425 cGy.min-1 were calculated equal to 125, 123, 121 cGy, respectively. The measured values of delivering 80, 120, and 150 cGy prescribed doses at 6 MV, 10 MV, and 15 MV were accurate at 6 MV and about 1.5 times higher than the prescribed doses at 10 and 15 MV. Thermoluminescence response in diagnostic energy range showed an uprising trend with increasing energy. Conclusion: The raising thermoluminescence response with increasing energy contradicts with the findings of Nunn. Due to the reproducibility and linear response of dosimeters in an acceptable dose range, they could be used in diagnostic and therapeutic fields. Effects of absorbed doses from background in low-dose studies, mainly in diagnostic radiology range, could be evaluated in more detail in future surveys.

Electron beam water calorimetry measurements to obtain beam quality conversion factors.

To provide results of water calorimetry and ion chamber measurements in high-energy electron beams carried out at the National Research Council Canada (NRC). There are three main aspects to this work: (a) investigation of the behavior of ionization chambers in electron beams of different energies with focus on long-term stability, (b) water calorimetry measurements to determine absorbed dose to water in high-energy beams for direct calibration of ion chambers, and (c) using measurements of chamber response relative to reference ion chambers, determination of beam quality conversion factors, kQ , for several ion chamber types. Measurements are made in electron beams with energies between 8MeV and 22MeV from the NRC Elekta Precise clinical linear accelerator. Ion chamber measurements are made as a function of depth for cylindrical and plane-parallel ion chambers over a period of five years to investigate the stability of ion chamber response and for indirect calibration. Water calorimetry measurements are made in 18MeV and 22MeV beams. An insulated enclosure with fine temperature control is used to maintain a constant temperature (drifts less than 0.1mK/min) of the calorimeter phantom at 4°C to minimize effects from convection. Two vessels of different designs are used with calibrated thermistor probes to measure radiation induced temperature rise. The vessels are filled with high-purity water and saturated with H2 or N2 gas to minimize the effect of radiochemical reactions on the measured temperature rise. A set of secondary standard ion chambers are calibrated directly against the calorimeter. Finally, several other ion chambers are calibrated in the NRC 60 Co reference field and then cross-calibrated against the secondary standard chambers in electron beams to realize kQ factors. The long-term stability of the cylindrical ion chambers in electron beams is better (always <0.15%) than plane-parallel chambers (0.2% to 0.4%). Calorimetry measurements made at 22MeV with two different vessel geometries are consistent within 0.2% after correction for the vessel perturbation. Measurements of absorbed dose calibration coefficients for the same secondary standard chamber separated in time by 10yr are within 0.2%. Drifts in linac output that would affect the transfer of the standard are mitigated to the 0.1% level by performing daily ion chamber normalization measurements. Calibration coefficients for secondary standard ion chambers can be achieved with uncertainties less than 0.4% (k=1) in high-energy electron beams. The additional uncertainty in deriving calibration coefficients for well-behaved chambers indirectly against the secondary standard reference chambers is negligible. The kQ factors measured here differ by up to 1.3% compared to those in TG-51, an important change for reference dosimetry measurements. The measurements made here of kQ factors for eight plane-parallel and six cylindrical ion chambers will impact future updates of reference dosimetry protocols by providing some of the highest quality measurements of this crucial dosimetric parameter.

Verification of an irregular field algorithm of a treatment planning system using a locally designed pelvic phantom: A simple design low-cost phantom suitable for quality assurance and control test in radiotherapy

BACKGROUND: Modern radiotherapy treatment machine today comes along side with sophisticated treatment planning systems (TPSs). The accuracy of any TPS depends on the mathematical algorithm it uses and can be well verified using a dedicated phantom. AIMS AND OBJECTIVES: To design a low-cost pelvic phantom and to use the designed phantom to verify whether the accuracy of an Irregular Field Algorithm of a Precise PLAN 2.16 TPS is within ±5% International Commission on Radiation Units and Measurements (ICRU) minimal limit. MATERIALS AND METHODS: Designed pelvic phantom was made of Plexiglas with six tissue equivalent inserts and an ion-chamber port. The mimicked organs for the inserts were: Prostate, bladder, adipose, muscle, rectum, and bone. A Hi-speed computed tomography (CT) simulator was used for acquiring images and CT numbers of the designed pelvic phantom, a Precise PLAN 2.16 TPS was used for image planning, an Elekta-Precise Clinical Linear Accelerator was used for prescription of the planned images and a precalibrated NE 2570/1 farmer-type ion-chamber with an electrometer was used to calculate the mean dose. Data analysis value was done using GraphPad Prism 5.0 statistics software. RESULTS: The maximum percentage deviation with large field sizes of 22 cm × 25 cm for six different inhomogeneous inserts was −3.95%, and bone only homogeneous inserts was 2.38%. The maximum percentage deviation with small field sizes of 5 cm × 5 cm with six different inhomogeneous inserts was −3.57%. The percentage deviation between the solid water phantom and the locally designed pelvic phantom was −3.46%. CONCLUSION: The irregular field algorithm showed an overall accuracy of approximately ±4% with the locally designed pelvic phantom for both large and small field sizes against ±5% ICRU minimal limit. Although there were significant differences in percentage deviation between inhomogeneity and homogeneous insert irrespective of field sizes.

Practical issues regarding angular and energy response in in vivo intraoperative electron radiotherapy dosimetry

AimTo estimate angular response deviation of MOSFETs in the realm of intraoperative electron radiotherapy (IOERT), review their energy dependence, and propose unambiguous names for detector rotations. BackgroundMOSFETs have been used in IOERT. Movement of the detector, namely rotations, can spoil results. Materials and methodsWe propose yaw, pitch, and roll to name the three possible rotations in space, as these unequivocally name aircraft rotations. Reinforced mobile MOSFETs (model TN-502RDM-H) and an Elekta Precise linear accelerator were used. Two detectors were placed in air for the angular response study and the whole set of five detectors was calibrated as usual to evaluate energy dependence. ResultsThe maximum readout was obtained with a roll of 90° and 4MeV. With regard to pitch movement, a substantial drop in readout was achieved at 90°. Significant overresponse was measured at 315° with 4MeV and at 45° with 15MeV. Energy response is not different for the following groups of energies: 4, 6, and 9MeV; and 12MeV, 15MeV, and 18MeV. ConclusionsOur proposal to name MOSFET rotations solves the problem of defining sensor orientations. Angular response could explain lower than expected results when the tip of the detector is lifted due to inadvertent movements. MOSFETs energy response is independent of several energies and differs by a maximum of 3.4% when dependent. This can limit dosimetry errors and makes it possible to calibrate the detectors only once for each group of energies, which saves time and optimizes lifespan of MOSFETs.

O2. Verification of an irregular field algorithm of a treatment planning system using a locally designed pelvic phantom: A simple design low cost phantom suitable for research and quality assurance

Introduction The accuracy of a treatment planning system (TPS) depends on the mathematical algorithm it uses. One major problem with most radiotherapy centers in Nigeria is the non-availability of QA phantoms for treatment planning system verification. The aim was to design a low cost phantom and to use the designed phantom to verify the accuracy of the Irregular Field Algorithm of a Precise PLAN 2.16 TPS. Materials and Methods The designed pelvic phantom was made of Plexiglas with six tissue equivalent inserts and an ion-chamber port. The pelvic phantom was designed by using elemental compositions as follows: prostate was C:MgO:H2O(56%:21%:23%), bladder was C:MgO:H2O(52%:25.5%:22.5%), adipose was C:MgO:H2O(70%:11%:19%), muscle was C:MgO:H2O(55%:23%:22%), rectum was C:MgO:H2O(56%:26.5%:27.5%) and bone was C:MgO:H2O:Ca (37%:25%:23%:15%). A Hi-Speed CT simulator was used for acquiring images and CT numbers of the designed pelvic phantom, a Precise PLAN TPS was used for the treatment planning. Absorbed dose measurements were carried out at the Elekta-Precise Linear Accelerator using a pre-calibrated NE 2570/1 farmer-type ion-chamber. Data analysis was done using GraphPad Prism 7.0 statistics software. Results The maximum percentage deviation with large field sizes of 22 × 25 cm2 for six different inhomogeneous inserts was −3.95% and bone only homogeneous inserts was 2.38%. The maximum percentage deviation with small field sizes of 5 × 5 cm2 with six different inhomogeneous inserts was −3.57%. The percentage deviation between the solid water phantom and the locally designed pelvic phantom was −3.35%. Conclusion The Irregular Field Algorithm showed an overall accuracy of approximately ± 4% with the locally designed pelvic phantom for both large and small field sizes.

SU-F-T-577: Comparison of Small Field Dosimetry Measurements in Fields Shaped with Conical Applicators On Two Different Accelerating Systems

Purpose: To investigate small field dosimetry measurements and associated uncertainties when conical applicators are used to shape treatment fields from two different accelerating systems. Methods: Output factor measurements are made in water in beams from the CyberKnife radiosurgery system, which uses conical applicators to shape fields from a (flattening filter-free) 6 MV beam, and in a 6 MV beam from the Elekta Precise linear accelerator (with flattening filter) with BrainLab external conical applicators fitted to shape the field. The measurements use various detectors: (i) an Exradin A16 ion chamber, (ii) two Exradin W1 plastic scintillation detectors, (iii) a Sun Nuclear Edge diode, and (iv) two PTW microDiamond synthetic diamond detectors. Profiles are used for accurate detector positioning and to specify field size (FWHM). Output factor measurements are corrected with detector specific correction factors taken from the literature where available and/or from Monte Carlo simulations using the EGSnrc code system. Results: Differences in measurements of up to 1.7% are observed with a given detector type in the same beam (i.e., intra-detector variability). Corrected results from different detectors in the same beam (inter-detector differences) show deviations up to 3 %. Combining data for all detectors and comparing results from the two accelerators results in a 5.9% maximum difference for the smallest field sizes (FWHM=5.2–5.6 mm), well outside the combined uncertainties (∼1% for the smallest beams) and/or differences among detectors. This suggests that the FWHM of a measured profile is not a good specifier to compare results from different small fields with the same nominal energy. Conclusion: Large differences in results for both intra-detector variability and inter-detector differences suggest potentially high uncertainties in detector-specific correction factors. Differences between the results measured in circular fields from different accelerating systems provide insight into sources of variability in small field dosimetric measurements reported in the literature.

SU‐F‐P‐09: A Global Medical Physics Collaboration for Implementation of Modern Radiotherapy in Botswana

Purpose:The global burden of cancer is considerable, particularly in low and middle‐income countries. Massachusetts General Hospital (MGH) and Botswana‐Harvard AIDS Institute have partnered with the oncology community and government of Botswana to form BOTSOGO (BOTSwana Oncology Global Outreach) to address the rising burden of cancer in Botswana. Currently, radiation therapy (RT) is only available at a single linear accelerator (LINAC) in Gaborone Private Hospital (GPH). BOTSOGO worked to limit the absence of RT during a LINAC upgrade and ensure a safe transition to modern radiotherapy techniques.Methods:The existing Elekta Precise LINAC was decommissioned in November 2015 and replaced with a new Elekta VERSA‐HD with IMRT/VMAT/CBCT capability. Upgraded treatment planning and record‐and‐verify systems were also installed. Physicists from GPH and MGH collaborated during an intensive on‐site visit in Botswana during the commissioning process. Measurements were performed using newly purchased Sun Nuclear equipment. Photon beams were matched with an existing model to minimize the time needed for beam modeling and machine down time. Additional remote peer review was also employed. Independent dosimetry was performed by irradiating OSLDs, which were subsequently analyzed at MGH.Results:Photon beam quality agreed with reference data within 0.2%. Electron beam data agreed with example clinical data within 3%. Absolute dose calibration was performed using both IAEA and AAPM protocols. Absolute dose measurements with OSLDs agreed within 5%. Quentry cloud‐based software was installed to facilitate remote review of treatment plans. Patient treatments resumed in February 2016. The time without RT was reduced, therefore likely resulting in reduced patient morbidity/mortality.Conclusion:A global physics collaboration was utilized to commission a modern LINAC in a resource‐constrained setting. This can be a useful model in other areas with limited resources. Further use of technology and on‐site exchanges will facilitate the introduction of more advanced techniques in Botswana.We acknowledge funding support from the AAPM International Educational Activities Committee and the NCI Federal Share Proton Beam Program Income Grant.

TU-F-CAMPUS-T-04: Variations in Nominally Identical Small Fields From Photon Jaw Reproducibility and Associated Effects On Small Field Dosimetric Parameters

Purpose: To investigate uncertainties in small field output factors and detector specific correction factors from variations in field size for nominally identical fields using measurements and Monte Carlo simulations. Methods: Repeated measurements of small field output factors are made with the Exradin W1 (plastic scintillation detector) and the PTW microDiamond (synthetic diamond detector) in beams from the Elekta Precise linear accelerator. We investigate corrections for a 0.6x0.6 cm2 nominal field size shaped with secondary photon jaws at 100 cm source to surface distance (SSD). Measurements of small field profiles are made in a water phantom at 10 cm depth using both detectors and are subsequently used for accurate detector positioning. Supplementary Monte Carlo simulations with EGSnrc are used to calculate the absorbed dose to the detector and absorbed dose to water under the same conditions when varying field size. The jaws in the BEAMnrc model of the accelerator are varied by a reasonable amount to investigate the same situation without the influence of measurements uncertainties (such as detector positioning or variation in beam output). Results: For both detectors, small field output factor measurements differ by up to 11 % when repeated measurements are made in nominally identical 0.6x0.6 cm2 fields. Variations in the FWHM of measured profiles are consistent with field size variations reported by the accelerator. Monte Carlo simulations of the dose to detector vary by up to 16 % under worst case variations in field size. These variations are also present in calculations of absorbed dose to water. However, calculated detector specific correction factors are within 1 % when varying field size because of cancellation of effects. Conclusion: Clinical physicists should be aware of potentially significant uncertainties in measured output factors required for dosimetry of small fields due to field size variations for nominally identical fields.

Treatment planning validation for symmetric and asymmetric motorized wedged fields

Purpose: Wedged beam are often used in clinical radiotherapy to compensate missing tissues and dose gradients. The Elekta Precise linear accelerator supports an internal motorized wedge, which is a single large, physical wedge on a motorized carriage. In this study, the dosimetric performance of Elekta precise three dimensional treatment planning system (3DTPS) is evaluated by comparing the calculated and measured doses. Methods: The calculations were performed by the 3DTPS for symmetric as well as asymmetric fields in a source to skin distance (SSD) setup at the depth of maximum dose (d max ) as well as at 5, 10, and 20 cm depths in water phantom using 60° motorized wedges for field sizes of 4 × 4, 10 × 10, and 20 × 20 cm 2 for 6 and 15 MV photon beams. Measurements were produced by Elekta Precise linear accelerator using 0.125 cc volume ionization chamber. Results: Good agreement between the measured and calculated isodose lines were found, with the maximum difference not exceed 5%. The difference between the calculated and measured data increases as the field size decreases, and the deviation in symmetric setting was less than that of asymmetric setting. The increase in wedge angle led to increase in the difference between calculated and measured data. Conclusion: The results from this study showed that the accuracy of Elekta Precise 3DTPS used with the motorized wedges for symmetric and asymmetric fields is adequate for the clinical applications under the studied experimental conditions.