Tissue Phantom Ratio Research Articles

Beam quality and the mystery behind the lower percentage depth dose in grid radiation therapy

Grid therapy recently has been picking momentum due to favorable outcomes in bulky tumors. This is being termed as Spatially Fractionated Radiation Therapy (SFRT) and lattice therapy. SFRT can be performed with specially designed blocks made with brass or cerrobend with repeated holes or using multi-leaf collimators where dosimetry is uncertain. The dosimetric challenge in grid therapy is the mystery behind the lower percentage depth dose (PDD) in grid fields. The knowledge about the beam quality, indexed by TPR20/10 (Tissue Phantom Ratio), is also necessary for absolute dosimetry of grid fields. Since the grid may change the quality of the primary photons, a new kq,q0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\mathbf{k}}_{\\mathbf{q},{\\mathbf{q}}_{0}}$$\\end{document} should be evaluated for absolute dosimetry of grid fields. A Monte Carlo (MC) approach is provided to resolving the dosimetric issues. Using 6 MV beam from a linear accelerator, MC simulation was performed using MCNPX code. Additionally, a commercial grid therapy device was used to simulate the grid fields. Beam parameters were validated with MC model for output factor, depth of maximum dose, PDDs, dose profiles, and TPR20/10. The electron and photon spectra were also compared between open and grid fields. The dmax is the same for open and grid fields. The PDD with grid is lower (~ 10%) than the open field. The difference in TPR20/10 of open and grid fields is observable (~ 5%). Accordingly, TPR20/10 is still a good index for the beam quality in grid fields and consequently choose the correct kq,q0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\mathbf{k}}_{\\mathbf{q},{\\mathbf{q}}_{0}}$$\\end{document} in measurements. The output factors for grid fields are 0.2 lower compared to open fields. The lower depth dose with grid therapy is due to lower depth fluence with scatter radiation but it does not impact the dosimetry as the calibration parameters are insensitive to the effective beam energies. Thus, standard dosimetry in open beam based on international protocol could be used.

Clinical reference dosimetry for the 0.5 T inline rotating biplanar Linac-MR.

The world's first clinical 0.5 T inline rotating biplanar Linac-MR system is commissioned for clinical use. For reference dosimetry, unique features to device, including an SAD=120cm, bore clearance of 60cm × 110cm, as well as 0.5 T inline magnetic field, provide some challenges to applying a standard dosimetry protocol (i.e., TG-51). In this work, we propose a simple and practical clinical reference dosimetry protocol for the 0.5T biplanar Linac-MR and validated its results. Our dosimetry protocol for this system is as follows: tissue phantom ratios at 20 and 10cm are first measured and converted into %dd10x beam quality specifier using equations provided and Kalach and Rogers. The converted %dd10x is used to determine the ion chamber correction factor, using the equations in the TG-51 addendum for the Exradin A12 farmer chamber used, which is cross-calibrated with one calibrated at a standards laboratory. For a 0.5 T parallel field, magnetic field effect on chamber response is assumed to have no effect and is not explicitly corrected for. Once the ion chamber correction factor for a non-standard SAD (kQ,msr) is determined, TG-51 is performed to obtain dose at a depth of 10cm at SAD=120cm. The dosimetry protocol is repeated with the magnetic field ramped down. To validate our dosimetry protocol, Monte Carlo (EGSnrc) simulations are performed to confirm the determined kQ,msr values. MC Simulations and magnetic Field On versus Field Off measurements are performed to confirm that the magnetic field has no effect. To validate our overall dosimetry protocol, external dose audits, based on optical simulated luminescent dosimeters, thermal luminescent dosimeters, and alanine dosimeters are performed on the 0.5 T Linac-MR system. Our EGSnrc results confirm our protocol-determined kQ,msr values, as well as our assumptions about magnetic field effects (kB=1) within statistical uncertainty for the A-12 chamber. Our external dosimetry procedures also validated our overall dosimetry protocol for the 0.5 T biplanar Linac-MR hybrid. Ramping down the magnetic field has resulted in a dosimetric difference of 0.1%, well within experimental uncertainty. With the 0.5 T parallel magnetic field having minimal effect on the ion chamber response, a TPR20,10 approach to determine beam quality provides an accurate method to perform clinical dosimetry for the 0.5 T biplanar Linac-MR.

Determination of Tissue Phantom Ratios of High Energy Photon Beams Using Monte Carlo Code

Monte Carlo codes are veritable tools in radiotherapy to understand the transport mechanism, dose distributions, and energy depositions of ionizing radiations traversing in different media. In some beam calibration protocols, a tissue-phantom-ratio at depths 20 cm and 10 cm (TPR20,10) in a phantom is used to determine the beam-quality conversion factor for different ion chambers. Thus, this study aims to evaluate the efficiency of Monte Carlo code in the determination of beam quality of high radiation energy beams similar to what is expected in clinical settings. Electron Gamma Shower National Research Council in Canada (EGSnrc) Monte Carlo Code was used to design complex ion chamber geometries according to the manufacturer’s specifications. Phase space files were used as x-ray beam radiation sources. Two set-ups (SAD and SSD) methods were used for the determination of TPR20,10 to conform to the accepted clinical procedures. Beam collimation was 10 cm by 10 cm field size with an ionization chamber placed at 20 cm and 10 cm in the water phantom to obtain doses at two points respectively. The TPR20,10 of each energy was obtained using the appropriate equations. The obtained results showed no significant difference between the measured and available TPR 20,10 data (p = 0.995). Ion chamber configurations and specifications were also found not to have a significant effect (p = 0.33) on the TPR 20,10 obtained values. The results obtained in this study are in agreement with the recommended standard values. The findings of this study show that the Monte Carlo codes can be used as a tool in determining x-ray beam quality indices of designed clinical linear accelerator (Linac) machines.

Comparison of the accuracy of Monte Carlo and Ray Tracing dose calculation algorithms for multiple target brain treatments on CyberKnife.

Single plan multiple brain targets (MBT) stereotactic radiosurgery dose difference between Monte Carlo (MC) and Ray Tracing (RT) algorithms has not been studied. A retrospective study and dose measurements were performed to access factors influencing dose differences. Fifty-three RT treatment plans with a total of 209 brain metastases were extracted from Precision Treatment Planning System (TPS). These plans were generated using fixed cones and were delivered using the CyberKnife M6 system. The same treatment plans were recalculated using MC algorithm and keeping the beam parameters unchanged. MC calculated plan parameters were extracted and dose differences were normalised to MC calculated dose. Correlations were investigated. RT and MC calculated off-centre-ratio (OCR) and tissue-phantom-ratio (TPRs) were exported from the TPS and compared with measured. Plans with 5 gross tumour volumes (GTVs) were created on a phantom and dose measured using a CC04 ionisation chamber and microdiamond detector for comparison with calculated doses. Calculated and measured TPR agreed within ± 1% beyond depth of maximum dose. The OCR showed differences up to 4.3% in the penumbra and out-of-field (OOF) regions. Largest RT and MC calculated GTV mean dose difference was -5.7%. An increase in the number of GTVs and reduction in the geometric separation of metastases were associated with increased differences between RT and MC calculated doses. In conclusion, calculated dose disagreement in MBT depends on the number of GTVs per plan, number of GTVs within a certain separation distance and plan complexity. MC dose calculation is recommended for complex CyberKnife SRS of MBT.

Dosimetric characterization of CyberKnife® robotic SRS using MOSFET-based "MOskin" detector

AimThis study assesses the implementation of MOSkin detectors for the SRS dosimetry of the CyberKnife® system. MethodThe performance of the MOSkin detectors was evaluated by using Iris collimators on water-equivalent phantoms in the CyberKnife® system. The dosimetric evaluation included measurements of the tissue–phantom ratio (TPR), output factor, and off-axis ratio. The results of the MOSkin sensors were compared with those of PTW 60016 and EDGE diodes. ResultsThe TPR measurements obtained by the MOSkin devices exhibited conformity lower than 2.9% compared with the results of the PTW60016 diodes across the field sizes considered. The output factor was consistent to within 2.8% of that of the EDGE detectors, following adjustment based on the appropriate field correction factors. The off-axis ratio of the MOSkin detectors corresponded to that of the EDGE diodes, with an FWHM of 4.3% and a penumbra narrower than 0.22 mm. ConclusionMOSkin sensors are strong contenders for Cyberknife® quality assurance of both machines and patients, as they are capable of reproducing measurements obtained by commercial sensors with an acceptable degree of precision.

Technical note: Determining equivalent squares of high-energetic photon fields.

We developed a method based on a physical pencil beam model for accurate equivalent square calculations for rectangular and irregular fields, for different definitions of equivalent squares, for beams with and without flattening filter, different photon energies, and depths in water. We considered two equivalent square definitions: equal dose at a point on the beam axis and equal depth dose, measured as tissue phantom ratio at 20 and 10cm depth ( ). As dose engine, we used an analytical pencil beam model. By integrating the pencil beam kernels, we assigned square fields to rectangular fields minimizing the dose, respectively, the difference. The results were compared with measurements at 100mm depth for nominal beam energies of 6 and 18MV, the Sterling equation, the geometric mean, and data from BJR Suppl 25 (British Institute of Radiology, 1996). Pencil beam results were closest to the measurements. An energy dependence of several millimeters for small field dimensions and depth dependencies for very elongated fields were observed. For the assignment of WFF square to FFF rectangular fields, using the equal- definition, our method agrees with previously published results. For circular fields approximated by leaves, we found deviations to the data from BJR Suppl. 25 below 1mm for diameters smaller than200mm. Our study shows that the validity range for geometric mean and Sterling equationis limited. Ergo, instead of specifying specific validity ranges, we suggest using the pencil beam method, valid for all aspect ratios, including elongated fields in the primary dose dominated regime. We published our method as python library and graphical user interface on GitHub. Users can choose between two definitions of equivalent square and between WFF and FFF fields. The implemented pencil beam method for irregular fields is also usable for quality assurance such as monitor unitchecks.

Verification of monitoring unit calculations for the 3D conformal radiotherapy treatment planning system

The present work aims to validate an Independent Calculation of Monitoring Units (ICMU) software developed to accurately calculate the number of Monitoring Units (MU) used in radiation treatment. The calculation was performed for the photon beam with a variety of nominal energies (6 MV − 18 MV) and filed size ranging from 3 × 3 cm2 to 40 × 40 cm2. Two set-up conventions of treatment techniques were tested, a constant source to surface distance (SSD) and an isocentric set-up with a constant source to axis distance (SAD). The ICMU software was written in C-sharp (C #), using the equivalent square field size concept to calculate the radiation field quantities and connects to a database containing measurements quantities such as Tissue Phantom ratio (TPR) and Output Factors (OFs). The ICMU was successfully validated by comparing it to the Treatment Planning System (TPS) or CMS Monitor Unit Calculations (CMS_MU). The required standards were satisfactorily met (5.0%).

Monte Carlo Simulation and Experimental Determination of Tissue Phantom Ratio for Photon Beams delivered from Medical Linear Accelerator

For an external radiotherapy procedure, the tissue phantom ratio (TPR20,10) is used as a quality index. This work presents an estimate of TPR20,10 using two cylindrical ionization chambers (NE2571 Farmer and PTW30013) in three high-energy photon modes (6, 10 and 15 MV) using both the Monte Carlo (MCNP) process and the experimental setup. The MCNP (version MCNP5) was used for the simulation of photon beams delivered by Varian-2300CD for the determination of TPR20,10 according to technical report series (TRS) 398. Again applying the same protocol TPR20,10 values were measured experimentally with NE2571 Farmer and PTW30013 chambers for the same medical linear accelerator (LINAC). The differences of TPR20, 10 between MCNP and experimental values were found for NE2571 Farmer chamber within 4.17 percent, 2.9 percent and 2.5 percent and similarly, these were within 3.89 percent, 2.71 percent and 1.98 percent at 6, 10 and 15 MV respectively for PTW30013. The TPR20,10 values simulated by our calculated MCNP demonstrated strong agreement with our experimental results.

Initial Beam Characteristics Calculations of 6 MV FF X-Ray Beam from Elekta Versa-HD Linac Using Topas MC Simulation Code

The focus of this research was to calculate initial beam characteristics of a 6 MV X-rays with flattening filter (FF) from Elekta Versa-HD linear accelerator (Linac) using the Tool for Particle Simulation (TOPAS) Monte Carlo (MC) simulation code. TOPAS 3.6 simulated data was compared with the measured data. During simulation, all the main component were simulated including Linac head, Multi-Leaf Collimators (MLCs), and uniform water phantom, while the SNC 3D scanner phantom and SNC125c Chamber were used for measurements. At collimator sizes of 5x5, 10x10, and 20x20 cm2, the depth dose scans (PDD) distribution and beam profiles (BP) were measured. The estimated tissue phantom ratio (TPR20,10) was 0.669, which was -1.1 percent lower than the measured TPR20,10 of 0.681. The obtained results show that the simulated and measured data were in good agreement, with gamma rate analysis of 3%/3 mm criteria reaching 95% and 99 % for PDD and BP respectively.

Efficient dose-rate correction of silicon diode relative dose measurements.

Silicon diodes are often the detector of choice for relative dose measurements, particularly in the context of radiotherapy involving small photon beams. However, a major drawback lies in their dose-rate dependency. Although ionization chambers are often too large for small field output factor (OF) measurements, they are valuable instruments to provide reliable percent-depth dose (PDD) curves in reference beams. The aim of this work is to propose a practical and accurate method for the characterization of silicon diode dose-rate dependence correction factors using ionization chamber measurements as a reference. The robustness of ionization chambers for PDD measurements is used to quantify the dose-rate dependency of a diode detector. A mathematical formalism, which exploits the error induced in percent-depth ionization (PDI) curves for diodes by their dose-rate dependency, is developed to derive a dose-rate correction factor applicable to diode relative measurements. The method is based on the definition of the recombination correction factor given in the addendum to TG 51 and is applied to experimental measurements performed on a CyberKnife M6 radiotherapy unit using a PTW 60012 diode detector. A measurement-based validation is provided by comparing corrected PDI curves to measurements performed with a PTW 60019 diamond detector, which does not exhibit dose-rate dependence. Results of dose-rate correction factors for PDI curves, off-axis ratios (OARs), tissue-phantom ratios, and small field OFs are coherent with the expected behavior of silicon diode detectors. For all considered setups and field sizes, the maximum correction and the maximum impact of the uncertainties induced by the correction are obtained for OARs for the 60 mm collimator, with a correction of 2.5% and an uncertainty of 0.34%. For OFs, corrections range from 0.33% to 0.82% for all field sizes considered, and increase with the reduction of the field size. Comparison of PDI curves corrected for dose-rate and for in-depth beam quality variations illustrates excellent agreement with measurements performed using the diamond detector. The proposed method allows the efficient and precise correction of the dose-rate dependence of silicon diode detectors in the context of clinical relative dosimetry.

A probabilistic approach for determining Monte Carlo beam source parameters: I. Modeling of a CyberKnife M6 unit

Objective. During Monte Carlo modeling of external radiotherapy beams, models must be adjusted to reproduce the experimental measurements of the linear accelerator being considered. The aim of this work is to propose a new method for the determination of the energy and spot size of the electron beam incident on the target of a linear accelerator using a maximum likelihood estimation. Approach. For that purpose, the method introduced by Francescon et al (2008 Med. Phys. 35 504–13) is expanded upon in this work. Simulated tissue-phantom ratios and uncorrected output factors using a set of different detector models are compared to experimental measurements. A probabilistic formalism is developed and a complete uncertainty budget, which includes a detailed simulation of positioning errors, is evaluated. The method is applied to a CyberKnife M6 unit using four detectors (PTW 60012, PTW 60019, Exradin A1SL and IBA CC04), with simulations being performed using the EGSnrc suite. Main results. The likelihood distributions of the electron beam energy and spot size are evaluated, leading to and . Using these results and a 95% confidence region, simulations reproduce measurements in 13 out of the 14 considered setups. Significance. The proposed method allows an accurate beam parameter optimization and uncertainty evaluation during the Monte Carlo modeling of a radiotherapy unit.

Determination of the Small-Field Output Factor for 6 MV Photon Beam Using EGSnrc Monte Carlo.

Accuracy of ionization chamber (IC) to measure the scatter output factor (Scp) of a linear accelerator (linac) is crucial, especially in small field (<4 cm × 4 cm). The common IC volume of 0.6 cc is not adequate for small-field measurement and not all radiotherapy centers can afford to purchase additional IC due to the additional cost. This study aimed to determine the efficiency of the EGSnrc Monte Carlo (MC) to calculate the Scp for various field sizes including small field in Elekta Synergy (Agility multileaf collimator) linac. The BEAMnrc and DOSXYZnrc user codes were used to simulate a 6 MV linac model for various field sizes and calculate the radiation dose output in water phantom. The modeled linac treatment head was validated by comparing the percentage depth dose (PDD), beam profile, and beam quality (Tissue Phantom Ratio (TPR)20,10) with the IC measurement. The validated linac model was simulated to calculate the Scp consisting of collimator scatter factor (Sc) and phantom scatter factor (Sp). The PDD and beam profile of the simulated field sizes were within a good agreement of ±2% compared with the measured data. The TPR20,10 value was 0.675 for field size 10 cm × 10 cm. The Scp, Sc, and Sp simulated values were close to the IC measurement within ±2% difference. The simulation for Sc and Sp in 3 cm × 3 cm field size was calculated to be 0.955 and 0.884, respectively. In conclusion, this study validated the efficiency of the MC simulation as a promising tool for the Scp calculation including small-field size for linac.

Validation of Monte Carlo dose calculation algorithm for CyberKnife multileaf collimator.

PurposeTo commission and evaluate the Monte Carlo (MC) dose calculation algorithm for the CyberKnife equipped with a multileaf collimator (MLC).MethodsWe created a MC model for the MLC using an integrated module of the CyberKnife treatment planning software (TPS). Two parameters could be optimized: the maximum energy and the source full width at half‐maximum (FWHM). The optimization was performed by minimizing the differences between the measured and the MC calculated tissue phantom ratios and profiles. MLC plans were calculated in the TPS with the MC algorithm and irradiated on different phantoms. The dose was measured using an A1SL ionization chamber and EBT3 Gafchromic films, and then compared to the TPS dose to obtain dose differences (ΔD). Finally, patient‐specific quality assurances (QA) were performed with global gamma index criteria of 3%/1 mm.ResultsThe maximum energy and source FWHM showing the best agreement with measurements were 6.4 MeV and 1.8 mm. The output factors calculated with these parameters gave an agreement within ±1% with measurements. The ΔD showed that MC model systematically underestimated the dose with an average of −1.5% over all configurations tested. For depths deeper than 12 cm, the ΔD increased, up to −3.0% (maximum at 15.5 cm depth).ConclusionsThe MC model for MLC of CyberKnife is clinically acceptable but underestimates the delivered dose by an average of −1.5%. Therefore, we recommend using the MC algorithm with the MLC only in heterogeneous regions and for shallow‐seated tumors.

On the evaluation of edgeless diode detectors for patient-specific QA in high-dose stereotactic radiosurgery.

In this work, the potential of an innovative "edgeless" silicon diode was evaluated as a response to the still unmet need of a reliable tool for plan dosimetry verification of very high dose, non-coplanar, patient-specific radiosurgery treatments. In order to prove the effectiveness of the proposed technology, we focused on radiosurgical treatments for functional disease like tremor or pain. The edgeless diodes response has been validated with respect to clinical practice standard detectors by reproducing the reference dosimetry data adopted for the Treatment Planning System. In order to evaluate the potential for radiosurgery patient-specific treatment plan verification, the anthropomorphic phantom Alderson RANDO has been adopted along with three edgeless sensors, one placed in the centre of the Planning Target Volume, one superiorly and one inferiorly. The reference dosimetry data obtained from the edgeless detectors are within 2.6% for output factor, off-axis ratio and well within 2% for tissue phantom ratio when compared to PTW 60,018 diode. The edgeless detectors measure a dose discrepancy of approximately 3.6% from the mean value calculated by the TPS. Larger discrepancies are obtained in very steep gradient dose regions when the sensors are placed outside the PTV. The angular independent edgeless diode is proposed as an innovative dosimeter for patient quality assurance of brain functional disorders and other radiosurgery treatments. The comparison of the diode measurements with TPS calculations confirms that edgeless diodes are suitable candidates for patient-specific dosimetric verification in very high dose ranges delivered by non-isocentric stereotactic radiosurgery modalities.

Validation of polylactic acid polymer as soft tissue substitutive in radiotherapy

We report the experimental study on the physical and radiation properties of polylactic acid polymer (PLA-C3H4O2) combined with Monte Carlo simulations for application in radiotherapy. The suitability of PLA for phantoms construction as a substitute for soft human tissue in MeV photon radiation dosimetry and therapy is discussed. The elemental composition and radiation properties are described as recommended by ICRU 44. In addition, Hounsfield unit (HU), mass attenuation coefficient, mass stopping power and tissue phantom ratio were determined. Previous works on this subject presented mainly the properties of the raw material formula, neglecting above all the effects of the printing matrix and commercial raw materials used for PLA's production. The present upon irradiation properties of polylactic acid polymer were contrasted to those known of solid water and attested the viability and performance of PLA to bio-mimic soft tissue. The experimental and simulated data demonstrate the adequacy of PLA to printed phantoms provided that the printer infills result in a density of 1.05–1.09 g/cm3.

Dosimetric impacts of beam-hardening filter removal for the CyberKnife system.

Equipment refurbishment was performed to remove the beam-hardening filter (BHF) from the CyberKnife system (CK). This study aimed to confirm the change in the beam characteristics between the conventional CK (present-BHF CK) and CK after the BHF was removed (absent-BHF CK) and evaluate the impact of BHF removal on the beam quality correction factors kQ. The experimental measurements of the beam characteristics of the present- and absent-BHF CKs were compared. The CKs were modeled using Monte Carlo simulations (MCs). The energy fluence spectra were calculated using MCs. Finally, kQ were estimated by combining the MC results and analytic calculations based on the TRS-398 and TRS-483 approaches. All gamma values for percent depth doses and beam profiles between each CK were less than 0.5 following the 3%/1 mm criteria. The percentage differences for tissue-phantom ratios at depths of 20 and 10cm and percentage depth doses at 10cm between each CK were-1.20% and-0.97%, respectively. The MC results demonstrated that the photon energy fluence spectrum of the absent-BHF CK was softer than that of the present-BHF CK. The kQ values for the absent-BHF CK were in agreement within 0.02% with those for the present-BHF CK. The photon energy fluence spectrum was softened by the removal of BHF. However, no remarkable impact was observed for the measured beam characteristics and kQ. Therefore, the previous findings of the kQ values for the present-BHF CK can be directly used for the absent-BHF CK.

Dose-area product ratio in external small-beam radiotherapy: beam shape, size and energy dependencies in clinical photon beams

In small-field radiotherapy (RT), a significant challenge is to define the amount of radiation dose absorbed in the patient where the quality of the beam has to be measured with high accuracy. The properties of a proposed new beam quality specifier, namely the dose-area-product ratio at 20 and 10 cm depths in water or DAPR20,10, were studied to yield more information on its feasibility over the conventional quality specifier tissue-phantom ratio or TPR20,10. The DAPR20,10 may be measured with a large-area ionization chamber (LAC) instead of small volume chambers or semi-conductors where detector, beam and water phantom positioning and beam perturbations introduce uncertainties. The effects of beam shape, size and energy on the DAPR20,10 were studied and it was shown that the DAPR20,10 increases with increasing beam energy similarly to TPR20,10 but in contrast exhibits a small beam size and shape dependence. The beam profile outside the beam limiting devices has been shown to have a large contribution to the DAPR20,10. There is potential in large area chambers to be used in DAPR measurement and its use in dosimetry of small-beam RT for beam quality measurements.

Monte Carlo calculated detector-specific correction factors for Elekta radiosurgery cones

A radiation field is considered small if its dimension is lower than the range of secondary electrons and the collimating devices partially occlude the source. Different detector types, such as unshielded diodes, diamond detectors, and small-volume ion chambers, are used for small-field measurements. Although the active volumes of these detectors are small, their non-water equivalent materials cause response variations. Herein, we aim to calculate the correction factors for our clinical detectors, EDGE detector (Sun Nuclear), 60017 diode (PTW), and CC01 ion chamber (IBA), for stereotactic radiosurgery cones of diameters of 5–15 mm in an Elekta Synergy linear accelerator using a Monte Carlo simulation. An Elekta Synergy linear accelerator treatment head was simulated using BEAMnrc Monte Carlo code as per the manufacturer specification. All three detectors were simulated as per the manufacturer specification. Three EGSnrc user codes were used for the detector simulation based on the detector geometry. The Monte Carlo model of the treatment head was validated against the measured data for a standard field size of 10 × 10 cm2. The off-axis profile, percentage depth dose, and tissue phantom ratio were verified in the validation procedure. The measured and Monte Carlo calculated relative output factors (ROFs) were not consistent. In a 5 mm field size, EDGE diode overestimated the ROF by 7.06%, and 60017 diode to 4.611%. In a 7.5 mm field size, the variations were 4.295% and 3.691% for EDGE and 60017 diodes, respectively. CC01 ion chamber under-responded up to 10% because of its low-density active volume. The maximum corrections were obtained in the smallest field size, which were 0.939(0.007), 0.962(0.006), and 1.117(0.008) for EDGE, PTW T60017, and CC01 detectors, respectively. After applying the Monte Carlo calculated correction factor to the measured ROF, it became consistent with the Monte Carlo calculated ROF.

Commissioning Measurements of Flattening Filter and Flattening Filter Free Photon Beams Using a TrueBeam Stx® Linear Accelerator

Introduction: TrueBeam STx® latest generation linear accelerators (linacs) were installed at Sheikh Khalifa International University Hospital Casablanca, Morocco, this study aimed to present and analyse the dosimetric characteristics obtained during the commissioning. Material and Methods: Dosimetric parameters, including percentage depth dose, profiles, output factor, multileaf collimator (MLC) transmission, and dosimetric leaf gaps (DLG) factors were systematically measured for commissioning. Moreover, six photons beams (i.e., X6MV, X6FFFMV, X10MV, X10FFFMV, X15MV, and X18MV) were examined in this study, and a comparison was made between flattening filter (FF) and flattening filter free (FFF) beams. Results: According to the results, the FF and FFF beams symmetry and flatness were in the tolerance intervals. The unflattness values were estimated at 1.1% and 1.2% for X6FFFMV and X10FFFMV, respectively. Furthermore, tissue phantom ratio(20/10)(TPR) values of the FF beams were X6MV, 0.664; X10MV, 0.738; X15MV, 0.761; and X18MV, 0.778, and the TPR (20/10) values of the FFF beams included 0.632 and 0.703 for 6FFFMV and 10FFFMV, respectively. The results also revealed that the output factor values increased with field size, the surface dose decreased with increasing energy, and the FFF obtained lower mean energy. The MLC transmissions factors were 0.0121, 0.0103, 0.0136, 0.0122, 0.0133, and 0.0121 for X6, X6FFF, X10, X10FFF, X15, and X18, respectively; additionally, the DLG factors were obtained at 0.32, 0.26, 0.41, 0.37, 0.42, and 0.38 mm for X6, X6FFF, X10, X10FFF, X15, and X18, respectively. Conclusion: Photon beams reference dosimetric characteristics were successfully matched with the international recommendations and vendor technical specifications.

Monte Carlo Simulation and Experimental Determination of Tissue Phantom Ratio for Megavoltage Photon Beam

Monte Carlo Simulation and Experimental Determination of Tissue Phantom Ratio for Megavoltage Photon Beam