Off-axis Ratio Research Articles

Research and correction for over-response phenomenon when using inorganic scintillator optical fiber X-ray sensor to measure off-axis ratio (OAR)

Research and correction for over-response phenomenon when using inorganic scintillator optical fiber X-ray sensor to measure off-axis ratio (OAR)

Validation of Acuros for total body irradiation at extended distance.

Standardized and accurately reported doses are essential in conventional total body irradiation (TBI), especially lung doses. This study evaluates the accuracy of the Acuros algorithm in predicting doses for extended-distance TBI. Measurements and calculations were done with both 6 and 18 MV. Tissue Maximum Ratio (TMR), output and off axis ratios (OAR) were measured at 200 and 500cm source to detector distance and compared to Acuros calculated values. Two end-to-end tests were carried out, one with an in-house phantom (solid water and Styrofoam) with inserted ion chambers and the other was with the Imaging and Radiation Oncology Core (IROC) TBI anthropomorphic phantom equipped with TLDs. The end-to-end test was done for 6 and 18 MV both with and without lung blocks. The source to midplane distance for both phantoms were at 518 and 508cm respectively. Lung blocks were placed at the phantom surface and a beam spoiler was positioned 30cm from the surface of the phantoms as per our clinical set up. The agreement between measured and calculated TMR, output and off axis ratios for both 6 and 18 MV were within 2%. Ion chamber measurements in both the Styrofoam and solid water for both energies carried out with and without lung blocks were within 2% of calculated values. TLD measured doses for both 6 and 18 MV in the IROC phantom were within 5% of calculated doses which is within the uncertainty of the TLD measurement. The results indicate that the clinical beam model for Acuros 16.1 commissioned at standard clinical distances is capable of calculating doses accurately at extended distances up to 500cm.

Effect of photons and electrons on the over-response of optical fiber X-ray sensors

Optical fiber X-ray sensors are prone to over-response compared to ionization chambers (ICs) when used to measure the percentage depth dose (PDD) curves and off axis ratio (OAR) curves. Investigating this phenomenon is crucial for the successful adoption of these sensors in clinical radiotherapy. Compared with ion chambers and other water equivalent dosimeters, the fiber X-ray sensors using high Zeff scintillators (Gd2O2S:Tb) have generally higher mass attenuation coefficients. In this case, the response of scintillators to photons needs to be considered. In this article, a Monte-Carlo (MC) software package BEAMnrc was used to simulate the distribution of photon and electron fluxes in a clinical based water phantom, besides the energy deposition of air and Gd2O2S:Tb in varying energy was also simulated. The distribution of the photon and electron fluxes in the horizontal (radial) direction at a fixed 10 cm water depth was studied, as well as the fluxes in the vertical (depth) direction. An embedded-structure fiber X-ray sensor was used for the experimental measurements. The simulation results indicate that, in the horizontal direction, there are many photons remain outside the radiation field. In the vertical direction, as the size of the radiation field increases, the maximum point of the photon flux shifts towards a deeper water depth. These behavior of photon flux is consistent with the fiber X-ray sensors measurement. However, the maximum point of the electron flux remains unchanged in the vertical direction. In the horizontal direction, the electron flux decreases rapidly, and this scenario is similar to the case when using the ion chamber. Results of analyzing the spectra inside and outside the radiation field show that there is a large number of photons with energy below 0.2 MeV outside the radiation field, and the proportion of these low-energy photons is greater than 60 %. The findings indicate that the fiber X-ray sensors' over-response can be primarily attributed to the lower energy scattered photons.

Validation of the geometric equivalent field concept in total scatter factor calculations, for half-, quarter- and off-isocenter asymmetric square fields.

Monitor unit (MU) verification for any symmetric or asymmetric field is performed using a total scatter factor (Scp ), that is calculated based on the geometric equivalent square field (GESF) concept. In this study, we measured the Scp of various asymmetric square fields (ASFs ) and their respective GESFs. Square half-fields (SHFs ), square quarter-fields (SQFs ) and square off-isocenter fields (SOFs ), with sizes ranging from 3×3 cm2 to 20×20 cm2 were created, by varying the collimator jaws of two Varian iX Linacs (6/18 and 6/23 MV). A semi-flex ion chamber was used to measure Scp at a depth of 10cm within a water phantom, at the effective field center (EFC) of all ASFs , and at the isocenter (IC) of their respective GESFs. The later Scp values were corrected by the off-axis ratio [OAR(r)] of the 40×40 cm2 field size, where r is the distance between EFC and IC. The results show that the Scp (EFC) is independent of the type of the ASF (SHF, SQF, or SOF) and no significant difference exists between the 18 and 23 MV beams. Compared with the Scp (IC), the Scp (EFC) increased with increasing r, by up to 2% and 4% for 18/23 and 6 MV, respectively. The GESF concept provides acceptable accuracy (<2%) for the calculation of Scp of the ASFs used in most clinical situations (except from SOF with EFC at large r), and thus can be used in MU verification calculations.

Development of the Independent Dose Verification Method for the Ring Gantry PET/CT Linac

The RefleXion Xi is a new external beam radiotherapy delivery modality combining kilovoltage fan-beam CT and on-board PET for Biology-guided Radiotherapy. Although the machine shares similar components with the Tomotherapy machine, it has unique hardware features such as stationary beam delivery system with couch increments. The goal of this study is to develop an in-house independent secondary dose calculation method for this ring gantry PET/CT Linac. The method aggregates the beam intensities at each discrete firing gantry angle and couch moving position. The non-uniform intensity map is decomposed into a series of segments of uniform beam intensities, and then coordinates of beam segments were modified based on the relative distance from the dose calculation point to keep the calculation point in the same spot along the gantry rotation. The dose to the calculation point of each segment is determined by using measured tissue-maximum-ratio, output factors, and off-axis ratio by independent binary collimators. Two-dimensional convolution method is applied to integrate all dose contribution components in segment areas for efficient calculation. The final dose is obtained by summing all dose to the calculation point of segments for all firing gantry angle and couch position. Twenty patients with different treatment sites including head and neck, prostate, and lung regions were analyzed. Comparison of the point dose calculated by the independent program to that calculated by the planning system has shown reasonable agreement within ±5%. The independent dose verification program has been developed as an initial patient specific QA for improving patient safety. Further study will be performed to enhance the accuracy and reliability by including additional leakage and scatter models.

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.

Development of a collapsed cone convolution/superposition dose calculation algorithm with a mass density-specific water kernel for magnetic resonance-guided radiotherapy.

This study aimed to develop and validate a collapsed cone convolution for magnetic resonance-guided radiotherapy (CCCMR). The 3D energy deposition kernels (EDKs) were generated in water in a 1.5-T transverse magnetic field. The CCCMR corrects the inhomogeneity in simulation geometry by referring to the EDKs according to the mass density between the interaction and energy deposition points in addition to density scaling. Dose distributions in a water phantom and in slab phantoms with inserted inhomogeneities were calculated using the Monte Carlo (MC) and CCCMR. The percentage depth dose (PDD) and off-axis ratio (OAR) were compared, and the gamma passing rate (3%/2mm) was evaluated. The CCCMR simulated asymmetric dose distributions in the simulation phantoms, especially the water phantom, and all PDD and OAR profiles were in good agreement with the findings of the MC. The gamma passing rates were >99% for each field size and for the entire region. In the inhomogeneity phantoms, although the CCCMR underestimated dose in the low mass density regions, it could reconstruct dose changes at mass density boundaries. The gamma passing rate for the entire region was >95% for the field size of 2 × 2cm2, but it was 68.9-86.7% for the field sizes of ≥5 × 5cm2. Conclusively, in water, the CCCMR can obtain dose distributions comparable to those with the MC. Although the dose differences between them were mainly in inhomogeneity regions, the possibility of the effective use of the CCCMR in small field sizes was demonstrated.

Calculation of Experimental Penumbral Width of X-ray Dose Distributions Using Multiple Detectors

X-ray penumbral width depends on the size of the detector in the off-axis ratio of medical linear accelerators. The use of detectors with appropriate sizes according to the irradiation field is recommended. However, when measuring dose distributions, the shape of the off-axis ratio differs depending on detectors even if the dose distribution remains unchanged. This study aims to calculate the penumbral width using multiple detectors and obtain the independent penumbral width according to irradiation size. Using the X-ray output from a medical linear accelerator, off-axis ratios in water were measured using nine types of detectors. Penumbral width was calculated from each measured off-axis ratio by linear approximation and determining the intercept for analysis. The experimental irradiation fields were square areas (1×1 cm2 to 10×10 cm2). Penumbral widths were obtained from three types of detectors with different sensitive region widths of at least 1.2 mm. The values estimated from the nine detectors were 2.51-4.07 and 2.93-4.70 mm for 6 and 10 MV X-rays, respectively. The penumbral width and variation due to detector size increased with the irradiation field. The results estimated from the three selected detectors varied within ±0.5 mm compared with those from the nine detectors and were generally consistent. The reliability of the results was evaluated by comparing with the results from past studies and Monte Carlo simulations. Calculation of the penumbral width can be done regardless of size for various detectors. Thus, dose distributions can be compared for the linear accelerator at different facilities.

Improving animal-specific radiotherapy quality assurance for kilovoltage X-ray radiotherapy using a 3D printed dog skull water phantom.

Accurate dose assessment during animal radiotherapy is beneficial for veterinary medicine and medical education. To visualize the radiation treatment distribution of orthovoltage X-ray equipment in clinical practice using Monte Carlo simulations and create a dog skull water phantom for animal-specific radiotherapy. EGSnrc-based BEAMnrc and DOSXYZnrc codes were used to simulate orthovoltage dose distributions. At 10, 20, 30, 40, 50, and 80 mm in a water phantom, the depth dose was measured with waterproof Farmer dosimetry chambers, and the diagonal off-axis ratio was measured with Gafchromic EBT3 film to simulate orthovoltage dose distributions. Energy differences between orthovoltage and linear accelerated radiotherapy were assessed with a heterogeneous bone and tissue virtual phantom. The animal-specific phantom for radiotherapy quality assurance (QA) was created from CT scans of a dog and printed with a three-dimensional printer using polyamide 12 nylon, with insertion points for dosimetry chambers and Gafchromic EBT3 film. Monte Carlo simulated and measured dose distributions differed by no more than 2.0% along the central axis up to a depth of 80 mm. The anode heel effect occurred in shallow areas. The orthovoltage radiotherapy percentage depth dose in bone was >40%. Build-up was >40%, with build-down after bone exit, whereas linear accelerator radiotherapy absorption changed little in the bone. A highly water-impermeable, animal-specific dog skull water phantom could be created to evaluate dose distribution. Animal-specific water phantoms and Monte Carlo simulated pre-treatment radiotherapy are useful QA for orthovoltage radiotherapy and yield a visually familiar phantom that will be useful for veterinary medical education.

Monte Carlo simulation of linac using PRIMO

BackgroundMonte Carlo simulation is considered as the most accurate method for dose calculation in radiotherapy. PRIMO is a Monte-Carlo program with a user-friendly graphical interface.Material and methodA VitalBeam with 6MV and 6MV flattening filter free (FFF), equipped with the 120 Millennium multileaf collimator was simulated by PRIMO. We adjusted initial energy, energy full width at half maximum (FWHM), focal spot FWHM, and beam divergence to match the measurements. The water tank and ion-chamber were used in the measurement. Percentage depth dose (PDD) and off axis ratio (OAR) were evaluated with gamma passing rates (GPRs) implemented in PRIMO. PDDs were matched at different widths of standard square fields. OARs were matched at five depths. Transmission factor and dose leaf gap (DLG) were simulated. DLG was measured by electronic portal imaging device using a sweeping gap method.ResultFor the criterion of 2%/2 mm, 1%/2 mm and 1%/1 mm, the GPRs of 6MV PDD were 99.33–100%, 99–100%, and 99–100%, respectively; the GPRs of 6MV FFF PDD were 99.33–100%, 98.99–99.66%, and 97.64–98.99%, respectively; the GPRs of 6MV OAR were 96.4–100%, 90.99–100%, and 85.12–98.62%, respectively; the GPRs of 6MV FFF OAR were 95.15–100%, 89.32–100%, and 87.02–99.74%, respectively. The calculated DLG matched well with the measurement (6MV: 1.36 mm vs. 1.41 mm; 6MV FFF: 1.07 mm vs. 1.03 mm, simulation vs measurement). The transmission factors were similar (6MV: 1.25% vs. 1.32%; 6MV FFF: 0.8% vs. 1.12%, simulation vs measurement).ConclusionThe calculated PDD, OAR, DLG and transmission factor were all in good agreement with measurements. PRIMO is an independent (with respect to analytical dose calculation algorithm) and accurate Monte Carlo tool.

The Study of Dosimetric Characteristics of the XHA600D Medical Linear Accelerator Based on a Monte Carlo Code

By investigating the influence of initial electrons on dosimetric characteristics, reasonable incident electron parameters for the nominal 6 MV photon beam of the XHA600D accelerator are finally established, i.e., a 6 MeV monoenergetic electron beam with a radial intensity FWHM of 2.5 mm and an angular divergency of 0.15°. Based on reasonable initial parameters, Percentage Depth Doses (PDDs), Off-Axis Ratios (OARs), total scatter factors, beam qualities, and penumbra widths of both flatteningfilter (FF) and flattening-filter-free (FFF) beams for fields ranging from 4 × 4 to 30 × 30 cm2 are simulated systematically with EGSnrc codes. Not only the simulated dosimetric properties are in excellent agreement with the measurements, but also the dosimetric discrepancies between FF and FFF beams are consistent with the laws of previous studies on other accelerators. Therefore, reasonable incident electron parameters are able to accurately verify the performance of the XHA600D accelerator and can be used for further dosimetry research.

Varian eclipse stereotactic 5mm cone data commissioning.

Conical collimators are effective and readily available accessories for the field shaping of small stereotactic fields, however the measurements required to accurately characterise the smallest radiation fields are difficult, prone to large errors, and there is little published commissioning data to compare measurements against. The aim of this investigation was to commission the cone dose calculation algorithm of a Varian Eclipse treatment planning system for a Varian 5mm cone attached to a Varian TrueBeam linear accelerator beam-matched to the Varian Golden Beam Data (GBD). Tissue maximum ratios (TMRs) and off-axis ratios (OARs) were measured using PTW 60019 microDiamond and PTW 60018 SRS Diode detectors for a flattening filter free 6MV beam. The output factor (OF) was measured with the microDiamond and EBT-XD film. Results were compared to the GBD for this cone and an OF measured by the Australian Clinical Dosimetry Service during an independent audit. Film dosimetry was used to evaluate Eclipse dose calculations in a solid water phantom and end-to-end accuracy with an anthropomorphic head phantom. Output correction factors were derived from IAEA TRS-483. Gamma analysis was used to compare measured TMRs and OARs, and to compare Eclipse dose planes with film dosimetry results. Comparisons between measured and GBD TMRs passed gamma analysis within the specified criteria, while differences between distances to agreement for OARs measured with different detectors was attributed to different volume averaging characteristics. The OFs measured with the microDiamond and film agreed within measurement uncertainty. It was decided to configure Eclipse with the microDiamond measured OF and the SRS Diode measured TMR and OAR data. This was validated with various comparisons carried out to confirm both measurement accuracy and Eclipse configuration.

Comparison between radiochromic film and Sun nuclear edge diode detector during the commissioning of small field size radiosurgery cones

Stereotactic Radiosurgery (SRS) is a procedure that uses ionizing radiation in a single fraction or few fractions to treat intracranial lesions. Usually, for these treatments, small photon fields, formed by circular collimators or micro multileaf collimators are used. The objective of the present work is to compare the measurements of the dose in depth (PDD), the off-axis ratio (OAR), and the output factor (OF) using Edge diode and radiochromic films and analyzing the agreement between the two detectors during these measures. In the gamma index analysis of the profiles, the minimum percentage reached was 98.3% of the points in the criterion 1% and 0.5 mm. The maximum percentage difference in the output factor was 2.79% for a cone 4 mm and 10FFF. In the gamma index analysis of the PDDs, the minimum percentage reached was 97% of the points in the criterion 1% and 1mm. The results show a good agreement between the edge detector and the film in small field measurements for radiosurgery.

Characterization of a Commercial PIN Diode for Stereotactic Radiosurgery Dosimetry

The stereotactic radiosurgery (SRS) is a technique that uses multiple beams extremely collimated of ionizingradiation to treat intracranial lesions and functional abnormalities, with high geometric precision and dosimetricaccuracy. The use of small fields is already a reality in modern radiotherapy techniques. However, the accuracyin small-field dosimetry is challenging because of several process physics and aspects related to the detector. Theaim of this paper is to evaluate the response of a dosimetric system developed by Nuclear Energy Department ofFederal University of Pernambuco (DEN/UFPE) for small field dosimetry. Measurements of, output factor (OF),off-axis ratio (OAR) and percentage depth dose (PDP) were performed with this dosimetric system andcompared with the results obtained with a commercial diode and treatment planning system data. The resultsshowed that the Diode-DEN presents a reliable and economical alternative for small fields dosimetry used intreatments of SRS. This detector can be used for validation of dates obtained in commissioning of linearaccelerator and inserted in Treatment Planning Systems (TPS).

Spatial compliance modeling and optimization of a translational joint using corrugated flexure units

Compliant mechanisms with corrugated flexure units have been attracting increasing attention due to their large range of motion. However, most research has considered in-plane compliance, whereas the out-of-plane performance, which affects the load-carrying ability, has seldom been studied. In this paper, an analytical model for the spatial compliance of a corrugated flexure translational joint is formulated. The in-plane compliance of the corrugated flexure translational joint is modeled by the Maxwell–Mohr method and compliance matrix method. For the out-of-plane compliance, a modification method is proposed for the closed-form analytical solution based on a finite element analysis with shell elements. The corrugated flexure translational joint is further optimized to achieve a high in-plane and out-of-plane off-axis stiffness ratio, low first-order natural frequency, and large range of motion. Experiments were conducted to verify the effectiveness of the method.

A publicly available dataset of out-of-field dose profiles of a 6 MV linear accelerator.

An increase in radiotherapy-induced secondary malignancies has led to recent developments in analytical modelling of out-of-field dose. These models must be validated against measurements, but currently available datasets are outdated or limited in scope. This study aimed to address these shortcomings by producing a large dataset of out-of-field dose profiles measured with modern equipment. A novel method was developed with the intention of allowing physicists in all clinics to perform these measurements themselves using commonly available dosimetry equipment. A standard 3D scanning water tank was used to collect 36 extended profiles. Each profile was measured in two sections, with the inner section measured with the beam directly incident on the tank, and the outer section with the beam incident on a water-equivalent phantom abutted next to the tank. The two sections were then stitched using a novel feature-matching approach. The profiles were compared against linac commissioning data and manually inspected for discontinuities in the overlap region. The dataset is presented as a publicly accessible comma separated variable file containing off-axis ratios at a range of off-axis distances. This dataset may be applied to the development and validation of analytical models of out-of-field dose. Additionally, it may be used to inform dose estimates to radiosensitive implants and anatomy. Physicists are encouraged to perform these out-of-field measurements in their own clinics and share their results with the community.

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.

First Beam Commissioning Report of a Novel Medical Linear Accelerator Designed for Biologically Guided Radiotherapy

<h3>Purpose/Objective(s)</h3> A biologically guided radiotherapy (BgRT) system was developed and equipped with a novel O-ring gantry Linac, positron emission tomography (PET) and computed tomography technologies. This architectural design is to support real-time PET-guided treatment, termed biology-guided radiotherapy. To reduce latency between PET data acquisition and radiation delivery, the BgRT system incorporates a fast-transitioning binary multi-leaf collimator (MLC) leaves sandwiched between a split-jaw. This study reports the dosimetric commissioning of the first clinical BgRT machines at our institution, and compared with the reference data provided by the manufacturer. <h3>Materials/Methods</h3> The dosimetric commissioning of the 6 MV flattening filter-free (FFF) photon beam was characterized using a 3D water phantom and diode detectors at a source to surface distance of 85 cm. In-air profiles of 40cm x 2cm field, 40cm x 1cm field, and various leaf combinations plus in water PDD data of 12 field sizes (1.25, 2.5, 5, 10, 20 and 40 cm MLC opened leaves with 1 and 2cm jaw settings) were acquired first for beam modeling. Additional longitudinal and transverse profile scans were performed for these 12 field sizes at the depth of 1.5, 5, 10, 15 and 20 cm to verify the beam model accuracy. The relative outputs factors were measured with the W2 1mmx1mm scintillator at the depth of 10cm for the field sizes from the smallest 0.6cm x 1cm to the maximum 40cm x 2cm. <h3>Results</h3> For the PDDs of reference field size of 10cm x 2cm measured at Stanford, the maximum doses (d_max) were 12.5mm within the manufacturer's reference range of 13 ± 0.75 mm. The percentage doses at 10cm depth (d₁₀) at reference field size were 56.9% and 57.6% for Stanford and RefleXion measurements, respectively. The longitudinal field sizes for all tested fields were agreed within 0.5mm and the transverse off-axis ratio is within 1% in the beam core (80% of nominal field size). The output factors measured at our institute were ranging from 0.706 to 1.012 relative to the reference 10cm x 2cm field. <h3>Conclusion</h3> This evaluation represents the first commissioning evaluation of the dosimetric characteristics of the BgRT system with architecture designed to accommodate both PET detector and LINAC subsystems. This data will serve as a clinical reference for the BgRT treatment planning system's beam model and can act as a standard for comparison and beam model improvement for units installed at future clinical sites.

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.

Small-field beam data acquisition, detector dependency, and film-based validation for a novel self-shielded stereotactic radiosurgery system.

This study reports a single-institution experience with beam data acquisition and film-based validation for a novel self-shielded sterotactic radiosurgery unit and investigates detector dependency on field output factors (OFs), off-axis ratios (OARs), and percent depth dose (PDD) measurements within the context of small-field dosimetry. The delivery platform for this unit consists of a 2.7-MV S-band linear accelerator mounted on coupled gimbals that rotate around a common isocenter (source-to-axis distance [SAD]=450mm), allowing for more than 260 noncoplanar beam angles. Beam collimation is achieved via a tungsten collimator wheel with eight circular apertures ranging from 4mm to 25mm in diameter. Three diodes (PTW 60012 Diode E, PTW 60018 SRS Diode, and Sun Nuclear EDGE) and a synthetic diamond detector (PTW 60019micro Diamond [µD] detector) were used for OAR, PDD, and OF measurements. OFs were also acquired with a PTW 31022 PinPoint ionization chamber. Beam scanning was performed using a 3D water tank at depths of 7, 50, 100, 200, and 250mm with a source-to-surface distance of 450mm. OFs were measured at the depth of maximum dose (dmax =7mm) with the SAD at 450mm. Gafchromic EBT3 film was used to validate OF and profile measurements and as a reference detector for estimating correction factors for active detector OFs. Deviations in field size, penumbra, and PDDs across the different detectors were quantified. Relative OFs (ROFs) for the diodes were within 1.4% for all collimators except for 5 and 7.5mm, for which SRS Diode measurements were higher by 1.6% and 2.6% versus Diode E. The µD ROFs were within 1.4% of the diode measurements. PinPoint ROFs were lower by >10% for the 4-mm and 5-mm collimators versus the Diode E and µD. Corrections to OFs using EBT3 film as a reference were within 1.2% for all diodes and the µD detector for collimators 10mm and greater and within 2.0%, 2.8%, and 1.1% for the 7.5-, 5-, and 4-mm collimators, respectively. The maximum difference in full width at half maximum (FWHM) between the Diode E and the other active detectors was for the 25-mm collimator and was 0.09mm (µD), 0.16mm (SRS Diode), and 0.65mm (EDGE). Differences seen in PDDs beyond the depth of dmax were <1% across the three diodes and the µD. FWHM and penumbra measurements made using EBT3 film were within 1.34% and 3.26%, respectively, of the processed profile data entered into the treatment planning system. Minimal differences were seen in OAR and PDD measurements acquired with the diodes and the µD. ROFs measured with the three diodes were within 2.6% and within 1.4% versus the µD. Gafchromic Film measurements provided independent verification of the OAR and OF measurements. Estimated corrections to OFs using film as a reference were <1.6% for the Diode E, EDGE, and µD detector.