Small Field Dosimetry Research Articles

P265] Small field dosimetry comparison of 6 MV beam profiles aquired with diodesrs vs 0.125cc ionization chamber

Purpose Compare TPS calculated profiles based on 0.125 cc ionization chamber measurements for linac commissioning with beam profiles acquired using IAEA small beam recommendations with DiodeSRS. Methods At Eclipse™ 11.0 we calculated dose distribution for 2 × 2 cm, 3 × 3 cm and 4 × 4 cm MLC collimated fields at 100 cm SSD with AAA algorithm based on data acquired with 0.125cc PTW ionization chamber. Using TPS dose profile tool, we got the dose values at 10 cm depth. 2 × 2 cm, 3 × 3 cm and 4 × 4 cm MLC collimated field profiles with SSD = 100 cm at 10 cm depth, were acquired following IAEA Technical Reports Series N° 483, in water phantom using DiodeSRS PTW and Mephysto™ mc2 PTW. Transverse beam profiles were compared measuring the full with at half maximum (FWHM). Results For each field, we found a decrease in the FWHM between 2% and 10% for the DiodeSRS measurements. Conclusions Using the IAEA protocol for small field measurements we found that DiodeSRS detectors are more accurate measuring small field relative dosimetry. We intend to recommission our linac small fields according to IAEA report and evaluate the impact on our IMRT/VMAT plans quality assurance.

Abstract 4153: Commissioning and radiobiological verification of the small animal radiotherapy research platform (SARRP)

Abstract Preclinical radiation biology has become increasingly sophisticated due to the implementation of advanced small animal image guided radiation platforms into laboratory investigation. Small animal radiotherapy devices enable state-of-the-art image guided therapy (IGRT) research to be performed in small animal cancer models by combining high-resolution cone beam computed tomography (CBCT) imaging with an isocentric irradiation system. The technological evolution from simple, broad field irradiation configurations, to more sophisticated dose deliveries for preclinical radiobiology experiments has introduced new dosimetry challenges for preclinical small animal cancer research. Because of this, robust QA and dosimetry techniques are a key part of using novel treatment platforms using very small irradiation fields. In this study, we present a dosimetric study of the small animal radiotherapy research platform (SARRP, Xstrahl Inc.) focusing on the small field dosimetry. Physical dosimetry was assessed using ion chambers and radiochromic film, investigating the impact the beam focus size has on the dose rate output as well as beam characteristics (beam shape and penumbra variations). Two film reading modalities have been used to assess the dose output using the 0.5 mm diameter aperture. This study establishes that FilmQA Pro is a suitable tool for small field dosimetry, with a sufficiently small sampling area (0.1 mm) to ensure an accurate measurement. Furthermore, all small field dosimetry data generated in this study are equally comparable with our Monte Carlo simulations for both focal spot sizes. Overall, it was observed that the small focal spot, while having a lower dose-rate output was shown to produce a more stable and homogenous beam with minimum penumbra over time when compared to the larger focal spot. These findings suggest that when preclinical stereotactic irradiation fields are used, a practical compromise (time to deliver a desired dose and field homogeneity) needs to be considered when deciding the optimum treatment plan and beam configuration used. Citation Format: Jonathan L. Kane. Commissioning and radiobiological verification of the small animal radiotherapy research platform (SARRP) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4153.

Dosimetric verification and commissioning for a small animal image-guided irradiator

In recent years, small animal image-guided irradiators have been widely utilized in preclinical studies involving rodent models. However, the dosimetry commissioning of such equipment involving kilovoltage small-field dosimetry has not received as much interest as the megavoltage small-field dosimetry used clinically. To date, a paucity of measured kilovoltage beam data, especially for field sizes less than 3 mm, can be found in the literature. For improvement of rodent treatments in the future, this work aims to provide comprehensive and accurate beam data for the small animal radiation research platform (SARRP, Xstrahl) using EBT3 Gafchromic films and Monte Carlo calculation, with submillimeter resolution and accuracy.This work includes three primary tasks: (1) establish an optimized film measurement protocol for small field dosimetry of kilovoltage photon beam. (2) Acquire dosimetric data including (a) depth dose curves from the surface to 6 cm depth (b) beam profiles, (c) penumbra, (d) cone factors and (e) 2D dose distribution. These tasks were undertaken for a 220 kVp photon beam with five different small field widths and 33 cm source to surface distance (0.5 mm and 1 mm circular fields, 3 × 3 mm2, 5 × 5 mm2, 10 × 10 mm2 square fields). Beam data was measured with EBT3 films. (3) Provide comparative dosimetry for film measurements, Monte Carlo calculations, and the dose calculations performed with the SARRP treatment planning system, Muriplan.For the majority of parameters, film measurement agreed with Monte Carlo simulation within 1%. There were, however, discrepancies between measured beam data and Muriplan treatment planning data. Specifically, for PDD, Muriplan underestimates the dose for field sizes of 0.5 mm and 1 mm. For beam profiles comparisons, the calculation from Muriplan predicts a smaller lateral distance between the 50% isodose lines compared to film measurement. There is a difference of 0.18, 0.72, 0.6 mm between Muriplan and film for field sizes of 3, 5, 10 mm, respectively.This work demonstrates that accurate and precise kilovoltage small-field dosimetry can be conducted using EBT3 Gafchromic film with an optimized protocol. In addition, discrepancies between measured beam data and Muriplan were identified.

A new reference-type ionization chamber with direction-independent response for use in small-field photon-beam dosimetry – An experimental and Monte Carlo study

The frequently applied narrow and non-standard transverse dose profiles of intensity modulated photon-beam radiotherapy, lacking lateral secondary electron equilibrium, require the use of high-resolution dosimetry detectors, and small air-filled detectors are recommended as the reference detectors for cross-calibration of the high-resolution detectors. The present study focuses on the dosimetric properties of a novel cylindrical ionization chamber, the PTW Semiflex 3D 31021. The chamber's effective point of measurement was found to lie at (0.41±0.04) r downstream the tip of the inner surface of the spherical front wall in the axial orientation and (0.46±0.04) r upstream the chamber axis in the radial orientation. Due to its symmetrical design, the sigma values of its lateral dose response functions for all chamber's orientations are the same (2.10±0.05mm). The polarity correction factors obtained in this work do not exceed 0.1% and the saturation correction factor was below 1% up to a dose-per-pulse value of 0.956mGy. The radiation quality correction factor kQ of the chamber as a function of the tissue-phantom-ratio, TPR20,10, has been calculated by Monte Carlo simulation and has been determined experimentally at the German Metrology Institute (Physikalisch-Technische Bundesanstalt, PTB). The values of the non-reference condition correction factor kNR have been Monte-Carlo-calculated for use of the chamber at various depths and field sizes.

Real-time high spatial resolution dose verification in stereotactic motion adaptive arc radiotherapy.

PurposeRadiation treatments delivered with real‐time multileaf collimator (MLC) tracking currently lack fast pretreatment or real‐time quality assurance. The purpose of this study is to test a 2D silicon detector, MagicPlate‐512 (MP512), in a complex clinical environment involving real‐time reconfiguration of the MLC leaves during target tracking.Methods MP512 was placed in the center of a solid water phantom and mounted on a motion platform used to simulate three different patient motions. Electromagnetic target tracking was implemented using the Calypso system (Varian Medical Systems, Palo Alto, CA, USA) and an MLC tracking software. A two‐arc VMAT plan was delivered and 2D dose distributions were reconstructed by MP512, EBT3 film, and the Eclipse treatment planning system (TPS). Dose maps were compared using gamma analysis with 2%/2 mm and 3%/3 mm acceptance criteria. Dose profiles were generated in sup‐inf and lateral directions to show the agreement of MP512 to EBT3 and to highlight the efficacy of the MLC tracking system in mitigating the effect of the simulated patient motion.ResultsUsing a 3%/3 mm acceptance criterion for 2D gamma analysis, MP512 to EBT3 film agreement was 99% and MP512 to TPS agreement was 100%. For a 2%/2 mm criterion, the agreement was 95% and 98%, respectively. Full width at half maximum and 80%/20% penumbral width of the MP512 and EBT3 dose profiles agreed within 1 mm and 0.5 mm, respectively. Patient motion increased the measured dose profile penumbral width by nearly 2 mm (with respect to the no‐motion case); however, the MLC tracking strategy was able to mitigate 80% of this effect.Conclusions MP512 is capable of high spatial resolution 2D dose reconstruction during adaptive MLC tracking, including arc deliveries. It shows potential as an effective tool for 2D small field dosimetry and pretreatment quality assurance for MLC tracking modalities. These results provide confidence that detector‐based pretreatment dosimetry is clinically feasible despite fast real‐time MLC reconfigurations.

Determination of the active volumes of solid-state photon-beam dosimetry detectors using the PTB proton microbeam.

This study aims at the experimental determination of the diameters and thicknesses of the active volumes of solid-state photon-beam detectors for clinical dosimetry. The 10MeV proton microbeam of the PTB (Physikalisch-Technische Bundesanstalt, Braunschweig) was used to examine two synthetic diamond detectors, type microDiamond (PTW Freiburg, Germany), and the silicon detectors Diode E (PTW Freiburg, Germany) and Razor Diode (Iba Dosimetry, Germany). The knowledge of the dimensions of their active volumes is essential for their Monte Carlo simulation and their applications in small-field photon-beam dosimetry. The diameter of the active detector volume was determined from the detector current profile recorded by radially scanning the proton microbeam across the detector. The thickness of the active detector volume was determined from the detector's electrical current, the number of protons incident per time interval and their mean stopping power in the active volume. The mean energy of the protons entering this volume was assessed by comparing the measured and the simulated influence of the thickness of a stack of aluminum preabsorber foils on the detector signal. For all detector types investigated, the diameters measured for the active volume closely agreed with the manufacturers' data. For the silicon Diode E detector, the thickness determined for the active volume agreed with the manufacturer's data, while for the microDiamond detectors and the Razor Diode, the thicknesses measured slightly exceeded those stated by the manufacturers. The PTB microbeam facility was used to analyze the diameters and thicknesses of the active volumes of photon dosimetry detectors for the first time. A new method of determining the thickness values with an uncertainty of ±10% was applied. The results appear useful for further consolidating detailed geometrical knowledge of the solid-state detectors investigated, which are used in clinical small-field photon-beam dosimetry.

An inexpensive method of small photon field dosimetry with EBT3 radiochromic film

The relative output factor for small field photon beams with EBT3 (External beam therapy, Gafchromic ™) Radiochromic film were obtained using indigenously developed a program in MATLAB. MLC and JAWS created square small field sizes for dosimetry. To calibrate Radiochromic film, we have cut the film into several small segments of 6 × 6 cm2. These small pieces of film were irradiated to known doses ranging from 25 to 300 cGy by SAD technique using 6 MV photon beam from Linear accelerator. A Program in MATLAB was written to analyze the scanned film data and all the images of the films were imported into this Program. To validate the accuracy of calibration curve, radiochromic films were irradiated with different field sizes of 10 × 10 cm2, 15 × 1 5 cm2, 20 × 20 cm2 to known dose of 200 cGy each by SAD technique. Field sizes were defined using two methods, in the first method fields were defined using X-Y collimator Jaws with the MLC retracted to their maximum position. In the Second method, MLCs were used to define square field sizes of 1 × 1 cm2 to 5 × 5 cm2, 10 × 10 cm2 and 15 × 15 cm2. Calibration curve created using gafchromic film show percentage variation between measured and delivered dose within 3% of standard field size. Output factors measured by Gafchromic EBT3 film showed close agreement with those measured using the other detector for field sizes of 2.0 × 2.0 cm2 and above. EBT3 film has properties such as linear response, energy, and dose-rate independence. It makes it suitable detector for small field dosimetry. Our Indigenous developed MATLAB Program for film dosimetry gave the desired results. From this study, we can conclude that EBT3 film can be used for relative output factors ranging from small field to large fields.

EP-1730: Small field dosimetry by the PTW microDiamond: multicenter experimental study and MC simulations

EP-1730: Small field dosimetry by the PTW microDiamond: multicenter experimental study and MC simulations

EP-1728: Small field dosimetry formalism implemented using various detectors on several linear accelerators

EP-1728: Small field dosimetry formalism implemented using various detectors on several linear accelerators

EP-1720: A silicon diode array detector for small field dosimetry with flattening filter free beams

EP-1720: A silicon diode array detector for small field dosimetry with flattening filter free beams

Determination of kQmsr,Q0fmsr,fref factors for ion chambers used in the calibration of Leksell Gamma Knife Perfexion model using EGSnrc and PENELOPE Monte Carlo codes.

To calculate the kQmsr,Q0fmsr,fref factors for nine common ionization chamber types following the small fields dosimetry formalism for the calibration of the Leksell Gamma Knife® (LGK) PerfexionTM using Monte Carlo simulation. This study also provides the first independent comparison of EGSnrc and PENELOPE for the calculation of kQmsr,Q0fmsr,fref correction factors and proposes a practical method to predict these factors based on chamber type, chamber orientation and phantom electron density. The ionization chambers are modeled using the EGSnrc and PENELOPE Monte Carlo codes based on the blueprints provided by the manufacturers. The chambers are placed in a half-sphere water phantom and five spherical phantoms made of liquid water, solid water, ABS, polystyrene, and PMMA, respectively. Dose averaged over the air cavity of the chambers and a small water volume are calculated using EGSnrc and PENELOPE Monte Carlo codes for both conventional and machine specific reference (msr) setups. Using the calculated dose ratio, the kQmsr,Q0fmsr,fref factor is determined for all phantom materials and two possible orientations of chamber. The calculated kQmsr,Q0fmsr,fref factors are compared to a previous Monte Carlo study. A relationship between the kQmsr,Q0fmsr,fref factor and the electron density of the phantom material is derived to predict the kQmsr,Q0fmsr,fref factor for any phantom material type. Applying the calculated kQmsr,Q0fmsr,fref factors to the measured dose rate of a recent round robin study improves consistency of reference dosimetry of the Leksell Gamma Knife® (LGK) PerfexionTM . Agreement within uncertainty is observed between kQmsr,Q0fmsr,fref values determined in this study and the previous PEGASOS/PENELOPE study in a liquid water phantom. The difference between kQmsr,Q0fmsr,fref values in parallel and perpendicular detector orientations is most significant for the PTW 31010 (1.8%) chamber. The percentage root-mean-square (%RMS) deviation between EGSnrc and PENELOPE calculated kQmsr,Q0fmsr,fref values for Exradin-A1SL, A14 and A14SL chambers studies in this work was found to be 0.4%. The kQmsr,Q0fmsr,fref values increase linearly with electron density of the phantom material for all chamber types mainly due to the linear dependency of photon energy fluence ratios on electron density. The average percentage difference between the calculated and predicted kQmsr,Q0fmsr,fref values using two methods is found to be 0.15% and 0.16%. Previously measured dose rates corrected with the kQmsr,Q0fmsr,fref values determined in this work leads to absorbed dose values consistent to within 0.8%. The calculated kQmsr,Q0fmsr,fref values in this work will enable users to apply the appropriate correction for their own specific phantom material only knowing the electron density of the phantom material.

Development and clinical characterization of a novel 2041 liquid-filled ionization chambers array for high-resolution verification of radiotherapy treatments.

The aim of this study was to present a novel 2041 liquid-filled ionization chamber array for high-resolution verification of radiotherapy treatments. The prototype has 2041 ionization chambers of 2.5 × 2.5 mm2 area filled with isooctane. The detection elements are arranged in a central square grid of 43 × 43, totally covering an area of 107.5 × 107.5 mm2 . The central inline and cross-line are extended to 227 mm and the diagonals to 321 mm to be able to perform profile measurements of large fields. We have studied stability, pixel response uniformity, dose rate dependence, depth and field size dependence and anisotropy. We present results for output factors, tongue-and-groove, garden fence, small field profiles, irregular fields, and verification of dose planes of patient treatments. Comparison with other detectors used for small field dosimetry (SFD, CC13, microDiamond) has shown good agreement. Output factors measured with the device for square fields ranging from 10 × 10 to 100 × 100 mm2 showed relative differences within 1%. The response of the detector shows a strong dependence on the angle of incident radiation that needs to be corrected for. On the other hand, inter-pixel relative response variations in the 0.95-1.08 range have been found and corrected for. The application of the device for the verification of dose planes of several treatments has shown gamma passing rates above 97% for tolerances of 2% and 2 mm. The verification of other clinical fields, like small fields and irregular fields used in the commissioning of the TPS, also showed large passing rates. The verification of garden fence and tongue-and-groove fields was affected by volume-averaging effects. The results show that the liquid filled ionization chamber prototype here presented is appropriate for the verification of radiotherapy treatments with high spatial resolution. Recombination effects do not affect very much the verification of relative dose distributions. However, verification of absolute dose distributions may require normalization to a radiation field which is representative of the dose rate of the treatment delivered.

Performance of a PTW 60019 microDiamond detector in a 1.5 T MRI-linac

Accurate small-field dosimetry is critical for a magnetic resonance linac (MRI-linac). The PTW 60019 microDiamond is close to an ideal detector for small field dosimetry due to its small physical size, high signal-to-noise ratio and approximate water equivalence. It is important to fully characterise the performance of the detector in a 1.5 T magnetic field prior to its use for MRI-linac commissioning and quality assurance.Standard techniques of detector testing have been implemented, or adapted where necessary to suit the capabilities of the MRI-linac. Detector warmup, constancy, dose linearity, dose rate linearity, field size dependence and leakage were within tolerance. Measurements with the detector were consistent with ion chamber measurements for medium sized fields. The effective point of measurement of the detector when used within a 1.5 T magnetic field was determined to be 0.80 ± 0.23 mm below the top surface of the device, consistent with the existing vendor recommendation and alignment mark at 1.0 mm. The angular dependence was assessed. Variations of up to 9.7% were observed, which are significantly greater than in a 0 T environment. Within the expected range of use, the maximum effect is approximately 0.6% which is within tolerance. However for large beams within a magnetic field, the divergence and consequent variation in angle of photon incidence means that the microDiamond would not be ideal for characterising the profiles and it would not be suitable for determining large-field beam parameters such as symmetry. It would also require a correction factor prior to use for patient-specific QA measurements where radiation is delivered from different gantry angles.The results of this study demonstrate that the PTW 60019 microDiamond detector is suitable for measuring small radiation fields within a 1.5 T magnetic field and thus is suitable for use in MRI-linac commissioning and quality assurance.

Refinement of MLC modeling improves commercial QA dosimetry system for SRS and SBRT patient-specific QA.

Mobius 3D (M3D) provides a volumetric dose verification of the treatment planning system's calculated dose using an independent beam model and a collapsed cone convolution superposition algorithm. However, there is a lack of investigation into M3D's accuracy and effectiveness for stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT) quality assurance (QA). Here, we collaborated with the vendor to develop a revised M3D beam model for SRS/SBRT cases treated with a 6X flattening filter-free (FFF) beam and high-definition multiple leaf collimator (HDMLC) on an Edge linear accelerator. Eighty SRS/SBRT cases, planned with AAA dose algorithm and validated with Gafchromic film, were compared to M3D dose calculations using 3D gamma analysis with 2%/2 mm gamma criteria and a 10% threshold. A revised beam model was developed by refining the HD-MLC model in M3D to improve small field dose calculation accuracy and beam profile agreement. All cases were reanalyzed using the revised beam model. The impact of heterogeneity corrections for lung cases was investigated by applying lung density overrides to five cases. For the standard and revised beam models, respectively, the mean gamma passing rates were 94.6% [standard deviation (SD): 6.1%] and 98.0% [SD: 1.7%] (for the overall patient), 88.2% [SD: 17.3%] and 93.8% [SD: 6.8%] (for the brain PTV), 71.4% [SD: 18.4%] and 81.5% [SD: 14.3%] (for the lung PTV), 83.3% [SD: 16.7%] and 67.9% [SD: 23.0%] (for the spine PTV), and 78.6% [SD: 14.0%] and 86.8% [SD: 12.5%] (for the PTV of all other sites). The lung PTV mean gamma passing rates improved from 74.1% [SD: 7.5%] to 89.3% [SD: 7.2%] with the lung density overridden. The revised beam model achieved an output factor within 3% of plastic scintillator measurements for 2 × 2 cm2 MLC field size, but larger discrepancies are still seen for smaller field sizes which necessitate further improvement of the beam model. Special attention needs to be paid to small field dosimetry, MLC modeling, and inhomogeneity corrections in the beam model for SRS/SBRT QA. The improvements noted in this study, and further collaborations between clinical physicists and the vendor to refine the M3D beam model could enable M3D to become a premier SRS/SBRT QA tool.

A 3D correction method for predicting the readings of a PinPoint chamber on the CyberKnife® M6™ machine

The use of small fields in radiation therapy techniques has increased substantially in particular in stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT). However, as field size reduces further still, the response of the detector changes more rapidly with field size, and the effects of measurement uncertainties become increasingly significant due to the lack of lateral charged particle equilibrium, spectral changes as a function of field size, detector choice, and subsequent perturbations of the charged particle fluence. This work presents a novel 3D dose volume-to-point correction method to predict the readings of a 0.015 cc PinPoint chamber (PTW 31014) for both small static-fields and composite-field dosimetry formed by fixed cones on the CyberKnife® M6™ machine.A 3D correction matrix is introduced to link the 3D dose distribution to the response of the PinPoint chamber in water. The parameters of the correction matrix are determined by modeling its 3D dose response in circular fields created using the 12 fixed cones (5 mm–60 mm) on a CyberKnife® M6™ machine. A penalized least-square optimization problem is defined by fitting the calculated detector reading to the experimental measurement data to generate the optimal correction matrix; the simulated annealing algorithm is used to solve the inverse optimization problem. All the experimental measurements are acquired for every 2 mm chamber shift in the horizontal planes for each field size. The 3D dose distributions for the measurements are calculated using the Monte Carlo calculation with the MultiPlan® treatment planning system (Accuray Inc., Sunnyvale, CA, USA). The performance evaluation of the 3D conversion matrix is carried out by comparing the predictions of the output factors (OFs), off-axis ratios (OARs) and percentage depth dose (PDD) data to the experimental measurement data. The discrepancy of the measurement and the prediction data for composite fields is also performed for clinical SRS plans.The optimization algorithm used for generating the optimal correction factors is stable, and the resulting correction factors were smooth in the spatial domain. The measurement and prediction of OFs agree closely with percentage differences of less than 1.9% for all the 12 cones. The discrepancies between the prediction and the measurement PDD readings at 50 mm and 80 mm depth are 1.7% and 1.9%, respectively. The percentage differences of OARs between measurement and prediction data are less than 2% in the low dose gradient region, and 2%/1 mm discrepancies are observed within the high dose gradient regions. The differences between the measurement and prediction data for all the CyberKnife based SRS plans are less than 1%.These results demonstrate the existence and efficiency of the novel 3D correction method for small field dosimetry. The 3D correction matrix links the 3D dose distribution and the reading of the PinPoint chamber. The comparison between the predicted reading and the measurement data for static small fields (OFs, OARs and PDDs) yield discrepancies within 2% for low dose gradient regions and 2%/1 mm for high dose gradient regions; the discrepancies between the predicted and the measurement data are less than 1% for all the SRS plans. The 3D correction method provides an access to evaluate the clinical measurement data and can be applied to non-standard composite fields intensity modulated radiation therapy point dose verification.

A multi-center output factor intercomparison to uncover systematic inaccuracies in small field dosimetry.

Large uncertainties in output factor (OF) small fields dosimetry motivated multicentric studies. The focus of the study was the determination of the OFs, for different linacs and radiosurgery units, using new-generation detectors. Intercomparison studies between radiotherapy centers improved quality dosimetry practices. Results confirmed the effectiveness of the studies to uncover large systematic inaccuracies in small field dosimetry.

Different Dosimeters/Detectors Used in Small-Field Dosimetry: Pros and Cons.

With the advent of complex and precise radiation therapy techniques, the use of relatively small fields is needed. Using such field sizes can cause uncertainty in dosimetry; therefore, special attention is required both in dose calculations and measurements. There are several challenges in small-field dosimetry such as the steep gradient of the radiation field, volume averaging effect, lack of charged particle equilibrium, partial occlusion of radiation source, beam alignment, and unable to use a reference dosimeter. Due to these challenges, special dosimeters are needed for small-field dosimetry, and this review article discusses this topic.

Dosimetric characterization of Elekta stereotactic cones

PurposeDosimetry of small fields defined by stereotactic cones remains a challenging task. In this work, we report the results of commissioning measurements for the new Elekta stereotactic conical collimator system attached to the Elekta VersaHD linac and present the comparison between the measured and Monte Carlo (MC) calculated data for the 6 MV FFF beam. In addition, relative output factor (ROF) dependence on the stereotactic cone aperture variation was studied and penumbra comparison for small MLC‐based and cone‐based fields was performed.MethodsCones with nominal diameters of 15 mm, 12.5 mm, 10 mm, 7.5 mm, and 5 mm were employed in our study. Percentage depth dose (PDD), off‐axis ratios (OAR), and ROF were measured using a stereotactic field diode (SFD). BEAMnrc code was used for MC simulations.ResultsMC calculated and measured PDDs for all cones agreed within 1%/0.5 mm, and OAR profiles agreed within 1%/0.5 mm. ROF obtained from the measurements and MC calculations agreed within 2% for all cone sizes. Small‐field correction factors for the SFD detector Kfield,3 × 3(SFD) were derived using MC calculations as a baseline and were found to be 0.982, 0.992, 0.997, 1.015, and 1.017 for the 5, 7.5, 10, 12.5, and 15‐mm cones respectively. The difference in ROF was about 10%, 6%, 3.5%, 3%, 2.5%, and 2% for ±0.3 mm variations in 5, 7.5, 10, 12.5, and 15‐mm cone aperture respectively. In case of single static field, cone‐based collimation produced a sharper penumbra compared to the MLC‐based.ConclusionsAccurate MC simulation can be an effective tool for verification of dosimetric measurements of small fields. Due to the very high sensitivity of output factors on the cone diameter, manufacture‐related variations in cone size may lead to considerable variations in dosimetric characteristics of stereotactic cones.

A novel high-resolution 2D silicon array detector for small field dosimetry with FFF photon beams

PurposeFlattening filter free (FFF) beams are increasingly being considered for stereotactic radiotherapy (SRT). For the first time, the performance of a monolithic silicon array detector under 6 and 10 MV FFF beams was evaluated. The dosimeter, named “Octa” and designed by the Centre for Medical Radiation Physics (CMRP), was tested also under flattened beams for comparison. MethodsOutput factors (OFs), percentage depth-dose (PDD), dose profiles (DPs) and dose per pulse (DPP) dependence were investigated. Results were benchmarked against commercially available detectors for small field dosimetry. ResultsThe dosimeter was shown to be a ‘correction-free’ silicon array detector for OFs and PDD measurements for all the beam qualities investigated. Measured OFs were accurate within 3% and PDD values within 2% compared against the benchmarks. Cross-plane, in-plane and diagonal DPs were measured simultaneously with high spatial resolution (0.3 mm) and real time read-out. A DPP dependence (24% at 0.021 mGy/pulse relative to 0.278 mGy/pulse) was found and could be easily corrected for in the case of machine specific quality assurance applications. ConclusionsResults were consistent with those for monolithic silicon array detectors designed by the CMRP and previously characterized under flattened beams only, supporting the robustness of this technology for relative dosimetry for a wide range of beam qualities and dose per pulses. In contrast to its predecessors, the design of the Octa offers an exhaustive high-resolution 2D dose map characterization, making it a unique real-time radiation detector for small field dosimetry for field sizes up to 3 cm side.

The ‘cutting away’ of potential secondary electron tracks explains the effects of beam size and detector wall density in small-field photon dosimetry

The well-known field-size dependent overresponse in small-field photon-beam dosimetry of solid-state detectors equipped with very thin sensitive volumes, such as the PTW microDiamond, cannot be caused by the photon and electron interactions within these sensitive layers because they are only a few micrometers thick. The alternative explanation is that their overresponse is caused by the combination of two effects, the modification of the secondary electron fluence profile (i) by a field size too small to warrant lateral secondary electron equilibrium and (ii) by the density-dependent electron ranges in the structural detector materials placed in front of or backing the sensitive layer. The present study aims at the numerical demonstration and visualization of this combined mechanism.The lateral fluence profiles of the secondary electrons hitting a 1 µm thick scoring layer were Monte-Carlo simulated by modelling their generation and transport in the upstream or downstream adjacent layers of thickness 0.6 mm and densities from 0.0012 to 3 g cm−3, whose atomic composition was constantly kept water-like. The scoring layer/adjacent layer sandwich was placed in an infinite water phantom irradiated by circular 60Co, 6 MV and 15 MV photon beams with diameters from 3 to 40 mm.The interpretation starts from the ideal case of lateral secondary electron equilibrium, where the Fano theorem excludes any density effect. If the field size is then reduced, electron tracks potentially originating from source points outside the field border will then be numerically ‘cut away’. This geometrical effect reduces the secondary electron fluence at the field center, but the magnitude of this reduction also varies with the density-dependent electron ranges in the adjacent layers. This combined mechanism, which strongly depends on the photon spectrum, explains the field size and material density effect on the response of detectors with very thin sensitive layers used in small-field photon-beam dosimetry.