Small Field Dosimetry Research Articles

Evaluation of calibration methods of Exradin W2 plastic scintillation detector for CyberKnife small-field dosimetry

PurposePlastic scintillation detectors (PSD) require corrections of Cerenkov light for accurate radiation dosimetry. The vendor-recommended calibration method of the Exradin W1 requires irradiation with a 40 × 40 cm2 field size, although the CyberKnife robotic radiosurgery system cannot generate such large field. Recently, an Exradin W2 PSD, succeeding Exradin W1, was characterized. The W2 PSD can be calibrated in water with small curvature using a new plastic holder, which has a short pole on the bottom to mount it on the arm of the water phantom. The purpose of this study was to evaluate the calibration method of Exradin W2 using the new plastic holder for calibration with small fields and compare the impact of the calibration methods on the beam data. MethodsAn Exradin W2-1x1 detector with active volume length 1 mm and diameter 1 mm was used. The Cerenkov light ratio (CLR) values were determined by two calibration methods: measurements in a 30 × 30 cm2 solid water and in water using a small plastic holder. Percent-depth-dose (PDD), beam profiles, and detector output factor (OFdet) of TrueBeam 6 MV photon beams with flattening filter (WFF) and flattening filter-free (FFF) beams were measured to compare these two calibration methods. For the CyberKnife photon beams, PDD, beam profiles, and OFdet were measured with the calibration in water, and the data were compared with those measured using a Diode SRS (PTW model 60018) and microDiamond (PTW model 60019). ResultsThe difference of the CLR between two calibration methods were within 3%. The effect of the calibration methods on the PDD, beam profiles, and OFdet of the TrueBeam were within 0.06%, 0.07%, and 0.8%, respectively. For the CyberKnife beams, the maximum difference of PDD in the dose fall-off region, full-width half-maximum (FWHM) of beam profiles, and penumbra among the three detectors were 1.3%, 0.25 mm, and 0.35 mm, respectively. The relative difference of the W2-measured OFdet from the microDiamond-measured field output factor were within 1.7%. ConclusionsThe calibration in water with a small-field size provided equivalent data compared to those obtained with a conventional calibration method. The Exradin W2 provided appropriate data for CyberKnife small-field dosimetry.

Evaluation of perturbation effects for various size TLDs in small field dosimetry

Small field treatments in radiotherapy have been increasingly developed during the last years due to the potential to produce more conformal dose in the target volume and to enhance the therapeutic ratio. On the other hand, dosimetric task for small field techniques is more challenging because of the production of small dimension high dose areas with rapid dose rate fall off margins. Dosimetry of such small fields necessitate a significant decrease in the size of routine radiation dosimeters. In this work we look at the effect of dosimeter size on the perturbation effect caused by the presence of the dosimeter in the radiation field. Monte Carlo simulations were used to characterize the radiation field and obtain distribution of energy deposition in a water phantom. Changes in electron spectra within various size (LiF and SiO2) TLDs in different size radiation fields 10 × 10 cm2, 2 × 2 cm2 and 0.5 × 0.5 cm2 were investigated and perturbation effect caused by the presence of dosimeter was observed. The results showed significant perturbations produced by conventional LiF TLDs that is especially important in small radiation fields. The use of small size silica fibre (SiO2) TL dosimeters for application in small field dosimetry was observed to be reasonably advantageous. Based on the simulation results, cylindrical silica fibres with 0.5 mm diameter do not produce a significant perturbation in the 10 × 10 cm2, 2 × 2 cm2 fields. The perturbation in the field by 0.125 mm diameter silica fibre was observed to be negligible even in 0.5 × 0.5 cm2 radiation field.

Dosimetry of small photon fields in the presence of bone heterogeneity using MAGIC polymer gel, Gafchromic film, and Monte Carlo simulation.

BackgroundThe presence of heterogeneity within the radiation field increases the challenges of small field dosimetry. In this study, the performance of MAGIC polymer gel was evaluated in the dosimetry of small fields beyond bone heterogeneity.Materials and methodsCircular field sizes of 5, 10, 20 and 30 mm were used and Polytetrafluoroethylene with density of 2.2 g/cm3 was used as the bone equivalent material. The PDD curves, beam profiles, and penumbra widths were measured using MAGIC polymer gel, EBT2 film, and Monte Carlo simulation.ResultsThe maximum differences between MAGIC and EBT2 are 6.1, 4.7, 2.4, and 2.2 for PDD curves at 5, 10, 20, and 30 mm circular fields, respectively. The dose differences and distance to agreement between MAGIC and MC were within 1.89%/0.46 mm, 1.66%/0.43 mm, 1.28%/0.77 mm, and 1.31%/0.81 mm for beam profile values behind bone heterogeneity at 5, 10, 20, and 30 mm field sizes, respectively.ConclusionThe results presented that the MAGIC polymer gel dosimeter is a proper instrument for dosimetry beyond high density heterogeneity.

Dose area product primary standards established by graphite calorimetry at the LNE-LNHB for small radiation fields in radiotherapy

PurposeTo present primary standards establishment in terms of Dose Area Product (DAP) for small field sizes. MethodsA large section graphite calorimeter and two plane-parallel ionization chambers were designed and built in-house. These chambers were calibrated in a 6MV FFF beam at the maximum dose rate of 1400 UM/min for fields defined by specifically designed circular collimators of 5, 7.5, 10, 13 and 15 mm diameter and jaws of 5, 7, 10, 13 and 15 mm side length on a Varian TrueBeam linac. ResultsThe two chambers show the same behaviour regardless of field shape and size. From 5 to 15 mm, calibration coefficients slightly increase with the field size with a magnitude of 1.8% and 1.1% respectively for the two chambers, and are independent of the field shape. This tendency was confirmed by Monte Carlo calculations. The average associated uncertainty of the calibration coefficients is around 0.6% at k=1. ConclusionsFor the first time, primary standards in terms of DAP were established by graphite calorimetry for an extended range of small field sizes. These promising results open the door for an alternative approach in small fields dosimetry.

Validity of equivalent square field concept in small field dosimetry.

The equivalent square (ES) concept has been used for traditional radiation fields defined by the machine collimating system. For small fields, the concept Sclin was introduced based on measuring dosimetric field width (full-width half maximum, FWHM) of the cardinal axis of the beam profiles. The pros and cons of this concept are evaluated in small fields and compared with the traditional ES using area and perimeter (4A/P) method based on geometric field size settings, for example, light field settings. One hundred thirty-seven square and rectangular fields from 5-50mm with every possible permutation (keeping one jaw fixed and varying other jaw from 5 to 50 mm) were utilized to measure FWHM for the validation of Sclin . Using a microSilicon detector and a scanning water tank, measurements were performed on an Elekta (Versa) machine with Agility head and a Varian TrueBeam with different MLC/Jaw design to evaluate the Sclin concept and to understand the effect of exchange factor in small fields. Field output factors were also measured for all 137 fields. The data fitting for fields ranging from 5-50mm between the traditional 4A/P method and Sclin shows differences and indicates a linear relationship with distinct separation of slope for Elekta and Varian machines. ForElekta Agility machine ES based on 4A/P<Sclin and for the VarianTrueBeam 4A/P>Sclin for square fields. Our measured data show that both methods are equally valid but does vary by the machine design. The field output factor is dependent on the elongation factor as well as machine design. For fields with sides ≥10mm, the exchange factor is nearly identical in both machines with magnitude up to 4%, which is close to measurement uncertainty (±3%), but for small fields (<10mm), the Elekta machine has higher exchange factors compared to the Varian machine. The results demonstrate that the two concepts for defining equivalent field (Sclin and 4A/P) are equivalent and can be directly related through an empirical equation. This study confirms that 4A/P is still valid for small fields except for very small fields (≤10mm) where source occlusion is a dominating factor. The Sclin method is potentially sensitive to measurement uncertainty due to measurement of FWHM which is machine-, detector- and user-dependent, while the 4A/P method relies mainly on geometry of the machine and has less dependency on type of machine, detector, and user. The exchange factors are comparable for both types of machines. The conclusion is based on data from an Elekta with Agility head and a Varian TrueBeam machine that may have potential for bias due to light field/collimator set up and alignment. Care should be taken in extrapolating these data to any other machine.

Treatment of ocular tumors through a novel applicator on a conventional proton pencil beam scanning beamline

Treatment of ocular tumors on dedicated scattering-based proton therapy systems is standard afforded due to sharp lateral and distal penumbras. However, most newer proton therapy centers provide pencil beam scanning treatments. In this paper, we present a pencil beam scanning (PBS)-based ocular treatment solution. The design, commissioning, and validation of an applicator mount for a conventional PBS snout to allow for ocular treatments are given. In contrast to scattering techniques, PBS-based ocular therapy allows for inverse planning, providing planners with additional flexibility to shape the radiation field, potentially sparing healthy tissues. PBS enables the use of commercial Monte Carlo algorithms resulting in accurate dose calculations in the presence of heterogeneities and fiducials. The validation consisted of small field dosimetry measurements of point doses, depth doses, and lateral profiles relevant to ocular therapy. A comparison of beam properties achieved through the applicator against published literature is presented. We successfully showed the feasibility of PBS-based ocular treatments.

Quantitative evaluation of dosimetric uncertainties associated with small electron fields

IntroductionAlthough many studies have investigated small electron fields, there are several dosimetric issues that are not well understood. This includes lack of charged particle equilibrium, lateral scatter, source occlusion and volume averaging of the detectors used in the measurement of the commissioning data. High energy electron beams are also associated with bremsstrahlung production that contributes to dose deposition, which is not well investigated, particularly for small electron fields. The goal of this work has been to investigate dosimetric uncertainties associated with small electron fields using dose measurements with different detectors as well as calculations with eMC dose calculation algorithm. MethodsDifferent dosimetric parameters including output factors, depth dose curves and dose profiles from small electron field cutouts were investigated quantitatively. These dosimetric parameters were measured using different detectors that included small ion chambers and diodes for small electron cutouts with diameters ranging from 15-50mm mounted on a 6 × 6cm2 cone with beam energies from 6-20MeV. ResultsLarge deviations existed between the output factors calculated with the eMC algorithm and measured with small detectors for small electron fields up to 55% for 6MeV. The discrepancy between the calculated and measured doses increased 10%-55% with decreasing electron beam energy from 20 MeV to 6 MeV for 15mm circular field. For electron fields with cutouts 20mm and larger, the measured and calculated doses agreed within 5% for all electron energies from 6-20MeV. For small electron fields, the maximal depth dose shifted upstream and becomes more superficial as the electron beam energy increases from 6-20MeV as measured with small detectors. DiscussionLarge dose discrepancies were found between the measured and calculated doses for small electron fields where the eMC underestimated output factors by 55% for small circular electron fields with a diameter of 15 mm, particularly for low energy electron beams. The measured entrance doses and dmax of the depth dose curves did not agree with the corresponding values calculated by eMC. Furthermore, the measured dose profiles showed enhanced dose deposition in the umbra region and outside the small fields, which mostly resulted from dose deposition from the bremsstrahlung produced by high energy electrons that was not accounted for by the eMC algorithm due to inaccurate modeling of the lateral dose deposition from bremsstrahlung. ConclusionElectron small field dosimetry require more consideration of variations in beam quality, lack of charged particle equilibrium, lateral scatter loss and dose deposition from bremsstrahlung produced by energetic electron beams in a comprehensive approach similar to photon small field dosimetry. Furthermore, most of the commercially available electron dose calculation algorithms are commissioned with large electron fields; therefore, vendors should provide tools for the modeling of electron dose calculation algorithms for small electron fields.

Characterization and implementation of the L-alanine detector for quality control of lung SBRT treatments with the VMAT technique

New treatments in radiotherapy have some difficulties, among them, the geometric and dosimetric characterization of the radiation beam and the use of small radiation fields. Determination of the prescribed dose in the target volume in cases of small fields is difficult due to the absence of lateral electronic equilibrium and the sharp dose gradient at the edges of the fields. In this way, the choice of the radiation detector becomes relevant when performing the dosimetry of small fields. Alanine dosimeters have proven to be a good option for measurements of high radiation doses in these field sizes. This work aims to characterize the alanine detector through dosimetric tests for the VMAT (Volumetric Modulated Arc Therapy) technique in cases of SBRT (Stereotactic Body Radiation Therapy) to demonstrate the feasibility of using the alanine dosimetric system for use in quality control. End-to-End test was performed in a phantom, simulating two situations: homogeneous and heterogeneous regions. The response of L-alanine showed a strong linear correlation with dose (R2 = 0.99997), and insignificant dependence to dose rate. In the End-to-End test for a planned dose of 18 Gy, the doses obtained in alanine presented expanded uncertainty of 4.60% at 95% confidence level. Therefore, this work demonstrated that alanine dosimeter is suitable for quality control of SBRT with the VMAT technique in routine applications.

The effect of ion chamber volume on intensity-modulated radiotherapy small field dosimetry

The use of appropriate equipment during the measurements required for a treatment planning system (TPS) directly affects the quality of treatment. The present study investigates the effect of small field output factors (OF) obtained with ion chambers of different volumes on intensity-modulated radiotherapy (IMRT) treatment plans. Three different beam models (BM65, BM13, BM04) were created at TPS using small fields OFs measured by ion chambers with three different volumes, i.e. 0.65, 0.13 and 0.04 cc respectively. IMRT plans with the same features were designed for these models. The difference between the calculated and measured doses at dose distributions for each IMRT plan was investigated using point-dose measurement and gamma index method. The OF values obtained with a 0.65 cc ion chamber were significantly different, especially in small fields. The greatest difference in point-dose measurements made in IMRT plans was observed in the plans produced in the BM65 beam model. There was no significant difference in gamma passing rate (GPR) values in 2D array measurements. Inaccurate OF values obtained during small-field measurements is one of the important parameters that affect dosimetric accuracy. Therefore, using different dosimetric methods in quality assurance during TPS commissioning may allow to detect measurement errors, especially in small-field measurements.

Monte Carlo study of small-field dosimetry for an ELEKTA Unity MR-Linac system

This study aims to investigate the magnetic field effect in small-field dosimetry for an ELEKTA Unity MR-Linac system using Monte Carlo (MC) simulations. The EGSnrc/BEAMnrc code was used to simulate the MR-Linac treatment head. The percentage depth-doses (PDDs) and off-axis dose profiles were calculated for field sizes of 2 cm × 2 cm and 10 cm × 10 cm using the DOSXYZnrc code, while the output factors were calculated with the egs_chamber code. The dose profiles and output factors were calculated at a depth of 10 cm in water with a 143.5 cm source-axis-distance. The output factors for the detectors with different sensitive volumes were compared with previously measured values. The kQ,B was also calculated. The accuracy of the simulated beam was validated by comparing the MC-calculated PDDs and dose profiles with previously measured values, which agreed within 2% except the penumbra region. The output factors of the ion chambers decreased rapidly depending on the sensitive volumes of small fields. The output factors of the PTW-31022 agreed within 2% with those in a water slab down to a 1 cm × 1 cm field. The output factors of PTW-31021 and PTW-31022 chambers and PTW-60018 diode agreed within 2% with previously measured values. The kQ,B values for PTW-30013, PTW-31010, PTW-31021, and PTW-31022 chambers were 0.974 ± 0.14%, 0.976 ± 0.26%, 0.977 ± 0.21%, and 0.995 ± 0.36%, respectively. The calculated output factors for the detectors showed good agreement with previously measured values. The magnetic field effect was small in ionization chambers with the small cavity radius.

Evaluation of MAGIC-f polymer gel dosimeter for dose profile measurement in small fields and stereotactic irradiation

Background and purposeThe use of small fields in modern radiotherapy treatment techniques is increasingly growing, which has aroused the interest of researchers to use more polymer gel in small field dosimetry. Polymer gel dosimeters contain radiation-sensitive materials that are polymerized under irradiation as a function of absorption dose, and the spatial information of the radiation dose distribution is extracted by imaging of polymer gel. In this study, the performance and capability of MAGIC-f polymer gel dosimeter were evaluated for small field dosimetry and the temporal stability was investigated. MethodsMAGIC-f polymer gel was manufactured and it was poured into phantoms and calibration vials, and then phantoms were irradiated with 6 MV photons with small fields. The values of R2 maps and dose distribution in irradiated gel phantoms were obtained using MRI images taken from the gel phantoms at 1 day and 14 days after irradiation. Also, EBT3 film was used for the measurement of dose profiles at the same field shapes. The transverse dose profiles measured with polymer gel dosimeter were compared with the results of EBT3 film. ResultsThe slope of fitted lines on dose-response data at 1 day and 14 days after irradiation were equal to 0.6005 and 0.6147 respectively. By comparing the results of MAGIC-f gel and EBT3 film, the relative difference in FWHM for profile curves was observed 12.1%, 6.1%, and 4.3% for 0.5, 1.0, and 1.5 cm field size respectively. The gamma index pass rates between dose profile of polymer gel and film with criteria of 1%/0.5 mm were more than 77.1%. In the case of temporal stability, the penumbra measured by MAGIC-f at 14 days after irradiation was almost equal to that at 1 day after irradiation but in terms of FWHM, it was greater at 14 days. ConclusionThe results of this study show that the MAGIC-f polymer gel has good reliability and stability characteristics for small field dosimetry but it must be benchmarked against other dosimeters to become a three-dimensional dosimeter in small field dosimetry.

Effect of inaccurate small field output factors on brain SRS plans

External beam radiotherapy often includes the use of field sizes 3 × 3 cm2 or less, which can be defined as small fields. Dosimetry is a difficult, yet important part of the radiotherapy process. The dosimetry of small fields has additional challenges, which can lead to treatment inconsistencies if not done properly. Most important is the use of an appropriate detector, as well as the application of the necessary corrections. The International Atomic Energy Agency and the American Association of Physicists in Medicine provide the International Code of Practice (CoP) TRS-483 for the dosimetry of small static fields used in external MV photon beams. It gives guidelines on how to apply small-field correction factors for small field dosimetry. The purpose of this study was to evaluate the impact of inaccurate small-field output factors on clinical brain stereotactic radiosurgery plans with and without applying the small-field correction factors as suggested in the CoP. Small-field correction factors for a Varian TrueBeam linear accelerator were applied to uncorrected relative dose factors. Uncorrected and corrected clinical plans were created with two different beam configurations, 6 MV with a flattening filter (6 WFF) and 6 MV without a flattening filter (6 FFF). For the corrected plans, the planning target volume mean dose was 1.6 ± 0.9% lower with p < 0.001 for 6 WFF and 1.8 ± 1.5% lower with p < 0.001 for 6 FFF. For brainstem, a major organ at risk, the corrected plans had a dose that was 1.6 ± 0.9% lower with p = 0.03 for 6 WFF and 1.8 ± 1.5% lower with p = 0.10 for 6 FFF. This represents a systematic error that should and can be corrected.

A portable magnet for radiation biology and dosimetry studies in magnetic fields.

In the current and rapidly evolving era of real-time MRI-guided radiotherapy, our radiation biology and dosimetry knowledge is being tested in a novel way. This paper presents the successful design and implementation of a portable device used to generate strong localized magnetic fields. These are ideally suited for small-scale experiments that mimic the magnetic field environment inside an MRI-linac system, or more broadly MRI-guided particle therapy aswell. A portable permanent magnet-based device employing an adjustable steel yoke and magnetic field focusing cones has been designed, constructed, and tested. The apparatus utilizes two banks of Nd Fe B permanent magnets totaling around 50 kg in mass to generate a strong magnetic field throughout a small volume between two pole tips. The yoke design allows adjustment of the pole tip gap and exchanging of the focusing cones. Further to this, beam portal holes are present in the yoke and focusing cones, allowing for radiation beams of up to 5 5 cm to pass through the region of high magnetic field between the focusing cone tips. Finite element magnetic modeling was performed to design and characterize the performance of the device. Automated physical measurements of the magnetic field components at various locations were measured to confirm the performance. The adjustable pole gap and interchangeable cones allows rapid changing of the experimental set-up to allow different styles of measurements to beperformed. A mostly uniform magnetic field of 1.2 T can be achieved over a volume of at least 3 3 3 cm . This can be reduced in strength to 0.3 T but increased in volume to 10 10 10 cm via removal of the cone tips and/or adjustment of the steel yoke. Although small, these volumes are sufficient to house radiation detectors, cell culture dishes, and various phantom arrangements targeted at examining small radiation field dosimetry inside magnetic field strengths that can be changed with ease. Most important is the ability to align the magnetic field both perpendicular to, or inline with, the radiation beam. To date, the system has been successfully used to conduct published research in the areas of radiation detector performance, lung phantom dosimetry, and how small clinical electron beams behave in these strong magneticfields. A portable, relatively inexpensive, and simple to operate device has successfully been constructed and used for performing radiation oncology studies around the theme of MRI-guided radiotherapy. This can be in either inline and perpendicular magnetic fields of up to 1.2 T with x-ray and particlebeams.

Investigation of 2D radiophotoluminescence films in radiotherapy: Multileaf collimator Quality Assurance and small field dosimetry

In radiotherapy, accurate dose determination and precise dose delivery to the tumor are directly associated with better treatment outcomes, such as better tumor control and lower post treatment complications. Challenges in Quality Assurance (QA) are a.o. (1) the varying amounts of primary and secondary scattered photons, the varying gantry angles and the energy heterogeneity and (2) the small field sizes used in Intensity Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT).As alternative to the commonly used GafChromic™ EBT films, radiophotoluminescence (RPL) films gained attention for high resolution 2D dosimetry in radiotherapy thanks to their insensitivity to ambient light, reusability, absence of fading and the few image corrections needed.In this work we characterize the response of improved Al2O3:C,Mg RPL films in a clinical 6 MV beam. Film response was evaluated in terms of uniformity, dose response, minimum detectable dose (MDD), small field dosimetry and its performance was evaluated for multileaf collimator (MLC) QA tests.The new films showed linear dose response up to 8 Gy, followed by supralinear response for higher doses up to 24 % at 20 Gy. QA checks for the MLC, including the chair and the alternating leaves, were compared with calculations from the treatment planning system (TPS) and showed an agreement within 5% for alternating leaves and a deviation in Full Width Half Maximum (FWHM) below 1.5 % for the chair test. The maximum FWHM discrepancy between RPL film results and reference data (diode) for small field sizes (30 × 30, 20 × 20, 10 × 10, 8 × 8, 6 × 6 mm2) was 1.9 %. Notwithstanding the variations in dose response, the studied films exhibit overall good agreement with reference data and demonstrate potential for QA in IMRT and small field radiotherapy

Comparison and Validation of Multiple Detectors against Monte Carlo Simulation for the Use of Small-Field Dosimetry.

The aim of this study was to compare the Exradin W2 scintillator, PTW microDiamond, IBA Razor Nano, and IBA Razor chamber detectors for small-field dose measurements and validate the measured data against the EGSnrc user code and observe the variation between daisy-chained and direct measurement methods for the above detectors. The W2 scintillator, microDiamond, Razor Nano, and Razor chamber detectors were used to measure the in-plane and cross-plane profiles and the output factors (OFs) at 10 cm depth, and 90 source-to-surface distance for 6MV X-rays (Elekta Versa HD). The field sizes ranged from 0.5 cm × 0.5 cm to 5 cm × 5 cm. The BEAMnrc/DOSXYZnrc user codes (EGSnrc) were used to simulate the reference profiles. Gamma analysis was performed to compare the measured and simulated dose distributions. The OFs measured with the W2 scintillator, microDiamond, Razor Nano chamber, Razor chamber, and the calculated Monte Carlo (MC) showed agreement to within 1% for the 3 cm × 3 cm field size. The uncertainty in the MC simulation was observed to be 0.4%. The percent difference in OFs measured using daisy-chained and direct measurement methods was within 0.15%, 0.4%, 1.4%, and 2.4% for microDiamond, W2 scintillator, Nano, and Razor chamber detectors, respectively. The lateral beam profiles and OFs of W2 scintillator, microDiamond, Razor Nano, and Razor chambers exhibit good agreement with the MC simulation within the nominal field sizes. Our results demonstrate that we can achieve considerable time-saving by directly measuring small-field OFs without daisy-chained methods using microDiamond and W2 scintillator. In terms of ease of use, sensitivity, reproducibility, and from a practical standpoint, we recommend microDiamond for small-field dosimetry.

Three-dimensional reproducibility assessment of radiochromic ClearView™ gel in custom vessel

Samples of the radiochromic hydrogel ClearView™ were characterized for 3D dosimeter performance in custom vessels and read with a modified Vista16 optical cone beam scanner as part of a small field dosimetry study. Uniform dose responses and small field depth doses revealed each sample was unique. As the gels aged over several weeks at 4 °C, the dose sensitivity decreased. Reconstructions of uniform dose distributions revealed unique features near the vessel interface. The central sub-volumes ranged from uniformity to within 2% throughout the sub-volume to a 12% gradient from base to top of gel. Comparisons of small field depth doses revealed that if the central volume had a uniform dose response, no correction was required. However, if a gradient in sensitivity from bottom to top was measured a correction may or may not be required. It is suspected that the filling protocol of the custom vessels is a factor in the observed variations in 3D dose response.

The current status and shortcomings of stereotactic radiosurgery.

BackgroundStereotactic radiosurgery (SRS) is a common treatment for intracranial lesions. This work explores the state of SRS treatment delivery to characterize current treatment accuracy based on treatment parameters.MethodsNCI clinical trials involving SRS rely on an end-to-end treatment delivery on a patient surrogate (credentialing phantom) from the Imaging and Radiation Oncology Core (IROC) to test their treatment accuracy. The results of 1072 SRS phantom irradiations between 2012 and 2020 were retrospectively analyzed. Univariate analysis and random forest models were used to associate irradiation conditions with phantom performance. The following categories were evaluated in terms of how they predicted outcomes: year of irradiation, TPS algorithm, machine model, energy, and delivered field size.ResultsOverall, only 84.6% of irradiations have met the IROC/NCI acceptability criteria. Pass rate has remained constant over time, while dose calculation accuracy has slightly improved. Dose calculation algorithm (P < .001), collimator (P = .024), and field size (P < .001) were statistically significant predictors of pass/fail. Specifically, pencil beam algorithms and cone collimators were more likely to be associated with failing phantom results. Random forest modeling identified the size of the field as the most important factor for passing or failing followed by algorithm.ConclusionConstant throughout this retrospective study, approximately 15% of institutions fail to meet IROC/NCI standards for SRS treatment. In current clinical practice, this is particularly associated with smaller fields that yielded less accurate results. There is ongoing need to improve small field dosimetry, beam modeling, and QA to ensure high treatment quality, patient safety, and optimal clinical trials.

Evaluation of a micro ionization chamber for dosimetric measurements in image-guided preclinical irradiation platforms

Image-guided small animal irradiation platforms deliver small radiation fields in the medium energy x-ray range. Commissioning of such platforms, followed by dosimetric verification of treatment planning, are mostly performed with radiochromic film. There is a need for independent measurement methods, traceable to primary standards, with the added advantage of immediacy in obtaining results. This investigation characterizes a small volume ionization chamber in medium energy x-rays for reference dosimetry in preclinical irradiation research platforms. The detector was exposed to a set of reference x-ray beams (0.5–4 mm Cu HVL). Leakage, reproducibility, linearity, response to detector’s orientation, dose rate, and energy dependence were determined for a 3D PinPoint ionization chamber (PTW 31022). Polarity and ion recombination were also studied. Absorbed doses at 2 cm depth were compared, derived either by applying the experimentally determined cross-calibration coefficient at a typical small animal radiation platform ‘user’s’ quality (0.84 mm Cu HVL) or by interpolation from air kerma calibration coefficients in a set of reference beam qualities. In the range of reference x-ray beams, correction for ion recombination was less than 0.1%. The largest polarity correction was 1.4% (for 4 mm Cu HVL). Calibration and correction factors were experimentally determined. Measurements of absorbed dose with the PTW 31022, in conditions different from reference were successfully compared to measurements with a secondary standard ionization chamber. The implementation of an End-to-End test for delivery of image-targeted small field plans resulted in differences smaller than 3% between measured and treatment planning calculated doses. The investigation of the properties and response of a PTW 31022 small volume ionization chamber in medium energy x-rays and small fields can contribute to improve measurement uncertainties evaluation for reference and relative dosimetry of small fields delivered by preclinical irradiators while maintaining the traceability chain to primary standards.

P05 - A beam monitor transmission detector for small and large fields dosimetry

P05 - A beam monitor transmission detector for small and large fields dosimetry

OD37 - Comparison of detectors performances in small field dosimetry of Versa HD flattened and flattening filter free beams

OD37 - Comparison of detectors performances in small field dosimetry of Versa HD flattened and flattening filter free beams