Experimental investigation of GafChromic® EBT3 intrinsic energy dependence with kilovoltage x rays, 137 Cs, and 60 Co

The delivery assessment for small targets on Halcyon radiotherapy system - Measured and calculated dose comparison

BACKGROUND

With the ever-increasing requirements of accuracy and personalization of radiotherapy treatments, stereotactic radiotherapy (SRT) with volumetric modulated arc therapy (VMAT) on O-ring Halcyon radiotherapy system could potentially provide a fast, safe, and feasible treatment option.

PURPOSE

The purpose of this study was to assess the delivery of Halcyon VMAT plans for small targets.

METHODS

Well-defined VMAT-SRT plans were created on Halcyon radiotherapy system with the stacked and staggered dual-layer MLC design for the film measurement set-up and the target sizes and shapes designed to emulate the targets of the stereotactic treatments. The planar dose distributions were acquired with film measurements and compared to a current clinical reference dose calculation with AcurosXB (v18.0, Varian Medical Systems) and to Monte Carlo simulations. With the collapsed arc versions of the VMAT-SRT plans, the uncertainty in dose delivery due to the multileaf collimator (MLC) without the gantry rotation could be separated and analyzed.

RESULTS

The target size was mainly limited by the resolution originated from the design of the MLC leaves. The results of the collapsed arc versions of the plans show good consistency among measured, calculated, and simulated dose distributions. With the full VMAT plans, the agreement between calculated and simulated dose distributions was consistent with the collapsed arc versions. The measured dose distribution agreed with the calculated and simulated dose distributions within the target regions, but considerable local differences were observed in the margins of the target. The largest differences located in the steep gradient regions presumably originating from the deviation of the isocenter.

CONCLUSIONS

The potential of the Halcyon radiotherapy system for VMAT-SRT delivery was evaluated and the study revealed valuable insights on the machine characteristics with the delivery.

Active and passive dosimetry for beta-emitting radiopharmaceutical therapy agents in a custom SPECT/CT compatible phantom

Objective. This work introduces a novel approach to performing active and passive dosimetry for beta-emitting radionuclides in solution using common dosimeters. The measurements are compared to absorbed dose to water (Dw) estimates from Monte Carlo (MC) simulations. We present a method for obtaining absorbed dose to water, measured with dosimeters, from beta-emitting radiopharmaceutical agents using a custom SPECT/CT compatible phantom for validation of Monte Carlo based absorbed dose to water estimates.Approach. A cylindrical, acrylic SPECT/CT compatible phantom capable of housing an IBA EFD diode, Exradin A20-375 parallel plate ion chamber, unlaminated EBT3 film, and thin TLD100 microcubes was constructed for the purpose of measuring absorbed dose to water from solutions of common beta-emitting radiopharmaceutical therapy agents. The phantom is equipped with removable detector inserts that allow for multiple configurations and is designed to be used for validation of image-based absorbed dose estimates with detector measurements. Two experiments with131I and one experiment with177Lu were conducted over extended measurement intervals with starting activities of approximately 150-350 MBq. Measurement data was compared to Monte Carlo simulations using the egs_chamber user code in EGSnrc 2019.Main results. Agreement withink= 1 uncertainty between measured and MC predictedDwwas observed for all dosimeters, except the A20-375 ion chamber during the second131I experiment. Despite the agreement, the measured values were generally lower than predicted values by 5%-15%. The uncertainties atk = 1 remain large (5%-30% depending on the dosimeter) relative to other forms of radiation therapy.Significance. Despite high uncertainties, the overall agreement between measured and simulated absorbed doses is promising for the use of dosimeter-based RPT measurements in the validation of MC predictedDw.

Dosimetric characterization of a rotating anode x-ray tube for FLASH radiotherapy research

PURPOSE

Most current research toward ultra-high dose rate (FLASH) radiation is conducted with advanced proton and electron accelerators, which are of limited accessibility to basic laboratory research. An economical alternative to charged particle accelerators is to employ high-capacity rotating anode x-ray tubes to produce kilovoltage x-rays at FLASH dose rates at short source-to-surface distances (SSD). This work describes a comprehensive dosimetric evaluation of a rotating anode x-ray tube for potential application in laboratory FLASH study.

METHODS AND MATERIALS

A commercially available high-capacity fluoroscopy x-ray tube with 75 kW input power was implemented as a potential FLASH irradiator. Radiochromic EBT3 film and thermoluminescent dosimeters (TLDs) were used to investigate the effects of SSD and field size on dose rates and depth-dose characteristics in kV-compatible solid water phantoms. Custom 3D printed accessories were developed to enable reproducible phantom setup at very short SSD. Open and collimated radiation fields were assessed.

RESULTS

Despite the lower x-ray energy and short SSD used, FLASH dose rates above 40 Gy/s were achieved for targets up to 10-mm depth in solid water. Maximum surface dose rates of 96 Gy/s were measured in the open field at 47 mm SSD. A non-uniform high-to-low dose gradient was observed in the planar dose distribution, characteristic of anode heel effects. With added collimation, beams up to 10-mm diameter with reasonable uniformity can be produced. Typical 80%-20% penumbra in the collimated x-ray FLASH beams were less than 1 mm at 5-mm depth in phantom. Ramp-up times at the maximum input current were less than 1 ms.

CONCLUSION

Our dosimetric characterization demonstrates that rotating anode x-ray tube technology is capable of producing radiation beams in support of preclinical FLASH radiobiology research.

FLASH Effects Induced by Orthovoltage X-Rays

PURPOSE

This work describes the first implementation and in vivo study of ultrahigh-dose-rate radiation (>37 Gy/s; FLASH) effects induced by kilovoltage (kV) x-ray from a rotating-anode x-ray source.

METHODS AND MATERIALS

A high-capacity rotating-anode x-ray tube with an 80-kW generator was implemented for preclinical FLASH radiation research. A custom 3-dimensionally printed immobilization and positioning tool was developed for reproducible irradiation of a mouse hind limb. Calibrated Gafchromic (EBT3) film and thermoluminescent dosimeters (LiF:Mg,Ti) were used for in-phantom and in vivo dosimetry. Healthy FVB/N and FVBN/C57BL/6 outbred mice were irradiated on 1 hind leg to doses up to 43 Gy at FLASH (87 Gy/s) and conventional (CONV; <0.05 Gy/s) dose rates. The radiation doses were delivered using a single pulse with the widths up to 500 ms and 15 minutes at FLASH and CONV dose rates. Histologic assessment of radiation-induced skin damage was performed at 8 weeks posttreatment. Tumor growth suppression was assessed using a B16F10 flank tumor model in C57BL6J mice irradiated to 35 Gy at both FLASH and CONV dose rates.

RESULTS

FLASH-irradiated mice experienced milder radiation-induced skin injuries than CONV-irradiated mice, visible by 4 weeks posttreatment. At 8 weeks posttreatment, normal tissue injury was significantly reduced in FLASH-irradiated animals compared with CONV-irradiated animals for histologic endpoints including inflammation, ulceration, hyperplasia, and fibrosis. No difference in tumor growth response was observed between FLASH and CONV irradiations at 35 Gy. The normal tissue sparing effects of FLASH irradiations were observed only for high-severity endpoint of ulceration at 43 Gy, which suggests the dependency of biologic endpoints to FLASH radiation dose.

CONCLUSIONS

Rotating-anode x-ray sources can achieve FLASH dose rates in a single pulse with dosimetric properties suitable for small-animal experiments. We observed FLASH normal tissue sparing of radiation toxicities in mouse skin irradiated at 35 Gy with no sacrifice to tumor growth suppression. This study highlights an accessible new modality for laboratory study of the FLASH effect.

A formalism for traceable dosimetry in superficial electronic brachytherapy (eBT)

Objective. Despite the number of treatments performed with electronic brachytherapy (eBT) there is no uniform methodology for reference dosimetry for international traceability to primary dosimetry standards in different eBT systems. The objective of this study is to propose a formalism for traceability reference dosimetry in superficial eBT, that is easy to apply in the clinic. This method was investigated for an Elekta Esteya with one applicator.Approach. The calibration x-ray spectrum at the primary standards dosimetry laboratory was matched to the measured eBT photon spectrum. Subsequently, two ionization chambers of different types were calibrated at the primary standard dosimetry laboratory (PSDL) in terms of air kerma against a primary standard. The chambers were used to measure ionization chamber reading ratios in-air at different distances from the applicator. Monte Carlo based air kerma ratios were calculated at different positions from the eBT applicator as well as backscatter factors in water and average mass energy absorption ratios in water and in air. Relative measurements with radiochromic films were performed in a water phantom to determine the ratio of absorbed dose to water,Dw, at the surface toDwat 1 cm depth in water. These were compared with Monte Carlo calculations.Main results. Calculations and measurements were combined to estimate theDwat the surface and at 1 cm depth in water. Ionization chamber agreement of the surface dose was 1.7%, within an uncertainty of 6.8% (k= 2). They agreed with the manufacturer dosimetry within 1.8%, with an uncertainty of 5.0% (k= 2). The feasibility of the formalism and methodology for the Esteya system was demonstrated.Significance. This study proposes a method for harmonization of traceable reference dosimetry for eBT contact treatments which does not involve a detailed simulation of the ionization chamber. The method demonstrated feasibility for one eBT system using one surface applicator. In the future the method could be applied for different eBT systems.

Monte Carlo modeling of Halcyon and Ethos radiotherapy beam using CAD geometry: validation and IAEA-compliant phase space

Objective.A Monte Carlo (MC) model of a Halcyon and Ethos (Varian Medical Systems, a Siemens Healthineers Company) radiotherapy beam was validated and field-independent phase space (PHSP) files were recorded above the dual-layer multileaf collimators (MLC).Approach.The treatment head geometry was modeled according to engineering drawings and the dual-layer MLC was imported from CAD (computer-aided design) files. The information for the incident electron beam was achieved from an iterative electromagnetic solver. The validation of the model was performed by comparing the dose delivered by the square MLC fields as well as complex field measurements.Main results.An electron phase space was generated from linac simulations and achieved improved MC results. The output factors for square fields were within 1% and the largest differences of 5% were found in the build-up region of PDDs and the penumbra region of profiles. With the more complicated MLC-shaped field (Fishbone), the largest differences of up to 8% were found in the MLC leaf tip region due to the uncertainty of the MLC positioning and the mechanical leaf gap value. The impact of the collimator rotation on the PHSP solution has been assessed with both small and large fields, confirming negligible effects on in-field and out-of-field dose distributions.Significance.A computational model of the Halcyon and Ethos radiotherapy beam with a high accuracy implementation of the MLC was shown to be able to reproduce the radiation beam characteristics with square fields and more complex MLC-shaped fields. The field-independent PHSP files that were produced can be used as an accurate treatment head model above the MLC, and reduce the time to simulate particle transport through treatment head components.

Characterization of Inorganic Scintillator Detectors for Dosimetry in Image-Guided Small Animal Radiotherapy Platforms

The purpose of the study was to characterize a detection system based on inorganic scintillators and determine its suitability for dosimetry in preclinical radiation research. Dose rate, linearity, and repeatability of the response (among others) were assessed for medium-energy X-ray beam qualities. The response's variation with temperature and beam angle incidence was also evaluated. Absorbed dose quality-dependent calibration coefficients, based on a cross-calibration against air kerma secondary standard ionization chambers, were determined. Relative output factors (ROF) for small, collimated fields (≤10 mm × 10 mm) were measured and compared with Gafchromic film and to a CMOS imaging sensor. Independently of the beam quality, the scintillator signal repeatability was adequate and linear with dose. Compared with EBT3 films and CMOS, ROF was within 5% (except for smaller circular fields). We demonstrated that when the detector is cross-calibrated in the user's beam, it is a useful tool for dosimetry in medium-energy X-rays with small fields delivered by Image-Guided Small Animal Radiotherapy Platforms. It supports the development of procedures for independent "live" dose verification of complex preclinical radiotherapy plans with the possibility to insert the detectors in phantoms.

Development of Standard X-Ray Beams for Calibration of Radiobiology Cabinet and Conformal Irradiators

This work seeks to develop standard X-ray beams that are matched to radiobiology X-ray irradiators. The calibration of detectors used for dose determination of these irradiators is performed with a set of standard X rays that are more heavily filtered and/or lower energy, which leads to a higher uncertainty in the dose measurement. Models of the XRad320, SARRP, and the X-ray tube at the University of Wisconsin Medical Radiation Research Center (UWMRRC) were created using the BEAMnrc user code of the EGSnrc Monte Carlo code system. These models were validated against measurements, and the resultant modeled spectra were used to determine the amount of added filtration needed to match the X-ray beams at the UWMRRC to those of the XRad320 and SARRP. The depth profiles and half-value layer (HVL) simulations performed using BEAMnrc agreed to measurements within 3% and 3.6%, respectively. A primary measurement device, a free-air chamber, was developed to measure air kerma in the medium energy range of X rays. The resultant spectra of the matched beams had HVL's that matched the HVL's of the radiobiology irradiators well within the 3% criteria recommended by the International Atomic Energy Agency (IAEA) and the average energies agreed within 2.4%. In conclusion, three standard X-ray beams were developed at the UWMRRC with spectra that more closely match the spectra of the XRad320 and SARRP radiobiology irradiators, which will aid in a more accurate dose determination during calibration of these irradiators.

How to design, fabricate, and validate a customized COMS-style eye plaque: Illustrated with a narrow-slotted plaque example

PURPOSE

A customized Collaborative Ocular Melanoma Study (COMS)-style eye plaque may provide superior dosimetric coverage compared with standard models for certain intraocular tumor locations and shapes. This work provides a recipe for developing and validating such customized plaques.

METHODS AND MATERIALS

The concept-into-clinical treatment process for a customized COMS-style eye plaque begins with a CAD model design that meets the specifications of the radiation oncologist and surgeon based on magnetic resonance, ultrasound, and clinical measurements, as well as a TG-43 hybrid heterogeneity-corrected dose prediction to model the dose distribution. Next, a 3D printed plastic prototype is created and reviewed. After design approval, a Modulay plaque is commercially fabricated. Quality assurance (QA) is subsequently performed to verify the physical measurements of the Modulay and Silastic and also includes dosimetric measurement of the calibration, depth dose, and dose profiles. Sterilization instructions are provided by the commercial fabricator. This customization procedure and QA methodology is demonstrated with a narrow-slotted plaque that was recently constructed for the treatment of a circumpapillary (e.g., surrounding the optic disk) ocular tumor.

RESULTS

The production of a customized COMS-style eye plaque is a multistep process. Dosimetric modeling is recommended to ensure that the design will meet the patient's needs, and QA is essential to confirm that the plaque has the proper dimensions and dose distribution. The customized narrow-slotted plaque presented herein was successfully implemented in the clinic, and provided superior dose coverage of juxtapapillary and circumpapillary tumors compared with standard or notched COMS-style plaques. Plaque development required approximately 30 h of physicist time and a fabrication cost of $1500.

CONCLUSION

Customized eye plaques may be used to treat intraocular tumors that cannot be adequately managed with standard models. The procedure by which a customized COMS-style plaque may be designed, fabricated, and validated was presented along with a clinical example.

Ultrahigh dose-rate (FLASH) x-ray irradiator for pre-clinical laboratory research

FLASH irradiation has been shown to reduce significantly normal tissue toxicity compared to conventional irradiation, while maintaining tumor control probability at similar level. Clinical translation of FLASH irradiation necessitates comprehensive laboratory studies to elucidate biological effects as well as pertinent technological and physical requirements. At present, FLASH research employs complex accelerator technologies of limited accessibilities. Here, we study the feasibility of a novel self-shielded x-ray irradiation cabinet system, as an enabling technology to enhance the preclinical research capabilities. The proposed system employs two commercially available high capacity 150 kVp fluoroscopy x-ray sources with rotating anode technology in a parallel-opposed arrangement. Simulation was performed with the GEANT4 Monte-Carlo platform. Simulated dosimetric properties of the x-ray beam for both FLASH and conventional dose-rate irradiations were characterized. Dose and dose rate from a single kV x-ray fluoroscopy source in solid water phantom were verified with measurements using Gafchromic films. The parallel-opposed x-ray sources can deliver over 50 Gy doses to a 20 mm thick water equivalent medium at ultrahigh dose-rates of 40-240 Gy s^-1. A uniform depth-dose rate (±5%) is achieved over 8-12 mm in the central region of the phantom. Mirrored beams minimize heel effect of the source and achieve reasonable cross-beam uniformity (±3%). Conventional dose-rate irradiation (≤0.1 Gy s^-1) can also be achieved by reducing the tube current and increasing the distance between the phantom and tubes. The rotating anode x-ray source can be used to deliver both FLASH and conventional dose-rate irradiations with the field dimensions well suitable for small animal and cell-culture irradiations. For FLASH irradiation using parallel-opposed sources, entrance and exit doses can be higher by 30% than the dose at the phantom center. Beam angling can be employed to minimize the high surface doses. Our proposed system is amendable to self-shielding and enhance research in regular laboratory setting.

Radiochromic Films for the Two-Dimensional Dose Distribution Assessment

Radiochromic films are mainly used for two-dimensional dose verification in photon, electron, and proton therapy treatments. Moreover, the radiochromic film types available today allow their use in a wide dose range, corresponding to applications from low-medical diagnostics to high-dose beam profile measurements in charged particle medical accelerators. An in-depth knowledge of the characteristics of radiochromic films, of their operating principles, and of the dose reading techniques is of paramount importance to exploit all the features of this interesting and versatile radiation detection system. This short review focuses on these main aspects by considering the most recent works on the subject.

Technical Note: Vendor miscalibration of preclinical orthovoltage irradiator identified through independent output check

PURPOSE

Accurate radiation dosimetry in radiobiological experiments is crucial for preclinical research in advancement of cancer treatment. Vendors of cell irradiators often perform calibration for end-users. However, calibration accuracy remains unclear due to missing detailed information on calibration equipment and procedures. In this study, we report our findings of a vender miscalibration of the radiation output and our investigation on the root cause of the discrepancy.

METHODS

Independent calibration verification for a commercial preclinical orthovoltage irradiator was conducted. Initially, in the absence of ionization chambers calibrated at kV energy, radiochromic films (EBT3) was first calibrated at MV energy. Energy correction factors from literature were used to create an in-house kV dosimetry system. The miscalibration identified with the in-house kV EBT3 dosimetry was later confirmed by ADCL calibrated ionization chambers (Exradin A1SL and PTW 30013) at kV energy. Ionization chambers were suspended in-air following TG-61 recommendation for output calibration. To investigate the root cause of the miscalibration, additional measurements were performed with ionization chambers placed on the shelf. A validated Monte Carlo simulation code was also used to investigate the impact of placing the ionization chamber on the shelf instead of suspending it in air during the vendor-performed calibration process.

RESULTS

Up to a 6% dosimetry error was observed when comparing the vendor calibrated output of the preclinical irradiator with our independent calibration check. Further investigation showed incorrect setups in the vendor's calibration procedure which may result in dose errors up to 11% from the backscatter of the shelf board during calibration, and up to 5% from omitting temperature and pressure corrections to ionization chamber readings.

CONCLUSION

Our study revealed large dose calibration errors caused by incorrect setup and the omission of temperature/pressure correction in the vendor's calibration procedure. The findings also highlighted the importance of performing an independent check of the dose calibration for preclinical kV irradiators. More absolute dosimetry training is needed for both vendors and end users for establishing accurate absolute dosimetry.

Monte Carlo simulations of EBT3 film dose deposition for percentage depth dose (PDD) curve evaluation

Purpose

To use Monte Carlo (MC) calculations to evaluate the effects of Gafchromic EBT3 film orientation on percentage depth dose (PDD) curves.

Methods

Dose deposition in films placed in a water phantom, and oriented either parallel or perpendicular with respect to beam axis, were simulated with MC and compared to PDDs scored in a homogenous water phantom. The effects of introducing 0.01–1.00 mm air gaps on each side of the film as well as a small 1°‐3° tilt for film placed in parallel orientation were studied. PDDs scored based on two published EBT3 film compositions were compared. Three photon beam energies of 120 kVp, 220 kVp, and 6 MV and three field sizes between 1 × 1 and 5 × 5 cm² were considered. Experimental PDDs for a 6‐MV 3 × 3 cm² beam were acquired.

Results

PDD curves for films in perpendicular orientation more closely agreed to water PDDs than films placed in parallel orientation. The maximum difference between film and water PDD for films in parallel orientation was −12.9% for the 220 kVp beam. For the perpendicular film orientation, the maximum difference decreased to 5.7% for the 120 kVp beam. The inclusion of an air gap had the largest effect on the 6‐MV 1 × 1 cm² beam, for which the dose in the buildup region was underestimated by 21.2% compared to the simulation with no air gap. A 2° film tilt decreased the difference between the parallel film and homogeneous water phantom PDDs from −5.0% to −0.5% for the 6 MV 3 × 3 cm² beam. The “newer” EBT3 film composition resulted in larger PDD discrepancies than the previous composition. Experimental film data qualitatively agreed with MC simulations.

Conclusions

PDD measurements with films should either be performed with film in perpendicular orientation to the beam axis or in parallel orientation with a ~ 2º tilt and no air gaps.

Positional and angular tracking of HDR 192 Ir source for brachytherapy quality assurance using radiochromic film dosimetry

PURPOSE

To quantify and verify the dosimetric impact of high-dose rate (HDR) source positional uncertainty in brachytherapy, and to introduce a model for three-dimensional (3D) position tracking of the HDR source based on a two-dimensional (2D) measurement. This model has been utilized for the development of a comprehensive source quality assurance (QA) method using radiochromic film (RCF) dosimetry including assessment of different digitization uncertainties.

METHODS

An algorithm was developed and verified to generate 2D dose maps of the mHDR-V2 ¹⁹² Ir source (Elekta, Veenendaal, Netherlands) based on the AAPM TG-43 formalism. The limits of the dosimetric error associated with source (0.9 mm diameter) positional uncertainty were evaluated and experimentally verified with EBT3 film measurements for 6F (2.0 mm diameter) and 4F (1.3 mm diameter) size catheters at the surface (4F, 6F) and 10 mm further (4F only). To quantify this uncertainty, a source tracking model was developed to incorporate the unique geometric features of all isodose lines (IDLs) within any given 2D dose map away from the source. The tracking model normalized the dose map to its maximum, then quantified the IDLs using blob analysis based on features such as area, perimeter, weighted centroid, elliptic orientation, and circularity. The Pearson correlation coefficients (PCCs) between these features and source coordinates (x, y, z, θ_y , θ_z ) were calculated. To experimentally verify the accuracy of the tracking model, EBT3 film pieces were positioned within a Solid Water® (SW) phantom above and below the source and they were exposed simultaneously.

RESULTS

The maximum measured dosimetric variations on the 6F and 4F catheter surfaces were 39.8% and 36.1%, respectively. At 10 mm further, the variation reduced to 2.6% for the 4F catheter which is in agreement with the calculations. The source center (x, y) was strongly correlated with the low IDL-weighted centroid (PCC = 0.99), while the distance to source (z) was correlated with the IDL areas (PCC = 0.96) and perimeters (PCC = 0.99). The source orientation θ_y was correlated with the difference between high and low IDL-weighted centroids (PCC = 0.98), while θ_z was correlated with the elliptic orientation of the 60-90% IDLs (PCC = 0.97) for a maximum distance of z = 5 mm. Beyond 5 mm, IDL circularity was significant, therefore limiting the determination of θ_z (PCC ≤ 0.48). The measured positional errors from the film sets above and below the source indicated a source position at the bottom of the catheter (-0.24 ± 0.07 mm).

CONCLUSIONS

Isodose line features of a 2D dose map away from the HDR source can reveal its spatial coordinates. RCF was shown to be a suitable dosimeter for source tracking and dosimetry. This technique offers a novel source QA method and has the potential to be used for QA of commercial and customized applicators.

On the use of EBT3 film for relative dosimetry of kilovoltage X ray beams

EBT3 films were evaluated for relative dosimetry in water, in the energy range of therapeutic kV X ray beams. A film batch was calibrated in air for all nine beam qualities of a clinical unit (XStrahl 200). Monte Carlo (MC) simulations using MCNP v.6 facilitated the calculation of the film absorbed dose (f), and beam quality (k_bq) energy dependences in air. Results were found in agreement with corresponding data in the literature. Film samples from the same batch were irradiated in water along the central beam axis for each beam quality. Experimental percentage depth dose (PDD) results obtained using calibration data in air showed quality and depth dependent differences from corresponding MC simulations. These differences increased beyond film dosimetry uncertainty (<3.3%), reaching up to 8% at increased depth. The observed differences reduced only slightly when spectral variation as a function of measurement point was accounted for, using photon effective energy. PDD measurements and corresponding MC results facilitated the determination of f and k_bq in water. Results showed that the origin of the observed differences between experimental and MC PDD results is the difference between film response in air and water, as a result of radiation field perturbation from the film oriented along the central beam axis. This implies a directional dependence of film response which necessitates that the angular distribution of photons impinging on the film is the same in the calibration and measurement geometries.

Microdosimetric modeling of the sensitometric curve of GafChromic films in the photon fields

The sensitometric curve of EBT3 GafChromic film located at a depth 5 cm of RW3 water-equivalent phantom exposed to 6 MV X-rays is investigated. Variation of optical density for absorbed doses less than 2 Gy is determined by the experimental measurement together with the microdosimetric one-hit detector model. It is found that this model needs two fitting parameters, a maximum optical density and a saturation parameter. Both of them depend on the film structure as well as the photon spectrum. Meanwhile, the saturation parameter is a function of the microdosimetric single-event distribution of specific energy, i.e., f₁(z). To calculate this distribution, irradiation of the films is simulated by the Geant4 toolkit. A sample of EBT3 film with 2 mm × 2 mm area is simulated. Active layer of the film is considered to contain 5000 cylindrical sensitive targets (SV) with 9.4 μm length and 1.62 μm diameter, located in random positions with different axial directions. The results obtained show that below an absorbed dose 1 Gy the maximum difference between the measured and the calculated optical densities is about 11%, while for the doses above 1 Gy the discrepancy is at most 3%. Eventually, it can be concluded that the sensitometric curve of EBT3 GafChromic film can satisfactorily be determined by the microdosimetric one-hit detector model, especially in the dose range above 1 Gy.

Energy and dose dependence of GafChromic EBT3-V3 film across a wide energy range

AIM

To determine the energy and dose dependence of GafChromic EBT3-V3 film over an energy range 0.2 mm Al HVL to 6 MV.

BACKGROUND

The decay scheme of a brachytherapy source may be complex and the spectrum of energy can be wide. LiF TLDs are the golden standard recommended for dosimetric measures in brachytherapy, for their energy independence, but TLDs could be not available in some centres. An alternative way to perform dose measurements is to use GafChromic films, but they show energy dependence.

METHODS AND MATERIALS

Films have been irradiated at increasing dose with three different beams: 6 MV beam, TPR20, 10 = (0.684 ± 0.01), HVL = (2.00 ± 0.01)mmAl and HVL = (0.20 ± 0.01)mmAl. Calibration curves were generated using the same dose range (0cGy to 850cGy) for the three energies. Using the 6 MV calibration curve as reference, the film response in terms of net optical density (OD) was evaluated.

RESULTS

The difference in the calibration curve obtained by irradiating the film with 6 MV and 2 mm Al HVL energy beams is less than 3 %, within the calibration uncertainty, in the dose range 500-850cGy. The OD of EBT3-V3 film is significantly lower at 0.2 mmAl HVL compared to 6 MV, showing differences up to 25 %.

CONCLUSION

Within the range 6 MV-2 mm Al HVL and dose higher than 500cGy, GafChromic EBT3-V3 films are energy independent. In this dose range, films can be calibrated in a simple geometry, using a 6 MV Linac beam, and can be used for brachytherapy sources dose measures. The use of EBT3 films can be extended to reference dosimetry in Ir-192 clinical brachytherapy.

Characterization of radiochromic films as a micrometer-resolution dosimeter by confocal Raman spectroscopy

PURPOSE

Micrometer spatial resolution dosimetry has become inevitable for advanced radiotherapy techniques. A new approach using radiochromic films was developed to measure a radiation dose at a micrometer spatial resolution by confocal Raman spectroscopy.

METHODS

The commercial radiochromic films (RCF), EBT3 and EBT-XD, were irradiated with known doses using 50, 100, 200, and 300 kVp, and 6-MV x rays. The dose levels ranged from 0.3 to 50 Gy. The Raman mapping technique developed in our early study was used to readout an area of 100 × 100 µm² on RCF with improved lateral and depth resolutions with confocal Raman spectrometry. The variation in Raman spectra of C-C-C deformation and C≡C stretching modes of diacetylene polymers around 676 and 2060 cm^-1 , respectively, as a function of therapeutic x-ray doses, was measured. The single peak (SP) of C≡C and the peak ratio (PR) of C≡C band height to C-C-C band height with a spatial resolution of 10 µm on both types of RCF were evaluated, averaged, and plotted as a function of dose. An achievable spatial resolution, clinically useful dose range, dosimetric sensitivity, dose uniformity, and postirradiation stability as well as the orientation, energy, and dose rate dependence, of both types of RCFs, were characterized by the technique developed in this study.

RESULTS

A spatial resolution on RCF achieved by SP and PR methods was ~4.5 and ~2.9 µm, respectively. Raman spectroscopy data showed dose nonuniformity of ~11% in SP method and <3% in PR method. The SP method provided dose ranges of up to ~10 and ~20 Gy for EBT3 and EBT-XD films, respectively while the PR method up to ~30 and ~50 Gy. The PR method diminished the orientation effect. The percent difference between landscape and portrait orientations for the EBT3 and the EBT-XD films at 4 Gy had an acceptable level of 1.2% and 2.4%, respectively. With both SP and PR methods, the EBT3 and the EBT-XD films showed weak energy (within ~10% and ~3% for SP and PR methods, respectively) and dose rate dependence (within ~5% and ~3% for SP and PR methods, respectively) and had a stable response after 24-h postirradiation.

CONCLUSIONS

A technique for micrometer-resolution dosimetry was successfully developed by detecting radiation-induced Raman shift on EBT3 and EBT-XD. Both types of RCFs were suitable for micrometer-resolution dosimetry using CRS. With CRS both lateral and depth resolutions on RCF were improved. The PR method provided superior characteristics in dose uniformity, dose ranges, orientation dependence, and laser effect for both types of RCFs. The overall dosimetric characteristics of the RCFs determined by this technique were similar to those known by optical density scanning. The CRS with the PR method is advantageous over other the traditional scanning systems as a spatial resolution of <10 µm on RCF can be achieved with less deviations.

Characterization of EBT3 radiochromic films for dosimetry of proton beams in the presence of magnetic fields

PURPOSE

Radiochromic film dosimetry is extensively used for quality assurance in photon and proton beam therapy. So far, GafchromicTM EBT3 film appears as a strong candidate to be used in future magnetic resonance (MR) based therapy systems. The response of Gafchromic EBT3 films in the presence of magnetic fields has already been addressed for different MR-linacs systems. However, a detailed evaluation of the influence of external magnetic fields on the film response and calibration curves for proton therapy has not yet been reported. This study aims to determine the dose responses of EBT3 films for clinical proton beams exposed to magnetic field strengths up to 1 T in order to investigate the feasibility of EBT3 film as an accurate dosimetric tool for a future MR particle therapy system (MRPT).

METHODS

The dosimetric characteristics of EBT3 films were studied for a proton beam passing through magnetic field strengths of B = 0, 0.5, and 1 T. Absorbed dose calibration and measurements were performed using clinical proton beams in the nominal energy range of 62.4-252.6 MeV. Irradiations were done using an in-house developed PMMA slab phantom placed in the center of a dipole research magnet. Monte Carlo (MC) simulations using the GATE/Geant4 toolkit were performed to predict the effect of magnetic fields on the energy deposited by proton beams in the phantom. Planned and measured doses from 3D box cube irradiations were compared to assess the accuracy of the dosimetric method using EBT3 films with/without the external magnetic field.

RESULTS

Neither for the mean pixel value nor for the net optical density, any significant deviations were observed due to the presence of an external magnetic field (B ≤ 1T) for doses up to 10 Gy. Dose-response curves for the red channel were fitted by a three-parameter function for the field-free case and for B = 1T, showing for both cases an R-square coefficient of unity and almost identical fitting parameters. Independently of the magnetic field, EBT3 films showed an under-response as high as 8% in the Bragg peak region, similarly to previously reported effects for particle therapy. No noticeable influence of the magnetic field strength was observed on the quenching effect of the EBT3 films.

CONCLUSIONS

For the first time detailed absorbed dose calibrations of EBT3 films for proton beams in magnetic field regions were performed. Results showed that EBT3 films represent an attractive solution for the dosimetry of a future MRPT system. As film response functions for protons are not affected by the magnetic field strenght, they can be used for further investigations to evaluate the dosimetric effects induced due to particle beams bending in magnetic fields regions.

Relative efficiency of Gafchromic EBT3 and MD-V3 films exposed to low-energy photons and its influence on the energy dependence

Energy-dependence of Gafchromic films exposed to low-energy photons has been reported to be a function of absorbed-dose. However, these studies are based on a relative-response, R, which considers the absorbed-dose in water and not within the film sensitive-volume. This work investigated the relative-efficiency, RE_film, (ratio of absorbed-dose required to produce the same net optical density (netOD) by ⁶⁰Co gamma and by x-ray) of Gafchromic EBT3 and MD-V3 films exposed to five x-ray beams from 20 kV to 160 kV and ⁶⁰Co gamma rays. A factor that accounts for the energy-dependence, f_x,Q,med, based on RE_film, phantom-material and depth at which the films are placed during irradiation was used to remove the influence of absorbed dose. Values of RE_film indicated that the absorbed dose from ⁶⁰Co gamma rays needs to be 4 and 3 times larger than those from 20 kV x-rays to produce the same netOD within the EBT3 and MD-V3 sensitive volumes, respectively. Thus, saturation could help explain why Gafchromic films show under-response to very low doses from low-energy photon beams, regardless of film model. Furthermore, RE_film, was found to be nearly independent of netOD and colour-channels. Consequently, f_x,Q,med is independent of the absorbed dose and colour-channels. In contrast, besides the variation with the photon energy, f_x,Q,med varied with film model, depth and phantom material used during the irradiation. Thus, the results suggest that f_x,Q,med is a more reliable wide-ranging parameter for evaluating the degree of energy-dependence of the film rather than the relative-response method commonly considered.

LET response variability of Gafchromic TM EBT3 film from a 60 Co calibration in clinical proton beam qualities

PURPOSE

To establish a method of accurate dosimetry required to quantify the expected linear energy transfer (LET) quenching effect of EBT3 film used to benchmark the dose distribution for a given treatment field and specified measurement depth. In order to facilitate this technique, a full analysis of film calibration which considers LET variability at the plane of measurement and as a function of proton beam quality is demonstrated. Additionally, the corresponding uncertainty from the process was quantified for several measurement scenarios.

MATERIALS AND METHODS

The net change in optical density (OD) from a single version of Gafchromic TM EBT3 film was measured using an Epson flatbed scanner and NIST-traceable OD filters. Film OD response was characterized with respect to the known dose to water at the point of measurement for both a NIST-traceable 60 Co beam at the UWADCL and several clinical single-energy and spread-out Bragg peak (SOBP) proton beam qualities at the Northwestern Medicine Chicago Proton Center. Increasing proton LET environments were acquired by placing film at increasing depths of Gammex HE Solid Water® whose water-equivalent thickness was characterized prior to measurement.

RESULTS

A strong LET dependence was observed near the Bragg peak (BP) consistent with previous studies performed with earlier versions of EBT3 film. The influence of range straggling on the film's LET response appears to have a uniform effect toward the BP regardless of the nominal beam energy. Proximal to this depth, the film's response decreased with decreasing energy at the same dose-average LET. The opposite trend was observed for depths past the BP. Changes in the SOBP energy modulation showed a linear relationship between the film's relative response and dose-averaged LET. Relative effectiveness factors (RE) were observed to range between 2%-7% depending on the width of the SOBP and depth of the film. Using the field-specific calibration technique, a total k = 1 uncertainty in the absorbed dose to water was estimated to range from 4.68%-5.21%.

CONCLUSION

While EBT3 film's strong LET dependence is a common problem in proton beam dosimetry, this work has shown that the LET dependence can be taken into account by carefully considering the depth and energy modulation across the film using field-specific corrections. RE factors were determined with a combined k = 1 uncertainty of 3.57% for SOBP environments and between 3.17%-4.69% for uniform, monoenergetic fields proximal to the distal 80% of the BP.

An investigation into the INTRABEAM miniature x-ray source dosimetry using ionization chamber and radiochromic film measurements

PURPOSE

Intraoperative radiotherapy using The INTRABEAM System (Carl Zeiss Meditec AG, Jena, Germany), a miniature low-energy x-ray source, has proven to be an effective modality in the treatment of breast cancer. However, some uncertainties remain in its dosimetry. In this work, we investigated the INTRABEAM system dosimetry by performing ionization chamber and radiochromic film measurements of absorbed dose in a water phantom.

METHODS

Ionization chamber measurements were performed with a PTW 34013 parallel-plate soft x-ray chamber at source to detector distances of 5 to 30 mm calculated using (a) the dose formula consistent with the TARGIT breast protocol (TARGIT), (b) the formula recommended by the manufacturer (Zeiss), and (c) the recently proposed CQ formalism of Watson et al. (Physics in Medicine & Biology, 2018;63:015016) EBT3 Gafchromic film measurements were made at the same depths in water. To account for the energy dependence of EBT3 film, multiple dose response calibration curves were employed across a range of photon beam qualities relevant to the INTRABEAM spectrum in water.

RESULTS

At all depths investigated, the TARGIT dose was significantly lower than that measured by the Zeiss and CQ methods, as well as film. These dose differences ranged from 14% to as large as 80%. In general, the doses measured by film, and the Zeiss and CQ methods were in good agreement to within measurement uncertainties (5-6%).

CONCLUSIONS

These results suggest that the TARGIT dose underestimates the physical dose to water from the INTRABEAM source. Understanding the correlation between the TARGIT and physical dose is important for any studies wishing to make dosimetric comparisons between the INTRABEAM and other radiation emitting devices.

Dosimetric characterization of a new directional low-dose rate brachytherapy source

PURPOSE

CivaTech Oncology Inc. (Durham, NC) has developed a novel low-dose rate (LDR) brachytherapy source called the CivaSheet.TM The source is a planar array of discrete elements ("CivaDots") which are directional in nature. The CivaDot geometry and design are considerably different than conventional LDR cylindrically symmetric sources. Thus, a thorough investigation is required to ascertain the dosimetric characteristics of the source. This work investigates the repeatability and reproducibility of a primary source strength standard for the CivaDot and characterizes the CivaDot dose distribution by performing in-phantom measurements and Monte Carlo (MC) simulations. Existing dosimetric formalisms were adapted to accommodate a directional source, and other distinguishing characteristics including the presence of gold shield x-ray fluorescence were addressed in this investigation.

METHODS

Primary air-kerma strength (SK ) measurements of the CivaDots were performed using two free-air chambers namely, the Variable-Aperture Free-Air Chamber (VAFAC) at the University of Wisconsin Medical Radiation Research Center (UWMRRC) and the National Institute of Standards and Technology (NIST) Wide-Angle Free-Air Chamber (WAFAC). An intercomparison of the two free-air chamber measurements was performed along with a comparison of the different assumed CivaDot energy spectra and associated correction factors. Dose distribution measurements of the source were performed in a custom polymethylmethacrylate (PMMA) phantom using GafchromicTM EBT3 film and thermoluminescent dosimeter (TLD) microcubes. Monte Carlo simulations of the source and the measurement setup were performed using MCNP6 radiation transport code.

RESULTS

The CivaDot SK was determined using the two free-air chambers for eight sources with an agreement of better than 1.1% for all sources. The NIST measured CivaDot energy spectrum intensity peaks were within 1.8% of the MC-predicted spectrum intensity peaks. The difference in the net source-specific correction factor determined for the CivaDot free-air chamber measurements for the NIST WAFAC and UW VAFAC was 0.7%. The dose-rate constant analog was determined to be 0.555 cGy h-1 U-1 . The average difference observed in the estimated CivaDot dose-rate constant analog using measurements and MCNP6-predicted value (0.558 cGy h-1 U-1 ) was 0.6% ± 2.3% for eight CivaDot sources using EBT3 film, and -2.6% ± 1.7% using TLD microcube measurements. The CivaDot two-dimensional dose-to-water distribution measured in phantom was compared to the corresponding MC predictions at six depths. The observed difference using a pixel-by-pixel subtraction map of the measured and the predicted dose-to-water distribution was generally within 2-3%, with maximum differences up to 5% of the dose prescribed at the depth of 1 cm.

CONCLUSION

Primary SK measurements of the CivaDot demonstrated good repeatability and reproducibility of the free-air chamber measurements. Measurements of the CivaDot dose distribution using the EBT3 film stack phantom and its subsequent comparison to Monte Carlo-predicted dose distributions were encouraging, given the overall uncertainties. This work will aid in the eventual realization of a clinically viable dosimetric framework for the CivaSheet based on the CivaDot dose distribution.