Characterization of the effect of MRI on Gafchromic film dosimetry

Clinical utility of Gafchromic film in an MRI-guided linear accelerator

Background

The purpose of this study is to comprehensively evaluate the suitability of Gafchromic EBT3 and EBT-XD film for dosimetric quality assurance in 0.35 T MR-guided radiotherapy.

Methods

A 0.35 T magnetic field strength was utilized to evaluate magnetic field effects on EBT3 and EBT-XD Gafchromic films by studying the effect of film exposure time within the magnetic field using two timing sequences and film not exposed to MR, the effect of magnetic field exposure on the crystalline structure of the film, and the effect of orientation of the film with respect to the bore within the magnetic field. The orientation of the monomer crystal was qualitatively evaluated using scanning electron microscopy (SEM) compared to unirradiated film. Additionally, dosimetric impact was evaluated through measurements of a series of open field irradiations (0.83 × 0.83-cm² to 19.92 × 19.92-cm²) and patient specific quality assurance measurements. Open fields were compared to planned dose and an independent dosimeter. Film dosimetry was applied to twenty conventional and twenty stereotactic body radiotherapy (SBRT) patient specific quality assurance cases.

Results

No visual changes in crystal orientation were observed in any evaluated SEM images nor were any optical density differences observed between films irradiated inside or outside the magnetic field for both EBT3 and EBT-XD film. At small field sizes, the average difference along dose profiles measured in film compared to the same points measured using an independent dosimeter and to predicted treatment planning system values was 1.23% and 1.56%, respectively. For large field sizes, the average differences were 1.91% and 1.21%, respectively. In open field tests, the average gamma pass rates were 99.8% and 97.2%, for 3%/3 mm and 3%/1 mm, respectively. The median (interquartile range) 3%/3 mm gamma pass rates in conventional QA cases were 98.4% (96.3 to 99.2%), and 3%/1 mm in SBRT QA cases were 95.8% (95.0 to 97.3%).

Conclusions

MR exposure at 0.35 T had negligible effects on EBT3 and EBT-XD Gafchromic film. Dosimetric film results were comparable to planned dose, ion chamber and diode measurements.

MR-guided proton therapy: Impact of magnetic fields on the detector response

PURPOSE

To investigate the response of detectors for proton dosimetry in the presence of magnetic fields.

MATERIAL AND METHODS

Four ionization chambers (ICs), two thimble-type and two plane-parallel-type, and a diamond detector were investigated. All detectors were irradiated with homogeneous single-energy-layer fields, using 252.7 MeV proton beams. A Farmer IC was additionally irradiated in the same geometrical configuration, but with a lower nominal energy of 97.4 MeV. The beams were subjected to magnetic field strengths of 0, 0.25, 0.5, 0.75, and 1 T produced by a research dipole magnet placed at the room's isocenter. Detectors were positioned at 2 cm water equivalent depth, with their stem perpendicular to both the magnetic field lines and the proton beam's central axis, in the direction of the Lorentz force. Normality and two sample statistical Student's t tests were performed to assess the influence of the magnetic field on the detectors' responses.

RESULTS

For all detectors, a small but significant magnetic field-dependent change of their response was found. Observed differences compared to the no magnetic field case ranged from +0.5% to -0.7%. The magnetic field dependence was found to be nonlinear and highest between 0.25 and 0.5 T for 252.7 MeV proton beams. A different variation of the Farmer chamber response with magnetic field strength was observed for irradiations using lower energy (97.4 MeV) protons. The largest magnetic field effects were observed for plane-parallel ionization chambers.

CONCLUSION

Small magnetic field-dependent changes in the detector response were identified, which should be corrected for dosimetric applications.

Machine QA for the Elekta Unity system: A Report from the Elekta MR-linac consortium

Over the last few years, magnetic resonance image‐guided radiotherapy systems have been introduced into the clinic, allowing for daily online plan adaption. While quality assurance (QA) is similar to conventional radiotherapy systems, there is a need to introduce or modify measurement techniques. As yet, there is no consensus guidance on the QA equipment and test requirements for such systems. Therefore, this report provides an overview of QA equipment and techniques for mechanical, dosimetric, and imaging performance of such systems and recommendation of the QA procedures, particularly for a 1.5T MR‐linac device. An overview of the system design and considerations for QA measurements, particularly the effect of the machine geometry and magnetic field on the radiation beam measurements is given. The effect of the magnetic field on measurement equipment and methods is reviewed to provide a foundation for interpreting measurement results and devising appropriate methods. And lastly, a consensus overview of recommended QA, appropriate methods, and tolerances is provided based on conventional QA protocols. The aim of this consensus work was to provide a foundation for QA protocols, comparative studies of system performance, and for future development of QA protocols and measurement methods.

An end-to-end assessment on the accuracy of adaptive radiotherapy in an MR-linac

PURPOSE

To develop and demonstrate an end-to-end assessment procedure for adaptive radiotherapy (ART) within an MR-guided system.

METHODS AND MATERIALS

A 3D printed pelvic phantom was designed and constructed for use in this study. The phantom was put through the complete radiotherapy treatment chain, with planned internal changes made to model prostate translations and shape changes, allowing an investigation into three ART techniques commonly used. Absolute dosimetry measurements were made within the phantom using both gafchromic film and alanine. Comparisons between treatment planning system (TPS) calculations and measured dose values were made using the gamma evaluation with criteria of 3 mm/3% and 2 mm/2%.

RESULTS

Gamma analysis evaluations for each type of treatment plan adaptation investigated showed a very high agreement with pass rates for each experiment ranging from 98.10% to 99.70% and 92.60% to 97.55%, for criteria of 3%/3 mm and 2%/2 mm respectively. These pass rates were consistent for both shape and position changes. Alanine measurements further supported the results, showing an average difference of 1.98% from the TPS.

CONCLUSION

The end-to-end assessment procedure provided demanding challenges for treatment plan adaptations to demonstrate the capabilities and achieved high consistency in all findings.

Quality assurance of IMRT treatment plans for a 1.5 T MR-linac using a 2D ionization chamber array and a static solid phantom

MR-guided radiotherapy requires novel quality assurance (QA) methods for intensity-modulated radiotherapy treatment plans (TPs). Here, an optimized method for TPs for a 1.5 T MR-linac was developed and implemented clinically. A static solid phantom and an MR-compatible 2D ionization chamber array were used. The array's response with respect to the incident beam gantry angles was characterized for four different orientations of the array relative to the beam. A lookup table was created identifying the optimum orientation for each gantry angle. For the QA of clinical MR-linac TPs, beams were grouped according to their gantry angles and measured with up to four setups. The method was applied to n = 106 clinical TPs of 54 patients for different tumour entities. Reference plans and plans created in the online adaptive workflow were analysed, using a local 3%/3 mm gamma criterion for dose values larger than 30% of the maximum. Pass rates were averaged over all beam groups. The array's response strongly depends on the beam incidence angle. Optimum angles typically range from -10° to 80° around the phantom setup angle. Consequently, plan verification required up to four setups. For clinical MR-linac TPs, the overall median pass rate was 98.5% (range 88.6%-100%). Pass rates depended on the tumour entity. Median pass rates were for liver metastases stereotactic body radiotherapy 99.2%, prostate cancer 99%, pancreatic cancer 98.9%, lymph node metastases 98.7%, partial breast irradiation (PBI) 98%, head-and-neck cancer 97.7%, rectal cancer 94% and others 96.6%. 85% of plans were accepted straightaway, with pass rates above 95%. A single plan with a pass rate below 90% was subsequently verified with a modified method. Off-axis target volumes, e.g. PBI, were verified successfully using a lateral shift of the phantom. The method is suitable to verify reference and online adapted TPs for a 1.5 T MR-linac, including plans for off-axis target volumes.

Influence of 0.35 T magnetic field on the response of EBT3 and EBT-XD radiochromic films

PURPOSE

To investigate the inconsistency of recent literature on the effect of magnetic field on the response of radiochromic films, we studied the influence of 0.35 T magnetic field on dosimetric response of EBT3 and EBT-XD Gafchromic^TM films.

METHODS

Two different models of radiochromic films, EBT3 and EBT-XD, were investigated. Pieces of films samples from two different batches for each model were irradiated at different dose levels ranging from 1 to 20 Gy using 6 MV flattening filter free (FFF) x-rays generated by a clinical MR-guided radiotherapy system (B = 0.35 T). Film samples from the same batch were irradiated at corresponding dose levels using 6 MV FFF beam from a conventional linac (B = 0) for comparison. The net optical density was measured 48 h postirradiation using a flatbed scanner. The absorbance spectra were also measured over 500-700 nm wavelength range using a fiber-coupled spectrometer with 2.5 nm resolution. To study the effect of fractionated dose delivery to EBT3 (/EBT-XD) films, 8 (/16) Gy dose was delivered in four 2 (/4) Gy fractions with 24 h interval between fractions.

RESULTS

No significant difference was found in the net optical density and net absorbance of the films irradiated with or without the presence of magnetic field. No dependency on the orientation of the film during irradiation with respect to the magnetic field was observed. The fractionated dose delivery resulted in the same optical density as delivering the whole dose in a single fraction.

CONCLUSIONS

The 0.35 T magnetic field employed in the ViewRay^® MR-guided radiotherapy system did not show any significant influence on the response of EBT3 and EBT-XD Gafchromic^TM films.

Multichannel Film Dosimetry for Quality Assurance of Intensity Modulated Radiotherapy Treatment Plans Under 0.35 T Magnetic Field

Purpose

To evaluate the intensity modulated radiotherapy (IMRT) quality assurance (QA) results of the multichannel film dosimetry analysis with single scan method by using Gafchromic™ EBT3 (Ashland Inc., Covington, KY, USA) film under 0.35 T magnetic field.

Methods

Between September 2018 and June 2019, 70 patients were treated with ViewRay MRIdian^® (ViewRay Inc., Mountain View, CA) linear accelerator (Linac). Film dosimetry QA plans were generated for all IMRT treatments. Multichannel film dosimetry for red, green and blue (RGB) channels were compared with treatment planning system (TPS) dose maps by gamma evaluation analysis.

Results

The mean gamma passing rates of RGB channels are 97.3% ± 2.26%, 96.0% ± 3.27% and 96.2% ± 3.14% for gamma evaluation with 2% DD/2 mm distance to agreement (DTA), respectively. Moreover, the mean gamma passing rates of RGB channels are 99.7% ± 0.41%, 99.6% ± 0.59% and 99.5% ± 0.67% for gamma evaluation with 3% DD/3 mm DTA, respectively.

Conclusion

The patient specific QA using Gafchromic™ EBT3 film with multichannel film dosimetry seems to be a suitable tool to implement for MR-guided IMRT treatments under 0.35 T magnetic field. Multichannel film dosimetry with Gafchromic™ EBT3 is a consistent QA tool for gamma evaluation of the treatment plans even with 2% DD/2 mm DTA under 0.35 T magnetic field presence.

Optical imaging method to quantify spatial dose variation due to the electron return effect in an MR-linac

PURPOSE

Treatment planning systems (TPSs) for MR-linacs must employ Monte Carlo-based simulations of dose deposition to model the effects of the primary magnetic field on dose. However, the accuracy of these simulations, especially for areas of tissue-air interfaces where the electron return effect (ERE) is expected, is difficult to validate due to physical constraints and magnetic field compatibility of available detectors. This study employs a novel dosimetric method based on remotely captured, real-time optical Cherenkov and scintillation imaging to visualize and quantify the ERE.

METHODS

An intensified CMOS camera was used to image two phantoms with designed ERE cavities. Phantom A was a 40 cm × 10 cm × 10 cm clear acrylic block drilled with five holes of increasing diameters (0.5, 1, 2, 3, 4 cm). Phantom B was a clear acrylic block (25 cm × 20 cm × 5 cm) with three cavities of increasing diameter (3, 2, 1 cm) split into two halves in the transverse plane to accommodate radiochromic film. Both phantoms were imaged while being irradiated by 6 MV flattening filter free (FFF) beams within a MRIdian Viewray (Viewray, Cleveland, OH) MR-linac (0.34 T primary field). Phantom A was imaged while being irradiated by 6 MV FFF beams on a conventional linac (TrueBeam, Varian Medical Systems, San Jose, CA) to serve as a control. Images were post processed in Matlab (Mathworks Inc., Natick, MA) and compared to TPS dose volumes.

RESULTS

Control imaging of Phantom A without the presence of a magnetic field supports the validity of the optical image data to a depth of 6 cm. In the presence of the magnetic field, the optical data shows deviations from the commissioned TPS dose in both intensity and localization. The largest air cavity examined (3 cm) indicated the largest dose differences, which were above 20% at some locations. Experiments with Phantom B illustrated similar agreement between optical and film dosimetry comparisons with TPS data in areas not affected by ERE.

CONCLUSION

There are some appreciable differences in dose intensity and spatial dose distribution observed between the novel experimental data set and the dose models produced by the current clinically implemented MR-IGRT TPS.

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.

Electron streams in air during magnetic-resonance image-guided radiation therapy

To investigate the undesired irradiations outside of the treatment field by electron streams in air (air-electron-stream) during magnetic-resonance image-guided radiation therapy (MR-IGRT). A custom-made support phantom adjusting angles between the beam central axis (CAX) and the phantom surface (termed phantom-angles), were used. Using the ViewRay system, a rectangular parallelepiped phantom placed on the support phantom, was irradiated with field sizes of 6.3 cm × 6.3 cm (FS6.3) and 12.6 cm × 12.6 cm (FS12.6) at gantry angles of 0°, 30°, and 330°, and phantom-angles of 10°, 20°, and 30°. For each beam delivery, the isocenter was located at the center of mass of the phantom and 3 Gy was delivered to the isocenter (prescription dose = 3 Gy). The doses given by the air-electron-streams were measured using the EBT3 films on the panels placed orthogonal to the direction of the magnetic field at distances of 10 and 17 cm from CAX. Two dose distributions per irradiation were measured on the panel facing the phantom surface of the incident beam (front panel) and on the panel facing the phantom surface of the beam exit (end panel). We investigated the doses by the air-electron-streams by calculating the average doses inside the circles drawn around a point of the maximum dose with radii of x cm (DRx) from the dose distributions on the panels (x = 1-5 cm). The largest value of DRx was DR1 (1.64 Gy, 55% of the prescription dose) at 10 cm distance from CAX, with FS12.6, at 30° phantom-angle and 330° gantry angle. The average difference of the DR1 at the end panels (FS12.6) between the calculations and measurements was 1.36 Gy. The average global gamma passing rate with 3%/3 mm on the dose distributions at the end panels (FS12.6) was 40.3%. The calculated dose distributions on both panels were not coincident with the measured dose distributions. The Spearman's rank correlation coefficients between the projected areas and the DRx values were always higher than 0.75 (all with p < 0.001). The doses by the air-electron-streams increased with the projected areas of the cross-sections of the treatment beams on the panels.

Investigation of TLD and EBT3 performance under the presence of 1.5T, 0.35T, and 0T magnetic field strengths in MR/CT visible materials

PURPOSE

The aim of this study was to investigate thermoluminescent dosimeters (TLD) and radiochromic EBT3 film inside MR/CT visible geometric head and thorax phantoms in the presence of: 0, 0.35, and 1.5 T magnetic fields.

METHODS

Thermoluminescent Dosimeters reproducibility studies were examined by irradiating IROC-Houston's TLD acrylic block five times under 0 and 1.5 T configurations of Elekta's Unity system and three times under 0 and 0.35 T configurations of ViewRay's MRIdian Cobalt-60 (⁶⁰ Co) system. Both systems were irradiated with an equivalent 10 × 10 cm² field size, and a prescribed dose of 3 Gy to the maximum depth deposition (dmax). EBT3 film and TLDs were investigated using two geometrical Magnetic Resonance (MR)-guided Radiation Therapy (MRgRT) head and thorax phantoms. Each geometrical phantom had eight quadrants that combined to create a centrally located rectangular tumor (3 × 3 × 5 cm³ ) surrounded by tissue to form a 15 × 15 × 15 cm³ cubic phantom. Liquid polyvinyl chloride plastic and Superflab were used to simulate the tumor and surrounding tissue in the head phantom, respectively. Synthetic ballistic gel and a heterogeneous in-house mixture were used to construct the tumor and surrounding tissue in the thorax phantom, respectively. EBT3 and double-loaded TLDs were used in the phantoms to compare beam profiles and point dose measurements with and without magnetic fields. GEANT4 Monte Carlo simulations were performed to validate the detectors for both Unity 0 T/1.5 T and MRIdian 0 T/0.35 T configurations.

RESULTS

Average TLD block measurements which, compared the magnetic field effects (magnetic field vs 0 T) on the Unity and MRIdian systems, were 0.5% and 0.6%, respectively. The average ratios between magnetic field effects for the geometric thorax and head phantoms under the Unity system were -0.2% and 1.6% and for the MRIdian system were 0.2% and -0.3%, respectively. Beam profiles generated with both systems agreed with Monte Carlo measurements and previous literature findings.

CONCLUSIONS

TLDs and EBT3 film dosimeters could potentially be used in MR/CT visible tissue equivalent phantoms that will experience a magnetic field environment.

Microstructure changes in radiochromic films due to magnetic field and radiation

PURPOSE

To correlate the dose response and changes in microscopic structures of the radiochromic films exposed to the clinical magnetic field in the range 1.5-3 T with standard and flattening filter-free (FFF) photon beams.

METHODS

The radiochromic film was cut into 5 × 5 cm² sheets/samples from one batch. These samples were exposed to a 1.5-T and/or 3-T B-fields from an MRI scanner using an abdominal sequence for 7 min before and after irradiation with 6 MV and/or 6 MV FFF beams. Films were placed in a reference condition at 5 cm depth in a solid water phantom and exposed up to 20 Gy. The sample orientation was maintained the same during exposure, readout, and scanning electron microscopic (SEM) analysis. The samples were scanned with an Epson Expression 11000XL in a 48-bit RGB color mode at 300 dpi with red channel. Scanned images were processed in Image J and red channel mean intensity values were recorded. The samples were then coated with 6 nm gold and imaged by SEM Teneo (5 kV, 13 pA) under 2000, 2500, and 3000 magnifications for texture analysis.

RESULTS

The changes in the microstructure of the films in magnetic fields (1.5- and 3.0-T) were dose dependent. The orientation and granular size of samples at higher doses were altered compared to the controls. Needle-shaped structures of the active layer were longer and aligned for samples exposed to higher doses and magnetic field. However, no significant changes in optical density due to the presence of a magnetic field pre/postirradiation up to 20 Gy were observed.

CONCLUSION

Fine structures of the film represent the polymerization characteristics that are affected by the radiation dose in the magnetic field. Upon exposure to radiation, diacetylene monomers undergo polymerization that forms longer chains with a temporal response. Even though this study did not notice any significant changes in optical density due to the presence of magnetic field, this should be studied in simultaneous application of the magnetic field during treatment in a dedicated MR-linac unit.

Remote Cherenkov imaging-based quality assurance of a magnetic resonance image-guided radiotherapy system

PURPOSE

Tools to perform regular quality assurance of magnetic resonance image-guided radiotherapy (MRIgRT) systems should ideally be independent of interference from the magnetic fields. Remotely acquired optical Cherenkov imaging-based dosimetry measurements in water were investigated for this purpose, comparing measures of dose accuracy, temporal dynamics, and overall integrated IMRT delivery.

METHODS

A 40 × 30.5 × 37.5 cm³ water tank doped with 1 g/L of quinine sulfate was imaged using an intensified charge-coupled device (ICCD) to capture the Cherenkov emission while being irradiated by a commercial MRIgRT system (ViewRay™). The ICCD was placed down-bore at the end of the couch, 4 m from treatment isocenter and behind the 5-Gauss line of the 0.35-T MRI. After establishing optimal camera acquisition settings, square beams of increasing size (4.2 × 4.2 cm² , 10.5 × 10.5 cm² , and 14.7 × 14.7 cm² ) were imaged at 0.93 frames per second, from an individual cobalt-60 treatment head, to develop projection measures related to percent depth dose (PDD) curves and cross beam profiles (CPB). These Cherenkov-derived measurements were compared to ionization chamber (IC) and radiographic film dosimetry data, as well as simulation data from the treatment planning system (TPS). An intensity-modulated radiotherapy (IMRT) commissioning plan from AAPM TG-119 (C4:C-Shape) was also imaged at 2.1 frames per second, and the single linear sum image from 509 s of plan delivery was compared to the dose volume prediction generated by the TPS using gamma index analysis.

RESULTS

Analysis of standardized test target images (1024 × 1024 pixels) yielded a pixel resolution of 0.37 mm/pixel. The beam width measured from the Cherenkov image-generated projection CBPs was within 1 mm accuracy when compared to film measurements for all beams. The 502 point measurements (i.e., pixels) of the Cherenkov image-based projection percent depth dose curves (pPDDs) were compared to pPDDs simulated by the treatment planning system (TPS), with an overall average error of 0.60%, 0.56%, and 0.65% for the 4.2, 10.5, and 14.7 cm square beams, respectively. The relationships between pPDDs and central axis PDDs derived from the TPS were used to apply a weighting factor to the Cherenkov pPDD, so that the Cherenkov data could be directly compared to IC PDDs (average error of -0.07%, 0.10%, and -0.01% for the same sized beams, respectively). Finally, the composite image of the TG-119 C4 treatment plan achieved a 95.1% passing rate using 4%/4 mm gamma index agreement criteria between Cherenkov intensity and TPS dose volume data.

CONCLUSIONS

This is the first examination of Cherenkov-generated pPDDs and pCBPs in an MR-IGRT system. Cherenkov imaging measurements were fast to acquire, and minimal error was observed overall. Cherenkov imaging also provided novel real-time data for IMRT QA. The strengths of this imaging are the rapid data capture ability providing real-time, high spatial resolution data, combined with the remote, noncontact nature of imaging. The biggest limitation of this method is the two-dimensional (2D) projection-based imaging of three-dimensional (3D) dose distributions through the transparent water tank.