Quantifying uncertainties associated with reference dosimetry in an MR-Linac.

BACKGROUND: Magnetic resonance (MR)-guided radiation therapy provides capabilities to utilize high-resolution and real-time MR imaging before and during treatment, which is critical for adaptive radiotherapy. This emerging modality has been promptly adopted in the clinic settings in advance of adaptations to reference dosimetry formalism that are needed to account for the presence of strong magnetic fields. In particular, the influence of magnetic field on the uncertainty of parameters in the reference dosimetry equation needs to be determined in order to fully characterize the uncertainty budget for reference dosimetry in MR-guided radiation therapy systems. PURPOSE: To identify and quantify key sources of uncertainty in the reference dosimetry of external high energy radiotherapy beams in the presence of a strong magnetic field. METHODS: In the absence of a formalized Task Group report for reference dosimetry in MR-integrated linacs, the currently suggested formalism follows the TG-51 protocol with the addition of a quality conversion factor kBQ accounting for the effects of the magnetic field on ionization chamber response. In this work, we quantify various sources of uncertainty that impact each of the parameters in the formalism, and evaluate their overall contribution to the final dose. Measurements are done in a 1.5 T MR-Linac (Unity, Elekta AB, Stockholm, Sweden) which integrates a 1.5 T Philips MR scanner and a 7 MVFFF linac. The responses of several reference-class small volume ionization chambers (Exradin:A1SL, IBA:CC13, PTW:Semiflex-3D) and Farmer type ionization chambers (Exradin:A19, IBA:FC65-G) were evaluated throughout this process. Long-term reproducibility and stability of beam quality, TPR 10 20 ${\mathrm{TPR}}_{10}^{20}$ , was also measured with an in-house built phantom. RESULTS: Relative to the conventional external high energy linacs, the uncertainty on overall reference dose in MR-linac is more significantly affected by the chamber setup: A translational displacement along y-axis of ± 3 mm results in dose variation of < |0.20| ± 0.02% (k = 1), while rotation of ± 5° in horizontal and vertical parallel planes relative to relative to the direction of magnetic field, did not exceed variation of < |0.44| ± 0.02% for all 5 ionization chambers. We measured a larger dose variation for xy-plane (horizontal) rotations (< |0.44| ± 0.02% (k = 1)) than for yz-plane (vertical) rotations (< ||0.28| ± 0.02% (k = 1)), which we associate with the gradient of kB,Q as a function of chamber orientation with respect to direction of the B0 -field. Uncertainty in Pion (for two depths), Ppol (with various sub-studies including effects of cable length, cable looping in the MRgRT bore, connector type in magnetic environment), and Prp were determined. Combined conversion factor kQ × kB,Q was provided for two reference depths at four cardinal angle orientations. Over a two-year period, beam quality was quite stable with TPR 10 20 ${\mathrm{TPR}}_{10}^{20}$ being 0.669 ± 0.01%. The actual magnitude of TPR 10 20 ${\mathrm{TPR}}_{10}^{20}$ was measured using identical equipment and compared between two different Elekta Unity MR-Linacs with results agreeing to within 0.21%. CONCLUSION: In this work, the uncertainty of a number of parameters influencing reference dosimetry was quantified. The results of this work can be used to identify best practice guidelines for reference dosimetry in the presence of magnetic fields, and to evaluate an uncertainty budget for future reference dosimetry protocols for MR-linac.

MR-linac daily semi-automated end-to-end quality control verification.

PURPOSE: Adaptive radiation therapy (ART) on the integrated Elekta Unity magnetic resonance (MR)-linac requires routine quality assurance to verify delivery accuracy and system data transfer. In this work, our objective was to develop and validate a novel automated end-to-end test suite that verifies data transfer between multiple software platforms and quantifies the performance of multiple machine subcomponents critical to the ART process. METHODS: We designed and implemented a software tool to quantify the MR and megavoltage (MV) isocenter coincidence, treatment couch positioning consistency, isocenter shift accuracy for the adapted plan as well as the MLC and jaw position accuracy following the beam aperture adaptation. Our tool employs a reference treatment plan with a simulated isocenter shift generated on an MR image of a readily available phantom with MR and MV visible fiducials. Execution of the test occurs within the standard adapt-to-position (ATP) clinical workflow with MV images collected of the delivered treatment fields. Using descriptive statistics, we quantified uncertainty in couch positioning, isocentre shift as well as the jaw and MLC positions of the adapted fields. We also executed sensitivity measurements to evaluate the detection algorithm's performance. RESULTS: We report the results of 301 daily testing instances. We demonstrated consistent tracking of the MR-to-MV alignment with respect to the established value and to detect small changes on the order of 0.2 mm following machine service events. We found couch position consistency relative to the test baseline value was within 95% CI [-0.31, 0.26 mm]. For phantom shifts that form the basis for the plan adaptation, we found agreement between MV-image-detected phantom shift and online image registration, within ± 1.5 mm in all directions with a 95% CI difference of [-1.29, 0.79 mm]. For beam aperture adaptation accuracy, we found differences between the planned and detected jaw positions had a mean value of 0.27 mm and 95% CI of [-0.29, 0.82 mm] and -0.17 mm and 95% CI of [-0.37, 0.05 mm] for the MLC positions. Automated fiducial detected accuracy was within 0.08 ± 0.20 mm of manual localization. Introduced jaw and MLC position errors (1-10 mm) were detected within 0.55 mm (within 1 mm for 15/256 instances for the jaws). Phantom shifts (1.3 or 5 mm in each cardinal direction) from a reference position were detected within 0.26 mm. CONCLUSIONS: We have demonstrated the accuracy and sensitivity of a daily end-to-end test suite capable of detecting errors in multiple machine subcomponents including system data transfer. Our test suite evaluates the entire treatment workflow and has captured system communication issues prior to patient treatment. With automated processing and the use of a standard vendor-provided phantom, it is possible to expand to other Unity sites.

Design and feasibility of a flexible, on-body, high impedance coil receive array for a 1.5 T MR-linac.

The lack of radiation-attenuating tuning capacitors in high impedance coils (HICs) make HICs an interesting building block of receive arrays for MRI-guided radiotherapy (MRIgRT). Additionally, their flexibility and limited channel coupling allow for low-density support materials, which are likely to be more radiation transparent (radiolucent). In this work, we introduce the use of HICs in receive arrays for MRIgRT treatments. We discuss the design and show the dosimetric feasibility of a HIC receive array that has a high channel count and aims to improve the imaging performance of the 1.5 T MR-linac. Our on-body design comprises an anterior and posterior element, which each feature a [Formula: see text] channel layout (32 channels total). The anterior element is flexible, while the posterior element is rigid to support the patient. Mockups consisting of support materials and conductors were built, irradiated, and optimized to minimize impact on the surface dose (7% of the dose maximum) and dose at depth ([Formula: see text]0.8% under a single conductor and [Formula: see text]1.4% under a conductor crossing). Anatomical motion and the use of multiple beam angles will ensure that these slight dose changes at depth are clinically insignificant. Subsequently, several functional, single-channel HIC imaging prototypes and a 5-channel array were built to assess the performance in terms of signal-to-noise ratio (SNR). The performance was compared to the clinical MR-linac array and showed that the 5-channel imaging prototype outperformed the clinical array in terms of SNR and channel coupling. Imaging performance was not affected by the radiation beam. In conclusion, the use of HICs allowed for the design of our flexible, on-body receive array for MRIgRT. The design was shown to be dosimetrically feasible and improved the SNR. Future research with a full array will need to show the gain in parallel imaging performance and thus acceleration.

Monte Carlo simulations of out-of-field surface doses due to the electron streaming effect in orthogonal magnetic fields.

The out-of-field surface dose contribution due to backscattered or ejected electrons, focused by the magnetic field, is evaluated in this work. This electron streaming effect (ESE) can contribute to out-of-field skin doses in orthogonal magnetic resonance guided radiation therapy machines. Using the EGSnrc Monte Carlo package, a phantom is set-up along the central axis of an incident 10 [Formula: see text] 10 cm2 7 MV FFF photon beam. The phantom exit or entry surface is inclined with respect to the magnetic field, and an out-of-field water panel is positioned 10 cm away from, and centered on, the isocenter. The doses from streaming backscattered or ejected electrons, for either a 0.35 T or 1.5 T magnetic field, are evaluated in the out-of-field water panel for surface inclines of 10, 30, and 45°. The magnetic field focuses electrons emitted from the inclined phantom. Dose distributions at the surface of the out-of-field water panel are sharper in the 1.5 T magnetic field as compared to 0.35 T. The maximum doses for the 0.35 T simulations are 23.2%, 37.8%, and 39.0% for the respective 10, 30, and 45° simulations. For 1.5 T, for the same angles, the maximum values are 17.1%, 29.8%, and 35.8%. Dose values drop to below 2% within the first 1 cm of the out-of-field water phantom. The phantom thickness is an important variable in the magnitude of the ESE dose. The ESE can produce large out-of-field skin doses and must be a consideration in treatment planning in the MRgRT work-flow. Treatments often include multiple beams which will serve to spread out the effect, and many beams, such as anterior-posterior, will reduce the skin dose due to the ESE. A 1 cm thick shielding of either a bolus placed on the patient or mounted on the present RF coils would greatly reduce the ESE dose contributions. Further exploration of the capabilities of treatment planning systems to screen for this effect is required.

Monte Carlo simulations of out-of-field skin dose due to spiralling contaminant electrons in a perpendicular magnetic field.

PURPOSE: The purpose of this study was to evaluate the potential skin dose toxicity contribution of spiralling contaminant electrons (SCE) generated in the air in an MR-linac with a 0.35 or 1.5 T magnetic field using the EGSnrc Monte Carlo (MC) code. Comparisons to experimental results at 1.5 T are also performed. METHODS: An Elekta generated phase space file for the Unity MR-linac is used in conjunction with the EGSnrc enhanced electric and magnetic field transport macros to simulate surface dose profiles and depth-dose curves in panels located 5 cm away from the beam edge and positioned either parallel or perpendicular to the magnetic field. Electrons generated in the air will spiral along the magnetic field lines, and though surface doses within the field will be reduced, the electrons can contribute to out-of-field surface doses. RESULTS: Surface dose profiles showed good agreement with experimental findings and the maximum simulated doses at surfaces perpendicular to the magnetic field were 3.77 ± 0.01% and 3.55 ± 0.01% for 1.5 and 0.35 T. These results are expressed as a percentage of the maximum dose to water delivered by the photon beam. The surface dose variations in the out-of-field region converge to the 0 T doses within the first 0.5 cm of material. An asymmetry in the dose distribution in surfaces positioned on either side of the photon beam and aligned parallel to the magnetic field is determined to be due to the magnetic field directing electrons deeper into, or localizing them to the surface of, the measurement panel. CONCLUSIONS: These results confirm the SCE dose contribution in surfaces perpendicular to the magnetic field and show these doses to be of the order of a few percentage of the maximum dose to water of the beam. Good agreement in the dose profiles is seen in comparisons between the MC simulations and experimental work. The effect is apparent in 0.35 and 1.5 T magnetic fields and dissipates within the first few millimeters of material. It should be noted that only SCEs from beam anteriorly incident on the patient will influence the patient surface dose, and the use of beams incident over different angles will reduce the dose to any particular patient surface.

Monte Carlo study of ionization chamber magnetic field correction factors as a function of angle and beam quality.

PURPOSE: To use EGSnrc Monte Carlo simulations for magnetic field dosimetry to determine optimal measurement orientations, calculate beam quality conversion factors for 32 cylindrical and three parallel-plate (PP) ion chambers, evaluate the beam quality and angular dependence of these factors, and examine the magnetic field effects on %dd(10)x and TPR1020. METHODS: Beam quality conversion factors, kQmag, and magnetic field conversion factors, kB  = kQmag/kQ , are calculated as a function of chamber rotation for six cylindrical ionization chamber in either a 60 Co beam with a 0.35 T magnetic field or a 7 MV beam with a 1.5 T field, both magnetic fields are perpendicular to the photon beam. The chambers' sensitive air volumes are varied by either using the entire geometric volume or excluding the air volume associated with the first 1 mm away from the stem. The kB and kQmag factors are evaluated using four clinical photon spectra. The variation in %dd(10)x and TPR1020 as a function of magnetic field for six photon spectra are studied using DOSXYZnrc. RESULTS: When the magnetic field is perpendicular to the photon beam, orienting the chamber parallel with the magnetic field reduces the magnetic field effect on chamber response (i.e., dose to air per water dose) and variations due to the unknown sensitive volume are essentially eliminated. Calculated kB factors are within 1% of unity for the majority of cylindrical chambers, although larger kB values are associated with chambers with high-Z electrodes. PP chambers have kB corrections as large as 8.9% and have a larger angular sensitivity compared to cylindrical chambers. Values of kB for cylindrical ion chambers are independent of beam quality, except for chambers with high-Z electrodes. For %dd(10)x values between 63.3% and 73.8%, kB varies by at most (0.26 ± 0.15)% when the magnetic field is perpendicular to the photon beam and parallel to the chamber. Differences in %dd(10)x , between no magnetic field and with a 1.5 T field perpendicular to the photon beam are (0.04 ± 0.10)%, (1.89 ± 0.10)%, and (6.20 ± 0.10)% for a 60 Co, 7, and 25 MV photon beam, respectively, while TPR1020 shows less than (0.36 ± 0.10)% change. Applying the ICRU-90 recommendations for stopping powers instead of ICRU-37 is found to change kQ (and hence kB ) by less than 0.1%. CONCLUSIONS: Orienting the chamber parallel to the magnetic field when the field is perpendicular to the photon beam will minimize the effect of the magnetic field on chamber response, and eliminate the problem of the unknown sensitive volume. Values of kB and kQmag can bring ion chamber dosimetry in magnetic fields in-line with the TG-51 protocol. PP chamber are sensitive to the magnetic field and variation in chamber response due to small angular changes makes them unlikely candidates for clinical reference dosimetry in magnetic fields. The stability in TPR1020, as a function of magnetic fields and beam qualities, makes it the best beam quality specifier in magnetic fields.

Sensitive volume effects on Monte Carlo calculated ion chamber response in magnetic fields.

PURPOSE: The development of magnetic resonance-guided radiation therapy (MRgRT) necessitates accurate Monte Carlo (MC) models of ion chambers for computing ion chamber corrections to compensate for the presence of the magnetic field. This study evaluates the sensitivity of the ion chamber dose response in a magnetic field on the collection volume used in the MC simulation. METHODS: The EGSnrc system's egs_chamber application is used with a recently developed and validated magnetic field transport code. The calculated dose to the sensitive volume of the chamber per unit incident photon fluence, normalized to that at 0 T, is evaluated as a function of magnetic field for the PTW 30013, PTW 31006, PTW 31010, Exradin A12S, and Exradin A1SL chambers. The sensitive region is varied by excluding the volume corresponding to either 0, 0.5, or 1 mm of distance away from the stem. The photon field, magnetic field, and ion chamber are all oriented perpendicular to each other as in the majority of published experimental works. RESULTS: The calculations for a Co-60 source demonstrate that variations from the 0 mm simulations are on the order of several percent with a maximum deviation, occurring at 0.5 T, of 1.75 ± 0.03% and 3.39 ± 0.06% for the 0.5 mm or 1 mm simulations, respectively, for a 0.057 cm3 A1SL chamber. Larger volume chambers showed smaller, but still non-negligible, variations. Simulations of the A1SL chamber with a 7 MV photon source, corresponding to the Elekta MR-linac machine, demonstrate that the effect is slightly reduced but still persists with a maximum deviation of 1.97 ± 0.08% for the 1 mm reduction. CONCLUSIONS: Usually, the geometric sensitive volume of the ion chamber is used in MC calculation as a substitute for the potentially unknown, smaller, true collection volume (governed by the complex electric field distribution inside the chamber). The calculations in this study demonstrate that even a small variation in simulated volume can lead to fairly large variations in the MC calculated ion chamber response in a magnetic field. This is an important effect that must be addressed to ensure proper calibration of MRgRT machines using MC ion chamber correction factors. This effect may play a role, even where there is no magnetic field, in small-field dosimetry when volume averaging effect are important.