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.

Local control and patterns of failure for "Radioresistant" spinal metastases following stereotactic body radiotherapy compared to a "Radiosensitive" reference.

PURPOSE: The concept of a radioresistant (RR) phenotype has been challenged with use of stereotactic body radiotherapy (SBRT). We compared outcomes following SBRT to RR spinal metastases to a radiosensitive cohort. METHODS: Renal cell, melanoma, sarcoma, gastro-intestinal, and thyroid spinal metastases were identified as RR and prostate cancer (PCA) as radiosensitive. The primary endpoint was MRI-based local failure (LF). Secondary endpoints included overall survival (OS) and vertebral compression fracture (VCF). RESULTS: From a prospectively maintained database of 1394 spinal segments in 605 patients treated with spine SBRT, 173 patients/395 RR spinal segments were compared to 94 patients/185 PCA segments. Most received 24-28 Gy in 2 fractions (68.9%) and median follow-up was 15.5 months (range, 1.4-84.2 months). 1- and 2-year LF rates were 19.2% and 22.4% for RR metastases, respectively, which were significantly greater (p < 0.001) than PCA (3.2% and 8.4%, respectively). Epidural disease (HR: 2.47, 95% CI 1.65-3.71, p < 0.001) and RR histology (HR: 2.41, 95% CI 1.45-3.99, p < 0.001) predicted for greater LF. Median OS was 17.4 and 61.0 months for RR and PCA cohorts, respectively. Lung/liver metastases, polymetastatic disease and epidural disease predicted for worse OS. 2-year VCF rates were ~ 13% in both cohorts. Coverage of the CTV V90 (clinical target volume receiving 90% of prescription dose) by ≥ 87% (HR: 2.32, 95% CI 1.29-4.18, p = 0.005), no prior spine radiotherapy (HR: 1.96, 95% CI 1.09-3.55, p = 0.025), and a greater Spinal Instability Neoplasia Score (p = 0.013) predicted for VCF. CONCLUSIONS: Higher rates of LF were observed after spine SBRT in RR metastases. Optimization strategies include dose escalation and aggressive management of epidural disease.

Insight gained from responses to surveys on reference dosimetry practices.

PURPOSE: To present the results and discuss potential insights gained through surveys on reference dosimetry practices. METHODS: Two surveys were sent to medical physicists to learn about the current state of reference dosimetry practices at radiation oncology clinics worldwide. A short survey designed to maximize response rate was made publicly available and distributed via the AAPM website and a medical physics list server. Another, much more involved survey, was sent to a smaller group of physicists to gain insight on detailed dosimetry practices. The questions were diverse, covering reference dosimetry practices on topics like measurements required for beam quality specification, the actual measurement of absorbed dose and ancillary equipment required like electrometers and environment monitoring measurements. RESULTS: There were 190 respondents to the short survey and seven respondents to the detailed survey. The diversity of responses indicates nonuniformity in reference dosimetry practices and differences in interpretation of reference dosimetry protocols. CONCLUSIONS: The results of these surveys offer insight on clinical reference dosimetry practices and will be useful in identifying current and future needs for reference dosimetry.

Experimental evaluation of a GPU-based Monte Carlo dose calculation algorithm in the Monaco treatment planning system.

A new GPU-based Monte Carlo dose calculation algorithm (GPUMCD), devel-oped by the vendor Elekta for the Monaco treatment planning system (TPS), is capable of modeling dose for both a standard linear accelerator and an Elekta MRI linear accelerator. We have experimentally evaluated this algorithm for a standard Elekta Agility linear accelerator. A beam model was developed in the Monaco TPS (research version 5.09.06) using the commissioned beam data for a 6 MV Agility linac. A heterogeneous phantom representing several scenarios - tumor-in-lung, lung, and bone-in-tissue - was designed and built. Dose calculations in Monaco were done using both the current clinical Monte Carlo algorithm, XVMC, and the new GPUMCD algorithm. Dose calculations in a Pinnacle TPS were also produced using the collapsed cone convolution (CCC) algorithm with heterogeneity correc-tion. Calculations were compared with the measured doses using an ionization chamber (A1SL) and Gafchromic EBT3 films for 2 × 2 cm2, 5 × 5 cm2, and 10 × 10 cm2 field sizes. The percentage depth doses (PDDs) calculated by XVMC and GPUMCD in a homogeneous solid water phantom were within 2%/2 mm of film measurements and within 1% of ion chamber measurements. For the tumor-in-lung phantom, the calculated doses were within 2.5%/2.5 mm of film measurements for GPUMCD. For the lung phantom, doses calculated by all of the algorithms were within 3%/3 mm of film measurements, except for the 2 × 2 cm2 field size where the CCC algorithm underestimated the depth dose by ~ 5% in a larger extent of the lung region. For the bone phantom, all of the algorithms were equivalent and calculated dose to within 2%/2 mm of film measurements, except at the interfaces. Both GPUMCD and XVMC showed interface effects, which were more pronounced for GPUMCD and were comparable to film measurements, whereas the CCC algorithm showed these effects poorly.

Suitability of novel 3D-printed and coated vessels for water calorimetry.

BACKGROUND: In water calorimetry, absolute dose to water is determined by measuring radiation-induced temperature rises. In conventional water calorimeters, temperature detectors are housed in handmade glass vessels that are filled with high-purity water, thus mitigating radiation-induced exo/endothermic chemical reactions of impurities that would otherwise introduce additional heat gain/loss, known as heat defect. Being hand-crafted, these glass vessels may suffer from imperfections, have shape and design constraints, are often backordered, and can be prohibitively expensive. PURPOSE: The purpose of this work is to determine suitability of 3D-printed plastic vessels that are further coated for use in water calorimetry applications, and to study their stability and characterize their associated heat defect correction factor ( k hd ) ${k_{{\mathrm{hd}}}})$ . This novel vessel production technique would allow for cost-effective rapid construction of vessels that can be produced with high accuracy and designs that are simply not practical with current glass vessel construction techniques. This in turn enables water calorimetry applications in many novel radiation delivery modalities, which may include spherical vessels in GammaKnife ICON water calorimetry as an example. METHODS: Eight vessels were 3D-printed using Accura ClearVue in an SLA 3D-printer. Two vessels were coated with Parylene C and four were coated with Parylene N. The water calorimetry preparation procedures followed for these vessels was identical to that of our traditional glass-vessels (i.e., same cleaning procedures, same high purity water, and same saturation procedures with high purity hydrogen gas). The performance of each vessel was characterized using our in-house built water calorimeter in an Elekta Versa using both 6 MV flattening filter-free (FFF) and 18 MV beams. The stability of the coating as function of time and accumulated dose was evaluated through repeated measurements. k hd ${k_{{\mathrm{hd}}}}\;$ of each vessel was determined through cross-comparisons against an Exradin A1SL ionization chamber with direction calibration link to Canada's primary standard laboratory. RESULTS: k HD ${k_{{\mathrm{HD}}}}\;$ of the two uncoated vessels differed by 2.8% under a 6 MV FFF beam. Vessels coated with Parylenes resulted in a stable and reproducible heat defect for both energies. An overall k hd ${k_{{\mathrm{hd}}}}$ of 1.001 ± 0.010 and 1.005 ± 0.010 were obtained for Parylene N and Parylene C coated vessels respectively. All Parylene coated vessels showed agreement, within the established uncertainties, to the zero-heat defect observed in a hydrogen-saturated glass vessel system. An additional long-term study (17 days) of a Parylene N vessel showed no change in response with accumulated dose and time. Electron microscopy images of a Parylene N coated vessel showed a uniform intact coating after repeated irradiations. CONCLUSIONS: An uncoated 3D-printed vessel is not viable for water calorimetry because it exhibits an unstable vessel-dependent heat defect. However, applying a Parylene coating stabilizes the heat defect, suggesting that coated 3D-printed vessels may be suitable for use in water calorimetry. This method facilitates the creation of intricate vessel shapes, which can be efficiently manufactured using 3D printing.

Experimental determination of magnetic field quality conversion factors for eleven ionization chambers in 1.5 T and 0.35 T MR-linac systems.

BACKGROUND: The static magnetic field present in magnetic resonance (MR)-guided radiotherapy systems can influence dose deposition and charged particle collection in air-filled ionization chambers. Thus, accurately quantifying the effect of the magnetic field on ionization chamber response is critical for output calibration. Formalisms for reference dosimetry in a magnetic field have been proposed, whereby a magnetic field quality conversion factor kB,Q is defined to account for the combined effects of the magnetic field on the radiation detector. Determination of kB,Q in the literature has focused on Monte Carlo simulation studies, with experimental validation limited to only a few ionization chamber models. PURPOSE: The purpose of this study is to experimentally measure kB,Q for 11 ionization chamber models in two commercially available MR-guided radiotherapy systems: Elekta Unity and ViewRay MRIdian. METHODS: Eleven ionization chamber models were characterized in this study: Exradin A12, A12S, A28, and A26, PTW T31010, T31021, and T31022, and IBA FC23-C, CC25, CC13, and CC08. The experimental method to measure kB,Q utilized cross-calibration against a reference Exradin A1SL chamber. Absorbed dose to water was measured for the reference A1SL chamber positioned parallel to the magnetic field with its centroid placed at the machine isocenter at a depth of 10 cm in water for a 10 × 10 cm2 field size at that depth. Output was subsequently measured with the test chamber at the same point of measurement. kB,Q for the test chamber was computed as the ratio of reference dose to test chamber output, with this procedure repeated for each chamber in each MR-guided radiotherapy system. For the high-field 1.5 T Elekta Unity system, the dependence of kB,Q on the chamber orientation relative to the magnetic field was quantified by rotating the chamber about the machine isocenter. RESULTS: Measured kB,Q values for our test dataset of ionization chamber models ranged from 0.991 to 1.002, and 0.995 to 1.004 for the Elekta Unity and ViewRay MRIdian, respectively, with kB,Q tending to increase as the chamber sensitive volume increased. Measured kB,Q values largely agreed within uncertainty to published Monte Carlo simulation data and available experimental data. kB,Q deviation from unity was minimized for ionization chamber orientation parallel or antiparallel to the magnetic field, with increased deviations observed at perpendicular orientations. Overall (k = 1) uncertainty in the experimental determination of the magnetic field quality conversion factor, kB,Q was 0.71% and 0.72% for the Elekta Unity and ViewRay MRIdian systems, respectively. CONCLUSIONS: For a high-field MR-linac, the characterization of ionization chamber performance as angular orientation varied relative to the magnetic field confirmed that the ideal orientation for output calibration is parallel. For most of these chamber models, this study represents the first experimental characterization of chamber performance in clinical MR-linac beams. This is a critical step toward accurate output calibration for MR-guided radiotherapy systems and the measured kB,Q values will be an important reference data source for forthcoming MR-linac reference dosimetry protocols.

AAPM WGTG51 Report 385: Addendum to the AAPM's TG-51 protocol for clinical reference dosimetry of high-energy electron beams.

An Addendum to the AAPM's TG-51 protocol for the determination of absorbed dose to water is presented for electron beams with energies between 4 MeV and 22 MeV ( 1.70 cm ≤ R 50 ≤ 8.70 cm $1.70\nobreakspace {\rm cm} \le R_{\text{50}} \le 8.70\nobreakspace {\rm cm}$ ). This updated formalism allows simplified calibration procedures, including the use of calibrated cylindrical ionization chambers in all electron beams without the use of a gradient correction. New k Q $k_{Q}$ data are provided for electron beams based on Monte Carlo simulations. Implementation guidance is provided. Components of the uncertainty budget in determining absorbed dose to water at the reference depth are discussed. Specifications for a reference-class chamber in electron beams include chamber stability, settling, ion recombination behavior, and polarity dependence. Progress in electron beam reference dosimetry is reviewed. Although this report introduces some major changes (e.g., gradient corrections are implicitly included in the electron beam quality conversion factors), they serve to simplify the calibration procedure. Results for absorbed dose per linac monitor unit are expected to be up to approximately 2 % higher using this Addendum compared to using the original TG-51 protocol.

Monte Carlo optimization and experimental validation of a prototype ionization chamber for accurate magnetic resonance image guided radiation therapy (MRgRT) daily output constancy measurements in solid phantoms.

PURPOSE: To optimize the design, develop and test a prototype ionization chamber for accurate daily output constancy measurements in solid phantoms in clinical magnetic resonance-guided radiation therapy (MRgRT) radiotherapy beams. Up to 4% variations in response using commercial ionization chambers have been previously reported; the prototype ionization chamber developed here aims to minimize these variations. METHODS: Monte Carlo simulations with the EGSnrc code system are used to optimize an ionization chamber design by increasing the thickness of a brass (high-density, nonferromagnetic, easy-to-machine) wall until results consistent with no air gap are produced for simulations with a 1.5 T and 0.35 T magnetic field, with a 0.2 mm air gap and varying the placement of the chamber model within the air gap. Based on the results of these simulations, prototype ionization chambers are manufactured and tested in conventional linac beams and in a 7 MV Elekta Unity MR-linac. The chambers are rotated about their axes, both parallel and perpendicular to the 1.5 T magnetic field, through 360º in a plastic phantom with measurements made at each cardinal angle. This reveals any variation in chamber response by varying the thickness of the air gap between the chamber and the phantom. RESULTS: Monte Carlo simulations demonstrate that the optimal thickness of the chamber wall to mitigate the effect of an asymmetric air gap between the chamber and the plastic phantom is 1.1 mm of brass. With this thickness, the differences between simulations with and without an air gap and with asymmetric placement of the chamber within the air gap are less than 0.2%. A prototype chamber constructed with a 1.1 mm brass wall thickness exhibits less than 0.3% variation in response when rotated about its axis in the plastic phantom in a beam from an MR-linac, independent of whether its axis is parallel or perpendicular to the magnetic field. CONCLUSION: The optimized ionization chamber design and validated prototype for accurate MR-linac daily output constancy measurements allows utilization of conventional phantoms and procedures in MRgRT systems. This can minimize disruption to clinical workflow for MR-linac quality assurance measurements.

AAPM WGTG51 Report 374: Guidance for TG-51 reference dosimetry.

Practical guidelines that are not explicit in the TG-51 protocol and its Addendum for photon beam dosimetry are presented for the implementation of the TG-51 protocol for reference dosimetry of external high-energy photon and electron beams. These guidelines pertain to: (i) measurement of depth-ionization curves required to obtain beam quality specifiers for the selection of beam quality conversion factors, (ii) considerations for the dosimetry system and specifications of a reference-class ionization chamber, (iii) commissioning a dosimetry system and frequency of measurements, (iv) positioning/aligning the water tank and ionization chamber for depth ionization and reference dose measurements, (v) requirements for ancillary equipment needed to measure charge (triaxial cables and electrometers) and to correct for environmental conditions, and (vi) translation from dose at the reference depth to that at the depth required by the treatment planning system. Procedures are identified to achieve the most accurate results (errors up to 8% have been observed) and, where applicable, a commonly used simplified procedure is described and the impact on reference dosimetry measurements is discussed so that the medical physicist can be informed on where to allocate resources.

Skull phantom-based methodology to validate MRI co-registration accuracy for Gamma Knife radiosurgery.

PURPOSE: Target localization, for stereotactic radiosurgery (SRS) treatment with Gamma Knife, has become increasingly reliant on the co-registration between the planning MRI and the stereotactic cone-beam computed tomography (CBCT). Validating image registration between modalities would be particularly beneficial when considering the emergence of novel functional and metabolic MRI pulse sequences for target delineation. This study aimed to develop a phantom-based methodology to quantitatively compare the co-registration accuracy of the standard clinical imaging protocol to a representative MRI sequence that was likely to fail co-registration. The comparative methodology presented in this study may serve as a useful tool to evaluate the clinical translatability of novel MRI sequences. METHODS: A realistic human skull phantom with fiducial marker columns was designed and manufactured to fit into a typical MRI head coil and the Gamma Knife patient positioning system. A series of "optimized" 3D MRI sequences-T1 -weighted Dixon, T1 -weighted fast field echo (FFE), and T2 -weighted fluid-attenuated inversion recovery (FLAIR)-were acquired and co-registered to the CBCT. The same sequences were "compromised" by reconstructing without geometric distortion correction and re-collecting with lower signal-to-noise-ratio (SNR) to simulate a novel MRI sequence with poor co-registration accuracy. Image similarity metrics-structural similarity (SSIM) index, mean squared error (MSE), and peak SNR (PSNR)-were used to quantitatively compare the co-registration of the optimized and compromised MR images. RESULTS: The ground truth fiducial positions were compared to positions measured from each optimized image volume revealing a maximum median geometric uncertainty of 0.39 mm (LR), 0.92 mm (AP), and 0.13 mm (SI) between the CT and CBCT, 0.60 mm (LR), 0.36 mm (AP), and 0.07 mm (SI) between the CT and T1 -weighted Dixon, 0.42 mm (LR), 0.23 mm (AP), and 0.08 mm (SI) between the CT and T1 -weighted FFE, and 0.45 mm (LR), 0.19 mm (AP), and 1.04 mm (SI) between the CT and T2 -weighted FLAIR. Qualitatively, pairs of optimized and compromised image slices were compared using a fusion image where separable colors were used to differentiate between images. Quantitatively, MSE was the most predictive and SSIM the second most predictive metric for evaluating co-registration similarity. A clinically relevant threshold of MSE, SSIM, and/or PSNR may be defined beyond which point an MRI sequence should be rejected for target delineation based on its dissimilarity to an optimized sequence co-registration. All dissimilarity thresholds calculated using correlation coefficients with in-plane geometric uncertainty would need to be defined on a sequence-by-sequence basis and validated with patient data. CONCLUSION: This study utilized a realistic skull phantom and image similarity metrics to develop a methodology capable of quantitatively assessing whether a modern research-based MRI sequence can be co-registered to the Gamma Knife CBCT with equal or less than equal accuracy when compared to a clinically accepted protocol.

Radiochromic film dosimetry of HDR (192)Ir source radiation fields.

PURPOSE: A radiochromic film based dosimetry system for high dose rate (HDR) Iridium-192 brachytherapy source was described. A comparison between calibration curves established in water and Solid Water™ was provided. METHODS: Pieces of EBT-2 model GAFCHROMIC™ film were irradiated in both water and Solid Water™ with HDR (192)Ir brachytherapy source in a dose range from 0 to 50 Gy. Responses of EBT-2 GAFCHROMIC™ film were compared for irradiations in water and Solid Water™ by scaling the dose between media through Monte Carlo calculated conversion factor for both setups. To decrease uncertainty in dose delivery due to positioning of the film piece with respect to the radiation source, traceable calibration irradiations were performed in a parallel-opposed beam setup. RESULTS: The EBT-2 GAFCHROMIC™ film based dosimetry system described in this work can provide an overall one-sigma dose uncertainty of 4.12% for doses above 1 Gy. The ratio of dose delivered to the sensitive layer of the film in water to the dose delivered to the sensitive layer of the film in Solid Water™ was calculated using Monte Carlo simulations to be 0.9941 ± 0.0007. CONCLUSIONS: A radiochromic film based dosimetry system using only the green color channel of a flatbed document scanner showed superior precision if used alone in a dose range that extends up to 50 Gy, which greatly decreases the complexity of work. In addition, Solid Water™ material was shown to be a viable alternative to water in performing radiochromic film based dosimetry with HDR (192)Ir brachytherapy sources.

Feasibility of operating a millimeter-scale graphite calorimeter for absolute dosimetry of small-field photon beams in the clinic.

PURPOSE: To characterize and build a cylindrically layered graphite calorimeter the size of a thimble ionization chamber for absolute dosimetry of small fields. This detector has been designed in a familiar probe format to facilitate integration into the clinical workflow. The feasibility of operating this absorbed dose calorimeter in quasi-adiabatic mode is assessed for high-energy accelerator-based photon beams. METHODS: This detector, herein referred to as Aerrow MK7, is a miniaturized version of a previously validated aerogel-insulated graphite calorimeter known as Aerrow. The new model was designed and developed using numerical methods. Medium conversion factors from graphite to water, small-field output correction factors, and layer perturbation factors for this dosimeter were calculated using the EGSnrc Monte Carlo code system. A range of commercially available aerogel densities were studied for the insulating layers, and an optimal density was selected by minimizing the small-field output correction factors. Heat exchange within the detector was simulated using a five-body compartmental heat transfer model. In quasi-adiabatic mode, the sensitive volume (a 3 mm diameter cylindrical graphite core) experiences a temperature rise during irradiation on the order of 1.3 mK·Gy-1 . The absorbed dose is obtained by calculating the product of this temperature rise with the specific heat capacity of the graphite. The detector was irradiated with 6 MV ( % dd ( 10 ) x  = 63.5%) and 10 MV ( % dd ( 10 ) x  = 71.1%) flattening filter-free (FFF) photon beams for two field sizes, characterized by S clin dimensions of 2.16 and 11.0 cm. The dose readings were compared against a calibrated Exradin A1SL ionization chamber. All dose values are reported at d max in water. RESULTS: The field output correction factors for this dosimeter design were computed for field sizes ranging from S clin  = 0.54 to 11.0 cm. For all aerogel densities studied, these correction factors did not exceed 1.5%. The relative dose difference between the two dosimeters ranged between 0.3% and 0.7% for all beams and field sizes. The smallest field size experimentally investigated, S clin  = 2.16 cm, which was irradiated with the 10 MV FFF beam, produced readings of 84.4 cGy (±1.3%) in the calorimeter and 84.5 cGy (±1.3%) in the ionization chamber. CONCLUSION: The median relative difference in absorbed dose values between a calibrated A1SL ionization chamber and the proposed novel graphite calorimeter was 0.6%. This preliminary experimental validation demonstrates that Aerrow MK7 is capable of accurate and reproducible absorbed dose measurements in quasi-adiabatic mode.

Development of a water calorimetry-based standard for absorbed dose to water in HDR 192Ir brachytherapy.

PURPOSE: The aim of this article is to develop and evaluate a primary standard for HDR 192Ir brachytherapy based on 4 degrees C stagnant water calorimetry. METHODS: The absolute absorbed dose to water was directly measured for several different Nucletron microSelectron 192Ir sources of air kerma strength ranging between 21,000 and 38,000 U and for source-to-detector separations ranging between 25 and 70 mm. The COMSOL MULTIPHYSICS software was used to accurately calculate the heat transport in a detailed model geometry. Through a coupling of the "conduction and convection" module with the "Navier-Stokes incompressible fluid" module in the software, both the conductive and convective effects were modeled. RESULTS: A detailed uncertainty analysis resulted in an overall uncertainty in the absorbed dose of 1.90% (1 sigma). However, this includes a 1.5% uncertainty associated with a nonlinear predrift correction which can be substantially reduced if sufficient time is provided for the system to come to a new equilibrium in between successive calorimetric runs, an opportunity not available to the authors in their clinical setting due to time constraints on the machine. An average normalized dose rate of 361 +/- 7 microGy/(h U) at a source-to-detector separation of 55 mm was measured for the microSelectron 192Ir source based on water calorimetry. The measured absorbed dose per air kerma strength agreed to better than 0.8% (1 sigma) with independent ionization chamber and EBT-1 Gafchromic film reference dosimetry as well as with the currently accepted AAPM TG-43 protocol measurements. CONCLUSIONS: This work paves the way toward a primary absorbed dose to water standard in 192Ir brachytherapy.

Direct measurement of absorbed dose to water in HDR 192Ir brachytherapy: water calorimetry, ionization chamber, Gafchromic film, and TG-43.

PURPOSE: Gafchromic film and ionometric calibration procedures for HDR 192Ir brachytherapy sources in terms of dose rate to water are presented and the experimental results are compared to the TG-43 protocol as well as with the absolute dose measurement results from a water calorimetry-based primary standard. METHODS: EBT-1 Gafchromic films, an A1SL Exradin miniature Shonka thimble type chamber, and an SI HDR 1000 Plus well-type chamber (Standard Imaging, Inc., Middleton, WI) with an ADCL traceable Sk calibration coefficient (following the AAPM TG-43 protocol) were used. The Farmer chamber and Gafchromic film measurements were performed directly in water. All results were compared to direct and absolute absorbed dose to water measurements from a 4 degrees C stagnant water calorimeter. RESULTS: Based on water calorimetry, the authors measured the dose rate to water to be 361 +/- 7 microGy/(h U) at a 55 mm source-to-detector separation. The dose rate normalized to air-kerma strength for all the techniques agree with the water calorimetry results to within 0.83%. The overall 1-sigma uncertainty on water calorimetry, ionization chamber, Gafchromic film, and TG-43 dose rate measurement amounts to 1.90%, 1.44%, 1.78%, and 2.50%, respectively. CONCLUSIONS: This work allows us to build a more realistic uncertainty estimate for absorbed dose to water determination using the TG-43 protocol. Furthermore, it provides the framework necessary for a shift from indirect HDR 192Ir brachytherapy dosimetry to a more accurate, direct, and absolute measurement of absorbed dose to water.

Experimental validation of recommended msr-correction factors for the calibration of Leksell Gamma Knife® Icontm unit following IAEA TRS-483.

Currently, the American Association of Physicists in Medicine (AAPM) TG-21 is the conventional protocol currently used for the calibration of the Leksell Gamma Knife® (LGK) (despite the publication of the AAPM TG-51 protocol). However, this protocol is based on the air-kerma standards requiring an elaborate conversion process resulting in an increase in the possibility of errors in the clinic. The International Atomic Energy Agency (IAEA) Technical Reports Series (TRS)-483 Code of Practice provides new recommendations on the dosimetry of small static fields and correction factor data for the calibration of the LGK unit. The purpose of this study is to experimentally validate previously calculated [Formula: see text] factors for the calibration of the LGK Perfexion/Icon unit in the context of the TRS-483 protocol. An experimental comparison between three protocols (TG-51, TG-21 and TRS-483 with the aforementioned correction factors) for calibration of the LGK unit is provided. Dose-rate measurements were performed on a LGK Icon unit using three ionization chambers and three phantoms with different orientations of the chambers with respect to the LGK unit. The dose rate was determined following the three calibration protocols. The standard deviation on the mean dose rate over all phantoms and chambers in different orientations determined using TG-51, TG-21 and TRS-483 protocols were 0.9%, 0.5% and 0.4%, respectively. The mean dose rate calculated using TG-51 protocol was 1.6% and 1.2% lower comparing to the TG-21 and TRS-483 protocols respectively. Applying the [Formula: see text] values calculated in Mirzakhanian et al (2018) to the measured dose rates in LGK unit for all chambers and phantoms resulted in dose rates that are consistent to within 0.4%. The TRS-483 protocol improves the consistency of the results especially when the chamber was positioned in different orientations with respect to the LGK (from 1.6% when using TG-51 or TG-21 protocols to 0.2% when using TRS-483 protocol) since the other protocols do not correct for the different chamber orientations.

Absolute dosimetry of a 1.5 T MR-guided accelerator-based high-energy photon beam in water and solid phantoms using Aerrow.

PURPOSE: In this work, the fabrication, operation, and evaluation of a probe-format graphite calorimeter - herein referred to as Aerrow - as an absolute clinical dosimeter of high-energy photon beams while in the presence of a B = 1.5 T magnetic field is described. Comparable to a cylindrical ionization chamber (IC) in terms of utility and usability, Aerrow has been developed for the purpose of accurately measuring absorbed dose to water in the clinic with a minimum disruption to the existing clinical workflow. To our knowledge, this is the first reported application of graphite calorimetry to magnetic resonance imaging (MRI)-guided radiotherapy. METHODS: Based on a previously numerically optimized and experimentally validated design, an Aerrow prototype capable of isothermal operation was constructed in-house. Graphite-to-water dose conversions as well as magnetic field perturbation factors were calculated using Monte Carlo, while heat transfer and mass impurity corrections and uncertainties were assessed analytically. Reference dose measurements were performed in the absence and presence of a B = 1.5 T magnetic field using Aerrow in the 7 MV FFF photon beam of an Elekta MRI-linac and were directly compared to the results obtained using two calibrated reference-class IC types. The feasibility of performing solid phantom-based dosimetry with Aerrow and the possible influence of clearance gaps is also investigated by performing reference-type dosimetry measurements for multiple rotational positions of the detector and comparing the results to those obtained in water. RESULTS: In the absence of the B-field, as well as in the parallel orientation while in the presence of the B-field, the absorbed dose to water measured using Aerrow was found to agree within combined uncertainties with those derived from TG-51 using calibrated reference-class ICs. Statistically significant differences on the order of (2-4)%, however, were observed when measuring absorbed dose to water using the ICs in the perpendicular orientation in the presence of the B-field. Aerrow had a peak-to-peak response of about 0.5% when rotated within the solid phantom regardless of whether the B-field was present or not. CONCLUSIONS: This work describes the successful use of Aerrow as a straightforward means of measuring absolute dose to water for large high-energy photon fields in the presence of a 1.5 T B-field to a greater accuracy than currently achievable with ICs. The detector-phantom air gap does not appear to significantly influence the response of Aerrow in absolute terms, nor does it contribute to its rotational dependence. This work suggests that the accurate use of solid phantoms for absolute point dose measurement is possible with Aerrow.

Experimental measurement of ionization chamber angular response and associated magnetic field correction factors in MR-linac.

PURPOSE: To measure ionization chamber dose response as a function of the angle between magnetic field direction and ionization chamber orientation in magnetic resonance-guided radiation therapy (MRgRT) system, and to evaluate angular dependence of magnetic field correction factor for reference dosimetry. METHODS: Measurements were performed on an Elekta MR-linac that integrates a 1.5-T Philips MRI and a 7-MV FFF photon beam accelerator. The response of four reference class chambers (Exradin-A19, A1SL, IBA FC65-G, and CC13, paired with a PTW UE electrometer) was studied. An in-house built MR-compatible water tank and an accompanying cylindrical insert that allowed chamber rotation around the cylinder's axis was used. The EPID onboard imaging was used to center chamber at the MR-linac isocenter (143.5 cm, SAD), as well as to verify position at each datapoint. RESULTS: A clear angular dependence of dose response for all chambers has been measured. The most significant effect of magnetic field on relative chamber response in the presence of magnetic field was observed in the orientation when chamber axis is perpendicular to the direction of magnetic field with the tip pointing in the same direction as Lorentz force. This effect is more pronounced for larger volume chambers; the maximum relative variation in the chamber response (between the setup described above and the one where chamber and magnetic field are parallel) is a 5.3% and 4.6% increase for A19 and FC65-G, respectively, and only 2.0% and 1.9% for smaller volume A1SL and CC13 chamber, respectively. We measured the absolute magnitude of the magnetic field correction factor k Q mag for the Exradin-A19, A1SL, IBA FC65-G, and CC13 to be 0.938 ± 1.13%, 0.968 ± 0.99%, 0.950 ± 1.13%, and 0.975 ± 1.13%, respectively. The values are for perpendicular orientation of the chamber relative to magnetic field and parallel to the Lorentz force. CONCLUSIONS: Experimental measurements carried out in this study have verified the optimal orientation of ionization chamber in terms of minimizing effect of magnetic field on the chamber dose response. This study provides a detailed high-resolution measurement of absolute k Q mag values for four reference class chambers as a function of the angle between ionization chamber's central axis and the direction of strong magnetic field over a full 360° rotation. The experimental results of this study can further be used for optimization of the actual sensitive volume of the chamber (and analysis of dead volume) in future Monte Carlo chamber simulations in the presence of strong magnetic fields. In addition, it will provide some necessary data for future reference dosimetry protocols for MR-linac.

First-stage validation of a portable imageable MR-compatible water calorimeter.

PURPOSE: The purpose of this study is to design a water calorimeter with three goals in mind: (a) To be fully magnetic resonance (MR)-compatible; (b) To be imaged using kV cone beam computed tomography (CBCT), MV portal imaging or MRI for accurate positioning; (c) To accommodate both vertical and horizontal beam incidence, as well as volumetric deliveries or Gamma Knife®. Following this, the calorimeter performance will be measured using an accelerator-based high-energy photon beam. METHODS: A portable 4°C cooled stagnant water calorimeter was built using MR-compatible materials. The walls consist of layers of acrylic plastic, aerogel-based material acting as thermal insulation, as well as tubing for coolant to flow to keep the calorimeter temperature stable at 4°C. The lid contains additional pathways for coolant to flow through as well as two hydraulically driven stirrers. The water calorimeter was positioned in an Elekta Versa using kV CBCT imaging as well as orthogonal MV image pairs. Absolute absorbed dose to water was then determined under a 6 MV flattening filter-free (FFF) beam. This was compared against reference dosimetry results that were measured under identical conditions with an Exradin A1SL ionization chamber with a calibration coefficient directly traceable to the National Research Council Canada. RESULTS: The dose to water determined with the calorimeter (n = 30) agreed with the A1SL ionization chamber reference dose measurements (n = 15) to within 0.25%. The uncertainty associated with the water calorimeter absorbed dose measurement was estimated to be 0.54% (k = 1). CONCLUSIONS: An MR-compatible water calorimeter was successfully built and absolute absorbed dose to water under a conventional 6 MV FFF beam was determined successfully as a first-stage validation of the system.

Maximum RBE change in 192Ir, 125I, and 169Yb brachytherapy and the corresponding effect on treatment planning.

PURPOSE: The purpose of this study was to examine RBE variation as a function of distance from the radioactive source, and the potential impact of this variation on a realistic prostate brachytherapy treatment plan. METHODS: Three brachytherapy sources (125I, 192Ir, and 169Yb) were modelled in Geant4 Monte Carlo code, and the resulting electron energy spectrum in water in 3D space around these sources was scored (voxel size of 2 mm3). With this energy spectrum, microdosimetric techniques were used to calculate the maximum RBE, RBEM, as a function of distance from the source. RBEM of 125I relative to 192Ir was calculated in order to validate simulations against literature; all other RBEM calculations were done by normalizing electron fluence at various distances to the source position. In order to examine the impact of RBEM variation in treatment planning, a realistic 192Ir prostate plan was re-evaluated in terms of RBE instead of absorbed dose. RESULTS: The RBEM of 125I, 192Ir, and 169Yb at 8 cm away from the source was 0.994 (+/-0.002), 1.030 (+/-0.003), and 1.066 (+/-0.008), respectively. RBEM in the HDR prostate treatment plan exhibited several hot (+3.6% in RBEM) spots. CONCLUSIONS: The large increase RBEM observed in 169Yb has not yet been described in the literature. Despite the presence of radiobiological hotspots in the HDR treatment, these variations are likely nominal and clinically insignificant.

The effect of magnetic field on Linac based Stereotactic Radiosurgery dosimetric parameters.

Objective: MR-linac machines are being developed for image-guided radiation therapy but the magnetic field of such machines could affect dose distributions. The purpose of this work was to evaluate the effect of a magnetic field on linac beam dosimetric parameters including penumbra for circular cones used in radiosurgery.Methods: Monte Carlo simulation was conducted for a linac machine with circular cones at 6 MV beam. A homogenous magnetic field of 1.5 T was applied transversely and parallel to the radiation beam. Percentage depth dose (PDD) and beam profiles in a water phantom with and without the magnetic field were calculated.Results: The results have shown that when the magnetic field is applied transversely, the PDDs in the water phantom differ in the buildup region and distant part of PDD curves. The beam profiles at three different depths are all significantly different from those without the magnetic field. The penumbra is greater when a magnetic field has been applied.Conclusion: Linear accelerator-based SRT and SRS use small circular cones. The beam penumbra for these cones can change in the presence of a magnetic field. The perturbation of dose distribution has been also observed in a patient plan due to the presence of a magnetic field. The results of this study show that dose distributions in the presence of a magnetic field must be considered for MR-guided radiotherapy treatments.