MRI Linac Research Articles

Radioluminescence-based fibre-optic dosimeters in radiotherapy: a review

Since their conception in the early 1990s, radioluminescence-based fibre-optic dosimeters (FODs) have attracted great interest for dosimetric applications in radiotherapy (RT). Over the years, many scintillating materials, both organic and inorganic, have been developed and tested by multiple research groups around the world. In parallel, reliable techniques for removing or subtracting the stem effect, one of the main drawbacks of FODs, have been proposed in the literature. To date, several prototype systems are widespread, some of which prove to be viable commercial solutions. This review aims to trace the efforts over the years that have enabled FODs to become reliable tools for dosimetry in the current RT procedures and promising options for future scenarios. After a first section devoted to a thorough discussion of the stem effect issue, the use of FODs in various applications of interest to RT, primarily small-field dosimetry and in vivo dosimetry, are addressed. Their use both in external beam RT, including hadrontherapy, and in brachytherapy are considered. A special focus is given to the peculiarities of MRI LINAC dosimetry and the contributions of FODs in this context. Finally, the state of the art concerning the development and characterization of FODs for monitoring ultra-high dose rate radiation beams, typical of FLASH therapy and microbeam RT are presented, highlighting the challenges still open.

Novel Virtual Reality App for Training Patients on MRI-guided Radiation Therapy

PurposePatients receiving respiratory gated magnetic resonance imaging-guided radiation therapy (MRIgRT) for abdominal targets must hold their breath for ≥25 seconds at a time. Virtual reality (VR) has shown promise for improving patient education and experience for diagnostic MRI scan acquisition. We aimed to develop and pilot-test the first VR app to educate, train, and reduce anxiety and discomfort in patients preparing to receive MRIgRT. Methods and MaterialsA multidisciplinary team iteratively developed a new VR app with patient input. The app begins with minigames to help orient patients to using the VR device and to train patients on breath-holding. Next, app users are introduced to the MRI linear accelerator vault and practice breath-holding during MRIgRT. In this quality improvement project, clinic personnel and MRIgRT-eligible patients with pancreatic cancer tested the VR app for feasibility, acceptability, and potential efficacy for training patients on using breath-holding during MRIgRT. ResultsThe new VR app experience was tested by 19 patients and 67 clinic personnel. The experience was completed on average in 18.6 minutes (SD = 5.4) by patients and in 14.9 (SD = 3.5) minutes by clinic personnel. Patients reported the app was “extremely helpful” (58%) or “very helpful” (32%) for learning breath-holding used in MRIgRT and “extremely helpful” (28%) or “very helpful (50%) for reducing anxiety. Patients and clinic personnel also provided qualitative feedback on improving future versions of the VR app. ConclusionThe VR app was feasible and acceptable for training patients on breath-holding for MRIgRT. Patients eligible for MRIgRT for pancreatic cancer and clinic personnel reported on future improvements to the app to enhance its usability and efficacy.

Dosimetric Implications of Weekly On-Line MR-Guided Adaptive Radiotherapy (RT) for Glioblastomas (GBM) Growing during RT

The UNITED trial (NCT04726397) involves once-weekly on-line adaptive RT for patients with GBM on a 1.5T MRI-Linac (MRL). For tumors that continue to enlarge during the course of RT, we hypothesize that the adaptive strategy improves the dosimetric coverage of the target relative to a non-adaptive strategy. As per the trial protocol, T1+contrast (T1c) and FLAIR MRI sequences were acquired once per week during RT to re-define and adapt the gross tumor volume (GTV) as the contrast enhanced volume, the clinical target volume (CTV) as a 5mm expansion around the GTV plus adjacent FLAIR hyperintense regions considered at-risk by the radiation oncologist, and a 3 mm PTV around the CTV. Nine UNITED patients with tumors that grew throughout RT (GTV and/or CTV) were identified. 5/9 patients were treated with 60 Gy in 30 fractions (6 weekly adaptions) and 4/9 with 40 Gy in 15 fractions (3 weekly adaptions). For the final week's GTV and CTV (GTVfinal and CTVfinal), the dosimetric outcomes of the delivered and adaptive summative plans (Dsum) were compared to dose of the baseline plan (Dbaseline) generated on Fraction 1 of treatment, the latter being indicative of the theoretical situation where no further adaption was taken. We measured the dose to 99% of the GTVfinal and CTVfinal: Dsum99 and Dbaseline99. For each adaptive fraction, the plan was optimized to achieve an objective of D99 greater than 95% of the prescription (D99>95%) for both the GTV and CTV. The relative increase in GTVfinal and CTVfinal from baseline was on average 119% (range: 98% to 144.7%) and 128% (range: 109% to 176%), respectively. The proportion of CTVfinal that was outside the baseline plan's PTV was on average 11.5% (range: 0% to 32%). The GTVfinal did not extend beyond the baseline PTV for any of the 9 cases. GTVfinal Dsum99 was >95% for all cases while CTVfinal Dsum99 was < 95% in 2 of 9 cases (74%, and 87%). By contrast, the baseline plan, if given for all fractions with no further adaptation, yielded a D99 for CTVfinal of <95% in 5 of 9 cases (28%, 52%, 75%, 84%, and 87%). In general, coverage of the CTVfinal decreased with increasing levels of CTVfinal outside of the baseline PTV. For all 5 cases where CTVfinal D99<95% on the baseline plan, more than 10% of CTVfinal was outside of the baseline PTV. Small margin, weekly adaptive RT on an MRL for GBM maintains coverage of the GTV in the presence of tumor growth while minimizing the degree of normal brain tissue irradiated. Dosimetric impact on non-GTV/CTV brain is outside the scope of the present study. Preliminary results indicate that a once weekly adaptive approach for small margin MR-guided RT improves tumor coverage for progressive tumors compared to a static (baseline) plan without adaptation.

Comparing Predicted Toxicities between Hypofractionated Proton and Photon Radiotherapy of Liver Cancer Patients with Different Adaptive Schemes.

With the availability of MRI linacs, online adaptive intensity modulated radiotherapy (IMRT) has become a treatment option for liver cancer patients, often combined with hypofractionation. Intensity modulated proton therapy (IMPT) has the potential to reduce the dose to healthy tissue, but it is particularly sensitive to changes in the beam path and might therefore benefit from online adaptation. This study compares the normal tissue complication probabilities (NTCPs) for liver and duodenal toxicity for adaptive and non-adaptive IMRT and IMPT treatments of liver cancer patients. Adaptive and non-adaptive IMRT and IMPT plans were optimized to 50 Gy (RBE = 1.1 for IMPT) in five fractions for 10 liver cancer patients, using the original MRI linac images and physician-drawn structures. Three liver NTCP models were used to predict radiation-induced liver disease, an increase in albumin-bilirubin level, and a Child-Pugh score increase of more than 2. Additionally, three duodenal NTCP models were used to predict gastric bleeding, gastrointestinal (GI) toxicity with grades >3, and duodenal toxicity grades 2-4. NTCPs were calculated for adaptive and non-adaptive IMRT and IMPT treatments. In general, IMRT showed higher NTCP values than IMPT and the differences were often significant. However, the differences between adaptive and non-adaptive treatment schemes were not significant, indicating that the NTCP benefit of adaptive treatment regimens is expected to be smaller than the expected difference between IMRT and IMPT.

ACPSEM position paper on ROMP scope of practice and staffing levels for magnetic resonance linear accelerators

The purpose of this position paper is to outline the ACPSEM recommendations on Medical Physicist scope of practice and staffing levels, as they relate to the use of dedicated MRI-Linacs in the treatment of patients. A core function of Medical Physicists is to safely implement changes in medical practice via the introduction of new technology and to ensure high quality radiation oncology services are provided to patients. Determining the feasibility of MRI-Linacs in any existing setting, or in establishing a new site, mandates the knowledge and services of Radiation Oncology Medical Physicists (ROMPs) as the Qualified Experts within this setting. ROMPs are key members of the multi-disciplinary team which will be required to steer the successful establishment of MRI Linac infrastructure within departments. To support efficient implementation, ROMPs must be embedded in the process from the start, including any feasibility study, initiation of the project, and development of the business case. ROMPs must be retained throughout all stages of acquisition, service development, and ongoing clinical use and expansion. The number of MRI-Linacs in Australia and New Zealand is growing. This expansion is occurring in parallel with rapid technological evolution, expanding tumour stream applications, and increasing consumer uptake. Growth and applications of MRI-Linac therapy will continue to occur beyond current known horizons, via development on the MR-Linac platform itself and through the migration of learning from this platform to conventional Linacs (known horizons for example include the use of daily, online image guided adaptive radiotherapy and MRI data informing decision making for planning and treatment before and throughout treatment courses). Clinical use, research and development will be a significant component of expanding patient access to MRI-Linac treatment and there will be an ongoing need to attract and retain ROMPs to initially establish services and in particular to drive service development and delivery for the life of the Linacs. MRI and Linac technologies mean it is necessary to perform a specialized workforce assessment for these devices, distinct from those employed for conventional Linacs and associated services. MRI-Linacs are complex, have a heightened risk profile compared to standard Linacs, and are unique in their treatment of patients. Accordingly, the workforce needs for MRI-Linacs are greater than for standard Linacs. To ensure safe and high-quality Radiation Oncology patient services are provided, it is recommended that staffing levels should be based on the 2021 ACPSEM Australian Radiation Workforce model and calculator using the MRI-Linac specific ROMP workforce modelling guidelines outlined in this paper. The ACPSEM workforce model and calculator are closely aligned with other Australian/New Zealand and international benchmarks.

Evaluation of an MRI linac magnetic isocenter walkout with gantry rotation in the presence of angle-specific corrections

Objective. To reduce the magnetic isocenter position variation with gantry rotation on an 0.35 T MRI-guided linac to a practically negligible level. Approach. Central fRequency (CF) offset, eddy current calibration, cross-term calibration, gradient delay, and gradient offsets are tuned for each MR linac installation at every 30° of gantry rotation and stored in a look-up table (LUT). During treatment, the CF is tuned only once in the beginning at an arbitrary gantry angle. After that, imaging paramters are offset based on the stored LUT values for any given gantry angle. Main results. For the same hardware configuration, the implementation of the gantry-angle-specific parameter corrections reduced the total isocenters range of travel in the transverse plane from 1.1 to 0.3 mm and from 0.8 to 0.2 mm in horizontal and vertical directions, respectively. With the longitudinal shift always being negligible (≤0.2 mm), the radius of the sphere encompassing the isocenter locations was reduced from 0.6 to 0.2 mm. Geometric distortion improved as well; in particular, the gantry-angle-averaged maximum longitudinal distortion within a 35 cm diameter sphere was reduced from 1.4 to 0.8 mm. Since the CF is tuned only once during treatment, imaging may resume promptly after the gantry reaches the next target position. Significance. The MRI-guided linear accelerator was conceived primarily as an instrument for precision image-guided therapy. Thus, it is important to keep the treatment and imaging isocentres as close as possible while minimizing the geometric distortion. The described solution reduces the walkout of the imaging isocenter to a fraction of 1 mm, while keeping geometric distortion in a substantial volume below 1 mm. The approach is robust and does not increase the overall procedure time.

Dosimetric feasibility of direct post-operative MR-Linac-based stereotactic radiosurgery for resection cavities of brain metastases

BackgroundPost-operative radiosurgery (SRS) of brain metastases patients is typically planned on a post-recovery MRI, 2–4 weeks after resection. However, the intracranial metastasis may (re-)grow in this period. Planning SRS directly on the post-operative MRI enables shortening this time interval, anticipating the start of adjuvant systemic therapy, and so decreasing the chance of extracranial progression. The MRI-Linac (MRL) allows the simultaneous execution of the post-operative MRI and SRS treatment. The aim of this work was investigating the dosimetric feasibility of MRL-based post-operative SRS. MethodsMRL treatments based on the direct post-operative MRI were simulated, including thirteen patients with resectable single brain metastases. The gross tumor volume (GTV) was contoured on the direct post-operative scans and compared to the post-recovery MRI GTV.Three plans for each patient were created: a non-coplanar VMAT CT-Linac plan (ncVMAT) and a coplanar IMRT MRL plan (cIMRT) on the direct post-operative MRI, and a ncVMAT plan on the post-recovery MRI as the current clinical standard. ResultsBetween the direct post-operative and post-recovery MRI, 15.5 % of the cavities shrunk by > 2 cc, and 46 % expanded by ≥ 2 cc. Although the direct post-operative cIMRT plans had a higher median gradient index (3.6 vs 2.7) and median V3Gy of the skin (18.4 vs 1.1 cc) compared to ncVMAT plans, they were clinically acceptable. ConclusionDirect post-operative MRL-based SRS for resection cavities of brain metastases is dosimetrically acceptable, with the advantages of increased patient comfort and logistics.Clinical benefit of this workflow should be investigated given the dosimetric plausibility.

RADT-08. DOSIMETRIC FEASIBILITY OF DIRECT POST-OPERATIVE MRI-LINAC-BASED STEREOTACTIC RADIOSURGERY FOR RESECTION CAVITIES OF BRAIN METASTASES

Abstract Background Post-operative stereotactic radiosurgery (SRS) of patients with brain metastases with single resection cavities is typically planned on a post-recovery MRI, 4-6 weeks after resection. However, meanwhile the intracranial metastasis may (re-)grow, and postponing adjuvant systemic treatment increases chance on extracranial progression as well. Anticipating direct post-operative SRS to minimize this interval would enable rapid start of systemic therapy. In this study, we considered treatment with MRI-Linac (MRL) SRS, because of the possibility to execute the post-operative MRI and SRS treatment combined on the MRL instead of on two separate systems, improving logistics and increasing patient comfort. However, it is unclear whether MRL-based SRS may be feasible from a dosimetric perspective. This study aims to shed light on the dosimetric feasibility of MRL-based SRS. METHODS We simulated MRL treatments including thirteen patients with resectable single brain metastases treated with single fraction CT-Linac (CTL) SRS. We therefore contoured direct post-operative gross tumor volumes (GTV) and compared them to post-recovery MRI GTV. Next, we compared a non-coplanar VMAT technique for CTL (ncVMAT) to a coplanar IMRT technique for MRL (cIMRT), creating three plans per patient: a ncVMAT plan and a cIMRT plan for the direct post-operative GTV, and a post-recovery ncVMAT plan as current clinical standard. RESULTSCompared to GTVs defined on direct post-operative MRI, on post-recovery MRI 15.5% of cavities shrunk by &amp;gt; 2cc, and 46% expanded by &amp;gt; 2cc. Although direct post-operative ncVMAT plans had lower median gradient index and higher median V3Gy of the skin, they were clinically acceptable according to clinical guidelines. CONCLUSION Although slightly inferior to non-coplanar CTL plans, direct post-operative MRL-based SRS for resection cavities of brain metastases is dosimetrically acceptable, at the trade-off between increased patient comfort and logistics. Additionally, MRL-based SRS enables substantially earlier start with adjuvant systemic therapies, thereby maximizing tumor control.

Oxygen Enhanced-MRI for Hypoxia Imaging in Head and Neck Cancer

<h3>Purpose/Objective(s)</h3> Hypoxia is an important predictor of treatment efficacy in head and neck (H&N) cancer. Oxygen-enhanced (OE)-MRI is a non-invasive method for mapping and quantifying the extent of regional hypoxia, by identifying tumor regions that are perfused but refractory to oxygen change (termed perfused oxygen refractory, or pOxy-R). The aim of this work was to develop OE-MRI in H&N carcinoma and measure pOxy-R during (chemo)radiotherapy ((C)RT) and to then translate OE-MRI onto an MRI-Linac (MRL), with a longer term aim of using the technique for biologically adaptive radiotherapy. We hypothesized that pOxy-R proportion would reduce during the treatment course, suggesting a reduction in regional tumor hypoxia. <h3>Materials/Methods</h3> The study received ethics approval. Imaging was performed on a 1.5 T MRI-RT simulator (MRSim) and a 1.5 T MRL system. OE-MRI gas delivery was administered during a dynamic series: scans 1-25 (21% O₂), 26-70 (100% O₂) and 71-91 (21% O₂). A low-flow gas blender ensured 15 L/min flow via a high concentration mask. In patients, DCE-MRI was acquired after OE-MRI, at the 8^th dynamic measurement (gadoterate meglumine, 0.1 mol/kg at 3 ml/s). The protocol was initially run in 6 healthy volunteers, each imaged twice on the MRSim. Next, 8 patients with H&N carcinoma (15 lesions in total, including primary tumor and lymph nodes) were imaged on the MRSim 1-2 times pre-treatment and then during standard-of-care (C)RT at weeks 2 and/or 4. Finally, 3 additional patients were imaged on the MRL system. <h3>Results</h3> The technique developed in 6 volunteers identified the nasal conchae as a suitable reference tissue to confirm oxygen delivery, as signals changed between air and 100% O₂ breathing (all p<0.001). In the 8 patients recruited to date (table 1), we observed a significant reduction in median cohort pOxy-R proportion at week 2 (41% to 25%, p<0.01, N=14 lesions, 7 patients). pOxy-R proportion was reduced at week 4 (43% to 31%, p=0.06, N=5 lesions, 3 patients). Coefficient of variation (wCV), was found to be 24% (N=6 lesions). Technique feasibility was demonstrated on the MRL, with all tumors showing signal change between air and 100% O₂ breathing (all p<0.001). <h3>Conclusion</h3> OE-MRI can identify and map oxygen refractory regions in patients with H&N carcinoma and may detect cohort-wide hypoxia modification induced by (C)RT. The technique can also be translated onto an MRL system, suggesting the potential feasibility of using OE-MRI for online adaptive radiotherapy.

Real-time B0 compensation during gantry rotation in a 0.35-T MRI-Linac.

Rotation of the ferromagnetic gantry of a low magnetic field MRI-Linac was previously demonstrated to cause large center frequency offsets of ±400Hz. The B0 off-resonances cause image artifacts and imaging isocenter shifts that would preclude MRI-guided arc therapy. The purpose of this study was to measure and compensate for center frequency offsets in real time during gantry rotation on a 0.35-T MRI-Linac using a free induction decay (FID) navigator. A nonselective FID navigator was added before each 2D balanced steady-state free precession cine image acquisition on a 0.35-T MRI-Linac. Images were acquired at 7.3 frames per second. Phase data from the initial FID navigator (while the gantry was stationary) was used as a reference. The phase data from each subsequent FID navigator was used to calculate the real-time B0 off-resonance. The transmitter/receiver phase and the phase accrual over the adjacent image acquisition were adjusted to correct for the center frequency offset. Measurements were performed using an MRI-Linac dynamic phantom prior to and while the gantry rotated clockwise and counterclockwise. Image quality and signal-to-noise ratio (SNR) were compared between uncorrected and B0 -corrected MRIs using a reference image acquired while the gantry was stationary. Four targets in the phantom were manually contoured on the first image frame, and an active contouring algorithm was used retrospectively on each subsequent frame to assess image variations and calculate Dice coefficients. Additionally, three healthy volunteers were imaged using the same pulse sequences with and without real-time B0 compensation during gantry rotation. Normalized root mean square errors (nRMSEs) were calculated for the phantom and in vivo to assess the efficacy of the B0 compensation on image quality. The measured center frequency offsets from the volunteer and MRI dynamic phantom navigator data were also compared. The sinusoidal behavior of the center frequency offsets was modeled based on the gantry layout and long-time constant eddy currents resulting from gantry rotation. The duration of the FID navigator and processing was 4.5ms. The FID navigator resulted in a ≤11% drop in SNR in the phantom and in vivo (liver). Dice coefficients from the MRI-guided radiation therapy (MR-IGRT) phantom contour measurements remained above 0.8 with B0 compensation. Without B0 compensation, the Dice coefficients dropped below 0.8 for up to 21% of the time depending on the contour. Real-time B0 compensation resulted in mean reductions in nRMSE of 51% and 16% for the MR-IGRT phantom and in vivo, respectively. Peak-to-peak center frequency offsets ranged from 757 to 773Hz in the phantom and 760 to 871Hz in vivo. Dynamic real-time B0 compensation significantly improved image quality and reduced artifacts during gantry rotation in the phantom and in vivo. However, the FID navigator resulted in a small drop in the imaging duty cycle and SNR.

Assessment of MRI-Linac Economics under the RO-APM.

The implementation of the radiation oncology alternative payment model (RO-APM) has raised concerns regarding the development of MRI-guided adaptive radiotherapy (MRgART). We sought to compare technical fee reimbursement under Fee-For-Service (FFS) to the proposed RO-APM for a typical MRI-Linac (MRL) patient load and distribution of 200 patients. In an exploratory aim, a modifier was added to the RO-APM (mRO-APM) to account for the resources necessary to provide this care. Traditional Medicare FFS reimbursement rates were compared to the diagnosis-based reimbursement in the RO-APM. Reimbursement for all selected diagnoses were lower in the RO-APM compared to FFS, with the largest differences in the adaptive treatments for lung cancer (−89%) and pancreatic cancer (−83%). The total annual reimbursement discrepancy amounted to −78%. Without implementation of adaptive replanning there was no difference in reimbursement in breast, colorectal and prostate cancer between RO-APM and mRO-APM. Accommodating online adaptive treatments in the mRO-APM would result in a reimbursement difference from the FFS model of −47% for lung cancer and −46% for pancreatic cancer, mitigating the overall annual reimbursement difference to −54%. Even with adjustment, the implementation of MRgART as a new treatment strategy is susceptible under the RO-APM.

Geometrical imaging accuracy, image quality and plan quality for prostate cancer treatments on a 1.5 T MRLinac in patients with a unilateral hip implant

Purpose. To assess the feasibility of prostate cancer radiotherapy for patients with a hip implant on an 1.5 T MRI-Linac (MRL) in terms of geometrical image accuracy, image quality, and plan quality. Methods. Pretreatment MRI images on a 1.5 T MRL and 3 T MRI consisting of a T2-weighted 3D delineation scan and main magnetic field homogeneity (B 0) scan were performed in six patients with a unilateral hip implant. System specific geometrical errors due to gradient nonlinearity were determined for the MRL. Within the prostate and skin contour, B 0 inhomogeneity, gradient nonlinearity error and the total geometrical error (vector summation of the prior two) was determined. Image quality was determined by visually scoring the extent of implant-born image artifacts. A treatment planning study was performed on five patients to quantify the impact of the implant on plan quality, in which conventional MRL IMRT plans were created, as well as plans which avoid radiation through the left or right femur. Results. The total maximum geometrical error in the prostate was <1 mm and the skin contour <1.7 mm; in all cases the machine-specific gradient error was most dominant. The B 0 error for the MRlinac MRI could partly be predicted based on the pre-treatment 3 T scan. Image quality for all patients was sufficient at 1.5 T MRL. Plan comparison showed that, even with avoidance of the hips, in all cases sufficient target coverage could be obtained with similar D1cc and D5cc to rectum and bladder, while V28Gy was slightly poorer in only the rectum for femur avoidance. Conclusion. We showed that geometrical accuracy, image quality and plan quality for six prostate patients with a hip implant or hip fixation treated on a 1.5 T MRL did not show relevant deterioration for the used image settings, which allowed safe treatment.

Potential for focal magnetic resonance-guided stereotactic body radiotherapy for prostate cancer: A review

Prostate cancer is the most commonly diagnosed cancer among Australian men. Current whole gland radiotherapy treatment regimens are associated with known toxicities. The MR-Linac has the capability to deliver real-time visually guided radiation to enable focal therapy and reduce toxicity through decreasing radiation doses to organs at risk. This review article discusses the rationale, potential benefits and limitations of the MR-Linac in focal prostate stereotactic body radiotherapy (SBRT), in an effort to reduce toxicity-related side effects for men with low to favourable-intermediate risk prostate cancer. Pubmed was systemically for all published and ongoing trials using the search terms ‘Prostate’ and ‘MRI Linac or MR Linac’. 8 articles were reviewed, of those 1 was deemed relevant, additions were made and expert opinions in the field were sought regarding the most relevant research. Real-time MRI imaging during the delivery of each fraction with daily plan adaption is now a reality due to the development of the MR-Linac system. It is hoped that with improved real-time imaging, treatment accuracy can be improved, increasing the percentage of the planning target volume receiving the prescribed dose while reducing radiation to the surrounding organs at risk. Early results of prostate SBRT are promising but further research is needed into long term survival benefits and toxicity related outcomes. Focal stereotactic radiotherapy for low-risk intermediate prostate cancer using the MR-Linac has the potential to provide adequate tumour control while decreasing the toxicity and quality of life impact of whole gland treatment for men with localised prostate cancer.

Accuracy of automatic structure propagation for daily magnetic resonance image-guided head and neck radiotherapy

Background and purpose Deformable image registration (DIR) and contour propagation are used in daily online adaptation for hybrid MRI linac (MRL) treatments. The accuracy of the propagated contours may vary depending on the chosen workflow (WF), affecting the amount of required manual corrections. This study investigated the impact of three different WFs of contour propagations produced by a clinical treatment planning system for a high-field MRL on head and neck cancer patients. Methods Seventeen patients referred for curative radiotherapy for oropharyngeal cancer underwent standard CT-based dose planning and MR scans in the treatment position for planning (pMR), and at the 10th (MR10), 20th (MR20) and 30th (MR30) fraction (±2). The primary tumour, a metastatic lymph node and 8 organs at risk were manually delineated on each set of T2 weighted images. Delineations were repeated one month later on the pMR by the same observer to determine the intra-observer variation (IOV). Three WFs were used to deform images in the treatment planning system for the high-field MRL: In WF1, only the planning image and contours were used as a reference for DIR and propagation to MR10,20,30. The most recently acquired image set prior to the daily images was deformed and uncorrected (WF2) versus manually corrected (WF3) structures propagated to the session image. Dice similarity coefficient (DSC), mean surface distance (MSD) and Hausdorff distance (HD) were calculated for each structure in each model. Results Population median DSC, MSD and HD for WF1 and WF3 were similar and slightly better than for WF2. WF3 provided higher accuracy than WF1 for structures that are likely to shrink. All DIR workflows were less accurate than the IOV. Conclusions WF1 and WF3 provide higher accuracy in structure propagation than WF2. Manual revision and correction of propagated structures are required for all evaluated workflows.

Technical feasibility of magnetic resonance fingerprinting on a 1.5T MRI-linac

Hybrid MRI-linac (MRL) systems enable daily multiparametric quantitative MRI to assess tumor response to radiotherapy. Magnetic resonance fingerprinting (MRF) may provide time efficient means of rapid multiparametric quantitative MRI. The accuracy of MRF, however, relies on adequate control over system imperfections, such as eddy currents and , which are different and not as well established on MRL systems compared to diagnostic systems. In this study we investigate the technical feasibility of gradient spoiled 2D MRF on a 1.5T MRL. We show with phantom experiments that the MRL generates reliable MRF signals that are temporally stable during the day and have good agreement with spin-echo reference measurements. Subsequent in-vivo MRF scans in healthy volunteers and a patient with a colorectal liver metastasis showed good image quality, where the quantitative values of selected organs corresponded with the values reported in literature. Therefore we conclude that gradient spoiled 2D MRF is feasible on a 1.5T MRL with similar performance as on a diagnostic system. The precision and accuracy of the parametric maps are sufficient for further investigation of the clinical utility of MRF for online quantitatively MRI-guided radiotherapy.

PO-1784: Quality Assurance for the adaptive workflow on an MRI Linac

PO-1784: Quality Assurance for the adaptive workflow on an MRI Linac

Assessment of the Air-Electron Streaming Effect for MRI Guided Lung SBRT: A Treatment Planning Study

Electron streams outside the body are inevitably generated in the presence of magnetic field during MRI guided radiotherapy. This study aims to investigate the air-electron streaming effects for lung Stereotactic Body Radiotherapy (SBRT) treated on the 1.5T MRI Linac. Three lung SBRT cases with prescription dose ranging from 30Gy to 60Gy in four to five fractions were investigated in this study. The original curative treatment plan was designed by using step-and-shoot intensity modulated radiotherapy (IMRT) technique and then calculated with Monaco (v5.40.01) Monte Carlo dose engine under the 1.5 T magnetic field. To investigate the effect of air-electron streaming, another comparable treatment plan without 1.5T magnetic field was generated using the same planning parameters. In order to display the in-air dose distribution outside the body in Monaco system, an auxiliary structure was created by extending the external with margin of 8cm in anterior. The two plans with or without magnetic field were then compared in terms of in air dose distribution outside the body and dose to the chin. Electron stream effects outside the body were obviously observed for all 3 lung SBRT cases when in the presence of magnetic field, while which were not observed in the absence of magnetic field. From sagittal view, the electron stream can travel a long distance in air along the magnetic field direction, even can reach to the patient’s chin. But only two of the three cases have CT scan including the chin region. For that two cases, the chin surface skin doses were 1.47Gy (2.45% of 60Gy) and 2.24Gy (7.47% of 30Gy) respectively. While for the plans without magnetic field, the corresponding chin surface skin dose were 0.18Gy (0.3% of 60Gy) and 0.32Gy (1.07% of 30Gy) respectively. For the last case, the streaming electrons were expected to reach at the chin area, but due to which was not scanned in CT image, it’s hard to evaluate quantitatively. Electron streaming effects outside the body were also observed in MRI guided lung SBRT plans and which could lead to a higher chin dose compared to conventional scenario without magnetic field. It is therefore recommended for the MRI guided lung SBRT patients that the chin region should be scanned and included in planning CT image. Other suggestions for example shielding with bolus or keeping the patient’s head tilted to the opposite side should also be considered.

Creation of High-Quality Robust Pancreas Plans for Daily Adaptive Hypofractionated Stereotactic Radiotherapy on the 0.35T MRI Linac

Creation of High-Quality Robust Pancreas Plans for Daily Adaptive Hypofractionated Stereotactic Radiotherapy on the 0.35T MRI Linac

Co-60 MR Guided Adaptive Radiation Treatment Improves Target Coverage and Organs-At-Risk Sparing: Dosimetric Analysis of 1185 Adaptive Fractions and 5 Years’ Experience

Early data indicate MRI guided online adaptive radiotherapy (MRGRT) spares organs-at-risk (OARs) when inter-fractional motion of tumor targets and critical GI organs is significant. However, these studies were limited in patient numbers. This study systematically assesses all previously adapted treatment fractions of the MR imaging system Co-60 machine at the authors’ institution. The dosimetry metrics on patients’ treatment day anatomy between the adapted plans vs the un-adapted plans are compared. The aims are to determine the reasons (and corresponding rates) necessitating plan adaptations and quantifying improvement in target coverage. All abdominal cancer patients (N = 137) previously treated with MRGRT on the VR Co-60 machine from 2015 to 2019 are included. Treatment sites were pancreas (98), adrenal (11) and not otherwise specified abdomen (28). A total 961 (81.1%) out of 1185 fractions were adapted. Plan adaptation conditions were 1) the target was edited, or 2) any critical OAR constraints were violated, or 3) target coverage was significantly worse (V95%Rx reduction >10% for PTV-PRV) than the one achieved with the previous plan. For each adapted fraction, dose was calculated using a Monte Carlo dose calculation engine previously developed by authors for both 1) the plan before adaptation and 2) the adapted new plan. A suite of computer algorithm programs were developed to 1) process the treatment plan data files exported from TPS, 2) recognize the case-specific target structure and critical OARs, 3) compute treatment-day DVH metrics (V95%Rx and D95% for the targets, and V>constraint for OARs) on the calculated dose volumes, and 4) analyze the results arithmetically and statistically. Data of 961 adapted treatment fractions were processed and analyzed. The analysis shows that 1) critical OARs were edited for 100% cases, 2) tumor targets were edited for 90 (9.4%) cases, 3) critical OAR constraints were violated for 870 (90.5%) cases by the un-adapted plans, 4) plan optimization objectives were edited for 33 (3.4%) cases and 5) all critical OAR constraints were met in all (100%) adapted plans, 6) target coverages were improved for 888 (92.4%) cases with the adapted plans compared to un-adapted plans that were normalized to meet OAR constraints. After more than 5 years and 1100 fractions of experience, results continue to confirm that adapted plans provided significantly better target coverage while preventing violation of critical OAR constraints. Future work is necessary to establish the clinical significance of these data and validate these findings in the VR MRI Linac.

Impact of magnetic fields on dose measurement with small ion chambers illustrated in high-resolution response maps.

PurposeDosimetry of ionizing radiation in the presence of strong magnetic fields is gaining increased relevance in light of advances for MRI‐guided radiation therapy. While the impact of strong magnetic fields on the overall response of ionization chambers has been simulated and measured before, this work investigates the local impact of the magnetic field on dose response in an ion chamber. High‐resolution 1D and 2D response maps have been created for two small clinical thimble ionization chambers, the PinPoint chambers 31006 and 31014 (Physikalisch Technische Werkstaetten Freiburg, Germany).MethodsWorking on the Imaging and Medical Beam Line of the Australian Synchrotron an intense kilovoltage radiation beam with very low divergence, collimated to 0.1 mm was used to scan the chambers by moving them on a 2D motion platform. Measured current and beam position were correlated to create the response maps. Small neodymium magnets were used to create a field of about 0.25 T. Chamber axis, magnetic field, and beam direction were perpendicular to each other. Measurements were performed with both orientations of the magnetic field as well as without it. Chamber biases of 5 and 250 V in both polarities were used.ResultsThe local distribution of the response of small thimble‐type ionization chambers was found to be impacted by a magnetic field. Depending on the orientation of the magnetic field, the chamber response near the stem was either enhanced or reduced with the response near the tip behaving the opposite way. Local changes were in the order of up to 40% compared to measurements without the magnetic field present. Bending of the central electrode was observed for the chamber with the steel electrode. The size of the volume of reduced collection near the guard electrode was impacted by the magnetic field.As the here investigated beam and field parameters differ from those of clinical systems, quantitatively different results would be expected for the latter. However, the gyroradii encountered here were similar to those of a 6–7 MV MRI linac with a 1.5 T magnet.ConclusionsMagnetic fields impact the performance of ionization chambers also on a local level. For practical measurements this might mean a change in the effective point of measurement, in addition to any global corrections. Further knowledge about the local response will help in selecting or constructing optimized chambers for use in magnetic fields.