Background and Purpose:Brachytherapy is a highly conformal and cost-effective radiotherapy modality, yet its clinical utilization has declined in multiple regions. This study investigated trends in brachytherapy utilization across different Canadian provinces from 2011 to 2020. Materials and Methods:A national survey was distributed to medical physicists in cancer centres across ten Canadian provinces, collecting data on types of brachytherapy, annual number of treatments, clinical indications, and logistical factors. In Québec, treated clinical indications were obtained through a complementary survey of all radiotherapy centres, extending previously published provincial brachytherapy data. Results:Out of 39 radiotherapy centres in Canada conducting brachytherapy treatments, 25 centres were included in this study. HDR accounted for the majority of treatments (60%-100%) and increased in most provinces, while LDR declined across reporting provinces; PDR comprised ≤ 10% of treatments and was limited to Alberta and Ontario. After adjusting for indication-specific cancer incidence and combining HDR and LDR brachytherapy, utilization trends showed decreases in Alberta and British Columbia, increases in Ontario, Nova Scotia, and Saskatchewan, and no significant change in Manitoba, Québec, or nationally. Conclusions:This study revealed significant regional variations in brachytherapy utilization and clinical practices across Canadian radiotherapy centres. Factors influencing these trends include reimbursement structures, personnel and infrastructure availability, and clinician preferences. The use of HDR modalities is increasing nationally, while LDR is declining. Despite significant regional differences, brachytherapy utilization in Canada was largely sustained over the decade from 2011 to 2020.

Cardiac radioablation (CRA) for refractory ventricular tachycardia (VT) was introduced in 2017 following the success of the ENCORE-VT trial (1). The current standard of care radiation therapy dose is 25 Gy in 1 fraction, however, the optimal therapeutic dose is being explored. Previous studies have shown fibrosis, reduced bipolar voltage, and scar formation in areas treated with radioablation at doses above 15 Gy. A study suggest that CRA upregulates connexin-43 and sodium channel, NaV1.5, thereby enhancing conduction velocity through cellular reprogramming, even at lower doses (2). In this interim analysis of a single institution, phase 2, dose de-escalation clinical trial, we report the early efficacy in managing refractory ventricular tachycardia. Eligible patients with ischemic or non-ischemic cardiomyopathy experiencing recurrent, monomorphic VT unresponsive to standard medical therapy were offered participation in the study. Patients unable to undergo or contraindicated to invasive electrophysiology study were included if their arrhythmic substrate could be identified non-invasively. Exclusion criteria included prior thoracic radiation therapy, active connective tissue disease, or interstitial pulmonary fibrosis. The CRA workflow is summarised in Figure A. Implantable defibrillators were programmed at the physician’s discretion, with an anti-tachycardia pacing (ATP) zone set, typically set 20 msec slower than the documented arrhythmia. Following treatment, a 6-week blanking period was allowed. During this blanking period, arrhythmia events were tracked but were not included in the efficacy analysis. Follow-ups, including device interrogation, were conducted at 6 weeks, 3 and 6 months. Five patients (4 males [80%], mean age 77 ± 3.2 years) have received CRA of 20 Gy in 1 fraction. The median left ventricular ejection fraction was 25% (range 19%-35%). Four patients had an ischemic substrate, and one had a non-ischemic substrate (valvular disease). The average planning target volume was 187.5 ml (IQR 95.9–209.3 ml). After a median follow-up of 6 months, four patients remained stable, and one patient died (secondary to septic shock). Two patients had recurrent VT, one of which presented as a VT storm within the 6-week blanking period but subsequently stabilised. The total VT burden, defined as any sustained VT episodes requiring ICD therapies (both ATP and/or shocks), decreased significantly from 14 episodes to 1 (p = 0.042; Graph A). Episodes requiring an ICD shock reduced from 6 to 0, while those requiring ATP decreased from 14 to 1. VT storm events decreased from 3 to 0. This interim analysis demonstrates that a 20 Gy dose effectively reduces VT burden. Preliminary efficacy results for a single 20 Gy dose of CRA are promising; however, data remain too limited for definitive conclusions.Figure A:Workflow for CRA Graph A:6-month ventricular arrhythmia

BACKGROUND:Traditional radiochromic film dosimetry requires batch-specific dose-response curve measurements, which are time-consuming and add complexity to clinical workflows. While relative dosimetry techniques have been proposed to streamline the process, they have neglected film non-uniformities, limiting their accuracy. PURPOSE:To develop and validate a relative optimized linearization (ROL) method for radiochromic film dosimetry that eliminates the need for dose-response curve measurements while incorporating non-uniformity corrections for improved accuracy. METHODS:The accuracy of the linearization method proposed by Devic et al. was first evaluated through simulations using EBT4 film dose-response data, with maximum dose values ranging from 1 to 10 Gy. Based on these results, the linearization was refined with an optimized power function to reduce errors across all dose ranges. The optimized linearization was then integrated into the multichannel dosimetry (MCD) framework of Micke et al. to correct for dose-independent variations, forming the ROL method. ROL was validated against MCD using measured film data from open field, wedge field, and volumetric modulated arc therapy (VMAT) plans. To assess robustness, the VMAT test case was further evaluated under induced positional and dose delivery errors. Sensitivity to treatment planning modeling errors was also examined. RESULTS:Simulations showed that optimized linearization using ROL reduced average errors from up to 3% in the green channel and 2% in the blue channel to below 1% across all channels and dose ranges. ROL produced dose distributions comparable to MCD (within 1%), particularly in the open and VMAT fields. Only small regions in the wedge field, specifically in the toe region, exceeded 1%, but remained below 1.5%. Sensitivity tests confirmed ROL's robustness to spatial errors and to more subtle treatment planning variations in MLC modeling. Partial plan deliveries, which effectively scale the measured dose distribution, showed expected deviations from MCD. However, gamma analysis of the ROL-computed dose successfully detected the partial delivery error. CONCLUSIONS:The ROL method provides an efficient alternative to traditional film dosimetry by removing the need for time-consuming calibration curves while maintaining high accuracy through non-uniformity corrections. Its streamlined workflow makes it particularly valuable for routine clinical quality assurance.

BACKGROUND:Ventricular tachycardia is a life-threatening cardiac arrhythmia for which radiation therapy is an emerging therapeutic option. Electroanatomic maps (EAMs) are used to define clinical target volumes (CTVs) in cardiac radioablation (CRA) treatment planning. Treatment planning systems are unable to integrate EAM data, thus many different workflows have been developed to guide clinicians in CTV creation. PURPOSE:To provide a review of existing CTV definition protocols involving EAM integration for CRA. METHODS:PubMed was searched on January 11, 2024, using appropriate search terms. Results were filtered according to inclusion and exclusion criteria following PRISMA guidelines. Results were manually sorted based on their workflow. RESULTS:The original literature search resulted in 271 search results, to which two hand-selected articles were added. 85 of the resulting articles met inclusion criteria and did not meet exclusion criteria. The reviewed protocols included side-by-side approaches for EAM integration into the treatment planning workflow as well as software-based protocols. Software-based protocols were further subcategorized based on whether the workflows used commercial or non-commercial software to aid in CTV definition. CONCLUSIONS:There is a strong desire to provide solutions for EAM integration into CRA CTV definition protocols. Although single center-specific approaches exist, there is no standardized workflow to address this problem. As the field of CRA grows, standardized workflows and guidelines will be necessary to perform meaningful analyses and comparisons of data between small data sets and to make recommendations for both technical and therapeutic indications.

BackgroundCardiac radioablation (CRA) is a new and promising treatment modality for patients with ventricular tachycardia refractory to standard-of-care treatment. Electroanatomic maps are used to define radiation target volumes; however, there is currently no native method to import electroanatomic maps into the treatment planning system (TPS).PurposeTo develop Edico, a semi-automated tool to enable electroanatomic map import into a TPS, by converting electroanatomic maps to a Digital Imaging and COmmunications in Medicine (DICOM) standard. The overall aim is to facilitate target volume delineation and improve workflow efficiency in treating patients.MethodsEdico imports voltage and spatial data from electroanatomic maps and sorts these into voxels to be exported in a DICOM format, with each voxel containing the average voltage value of the data that falls within it. Three different rectangular electroanatomic maps were created and processed using Edico to ensure that expected features are maintained through processing. A sensitivity analysis of voxel size was completed using 19 different electroanatomic maps processed at five different sets of voxel dimensions, for a total of 95 resulting voxelized datasets. The coefficient of variation in each populated voxel in the datasets was analyzed to determine which voxel sizes are necessary to ensure that data loss is kept to a minimum throughout processing, despite averaging. Five electroanatomic maps were used to re-contour clinical target volumes and planning target volumes for previously-treated patients with their electroanatomic maps now directly registered to their planning computed tomography (CT) scans.ResultsAll three rectangular test electroanatomic maps were processed as expected. All tested voxel sizes resulted in low coefficients of variation overall, with the exception of the largest voxel size of 1.8 x 1.8 x 8 mm. When using Edico, a user should choose voxel dimensions similar to or smaller than those of a planning CT. Of five pairs of clinical and planning target volumes from previously treated patients, adjustments were made to four (80%), retrospectively, using the electroanatomic maps generated using Edico, registered to the patients' planning CTs.ConclusionsEdico provides a reliable solution for electroanatomic map import into a TPS and facilitates clinical and planning target volume identification in CRA.

Despite increasing reports in the literature, there are no established criteria to predict success for SBRT in the context of treatment-refractory VT, or the optimal treatment dose for success, making it difficult to identify optimal patients. Current limited evidence suggests that this technique may be a relatively safe approach to provide an acute reduction in VT burden for those refractory to standard of care and has an acceptable acute toxicity profile however longer term follow-up is required. Long term toxicity, specifically to cardiac microstructures, and dose optimization is currently are the focus of ongoing study.

Purpose/Objective(s) Ventricular tachycardia (VT) is characterized by electrical re-entry within patches of heterogeneous myocardial fibrosis leading to sustained consecutive ventricular beats at a rate > 100 per minute. Catheter ablation is the standard of care adjunctive therapy for patients who are refractory to medical therapy to destroy the pathways responsible for these arrhythmias. Recently, a novel treatment approach using ablative radiation with stereotactic body radiation therapy (SBRT) to the arrhythmogenic scar regions defined by noninvasive cardiac mapping has been described for patients refractory to standard-of-care therapies. We describe our experience with 6 patients treated with this technique in our institution. Materials/Methods All 6 patients had refractory VT with previously failed ablations and at least one anti-arrhythmic drug. Patients were simulated with 4D computed tomography (4D-CT) and targets were defined using the combined information from cardiac mapping, diagnostic and simulation imaging with cardiologists, medical physicists, and radiation oncologists for each patient. An internal target volume was created based on the cardiac and respiratory motion. An isotropic margin of 3 mm was added to create the planning target volume (PTV). The PTVs were prescribed 25 Gy in 1 fraction normalized so 95% of the PTV was covered by the 25 Gy isodose. Radiation was delivered using volumetric modulated arc therapy. Patients were evaluated immediately following treatment for acute side effects, and then at 6 weeks, 3 months, 6 months, and then yearly. Implantable cardioverter defibrillator (ICD) interrogation was performed regularly by the treating cardiologist to assess the number of VT and ICD events. Results All 6 patients tolerated treatment with no immediate acute side effects. One patient experienced mild esophagitis in the first 3 weeks following treatment which resolved. 4 of 6 patients had immediate significant reduction in the number of VT and ICD events in the first 6 months after treatment (>90%), however, one patient did not respond and required an extracorporeal membrane oxygenation assisted ablation 3 months later. The first two patients treated have had longer follow-up and one remains VT-free and has stopped anti-arrhythmic drugs, however, another has relapsed 2 years following radiotherapy in an area of the arrhythmogenic substrate that was intentionally not irradiated due to organ at risk safety concerns. Conclusion Despite increasing reports in the literature, there are no established criteria to predict success for this treatment, making it difficult to identify optimal patients. Current limited evidence suggests that this technique may be a relatively safe approach that provides an acute reduction in VT burden for those that have run out of conventional treatment options.

Purpose/Objective(s)Ventricular tachycardia (VT) is characterized by electrical re-entry within patches of heterogeneous myocardial fibrosis leading to sustained consecutive ventricular beats at a rate > 100 per minute. Implantable cardioverter-defibrillators (ICD) are the main intervention for reducing mortality, however, they are exclusively a symptom-control therapy. Catheter ablation is the standard of care adjunctive therapy for patients who are refractory to medical therapy. Recently, a treatment approach with a stereotactic body radiation therapy (SBRT) to arrhythmogenic scar regions has been described. Initial results, using a single 25 Gy fraction, suggest this technique may improve morbidity for patient's refractory to standard of care therapy. The optimal dose for this therapy remains unclear and major adverse events with 25 Gy have been reported. This clinical trial hypothesizes that refractory VT treated with SBRT in a single fraction of 20 Gy is non-inferior to 25 Gy.Materials/MethodsInclusion criteria are age > 18 years, cardiomyopathy, recurrent episodes of monomorphic VT failing standard treatment with at least 1 antiarrhythmic drug and previous electrophysiologic ablation. Exclusion criteria are participants with previous thoracic radiation, connective tissue disease, interstitial pulmonary fibrosis, and pregnancy. Participants with contraindications to electrophysiology studies may be eligible for the study, provided the arrhythmic substrate can be defined through other non-invasive methods. We anticipate an incidence rate of approximately 5 VT events per person-year in participants treated with 25 Gy as a historical comparator based on a previous phase I/II trial. Based on a Poisson distribution, and using a non-inferiority margin of 8.5 events per-year (incident rate ratio of 1.70), recruiting 9 participants will provide 80% power when using a one-sided type I error, set at 0.05. Non-inferiority of 20 Gy relative to 25 Gy will be determined if the upper bound of the one-sided 95% confidence interval for the incidence rate ratio is below the pre-specified non-inferiority margin (incidence rate ratio=1.70). The primary efficacy endpoint is the reduction in arrhythmia burden measured by the total number of VT events and ICD treatments for VT comparing the 6 and 12-month periods after a single fraction of 20 Gy SBRT with a single fraction of 25 Gy in the published literature. Our primary safety endpoint is defined as the rate of severe treatment-related adverse events at ≤ 90 days as defined by the CTCAE v5.0. Secondary endpoints include overall survival, late adverse events, antiarrhythmic drug use, and quality of life.ResultsThe Research Ethics Board at the institution has approved this research study and will provide ongoing ethical oversight.ConclusionThis study is currently recruiting participants. This trial is registered at ClinicalTrials.gov, NCT05258422.

Purpose: Radiotherapy treatment planning based on magnetic resonance imaging (MRI) benefits from increased soft-tissue contrast and functional imaging. MRI-only planning is attractive but limited by the lack of electron density information required for dose calculation, and the difficulty to differentiate air and bone. MRI can map magnetic susceptibility to separate bone from air. A method is introduced to produce synthetic CT (sCT) through automatic voxel-wise assignment of CT numbers from an MRI dataset processed that includes magnetic susceptibility mapping. Methods: Volumetric multi-echo gradient echo datasets were acquired in the heads of five healthy volunteers and fourteen patients with cancer using a 3 T MRI system. An algorithm for CT synthesis was designed using the volunteer data, based on fuzzy c-means clustering and adaptive thresholding of the MR data (magnitude, fat, water, and magnetic susceptibility). Susceptibility mapping was performed using a modified version of the iterative phase replacement algorithm. On patient data, the algorithm was assessed by direct comparison to X-ray computed tomography (CT) scans. Results: The skull, spine, teeth, and major sinuses were clearly distinguished in all sCT, from healthy volunteers and patients. The mean absolute CT number error between X-ray CT and sCT in patients ranged from 78 and 134 HU. Conclusion: Susceptibility mapping using MRI can differentiate air and bone for CT synthesis. The proposed method is automated, fast, and based on a commercially available MRI pulse sequence. The method avoids registration errors and does not rely on a priori information, making it suitable for nonstandard anatomy.

Purpose: Radioisotopes such as Se-75, Yb-169, and Gd-153 have photon energy spectra and half-lives that make them excellent candidates as alternatives to Ir-192 for high-dose-rate brachytherapy. The aim of the present study was to evaluate the relative biological effectiveness (RBE) of current (Ir-192, I-125, Pd-103) and alternative (Se-75, Yb-169, Gd-153) brachytherapy radionuclides using Monte Carlo simulations of lineal energy distributions. Methods and Materials: Brachytherapy sources (microSelectron v2 [Ir-192, Se-75, Yb-169, Gd-153], SelectSeed [I-125], and TheraSeed [Pd-103]) were placed in the center of a spherical water phantom with a radius of 40 cm using the Geant4 Monte Carlo simulation toolkit. The kinetic energy of all primary, scattered, and fluorescence photons interacting in a scoring volume were tallied at various depths from the source. Electron tracks were generated by sampling the photon interaction spectrum and tracking all the interactions down to 10 eV using the event-by-event capabilities of the Geant4-DNA models. The dose mean lineal energy ((y) over bar (D)) values were obtained through random sampling of transfer points and overlaying spherical scoring volumes within the associated volume of the tracks. The scoring volume diameter was determined by fitting the (y) over bar (D) ratio for I-125 to its observed RBE. Results: (y) over bar (D) increased with the increasing distance from the source for Ir-192, Se-75, and Yb-169, remained constant for Gd-153 and I-125, and decreased for Pd-103. The diameter at which the (y) over bar (D) ratio coincided with the RBE of 1.15 to 1.20 for I-125 was similar to 25 to 40 nm. The RBE (reference 1 MeV photons) at high doses and dose rates for Ir-192, Se-75, Yb-169, Gd-153, I-125, and Pd-103 was 1.028 to 1.034, 1.05 to 1.07, 1.12 to 1.15, 1.16 to 1.21, 1.15 to 1.20, and 1.17 to 1.22, respectively. Conclusions: The radiation quality of the radionuclides under investigation was greater than that of high-energy photons. The present study has provided a set of values to modify the prescription doses for brachytherapy to account for the variation in radiation quality among radionuclides. (C) 2017 Elsevier Inc. All rights reserved.