Optimizing Gantry Breakpoint Angles in Proton Therapy: Enhancing Efficiency and Patient Experience.

Purpose: The breakpoint for a 360° radiotherapy gantry is typically positioned at 180°. This arbitrary setting has not been systematically evaluated for efficiency and may cause redundant gantry rotation and extended setup times. Our study aimed to identify an optimal gantry breakpoint angle for a full-gantry proton therapy system, with the goal of minimizing gantry movement. Materials and Methods: We analyzed 70 months of clinically delivered proton therapy plans (9152 plans, 131 883 fractions), categorizing them by treatment site and mapping the fields from a partial-gantry to full-gantry orientation. For each delivered fraction, we computed the minimum total gantry rotation angle as a function of gantry breakpoint position, which was varied between 0° and 360° in 1° steps. This analysis was performed separately within the entire plan cohort and individual treatment sites, both with and without the capability of over-rotating 10° past the breakpoint from either direction (20° overlap). The optimal gantry breakpoint was identified as one which resulted in a low average gantry rotation per fraction. Results: Considering mechanical constraints, 130° was identified as a reasonable balance between increased gantry-rotation efficiency and practical treatment considerations. With a 20° overlap, this selection reduced the average gantry rotation by 41.4° per fraction when compared to the standard 180° breakpoint. Disease site subgroups showed the following reduction in average gantry rotation: gastrointestinal 192.2°, thoracic 56.3°, pediatric 44.9°, genitourinary 19.9°, central nervous system 10.7°, breast 2.8°, and head and neck 0.1°. Conclusion: For a full-gantry system, a breakpoint of 130° generally outperforms the conventional 180° breakpoint. This reduction is particularly impactful for gastrointestinal, pediatric, and thoracic sites, which constitute a significant proportion of cases at our center. The adjusted breakpoint could potentially streamline patient delivery, alleviate mechanical wear, and enhance treatment precision by reducing the likelihood of patient movement during delivery.

Early Impact of Proton Beam Therapy on Electrophysiological Characteristics in a Porcine Model.

BACKGROUND: Particle therapy is a noninvasive, catheter-free modality for cardiac ablation. We previously demonstrated the efficacy for creating ablation lesions in the porcine heart. Despite several earlier studies, the exact mechanism of early biophysical effects of proton and photon beam delivery on the myocardium remain incompletely resolved. METHODS: Ten normal and 9 infarcted in situ porcine hearts received proton beam irradiation (40 Gy) delivered to the left ventricular myocardium with follow-up for 8 weeks. High-resolution electroanatomical mapping of the left ventricular was performed at baseline and follow-up. Bipolar voltage amplitude, conduction velocity, and connexin-43 were determined within the irradiated and nonirradiated areas. RESULTS: The irradiated area in normal hearts showed a significant reduction of bipolar voltage amplitude (10.1±4.9 mV versus 5.7±3.2, P<0.0001) and conduction velocity (85±26 versus 55±13 cm/s, P=0.03) beginning at 4 weeks after irradiation. In infarcted myocardium after irradiation, bipolar voltage amplitude of the infarct scar (2.0±2.9 versus 0.8±0.7 mV, P=0.008) was significantly reduced as well as the conduction velocity in the infarcted heart (43.7±15.7 versus 26.3±11.4 cm/s, P=0.02). There were no significant changes in bipolar voltage amplitude and conduction velocity in nonirradiated myocardium. Myocytolysis, capillary hyperplasia, and dilation were seen in the irradiated myocardium 8 weeks after irradiation. Active caspase-3 and reduction of connexin-43 expression began in irradiated myocardium 1 week after irradiation and decreased over 8 weeks. CONCLUSIONS: Irradiation of the myocardium with proton beams reduce connexin-43 expression, conduction velocity, and bipolar conducted electrogram amplitude in a large porcine model. The changes in biomarkers preceded electrophysiological changes after proton beam therapy.

Rotating Gantries Provide Individualized Beam Arrangements for Charged Particle Therapy.

PURPOSE: This study evaluates beam angles used to generate highly individualized proton therapy treatment plans for patients eligible for carbon ion radiotherapy (CIRT). METHODS AND MATERIALS: We retrospectively evaluated patients treated with pencil beam scanning intensity modulated proton therapy from 2015 to 2020 who had indications for CIRT. Patients were treated with a 190° rotating gantry with a robotic patient positioning system. Treatment plans were individualized to provide maximal prescription dose delivery to the tumor target volume while sparing organs at risk. The utilized beam angles were grouped, and anatomic sites with at least 10 different beam angles were sorted into histograms. RESULTS: A total of 467 patients with 484 plans and 1196 unique beam angles were evaluated and characterized by anatomic treatment site and the number of beam angles utilized. The most common beam angles used were 0° and 180°. A wide range of beam angles were used in treating almost all anatomic sites. Only esophageal cancers had a predominantly unimodal grouping of beam angles. Pancreas cancers showed a modest grouping of beam angles. CONCLUSIONS: The wide distribution of beam angles used to treat CIRT-eligible patients suggests that a rotating gantry is optimal to provide highly individualized beam arrangements.

Design and characterization of an aperture system and spot configuration for ocular treatments with a gantry-based spot scanning proton beam.

BACKGROUND AND PURPOSE: Proton therapy is a key modality used in the treatment of ocular melanoma. Traditionally ocular sites are treated using a dedicated eyeline with a passively scattered proton beam and a brass aperture. This work aims to design and characterize a beam-collimating aperture to treat ocular targets with a gantry-based spot scanning proton beam. METHODS: A plastic aperture system that slides into the gantry nozzle of a spot scanning proton beam was designed and constructed. It consists of an intermediate scraper layer to attenuate stray protons and a 3D-printed patient-specific aperture positioned 5.7 cm from the surface of the eye. The aperture system was modeled in TOPAS and Monte Carlo simulations were validated with film measurements. Two different spot configurations were investigated for treatment planning and characterized based on lateral penumbra, central axis (CAX) dose and relative efficiency. Alignment and leakage were investigated through experimental film measurements. Range was verified using a multi-layer ionization chamber. Reference dose measurements were made with a PinPoint 3D ion chamber. Neutron dose was evaluated through Monte Carlo simulations. RESULTS: Aperture alignment with radiation isocenter was determined to be within 0.31 mm at a gantry angle of 0°. A single-spot configuration with a 10 mm diameter aperture yielded film-measured lateral penumbras of 1 mm to 1.25 mm, depending on depth in the spread-out Bragg peak. TOPAS simulations found that a single spot configuration results in a flat dose distribution for a 10 mm diameter aperture and provides a CAX dose of less than 106% for apertures less than 14 mm in diameter. For larger targets, adding four corner spots to fill in the dose distribution is beneficial. Trade-offs between lateral penumbra, CAX dose and relative efficiency were characterized for different spot configurations and can be used for future clinical decision-making. The aperture was experimentally determined to not affect proton beam range, and no concerning leakage radiation or neutron dose was identified. Reference dose measurements with a PinPoint ion chamber were within 2.1% of Monte Carlo calculated doses. CONCLUSION: The aperture system developed in this work provides a method of treating ocular sites on a gantry-based spot scanning proton system. Additional work to develop compatible gaze tracking and gating infrastructure is ongoing.

Reduce Patient Treatment wait time in a Proton Beam Facility - A Gatekeeper Approach.

Patient wait time can negatively impact treatment quality in a proton therapy center, where multiple treatment rooms share one proton beam. Wait time increases patient discomfort that can lead to patient motion, dissatisfaction, and longer treatment delay. This study was to develop a patient call-back model that reduced patient wait while efficiently utilizing the proton beam. A "Gatekeeper" logic allowing therapists to adjust the time of a patient's call-back to the treatment room was developed. It uses a two-pronged approach to minimize overlap of long treatment and the possibility of excessive wait in the queue to receive the proton beam. The goal was to reduce the maximum wait time to less than eight minutes per field for a four-room facility. The effectiveness of this logic was evaluated through simulation, and five scenarios were compared. Four scenarios implementing various levels of gatekeeper logic were compared with the original scenario without the logic. The best performing model provided a reduction of the maximum field wait by 26% and met the predefined goal. Adjusting call-back extended the treatment day length by an average of 6 min and a maximum of 12 min in total. The use of this gatekeeper logic significantly reduces patient field wait with minimal impact on treatment day length for a four-room proton facility. A sample interface that adopts this logic for therapists to make informed decision on patient call-back time is demonstrated.

Ultra-low-dose (boom-boom) radiotherapy for management of recurrent ocular post-transplant lymphoproliferative disorder.

PURPOSE: To report a case of recurrent iris post-transplant lymphoproliferative disorder (PTLD) treated with ultra-low-dose (boom-boom) radiotherapy (RT). OBSERVATIONS: A 12-year-old Caucasian male with a history of bilateral, recurrent iris PTLD of the extranodal marginal zone lymphoma (MALT) type presented with persistent bilateral anterior chamber cellular infiltration, which was incompletely controlled on topical corticosteroids, and with elevated intraocular pressure (IOP) in the right eye secondary to steroid response. The patient was diagnosed with PTLD recurrence and was successfully treated with ultra-low-dose RT to both eyes in 2 fractions of 2 Gy. At 15 month follow-up the patient maintained complete disease control with normal IOP off all topical ophthalmic medications. CONCLUSIONS AND IMPORTANCE: Ultra-low-dose RT for ocular PTLD of the MALT subtype represents a novel therapeutic approach that may provide a durable treatment response and could be considered as either primary or adjuvant therapy for this rare condition.

Catheter-Free Arrhythmia Ablation Using Scanned Proton Beams: Electrophysiologic Outcomes, Biophysics, and Characterization of Lesion Formation in a Porcine Model.

BACKGROUND: Proton beam therapy offers radiophysical properties that are appealing for noninvasive arrhythmia elimination. This study was conducted to use scanned proton beams for ablation of cardiac tissue, investigate electrophysiological outcomes, and characterize the process of lesion formation in a porcine model using particle therapy. METHODS: Twenty-five animals received scanned proton beam irradiation. ECG-gated computed tomography scans were acquired at end-expiration breath hold. Structures (atrioventricular junction or left ventricular myocardium) and organs at risk were contoured. Doses of 30, 40, and 55 Gy were delivered during expiration to the atrioventricular junction (n=5) and left ventricular myocardium (n=20) of intact animals. RESULTS: In this study, procedural success was tracked by pacemaker interrogation in the atrioventricular junction group, time-course magnetic resonance imaging in the left ventricular group, and correlation of lesion outcomes displayed in gross and microscopic pathology. Protein extraction (active caspase-3) was performed to investigate tissue apoptosis. Doses of 40 and 55 Gy caused slowing and interruption of cardiac impulse propagation at the atrioventricular junction. In 40 left ventricular irradiated targets, all lesions were identified on magnetic resonance after 12 weeks, being consistent with outcomes from gross pathology. In the majority of cases, lesion size plateaued between 12 and 16 weeks. Active caspase-3 was seen in lesions 12 and 16 weeks after irradiation but not after 20 weeks. CONCLUSIONS: Scanned proton beams can be used as a tool for catheter-free ablation, and time-course of tissue apoptosis was consistent with lesion maturation.

Catheter-free ablation of infarct scar through proton beam therapy: Tissue effects in a porcine model.

BACKGROUND: Scar-related ventricular arrhythmias are common after myocardial infarction. Catheter ablation can improve prognosis, but the procedure is invasive and results are not always satisfactory. Noninvasive, catheter-free ablation using ionizing radiation has recently gained interest among electrophysiologists, but the tissue effects and physiological outcome have not been fully characterized. OBJECTIVE: The purpose of this study was to investigate the structural effects of cardiac scanned pencil beam proton therapy on infarct scar, the time course of imaging biomarkers, arrhythmias, and cardiac function in a porcine model. METHODS: Fourteen infarcted swine underwent proton beam treatment of the scar (40 or 30 Gy) and were followed for up to 30 weeks. Magnetic resonance imaging was performed every 4 weeks. RESULTS: Treated scar areas showed a significantly lower fraction of surviving myocytes at 30 weeks compared to untreated scar (30.1% ± 18.5% and 59.9% ± 10.1% in treated and untreated infarct, respectively), indicating scar homogenization. Four animals died suddenly during follow-up, all from documented monomorphic ventricular tachycardia. Cardiac function remained stable over the course of the study. Distinct imaging morphologies corresponded to certain tissue dose ranges and time points. CONCLUSION: Radioablation of cardiac infarct scar leads to significant homogenization of the scar, replicating the histologic effects of radiofrequency ablation. These changes correspond to distinct imaging morphologies on delayed contrast-enhanced cardiac magnetic resonance imaging, enabling noninvasive confirmation of tissue ablation effects The present study is the first to thoroughly investigate the structural effects of cardiac proton beam therapy in infarcted myocardium.

Technical Delivery Parameters of 2000 Proton Treatment Courses.

PURPOSE: To summarize the technical delivery parameters of proton plans delivered at the Mayo Clinic in Rochester, Minnesota. MATERIALS AND METHODS: The database of treated patient proton plans was queried to extract field parameters such as gantry angle, patient support angle, minimum and maximum water-equivalent depth (WED) treated, number of layers, field size, patient orientation, and monitor units. The plans were analyzed in aggregate, by disease site, and by fractionation. RESULTS: There were 2963 proton plans for 2023 distinct treatment sites delivered between June 2015 and September 2018. The mean number of fields per plan was 2.8. The mean number of energy layers per field was 51.9. The mean monitor unit per field was 117.4. The median maximum field dimension was 12.4 cm; 95% of the fields had a maximum dimension < 28.7 cm, and the maximum field dimension was 39.8 cm. The median maximum field WED was 16.4 cm; 95% of the fields reached a maximum WED of ≤ 26.4 cm, and the maximum field WED was 32.4 cm. CONCLUSION: A large variety of disease sites were treated using the maximum field size (40 cm) and WED (32.4 cm) capabilities of our half-gantry system.

Electrocardiogram-Gated Computed Tomography with Coronary Angiography for Cardiac Substructure Delineation and Sparing in Patients with Mediastinal Lymphomas Treated with Radiation Therapy.

PURPOSE: (1) Demonstrate feasibility of electrocardiogram-gated computed tomography with coronary angiography (E-CTA) in treatment planning for mediastinal lymphoma and (2) assess whether inclusion of cardiac substructures in the radiation plan optimization (CSS optimization) results in increased cardiac substructure sparing. METHODS AND MATERIALS: Patients with mediastinal lymphomas requiring radiation therapy were prospectively enrolled in an observational study. Patients completed a treatment planning computed tomography scan and E-CTA in the deep inspiration breath hold position. Avoidance structures (eg, coronary arteries and cardiac valves) were created in systole and diastole and then merged into a single planning organ-at-risk volume based on a cardiac substructure contouring atlas. In the photon cohort, 2 volumetric modulated arc therapy plans were created per patient with and without CSS optimization. Dosimetric endpoints were compared. RESULTS: In the photon cohort, 7 patients were enrolled. For all 7 patients, the treating physician elected to use the CSS optimization plan. At the individual level, 2 patients had reductions of 10.8% and 16.2% of the right coronary artery receiving at least 15 Gy, and 1 had a reduction of 9.6% of the left anterior descending artery receiving 30 Gy. No other differences for coronary arteries were detected between 15 and 30 Gy. Conversely, 5 of 7 patients had >10% reductions in dose between 15 to 30 Gy to at least 1 cardiac valve. The greatest reduction was 22.8% of the aortic valve receiving at least 30 Gy for 1 patient. At the cohort level, the maximum, mean, and 5-Gy increment analyses were nominally similar between planning techniques for all cardiac substructures and the lungs. CONCLUSIONS: Cardiac substructure delineation using E-CTA was feasible, and inclusion in optimization led to modest improvements in sparing of radiosensitive cardiac substructures for some patients.

Left ventricular function after noninvasive cardiac ablation using proton beam therapy in a porcine model.

BACKGROUND: Noninvasive cardiac ablation of ventricular tachycardia (VT) using radiotherapy has recently gained interest among electrophysiologists. The effects of left ventricular (LV) ablative radiation treatment on global LV function and volumes are unknown. OBJECTIVE: The purpose of this study was to investigate the effects of noninvasive ablation on LV function over time. METHODS: Twenty domestic swine underwent proton beam treatment of LV sites in a dose-finding design and were followed for up to 40 weeks by cardiac magnetic resonance imaging at 4-week intervals. Doses investigated were either 40 Gy at 1 site (n = 8) or 30 Gy at 2 sites (n = 4) in the low-dose group and 40 Gy at 3 sites (n = 8) in the high-dose group. RESULTS: LV mean dose (13.2 ± 1.8 Gy vs 4.6 ± 1.8 Gy) and the volume receiving at least 20 Gy (V20Gy) (24.7% ± 4.8% vs 6.4% ± 3.0%) differed significantly between groups. Dose-dependent effects on left ventricular ejection fraction (LVEF) and LV end-diastolic volume became manifest about 3 months after treatment. LVEF decline was correlated to mean dose (correlation coefficient ρ = -0.69; P = .008) and V20Gy (ρ = -0.66; P = .01), as was LV dilation (ρ = 0.72; P = .005; and ρ = 0.75, P = .003 respectively). CONCLUSION: Possible adverse effects on LV function, seen about 3 months after treatment, are dose dependent. Therefore, precise target definition and focused energy delivery are paramount in catheter-free ablation.

Clinical implementation of respiratory-gated spot-scanning proton therapy: An efficiency analysis of active motion management.

PURPOSE: The aim of this work is to describe the clinical implementation of respiratory-gated spot-scanning proton therapy (SSPT) for the treatment of thoracic and abdominal moving targets. The experience of our institution is summarized, from initial acceptance and commissioning tests to the development of standard clinical operating procedures for simulation, motion assessment, motion mitigation, treatment planning, and gated SSPT treatment delivery. MATERIALS AND METHODS: A custom respiratory gating interface incorporating the Real-Time Position Management System (RPM, Varian Medical Systems, Inc., Palo Alto, CA, USA) was developed in-house for our synchrotron-based delivery system. To assess gating performance, a motion phantom and radiochromic films were used to compare gated vs nongated delivery. Site-specific treatment planning protocols and conservative motion cutoffs were developed, allowing for free-breathing (FB), breath-holding (BH), or phase-gating (Ph-G). Room usage efficiency of BH and Ph-G treatments was retrospectively evaluated using beam delivery data retrieved from our record and verify system and DICOM files from patient-specific quality assurance (QA) procedures. RESULTS: More than 70 patients were treated using active motion management between the launch of our motion mitigation program in October 2015 and the end date of data collection of this study in January 2018. During acceptance procedures, we found that overall system latency is clinically-suitable for Ph-G. Regarding room usage efficiency, the average number of energy layers delivered per minute was <10 for Ph-G, 10-15 for BH and ≥15 for FB, making Ph-G the slowest treatment modality. When comparing to continuous delivery measured during pretreatment QA procedures, the median values of BH treatment time were extended from 6.6 to 9.3 min (+48%). Ph-G treatments were extended from 7.3 to 13.0 min (+82%). CONCLUSIONS: Active motion management has been crucial to the overall success of our SSPT program. Nevertheless, our conservative approach has come with an efficiency cost that is more noticeable in Ph-G treatments and should be considered in decision-making.

Theoretical and experimental analysis of photon counting detector CT for proton stopping power prediction.

PURPOSE: Photon counting detectors (PCDs) are being introduced in advanced x-ray computed tomography (CT) scanners. From a single PCD-CT acquisition, multiple images can be reconstructed, each based on only a part of the original x-ray spectrum. In this study, we investigated whether PCD-CT can be used to estimate stopping power ratios (SPRs) for proton therapy treatment planning, both by comparing to other SPR methods proposed for single energy CT (SECT) and dual energy CT (DECT) as well as to experimental measurements. METHODS: A previously developed DECT-based SPR estimation method was adapted to PCD-CT data, by adjusting the estimation equations to allow for more energy spectra. The method was calibrated directly on noisy data to increase the robustness toward image noise. The new PCD SPR estimation method was tested in theoretical calculations as well as in an experimental setup, using both four and two energy bin PCD-CT images, and through comparison to two other SPR methods proposed for SECT and DECT. These two methods were also evaluated on PCD-CT images, full spectrum (one-bin) or two-bin images, respectively. In a theoretical framework, we evaluated the effect of patient-specific tissue variations (density and elemental composition) and image noise on the SPR accuracy; the latter effect was assessed by applying three different noise levels (low, medium, and high noise). SPR estimates derived using real PCD-CT images were compared to experimentally measured SPRs in nine organic tissue samples, including fat, muscle, and bone tissues. RESULTS: For the theoretical calculations, the root-mean-square error (RMSE) of the SPR estimation was 0.1% for the new PCD method using both two and four energy bins, compared to 0.2% and 0.7% for the DECT- and SECT-based method, respectively. The PCD method was found to be very robust toward CT image noise, with a RMSE of 2.7% when high noise was added to the CT numbers. Introducing tissue variations, the RMSE only increased to 0.5%; even when adding high image noise to the changed tissues, the RMSE stayed within 3.1%. In the experimental measurements, the RMSE over the nine tissue samples was 0.8% when using two energy bins, and 1.0% for the four-bin images. CONCLUSIONS: In all tested cases, the new PCD method produced similar or better results than the SECT- and DECT-based methods, showing an overall improvement of the SPR accuracy. This study thus demonstrated that PCD-CT scans will be a qualified candidate for SPR estimations.

Inter-centre variability of CT-based stopping-power prediction in particle therapy: Survey-based evaluation.

BACKGROUND AND PURPOSE: Stopping-power ratios (SPRs) are used in particle therapy to calculate particle range in patients. The heuristic CT-to-SPR conversion (Hounsfield Look-Up-Table, HLUT), needed for treatment planning, depends on CT-scan and reconstruction parameters as well as the specific HLUT definition. To assess inter-centre differences in these parameters, we performed a survey-based qualitative evaluation, as a first step towards better standardisation of CT-based SPR derivation. MATERIALS AND METHODS: A questionnaire was sent to twelve particle therapy centres (ten from Europe and two from USA). It asked for details on CT scanners, image acquisition and reconstruction, definition of the HLUT, body-region specific HLUT selection, investigations of beam-hardening and experimental validations of the HLUT. Technological improvements were rated regarding their potential to improve SPR accuracy. RESULTS: Scan parameters and HLUT definition varied widely. Either the stoichiometric method (eight centres) or a tissue-substitute-only HLUT definition (three centres) was used. One centre combined both methods. The number of HLUT line segments varied widely between two and eleven. Nine centres had investigated influence of beam-hardening, often including patient-size dependence. Ten centres had validated their HLUT experimentally, with very different validation schemes. Most centres deemed dual-energy CT promising for improving SPR accuracy. CONCLUSIONS: Large inter-centre variability was found in implementation of CT scans, image reconstruction and especially in specification of the CT-to-SPR conversion. A future standardisation would reduce time-intensive institution-specific efforts and variations in treatment quality. Due to the interdependency of multiple parameters, no conclusion can be drawn on the derived SPR accuracy and its inter-centre variability.

Validation of proton stopping power ratio estimation based on dual energy CT using fresh tissue samples.

Dual energy CT (DECT) has been shown, in theoretical and phantom studies, to improve the stopping power ratio (SPR) determination used for proton treatment planning compared to the use of single energy CT (SECT). However, it has not been shown that this also extends to organic tissues. The purpose of this study was therefore to investigate the accuracy of SPR estimation for fresh pork and beef tissue samples used as surrogates of human tissues. The reference SPRs for fourteen tissue samples, which included fat, muscle and femur bone, were measured using proton pencil beams. The tissue samples were subsequently CT scanned using four different scanners with different dual energy acquisition modes, giving in total six DECT-based SPR estimations for each sample. The SPR was estimated using a proprietary algorithm (syngo.via DE Rho/Z Maps, Siemens Healthcare, Forchheim, Germany) for extracting the electron density and the effective atomic number. SECT images were also acquired and SECT-based SPR estimations were performed using a clinical Hounsfield look-up table. The mean and standard deviation of the SPR over large volume-of-interests were calculated. For the six different DECT acquisition methods, the root-mean-square errors (RMSEs) for the SPR estimates over all tissue samples were between 0.9% and 1.5%. For the SECT-based SPR estimation the RMSE was 2.8%. For one DECT acquisition method, a positive bias was seen in the SPR estimates, having a mean error of 1.3%. The largest errors were found in the very dense cortical bone from a beef femur. This study confirms the advantages of DECT-based SPR estimation although good results were also obtained using SECT for most tissues.

External Arrhythmia Ablation Using Photon Beams: Ablation of the Atrioventricular Junction in an Intact Animal Model.

BACKGROUND: This study sought to investigate external photon beam radiation for catheter-free ablation of the atrioventricular junction in intact pigs. METHODS AND RESULTS: Ten pigs were randomized to either sham irradiation or irradiation of the atrioventricular junction (55, 50, 40, and 25 Gy). Animals underwent baseline electrophysiological evaluation, cardiac gated multi-row computed tomographic imaging for beam delivery planning, and intensity-modulated radiation therapy. Doses to the coronary arteries were optimized. Invasive follow-up was conducted ≤4 months after the irradiation. A mean volume of 2.5±0.5 mL was irradiated with target dose. The mean follow-up length after irradiation was 124.8±30.8 days. Out of 7 irradiated animals, complete atrioventricular block was achieved in 6 animals of all 4 dose groups (86%). Using the same targeting margins, ablation lesion size notably increased with the delivered dose because of volumetric effects of isodose lines around the target volume. The mean macroscopically calculated atrial lesion volume for all 4 dose groups was 3.8±1.1 mL, lesions extended anteriorly into the interventricular septum. No short-term side effects were observed. No damage was observed in the tissues of the esophagus, phrenic nerves, or trachea. However, histology revealed in-field beam effects outside of the target volume. CONCLUSIONS: Single-fraction doses as low as 25 Gy caused a lesion with interruption of cardiac impulse propagation using this respective target volume. With doses of ≤55 Gy, maximal point-doses to coronary arteries could be kept <7Gy, but target conformity of lesions was not fully achieved using this approach.