SABR Patients Research Articles

Cardiac Substructure Dose and Survival in Stereotactic Radiotherapy for Lung Cancer: Results of the Multi-Centre SSBROC Trial

Background and PurposeStereotactic ablative body radiotherapy (SABR) is increasingly used for early-stage lung cancer, however the impact of dose to the heart and cardiac substructures remains largely unknown. The study investigated doses received by cardiac substructures in SABR patients and impact on survival. Materials and MethodsSSBROC is an Australian multi-centre phase II prospective study of SABR for stage I non-small cell lung cancer. Patients were treated between 2013 and 2019 across 9 centres. In this secondary analysis of the dataset, a previously published and locally developed open-source hybrid deep learning cardiac substructure automatic segmentation tool was deployed on the planning CTs of 117 trial patients. Physical doses to 18 cardiac structures and EQD2 converted doses (α/β = 3) were calculated. Endpoints evaluated include pericardial effusion and overall survival. Associations between cardiac doses and survival were analysed with the Kaplan–Meier method and Cox proportional hazards models. ResultsCardiac structures that received the highest physical mean doses were superior vena cava (22.5 Gy) and sinoatrial node (18.3 Gy). The highest physical maximum dose was received by the heart (51.7 Gy) and right atrium (45.3 Gy). Three patients developed grade 2, and one grade 3 pericardial effusion. The cohort receiving higher than median mean heart dose (MHD) had poorer survival compared to those who received below median MHD (p = 0.00004). On multivariable Cox analysis, male gender and maximum dose to ascending aorta were significant for worse survival. ConclusionsPatients treated with lung SABR may receive high doses to cardiac substructures. Dichotomising the patients according to median mean heart dose showed a clear difference in survival. On multivariable analyses gender and dose to ascending aorta were significant for survival, however cardiac substructure dosimetry and outcomes should be further explored in larger studies.

The first clinical implementation of real-time 6 degree-of-freedom image-guided radiotherapy for liver SABR patients

PurposeMultiple survey results have identified a demand for improved motion management for liver cancer IGRT. Until now, real-time IGRT for liver has been the domain of dedicated and expensive cancer radiotherapy systems. The purpose of this study was to clinically implement and characterise the performance of a novel real-time 6 degree-of-freedom (DoF) IGRT system, Kilovoltage Intrafraction Monitoring (KIM) for liver SABR patients. Methods/MaterialsThe KIM technology segmented gold fiducial markers in intra-fraction x-ray images as a surrogate for the liver tumour and converted the 2D segmented marker positions into a real-time 6DoF tumour position. Fifteen liver SABR patients were recruited and treated with KIM combined with external surrogate guidance at three radiotherapy centres in the TROG 17.03 LARK multi-institutional prospective clinical trial. Patients were either treated in breath-hold or in free breathing using the gating method. The KIM localisation accuracy and dosimetric accuracy achieved with KIM + external surrogate were measured and the results were compared to those with the estimated external surrogate alone. ResultsThe KIM localisation accuracy was 0.2±0.9 mm (left–right), 0.3±0.6 mm (superior-inferior) and 1.2±0.8 mm (anterior-posterior) for translations and −0.1◦±0.8◦ (left–right), 0.6◦±1.2◦ (superior-inferior) and 0.1◦±0.9◦ (anterior-posterior) for rotations. The cumulative dose to the GTV with KIM + external surrogate was always within 5% of the plan. In 2 out of 15 patients, >5% dose error would have occurred to the GTV and an organ-at-risk with external surrogate alone. ConclusionsThis work demonstrates that real-time 6DoF IGRT for liver can be implemented on standard radiotherapy systems to improve treatment accuracy and safety. The observations made during the treatments highlight the potential false assurance of using traditional external surrogates to assess tumour motion in patients and the need for ongoing improvement of IGRT technologies.

85 MR Distortion Artifact Correction for Spine SABR Patients with Pedicle Screws

85 MR Distortion Artifact Correction for Spine SABR Patients with Pedicle Screws

PO-2311 Factors affecting lung cancer SABR patients participating in a clinical trial

PO-2311 Factors affecting lung cancer SABR patients participating in a clinical trial

PO-1612 Inter and intrafraction motion in SABR patients treated for bone oligometastasis

PO-1612 Inter and intrafraction motion in SABR patients treated for bone oligometastasis

Normal tissue complication probabilities of lung SABR patients from a UK centre and its implication on personalised radiotherapy

Abstract Introduction: This work reports on the normal tissue complication probabilities (NTCP) from a UK cohort of previously treated peripheral lung SABR patients (n = 198) supplementing our previous publication on tumour control probabilities (TCP). Each patient was recalculated for alternative schedules. Materials and Methods: NTCP for 3 (54 Gy), 5 (55 and 60 Gy) and 8 (50 Gy) fraction (#) schemes were calculated with the Lyman Kutcher Burman (LKB) model in the software platform ‘Biosuite’ (Version 12·01) for lung and chest wall. Patients treated with 5 # or 8 # were then recomputed for alternative fractionations and doses (3 # and 5 #, for both 55 Gy and 60 Gy). Results: The mean lung NTCP (NTCPLUNG, for the outcome of radiation pneumonitis) was 2·8% (range 0·6 – 10·6). The mean chest wall NTCP (NTCPCW, for the outcome of rib fracture) was 1·4% (range 0·0–55·9). There were no statistically significant differences observed between male and female, tumour status or fractionation groups except for the NTCPLUNG between 5 # and 3 #. When recalculating NTCP and TCP individually, for 8 # patients, no differences were observed between mean TCP, NTCPLUNG or NTCPCW compared with 3 # or 5 # indicating that fractionation reduction is possible. Parity was observed between the 60 Gy group when recalculated for 55 Gy. For the 60 Gy in 5 # group, the NTCPCW increased significantly when recalculated for 3 #. Conclusion: NTCPs achievable with current UK planning techniques have been presented indicating SABR Consortium compliant centres are likely to have low complication population risks (< 3 %). 5 # schedules could be justified for 8 # patients, thereby reducing the number of treatment visits. Where there is a large overlap of PTV and chest wall, this indicates an NTCP/TCP calculation is required to investigate if fractionation reduction is individually appropriate.

Accounting for Serial and Parallel Functionality of the Lung May Minimize Post-Radiotherapy Loss of Respiratory Function in Lung Cancer Patients

Functional lung avoidance (FLA) radiotherapy (RT) aims to minimize post-RT respiratory function loss. Current FLA approaches spatially map lung function and preferentially avoid dose to high-functioning lung (HFL). However, current FLA methods do not adequately consider the conducting architecture responsible for gas exchange (airways/vasculature). Previously, we showed that excessive toxicity to airways results in airway collapse, leading to functional loss within downstream lung sub-volumes. We proposed the functionally weighted airway sparing (FWAS) method to spare airways connected to HFL. Here, we propose a novel method that combines both FLA and FWAS to minimize post-RT functional loss.We estimated post-RT decline in regional airflow and ventilation as a combined measure of functional loss. High-res breath-hold CTs (BHCTs) and simulation 4DCTs from seven lung SAbR patients were analyzed retrospectively. Three functional avoidance IMRT strategies were compared to conventional IMRT planning: (i) FLA - a 4DCT-based ventilation map was used to delineate the HFL, defined as the functional regions of the lung above the mean intensity in the map. FLA plans were optimized such that mean dose and V13.5 in HFL were as low as reasonably achievable while meeting clinical dose constraints. (ii) FWAS - a virtual bronchoscopy software was used to autosegment 12 airway generations on the BHCT. Each terminal airway was connected to a lung sub-volume and assigned a functional weight based on the relative ventilation contribution of the sub-volume based on the ventilation map. For non-terminal airways, the weight was calculated in terms of the cumulative ventilation supported by corresponding downstream airways. These weights were used in the optimization along with airway dose constraints, estimated as a function of airway diameter and 5% probability of collapse. (iii) FLA+FWAS - the two strategies were combined to reduce the dose to both HFL and corresponding upstream airways.Overall, FLA+FWAS provided the highest ventilation preservations with an average improvement, compared to conventional plans, of 12%, followed by FWAS (8%), and FLA (3%). In all cases, clinical objectives for planning target volume and organs at risk were met. Generalized estimated equations with identity link and normal distribution and exchangeable correlation matrix showed that all these improvements were statistically significant (P < 0.04).These initial results suggest that it is crucial to consider the serial as well as the parallel nature of the lung, being airway sparing likely more critical to minimize post-RT functional loss.

PD-0414: Correlating dosimetry with the outcomes – A analysis of UK’s largest cohort of lung SABR patients.

PD-0414: Correlating dosimetry with the outcomes – A analysis of UK’s largest cohort of lung SABR patients.

Tumour control probability of a UK cohort of lung SABR patients

AbstractAims:The aim of this work is to report on the tumour control probability (TCP) of a UK cohort of lung stereotactic ablative radiotherapy patients (n = 198) for a range of dose and fractionations common in the UK.Materials and methods:TCP values for 3 (54 Gy), 5 (55 and 60 Gy) and 8 (50 Gy) fraction (#) schemes were calculated with the linear-quadratic Marsden TCP model using the Biosuite software.Results:TCP values of 100% were computed for the 3 # and for 5 # (α/β = 10 Gy) cohorts; reduced to 99% (range 97–100) for the 5 # cohort only when an α/β of 20 Gy was used. The average TCP value for the 50 Gy in 8 # regime was 97% (range 92–99, α/β = 10 Gy) and 64% (range 48–79, α/β = 20 Gy). Statistical significant differences were observed between the α/β of 10 Gy versus 20 Gy groups and between all data grouped by fraction.Conclusion:TCPs achievable with current planning techniques in the UK have been presented. The ultra-conservative 50 Gy in 8 # scheme returns a significantly lower TCP than the other regimes.

Is multileaf collimator tracking or gating a better intrafraction motion adaptation strategy? An analysis of the TROG 15.01 stereotactic prostate ablative radiotherapy with KIM (SPARK) trial

PurposeStereotactic Ablative Radiotherapy (SABR) has recently emerged as a favourable treatment option for prostate cancer patients. With higher doses delivered over fewer fractions, motion adaptation is a requirement for accurate delivery of SABR. This study compared the efficacy of multileaf collimator (MLC) tracking vs. gating as a real-time motion adaptation strategy for prostate SABR patients enrolled in a clinical trial. MethodsForty-four prostate cancer patients treated over five fractions in the TROG 15.01 SPARK trial were analysed in this study. Forty-nine fractions were treated using MLC tracking and 166 fractions were treated using beam gating and couch shifts. A time-resolved motion-encoded dose reconstruction method was used to evaluate the dose delivered using each motion adaptation strategy and compared to an estimation of what would have been delivered with no motion adaptation strategy implemented. ResultsMLC tracking and gating both delivered doses closer to the plan compared to when no motion adaptation strategy was used. Differences between MLC tracking and gating were small with differences in the mean discrepancy from the plan of −0.3% (CTV D98%), 1.4% (CTV D2%), 0.4% (PTV D95%), 0.2% (rectum V30Gy) and 0.0% (bladder V30Gy). On average, 0.5 couch shifts were required per gated fractions with a mean interruption duration of 1.8 ± 2.6 min per fraction treated using gating. ConclusionBoth MLC tracking and gating were effective strategies at improving the accuracy of the dose delivered to the target and organs at risk. While dosimetric performance was comparable, gating resulted in interruptions to treatment.Clinical trial registration number: NCT02397317.

Functionally weighted airway sparing (FWAS): a functional avoidance method for preserving post-treatment ventilation in lung radiotherapy

Recent changes to the guidelines for screening and early diagnosis of lung cancer have increased the interest in preserving post-radiotherapy lung function. Current investigational approaches are based on spatially mapping functional regions and generating regional avoidance plans that preferentially spare highly ventilated/perfused lung. A potentially critical, yet overlooked, aspect of functional avoidance is radiation injury to peripheral airways, which serve as gas conduits to and from functional lung regions. Dose redistribution based solely on regional function may cause irreparable damage to the ‘supply chain’. To address this deficiency, we propose the functionally weighted airway sparing (FWAS) method. FWAS (i) maps the bronchial pathways to each functional sub-lobar lung volume; (ii) assigns a weighting factor to each airway based on the relative contribution of the sub-volume to overall lung function; and (iii) creates a treatment plan that aims to preserve these functional pathways. To evaluate it, we used four cases from a retrospective cohort of SAbR patients treated for lung cancer. Each patient’s airways were auto-segmented from a diagnostic-quality breath-hold CT using a research virtual bronchoscopy software. A ventilation map was generated from the planning 4DCT to map regional lung function. For each terminal airway, as resolved by the segmentation software, the total ventilation within the sub-lobar volume supported by that airway was estimated and used as a function-based weighting factor. Upstream airways were weighted based on the cumulative volumetric ventilation supported by corresponding downstream airways. Using a previously developed model for airway radiosensitivity, dose constraints were determined for each airway corresponding to a <5% probability of airway collapse. Airway dose constraints, ventilation scores, and clinical dose constraints were input to a swarm optimization–based inverse planning engine to create a 3D conformal SAbR plan (CRT). The FWAS plans were compared to the patients’ prescribed CRT clinical plans and the inverse-optimized clinical plans. Depending on the size and location of the tumour, the FWAS plan showed superior preservation of ventilation due to airflow preservation through open pathways (i.e. cumulative ventilation score from the sub-lobar volumes of open pathways). Improvements ranged between 3% and 23%, when comparing to the prescribed clinical plans, and between 3% and 35%, when comparing to the inverse-optimized clinical plans. The three plans satisfied clinical requirements for PTV coverage and OAR dose constraints. These initial results suggest that by sparing pathways to high-functioning lung subregions it is possible to reduce post-SAbR loss of respiratory function.

Normal Tissue Dose Constraints for Multiple Lung Stereotactic Radiotherapy Treatments

Introduction: The role and use of stereotactic radiotherapy (SABR) is evolving rapidly. A key article by Hanna et al. (2017) provides an excellent overview of current evidence and suggestion of sensible dose constraints. Given the topical nature of this discussion we present a short retrospective analysis of treating multiple lung SABR patients at our centre. Method: We retrospectively analysed toxicity, both early (within 3 months of SABR) and late, and normal tissue dose constraints on all patients who had multiple lung lesions treated with SABR (using volumetric modulated arc therapy (VMAT) technique) at our tertiary centre over a 25-month period from April 2016 until May 2018. Results: We have treated 78 lung lesions in 37 patients with a combination of synchronous lung cancer primaries and lung metastases diagnoses. Median follow-up was 9 months. Almost all patients received treatment on the same day for multiple lesions. We report no grade 3 toxicities in any patient nor any unexpected side effects. 5 patients (14.7%) developed grade 2 pneumonitis. In all 5 patients, lung V12.5 was &gt;20% (range 20.8-32.2%), yet only 1 patient exceeded acceptable lung V20 constraints. Regarding long-term toxicity, 66.6% of patients reported no treatment-related effects. Of 9 patients with long-term toxicity, 8 exceeded V12.5 constraint of &lt;15%, indeed of these 5 were &gt;20%. Lung V20 levels were acceptable for the majority of these. Local control of treated lesions at median follow-up in all comers was 86.2%. Discussion: Our findings show that multiple lung SABR is tolerable, safe with minimal long-term toxicity and acceptable early toxicity. Defining normal lung V12.5 of &lt;15% (optimal) and &lt;20% (acceptable) will significantly reduce the risk of pneumonitis and longer-term toxicity, proving itself more predictive than lung V20 levels for toxicity. Additionally, treating multiple lesions concurrently appears to bare no extra risk to patients.

Implementing patient specific arterial phase contrast time delays in radiotherapy planning scans for HCC SABR patients

Implementing patient specific arterial phase contrast time delays in radiotherapy planning scans for HCC SABR patients

The impact of baseline shifts towards the heart after image guidance on survival in lung SABR patients

Background and purposeA recent study of NSCLC patients showed small residual shifts of the high dose region towards/away from the heart after image-guidance were significantly related to overall survival. This study investigates whether the effect is observed in a SABR cohort, who have significantly different baseline outlook and are treated using an imaging protocol matching on the tumour rather than bony-anatomy alone. Materials and methods136 NSCLC patients treated with SABR were studied. The mean baseline shift of the tumour in the direction of the heart over the course of treatment was determined for each patient and used to categorise patients into risk groups. Kaplan-Meier survival curves were plotted and multivariable analysis performed to assess significance of the vector shift to the heart alongside common clinical variables. ResultsThe vector shift to the heart was independent of all tested clinical variables. A significant difference was seen in patient survival, with patients with shifts towards the heart having significantly worse prognosis as compared to patients with shifts away. Multivariable analysis found a hazard ratio of 1.262 per mm (p = 0.013) for the vector shift to the heart, i.e. for every 1 mm shift of the high dose region towards the heart there is a 1.262 higher chance of death. ConclusionsBaseline shifts towards the heart significantly correlate with overall survival in a cohort of NSCLC SABR patients, with increased risk with increasing shifts towards the heart. These results provide further evidence for the use of stricter heart dose planning constraints for thoracic radiotherapy and suggest a heart planning organ at risk volume may be required for SABR treatments to account for baseline shifts.

Clinical impact of removing respiratory motion during liver SABR

BackgroundLiver tumors are subject to motion with respiration, which is typically accounted for by increasing the target volume. The prescription dose is often reduced to keep the mean liver dose under a threshold level to limit the probability of radiation induced liver toxicity. A retrospective planning study was performed to determine the potential clinical gains of removal of respiratory motion from liver SABR treatment volumes, which may be achieved with gating or tumor tracking.MethodsTwenty consecutive liver SABR patients were analysed. The treated PTV included the GTV in all phases of respiration (ITV) with a 5 mm margin. The goal prescription was 50Gy/5# (BED 100 Gy10) but was reduced by 2.5 Gy increments to meet liver dose constraints. Elimination of motion was modelled by contouring the GTV in the expiration phase only, with a 5 mm PTV margin. All patients were replanned using the no-motion PTV and tumor dose was escalated to higher prescription levels where feasible given organ-at-risk constraints. For the cohort of patients with metastatic disease, BED gains were correlated to increases in tumour control probability (TCP). The effect of the gradient of the TCP curve on the magnitude of TCP increase was evaluated by repeating the study for an additional prescription structure, 54Gy/3# (BED 151 Gy10).ResultsCorrelation between PTV size and prescribed dose exists; PTVs encompassing < 10% of the liver could receive the highest prescription level. A monotonically increasing correlation (Spearman’s rho 0.771, p = 0.002) between the degree of PTV size reduction and motion vector magnitude was observed for GTV sizes <100cm3. For 11/13 patients initially planned to a decreased prescription, tumor dose escalation was possible (5.4Gy10–21.4Gy10 BED) using the no-motion PTV. Dose escalation in excess of 20 Gy10 increased the associated TCP by 5% or more. A comparison of TCP gains between the two fractionation schedules showed that, for the same patient geometry, the absolute increase in BED was the overarching factor rather than the gradient of the TCP curve.ConclusionsIn liver SABR treatments unable to be prescribed optimal dose due to exceeding mean liver thresholds, eliminating respiratory motion allowed dose escalation in the majority of patients studied and substantially increased TCP.

OC-0084 Baseline shifts towards the heart after IGRT are linked to overall survival in lung SABR patients

OC-0084 Baseline shifts towards the heart after IGRT are linked to overall survival in lung SABR patients

Both four-dimensional computed tomography and four-dimensional cone beam computed tomography under-predict lung target motion during radiotherapy

Background and purposeTo test the hypothesis that 4DCT and 4DCBCT-measured target motion ranges predict target motion ranges during lung cancer SABR. Materials and methodsTen lung SABR patients were implanted with Calypso beacons. 4DCBCT was reconstructed for 29 fractions (1–4fx/patient) from a 1 min CBCT scan. The beacon centroid motion segmented for all 4DCT and 4DCBCT bins was compared with the real-time imaging and treatment beacon centroid (“target”) motion range (4SDs) for each fraction. We tested the hypotheses that (1) 4DCT and 4CBCT predict treatment motion range and (2) there is no difference between 4DCT and 4DCBCT for predicting treatment motion range. Phase-wise root-mean-square errors (RMSEs) between imaging and treatment motion and reconstructed motion (4DCT, 4DCBCT) were calculated. Relationships between motion ranges in 4DCT and 4DCBCT and imaging and treatment motion ranges were investigated for the superior–inferior (SI), left–right (LR) and anterior–posterior (AP) directions. Baseline drifts and amplitude variability were investigated as potential factors leading to motion misrepresentation. ResultsSI 4DCT, 4DCBCT, imaging and treatment motion ranges were 6.3 ± 3.6 mm, 7.1 ± 4.5 mm, 11.1 ± 7.5 mm and 10.9 ± 6.9 mm, respectively. Similar 4DCT and 4DCBCT under-predictions were observed in the LR and AP directions. Hypothesis (1) was rejected (p < 0.0001). Treatment target motion range was under-predicted in 4DCT by factors of 1.7, 1.9 and 1.7 and in 4DCBCT by factors of 1.5, 1.6 and 1.6 in the SI, LR, and AP directions, respectively. RMSEs were generally lower for end-exhale than inhale. 4DCBCT showed higher correlations with the imaging and treatment target motion than 4DCT and testing hypothesis (2) a statistically significant difference between 4DCT and 4DCBCT was shown in the SI direction (p = 0.03). ConclusionFor lung SABR patients both 4DCT and 4DCBCT significantly under-predict treatment target motion ranges.

An augmented correlation framework for the estimation of tumour translational and rotational motion during external beam radiotherapy treatments using intermittent monoscopic x-ray imaging and an external respiratory signal

Increasing evidence shows that intrafraction tumour motion monitoring must include both six degrees of freedom (6DoF): 3D translations and 3D rotations. Existing real-time algorithms for 6DoF target motion estimation require continuous intrafraction fluoroscopic imaging at high frequency, thereby exposing patients to additional high imaging dose. This paper presents the first method capable of 6DoF motion monitoring using intermittent 2D kV imaging and a continuous external respiratory signal.Our approach is to optimise a state-augmented linear correlation model between an external signal and internal 6DoF motion. In standard treatments, the model can be built using information obtained during pre-treatment cone beam CT (CBCT). Real-time 6DoF tumor motion can then be estimated using just the external signal. Intermittent intrafraction kV images are used to update the model parameters, accounting for changes in correlation and baseline shifts.The method was evaluated in silico using data from 6 lung SABR patients, with the internal tumour motion recorded with electromagnetic beacons and the external signal from a bellows belt. Projection images from CBCT (10 Hz) and intermittent kV images were simulated by projecting the 3D Calypso beacon positions onto an imager. IMRT and VMAT treatments were simulated with increasing imaging update intervals: 0.1 s, 1 s, 3 s, 10 s and 30 s.For all the tested clinical scenarios, translational motion estimates with our method had sub-mm accuracy (mean) and precision (standard deviation) while rotational motion estimates were accurate to < and precise to . Motion estimation errors increased as the imaging update interval increased. With the largest imaging update interval (30 s), the errors were mm, mm and mm for translation in the left–right, superior–inferior and anterior–posterior directions, respectively, and , and for rotation around the aforementioned axes for both VMAT and IMRT treatments.In conclusion, we developed and evaluated a novel method for highly accurate real-time 6DoF motion monitoring on a standard linear accelerator without requiring continuous kV imaging. The proposed method achieved sub-mm and sub-degree accuracy on a lung cancer patient dataset.

The first clinical implementation of real-time image-guided adaptive radiotherapy using a standard linear accelerator

PurposeUntil now, real-time image guided adaptive radiation therapy (IGART) has been the domain of dedicated cancer radiotherapy systems. The purpose of this study was to clinically implement and investigate real-time IGART using a standard linear accelerator. Materials/methodsWe developed and implemented two real-time technologies for standard linear accelerators: (1) Kilovoltage Intrafraction Monitoring (KIM) that finds the target and (2) multileaf collimator (MLC) tracking that aligns the radiation beam to the target. Eight prostate SABR patients were treated with this real-time IGART technology. The feasibility, geometric accuracy and the dosimetric fidelity were measured. ResultsThirty-nine out of forty fractions with real-time IGART were successful (95% confidence interval 87–100%). The geometric accuracy of the KIM system was −0.1 ± 0.4, 0.2 ± 0.2 and −0.1 ± 0.6 mm in the LR, SI and AP directions, respectively. The dose reconstruction showed that real-time IGART more closely reproduced the planned dose than that without IGART. For the largest motion fraction, with real-time IGART 100% of the CTV received the prescribed dose; without real-time IGART only 95% of the CTV would have received the prescribed dose. ConclusionThe clinical implementation of real-time image-guided adaptive radiotherapy on a standard linear accelerator using KIM and MLC tracking is feasible. This achievement paves the way for real-time IGART to be a mainstream treatment option.

The First Clinical Implementation of Real-time Adaptive Radiation Therapy Using a Standard Linear Accelerator

Until now, real-time adaptive radiotherapy has been the domain of expensive dedicated cancer radiotherapy systems. The purpose of this study was to clinically implement and investigate real-time adaptive radiotherapy using a standard linear accelerator. The hypothesis is that real-time adaptive radiotherapy using a standard linear accelerator is feasible. We led an international consortium of universities and hospitals that developed two real-time image guidance technologies for standard linear accelerators: (1) Kilovoltage Intrafraction Monitoring (KIM) which finds the target position in real-time during radiotherapy and (2) multileaf collimator (MLC) tracking which aligns the radiation beam to the moving target. Together, KIM and MLC tracking enable real-time adaptive radiotherapy using a standard linear accelerator without any additional hardware. These two technologies have been clinically implemented independently, but have not been used together to treat patients. To test the real-time radiotherapy technology, prostate SABR patients enrolled on the TROG 15.01 SPARK (NCT02397317) clinical trial were treated. The feasibility hypothesis was tested using maximum likelihood estimates assuming a binomial distribution of success. The geometric accuracy of the real-time adaptive technology was measured by kV/MV triangulation. The dosimetric impact was assessed by dose reconstruction. Ten of 10 clinical treatments with real-time adaptation using the KIM and MLC tracking technologies were successful. The hypothesis test yielded a probability estimate of 1.0, with 95% confidence intervals of [0.69, 1.0]. For the fraction with the largest observed motion, up to 9mm, the geometric accuracy of the KIM system was -0.19±0.49, 0.17±0.26 and 0.08±0.64 mm in the LR, SI, and AP directions, respectively. The dose reconstruction results showed that real-time adaptation faithfully reproduced the planned dose in the presence of intrafraction motion. For the largest motion fraction, with real-time adaptation 98% of the CTV received the prescription dose; without real-time adaptation only 94% of the CTV would have received the prescription dose. With real-time tracking the rectal V70% increased from 10% planned to 12% delivered; without real-time adaptation the rectal V70% would have been 23%. The clinical implementation of real-time adaptive radiotherapy using a standard linear accelerator using KIM and MLC tracking is feasible. This milestone paves the way for this technology to be tested more broadly, enabling more accurate radiotherapy on widely available linear accelerators, and ushering in the era of real-time radiotherapy as a mainstream treatment option.