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Background — Several clinical trials indicate that intracoronary radiation is safe and effective for treatment of restenotic coronary arteries. We previously reported 6-month and 3-year clinical and angiographic follow-up demonstrating significant decreases in target lesion revascularization (TLR) and angiographic restenosis after γ radiation of restenotic lesions. The objective of this study was to document the clinical outcome 5 years after treatment of restenotic coronary arteries with catheter-based iridium-192 ( 192 Ir). Methods and Results — A double-blind, randomized trail compared 192 Ir to placebo sources in patients with restenosis after coronary angioplasty. Over a 9-month period, 55 patients were enrolled; 26 were randomized to 192 Ir and 29 to placebo. At 5-year follow-up, TLR was significantly lower in the 192 Ir group (23.1% versus 48.3%; P =0.05). There were 2 TLRs between years 3 and 5 in patients in the 192 Ir group and none in patients in the placebo group. The 5-year event-free survival rate (freedom from death, myocardial infarction, or TLR) was greater in 192 Ir-treated patients (61.5% versus 34.5%; P =0.02). Conclusions — Despite apparent mitigation of efficacy over time, there remains a significant reduction in TLR at 5 years and an improvement in event-free survival in patients treated with intracoronary 192 Ir. The early clinical benefits after intracoronary γ radiation with 192 Ir seem durable at 5-year clinical follow-up.

Although the frequency of restenosis after coronary angioplasty is reduced by stenting, when restenosis develops within a stent, the risk of subsequent restenosis is greater than 50 percent. We report on a multicenter, double-blind, randomized trial of intracoronary radiation therapy for the treatment of in-stent restenosis.Of 252 eligible patients in whom in-stent restenosis had developed, 131 were randomly assigned to receive an indwelling intracoronary ribbon containing a sealed source of iridium-192, and 121 were assigned to receive a similar-appearing nonradioactive ribbon (placebo).The primary end point, a composite of death, myocardial infarction, and the need for repeated revascularization of the target lesion during nine months of follow-up, occurred in 53 patients assigned to placebo (43.8 percent) and 37 patients assigned to iridium-192 (28.2 percent, P=0.02). However, the reduction in the incidence of major adverse cardiac events was determined solely by a diminished need for revascularization of the target lesion, not by reductions in the incidence of death or myocardial infarction. Late thrombosis occurred in 5.3 percent of the iridium-192 group, as compared with 0.8 percent of the placebo group (P=0.07), resulting in more late myocardial infarctions in the iridium-192 group (9.9 percent vs. 4.1 percent, P=0.09). Late thrombosis occurred in irradiated patients only after the discontinuation of oral antiplatelet therapy (with ticlopidine or clopidogrel) and only in patients who had received new stents at the time of radiation treatment.Intracoronary irradiation with iridium-192 resulted in lower rates of clinical and angiographic restenosis, although it was also associated with a higher rate of late thrombosis, resulting in an increased risk of myocardial infarction. If the problem of late thrombosis within the stent can be overcome, intracoronary irradiation with iridium-192 may become a useful approach to the treatment of in-stent restenosis.

The International Commission on Radiation Units and Measurement (IRCU) 50 has clearly defined treatment volumes in radiation therapy in the management of neoplasms. These concepts are applied to the field of endovascular brachytherapy (EVBT) for the prevention of postangioplasty restenosis. The following definitions are proposed: gross target length (GTL) is defined as the narrowed segment of the artery that requires intervention. Clinical target length (CTL) is defined as the intervened or injured length, which could be due to angioplasty, stent strut injury, stent deployment, or debulking procedures. Planning target length (PTL) is the CTL plus a margin to account for heart/catheter movement and uncertainty in target localization. The final treatment length (TL) is the PTL plus the effect of penumbra. The accurate specification of treatment length serves several important purposes. Based on an understanding of the different factors constituting the treatment length, adequate margins can be provided beyond the GTL; this will avoid geographic misses and minimize edge failures. These definitions of target length ensure treatment consistency and provide a standard terminology for communication among practitioners of EVBT, something of critical importance in the conduct of multi-institutional trials in this new and multidisciplinary therapy. Finally, since the efficacy of EVBT is critically dependent on the precision of radiation delivery, these guidelines ensure that the benefits of EVBT seen in prospective randomized trials can be translated into daily clinical practice at the community level.

To identify luminal dimension changes occurring within the stent alone and within the stent + margin segment, we reviewed the quantitative angiographic results obtained from the Scripps Coronary Radiation to Inhibit Proliferation Post Stenting (SCRIPPS) trial, a prospective randomized trial assessing the effect of iridium-192 (Ir-192) on the prevention of stent restenosis. Fifty-five patients were randomly assigned to receive Ir-192 or placebo sources after successful intervention. Procedural and 6-month follow-up cineangiograms were quantitatively reviewed in 52 patients to identify changes within the stent and the stent + margin segment. The percent diameter stenosis was lower within the stent than within the stent + margin segment after the procedure (6 +/- 22% vs 21+/- 15%, p0.0001) and at follow-up (28 +/- 29% vs 42 +/- 21%, p0.0001). As a result, a lower restenosis rate was found within the stent than within the stent + margin (25% vs 37%, p0.0001); isolated stent margin restenosis occurred in 11.5% of lesions. Treatment with Ir-192 reduced restenosis within the stent (8% vs 39%; p = 0.010) and within the stent + margin segment (17% vs 54%; p = 0.010); the reduction in restenosis at the margin only (8.3% vs 14.3%, p = 0.503) was not significant. The lowest relative risk for restenosis resulting from Ir-192 occurred within the stent (0.21; 95% confidence interval [CI] 0.05 to 0.86) compared with the stent + margin segment (0.31; 95% CI 0.12 to 0.81) or the stent margin (0.58; 95% CI 0.12 to 2.91). In the SCRIPPS trial, 32% of restenosis occurred at the stent margins. Treatment with Ir-192 reduced restenosis primarily within the stent rather than the margin. Whether extending the treatment length to fully include the stent margins will further reduce restenosis requires further study.

Recent preclinical and clinical studies indicate that irradiation using ionizing radiation in the dose range of 15 to 30 Gy may reduce the occurrence of restenosis in patients who have undergone an angioplasty. Several delivery systems of intravascular brachytherapy have been developed to deliver radiation doses in this range with minimal normal tissue toxicity. In late 1995 the American Association of Physicists in Medicine (AAPM) formed a task group to investigate these issues and to report the current state of the art of intravascular brachytherapy physics. The report of this task group is presented here.

Background. Recent clinical studies indicate that intravascular brachytherapy (IVB) can reduce the rate of restenosis substantially after angioplasty procedures. However, no clinical guidelines exist for optimal therapy. Methods. The members of the IVB Subcommittee of the American Brachytherapy Society (ABS) identified the areas of consensus and controversies in IVB to issue the ABS perspective on IVB, based on analysis of published reports and the clinical experience of the members in brachytherapy. Results. IVB is still experimental. The long-term efficacy, toxicity, the target tissue, and dose required for IVB are not established. The ABS recommends that IVB procedures must be performed, with careful attention to radiation-related issues, in the context of controlled multidisciplinary clinical trials with the approval of the institutional review board, the Nuclear Regulatory Commission, the Food and Drug Administration, and under an Investigational Device Exemption. The therapeutic radiologist, with a qualified radiation physicist, is responsible for dose prescription and delivery and needs to be present during the IVB procedure as part of this multidisciplinary team. The long-term outcome from these studies should be reviewed critically and published in peer-reviewed journals. The ABS endorsed the dosimetric guidelines of the American Association of Physicists in Medicine Task Group 60 (AAPM TG-60) report. The ABS recommends that dose specification be defined clearly; to allow comparisons between studies, the dose should be prescribed at 2 mm from the source for intracoronary brachytherapy and at an average luminal radius of +2 mm for peripheral vascular brachytherapy. The prescription doses at the above point is generally in the 12–18 Gy range. Comprehensive procedures for quality assurance, radiation protection, and emergencies should be in place before initiating an IVB program. Higher energy beta sources, lower energy gamma sources, dose-volume histograms, and correlation of three-dimensional reconstructions of delivered dose with patterns of failure are areas for further research. Conclusion. The ABS perspective on IVB is presented to assist the interventional team in developing protocols for the use of IVB in the prevention of restenosis. Long-term outcome data with a standardized reporting system are needed to establish the role of brachytherapy in preventing vascular restenosis. Endovascular brachytherapy is a new and evolving modality, and these recommendations are subject to modifications as new data become available.

Background. The debate over which radioisotope, gamma-emitting or beta-emitting, is ideal for vascular brachytherapy, is as old as the field itself. Gamma isotopes such as Ir-192 have been successfully used in large-scale clinical trials. However, radiation protection and safety remains a practical issue when high energy gamma rays are used in interventional suites. Methods. In this report, we have evaluated dosimetric properties of currently available radiation sources, gamma and beta, in terms of their utility in vascular brachytherapy. Published data have been analyzed in comparing the depth doses of these two radiation modalities. Results and Conclusions. Our evaluation shows that significant dosimetric differences do exist between gamma and beta sources. The superiority of one over the other in clinical utility will more likely be determined by the outcome of human trials.

In animal models of coronary restenosis, intracoronary radiotherapy has been shown to reduce the intimal hyperplasia that is a part of restenosis. We studied the safety and efficacy of catheter-based intracoronary gamma radiation plus stenting to reduce coronary restenosis in patients with previous restenosis.Patients with restenosis underwent coronary stenting, as required, and balloon dilation and were then randomly assigned to receive catheter-based irradiation with iridium-192 or placebo. Clinical follow-up was performed, with quantitative coronary angiographic and intravascular ultrasonographic measurements at six months.Fifty-five patients were enrolled; 26 were assigned to the iridium-192 group and 29 to the placebo group. Angiographic studies were performed in 53 patients (96 percent) at a mean (+/-SD) of 6.7+/-2.2 months. The mean minimal luminal diameter at follow-up was larger in the iridium-192 group than in the placebo group (2.43+/-0.78 mm vs. 1.85+/-0.89 mm, P=0.02). Late luminal loss was significantly lower in the iridium-192 group than in the placebo group (0.38+/-1.06 mm vs. 1.03+/-0.97 mm, P=0.03). Angiographically identified restenosis (stenosis of 50 percent or more of the luminal diameter at follow-up) occurred in 17 percent of the patients in the iridium-192 group, as compared with 54 percent of those in the placebo group (P= 0.01). There were no apparent complications of the treatment.In this preliminary, short-term study of patients with previous coronary restenosis, coronary stenting followed by catheter-based intracoronary radiotherapy substantially reduced the rate of subsequent restenosis.

The objective of this double-blind, randomized study was to determine the safety and efficacy of intracoronary radiation therapy (ICRT) with a dose of 17 Gray (Gy) compared to the currently recommended dose prescription of 14 Gy for the treatment of in-stent restenosis within bare metal stents.While gamma ICRT for in-stent restenosis has been proven efficacious, the optimal dose is unknown, and radiation failure due to recurrent neointimal hyperplasia remains a significant clinical problem for some patients. A higher radiation dose may improve outcomes, but may potentially increase adverse events.Following coronary intervention, 336 patients with in-stent restenosis were randomly assigned to receive ICRT with either 14 Gy or 17 Gy at 2 mm from an 192-iridium source.At 8-month follow up, fewer patients in the 17 Gy group underwent target lesion revascularization (TLR = 15.2% versus 27.2%; p = 0.01), target vessel revascularization (21.3% versus 33.1%; p = 0.02), or reached the composite endpoint of death, myocardial infarction, thrombosis, or TLR (17.1% versus 28.4%; p = 0.02). There were no differences in late thrombosis or mortality between treatment groups. There was a strong trend toward reduced in-lesion late loss (0.36 +/- 0.63 mm vs. 0.51 +/- 0.64 mm; p = 0.09) and a significantly lower rate of binary restenosis (23.9% versus 38.1%; p = 0.031) in the high dose group.Gamma ICRT with 17 Gy is safe and, compared to 14 Gy, reduces recurrent stenosis and clinical events at 8-month follow up. An increase in the currently recommended gamma radiation dose prescription from 14 Gy to 17 Gy should be strongly considered.

This year is the 100th anniversary of the use of ionizing radiation to treat disease. Since the turn of the century, many benign and malignant diseases have been treated with radiation therapy (RT). Through the decades, however, the application of RT in the treatment of benign diseases has largely disappeared-supplanted by other medical or surgical therapies. RT has most recently been committed to the treatment of malignancies to such a degree that the specialty was renamed radiation oncology. However, a continued. though very limited, role for RT has persisted in the treatment of certain benign hyperplastic disorders. RT significantly inhibits excessive fibroblast activity. Judicious clinical use of low-dose RT is highly effective in inhibiting such benign conditions as conjunctival pterygia. keloid scars. and aggressive fibromatosis. Excessive bone formation by the osteoblast, a derivative of the fibroblast. is successfully inhibited by low-dose RT and thereby inhibits heterotopic ossification. Although such benign hyperplasia disorders are significant clinical entities, the main thrust of RT continues to be the treatment of malignancies. With the coming millennium, a new therapeutic application of RT in benign disease promises to open new vistas previously unimagined. Studies have just begun in using RT to prevent vascular restenosis. RT may provide a powerful tool to solve what has become a vexing problem in vascular disease. In this review, we shall discuss early preclinical evidence that endovascular brachytherapy may inhibit coronary artery restenosis. We shall also introduce and describe the Scripps Clinic’s ongoing trial of endovascular RT in the prevention of coronary artery restenosis. The Scripps Coronary Radiation to Inhibit Proliferation Post Stenting trial (SCRIPPS) is the first attempt to use catheter-based intracoronary ionizing radiation in a prospective double-blinded randomized study. The study’s objective is to determine the initial safety and efficacy in reversing and preventing coronary artery restenosis by adding lowdose RT to today’s optima1 conventional cardiovascular therapy. This review will hopefully also provide a basis to understand the rationale and some of the potential benefits of applying radiation in the treatment of coronary artery restenosis.

Background— Although 192 Ir intracoronary brachytherapy has been demonstrated to dramatically reduce the recurrence of in-stent restenosis, up to 24% of these patients will still require repeat target-vessel revascularization. The short- and long-term outcomes of repeat percutaneous intervention in this population have not been characterized. Methods and Results— Analysis was performed of all patients enrolled in the GAMMA-I and GAMMA-II brachytherapy trials who underwent repeat percutaneous target lesion revascularization (TLR) because of restenosis. Subjects were divided into 2 cohorts: those who had received 192 Ir brachytherapy and those randomized to placebo. Forty-five (17.6%) of a total of 256 patients whose index treatment was intracoronary radiation therapy and 36 (29.8%) of 121 patients whose index treatment was placebo required repeat percutaneous TLR. The mean time to this first TLR was 295±206 days in the irradiated group and 202±167 days in the placebo group ( P =0.03). Acute procedural success occurred in 100% of irradiated patients and 94% of placebo controls ( P =0.19). After the first TLR, a subsequent TLR was required in 15 (33.3%) of 45 brachytherapy patients versus 17 (47.2%) of 36 placebo failure patients ( P =0.26). There was no significant difference in time to second TLR between the 2 groups. Other long-term major adverse event rates in both groups were comparable to those of other contemporary angioplasty/stenting series. Conclusions— In those patients who “fail” 192 Ir intracoronary brachytherapy for in-stent restenosis, treatment with 192 Ir delays the time to first TLR. Additionally, repeat percutaneous intervention in these patients is safe and efficacious in the short term, with acceptable long-term results.

Background. Early studies indicate brachytherapy is a potential clinical treamtent for restenosis after coronary angioplasty procedures. The objective of this study was to evaluate the safety and efficacy of gamma radiation plus stenting in patients' previous restenosis. Methods . In patients with previous coronary restenosis, balloon dilatation and/or coronary stenting was undertaken and then patients were randomly assigned to receive either Iridium-192 or placebo. Quantitative coronary angiographic, intravascular ultrasonographic, and clinical follow-up was obtained. Results . Of the 55 patients enrolled, 26 were treated with Iridium-192 and 29 were in the placebo group. Late luminal loss was significantly lower in the treated group compared to the placebo group (0.38 ± 1.06 mm vs. 1.03 ± 0.97 mm, p = 0.03). Restenosis (stenosis of ≥50% at follow-up) was found in 17% of treated patients compared to 54% of placebo patients ( p = 0.01). The need for target lesion revascularization was observed in 12% of patients in the treated group compared to 45% in the placebo group ( p = 0.01). Conclusions . In this initial trial, at 12 months follow-up, patients with previous restenosis were benefited by catheter-based gamma radiation therapy.

The purpose of this study was to compare the effects of balloon angioplasty versus repeat stenting on the early angiographic outcome in patients with in-stent restenosis. The treatment of in-stent restenosis using balloon angioplasty alone often yields excellent early results, but is associated with a high rate of late recurrence. In the SCRIPPS trial, patients with restenosis were treated either with balloon angioplasty alone or placement of additional stents to optimize angiographic results before randomization and exposure of the restenotic segment to gamma radiation or placebo. In patients undergoing repeat catheter based intervention for the treatment of in-stent restenosis, quantitative coronary angiography was used to compare the results of balloon angioplasty alone versus repeat stenting on early lumen loss. After a mean delay time interval of 71 min, the early loss was 0.35 +/- 0.34 mm in the balloon angioplasty alone group compared to 0.01 +/- 0.34 mm in the repeat-stenting group (P = 0.004). The early loss index in the balloon angioplasty alone group (12.8 +/- 12.9%) was significantly greater than in the repeat stenting group (0.7 +/- 12.1%; P = 0.003). Although balloon angioplasty for in-stent restenosis often provides excellent immediate angiographic results, luminal diameters are significantly reduced in the early time period after balloon dilatation. Repeat stenting nearly abolishes this early luminal loss.

Background —Although several early trials indicate treatment of restenosis with radiation therapy is safe and effective, the long-term impact of this new technology has been questioned. The objective of this report is to document angiographic and clinical outcome 3 years after treatment of restenotic stented coronary arteries with catheter-based 192 Ir. Methods and Results —A double-blind, randomized trial compared 192 Ir with placebo sources in patients with previous restenosis after coronary angioplasty. Over a 9-month period, 55 patients were enrolled; 26 were randomized to 192 Ir and 29 to placebo. At 3-year follow-up, target-lesion revascularization was significantly lower in the 192 Ir group (15.4% versus 48.3%; P 192 Ir patients (33% versus 64%; P 192 Ir patients but was unchanged in placebo patients. Conclusions —The early clinical benefits observed after treatment of coronary restenosis with 192 Ir appear durable at late follow-up. Angiographic restenosis continues to be significantly reduced in 192 Ir-treated patients, but a small amount of late loss was observed between the 6-month and 3-year follow-up time points. No events occurred in the 192 Ir group to suggest major untoward effects of vascular radiotherapy. At 3-year follow-up, vascular radiotherapy continues to be a promising new treatment for restenosis.

The most common cause of morbidity and mortality in the United States is vascular disease, which afflicts a wide spectrum of organs such as the heart (cardiovascular system), brain (cerebrovascular system), kidney (renal system), liver (hepatic system), and extremities (peripheral vascular system). The most common pathology of vascular diseases is occlusion. Percutaneous Transluminal Angioplasty (PTA) is currently the most common nonsurgical treatment for obstructive arteries. Unfortunately, the long-term effectiveness of PTA is limited by a high restenosis rate. Efforts to reduce post-PTA restenosis, including laser, mechanical atherectomy, intravascular stenting, and pharmacologic agents, have not been successful. With recent advances in the pathogenesis of restenosis, we have learned that the major problem is the intimal hyperplastic reaction in response to vessel injury. Encouraging animal data in the use of various radiotherapeutic approaches to prevent restenosis has led to a large number of multi-national, multicenter, randomized trials on coronary vascular systems. Because early results have been in favor of radiation therapy, and because the basic process of restenosis is similar for coronary and noncoronary vascular systems, many investigators extend the application of radiotherapy to the prevention of restenosis in peripheral vascular systems. However, the clinical scenarios are much different for peripheral vascular systems than for the coronary vascular system. This article discusses the current views of the pathophysiology of restenosis, major clinical trials, and perspectives on future studies. Experimental studies on animal models have documented the profound effects of endovascular brachytherapy in reducing restenosis caused by angioplasty and stenting. Early results of clinical trials are encouraging and confirm these positive results. Long-term follow-up data are needed to show that radiation does prevent, not merely delay, restenosis; Several areas of opportunity exist for both basic science research and clinical studies to enhance our knowledge of the pathophysiology. This would optimize the treatment strategy, maximizing the benefits and minimizing late complications.

Background —Although early trials indicate the treatment of restenosis with radiation therapy is safe and effective, the long-term impact of this new technology has been questioned. The possibility of late untoward consequences, such as aneurysm formation, perforation, and accelerated vascular disease, is of significant concern. Furthermore, it is not known whether the beneficial effects of radiation therapy will be durable or whether radiation will only delay restenosis. Methods and Results —A double-blind, randomized trial was undertaken to compare 192 Ir with placebo sources in patients with previous restenosis after coronary angioplasty. Patients were randomly assigned to receive a 0.76-mm (0.03-in) ribbon containing sealed sources of either 192 Ir or placebo. All patients underwent repeat coronary angiography at 6 months. All living patients were contacted 24 months after their index study procedure. Patients were assessed with respect to the need for target-lesion revascularization or nontarget-lesion revascularization, occurrence of myocardial infarction, or death. Over a 9-month period, 55 patients were enrolled; 26 were randomized to 192 Ir and 29 to placebo. Follow-up was obtained in 100% of living patients at a minimum of 24 months. Target-lesion revascularization was significantly lower in the 192 Ir group (15.4% versus 44.8%; P 192 Ir and placebo patients (19.2% versus 20.7%; P =NS). There were 2 deaths in each group. The composite end point of death, myocardial infarction, or target-lesion revascularization was significantly lower in 192 Ir-treated versus placebo-treated patients (23.1% versus 51.7%; P =0.03). No patient in the 192 Ir group sustained a target-lesion revascularization later than 10 months. Conclusions —At 2-year clinical follow-up, treatment with 192 Ir demonstrates significant clinical benefit. Although further follow-up (including late angiography) will be necessary, no clinical events have occurred to date in the 192 Ir group to suggest major untoward effects of vascular radiotherapy. At the intermediate follow-up time point, vascular radiotherapy continues to be a promising new treatment for restenosis.

In the Scripps Coronary Radiation to Inhibit Proliferation Poststenting (SCRIPPS) Trial, 192Ir significantly reduced angiographic, ultrasonographic, and clinical endpoints of restenosis. The objective of this analysis was to quantitate the impact of patient, lesion and technical characteristics on late angiographic outcome.Patients with restenotic, stented coronary lesions were randomized to receive either 192Ir or placebo sources. Late luminal loss and loss index were calculated for several patient subgroups, including patients with diabetes, in-stent restenosis, multiple previous percutaneous transluminal coronary angioplasty (PTCA) procedures, longer lesion lengths, saphenous vein grafts, small vessel diameters, and minimum dose exposures8.00 Gy. Two-factor analysis of variance was used to test for an interaction between patient characteristics and treatment effect.In the treated group, late loss was particularly low in patients with diabetes (0.19 mm), in-stent restenosis (0.17 mm), reference vessel diameters3.0 mm (0.07 mm), and patients who received a minimum radiation dose to the entire adventitial border of at least 8.00 Gy. The loss index in each of these subgroups was similarly low at -0.02, 0.03, -0.02, and 0.03, respectively. By 2-factor analysis of variance, a significant interaction between subgroup characteristic and treatment effect (late loss) was found in patients with in-stent restenosis (p = 0.035), and patients receiving a minimum dose of 8.00 Gy to the adventitial border (p = 0.009).In this pilot study, patient characteristics associated with a more aggressive proliferative response to injury appeared to confer an enhanced response to radiotherapy. Furthermore, a dose threshold response to 192Ir was found with an enhanced response occurring when the entire circumference of the adventitial border was exposed to at least 8.00 Gy.

Purpose/Objective: To establish a commissioning and acceptance test protocol for an ACelectromagnetic tracking system for use in target localization and tracking during radiation therapy. (Calypso® 4D Localization System, Calypso Medical, Seattle, WA). Materials/Methods: Following installation of infrared cameras and electromagnetic system, compatibility tests were made to ensure the system did not interfere with radiation output, MLCs or IMRT field fluence. The collision space between the linac and the Calypso System was evaluated. System verification included calibration using software and calibration tools, one of which has embedded RF transponders and optical reflectors. End‐to‐end testing to assess localization accuracy included CT scan of a radiographic phantom with RF transponders, creation of four Calypso Plans, data entry to the Calypso System and execution of treatment sessions. Tracking accuracy was measured using a precision translation table for orthogonal motions ± 5 mm. Results: Calypso System did not affect normal clinical operations of radiation output, MLC, or IMRT field segmentation. Overall accuracy on ten systems at five institutions and 40 treatment plans was 0.068 ± 0.027 cm, incorporating contributing errors from the CT scans, identification of transponders in the phantom, and effects of radiation dose delivery. With stable environment, no systematic drift was noticed over 30 minute. Translations of precise increments from −0.50 cm to +0.05cm were accurately tracked with error of 0.00cm, −0.02cm and 0.01cm (lat, long, vert). The system maintained accuracy with monthly optical calibrations. Changes in system readout of ±0.05 cm (readout quantization) were noticed at certain gantry angles. Conclusion: Evaluations demonstrate the ACelectromagnetic system with wireless transponders can be integrated into the radiotherapy environment with existing instrumentation and operates within the designed accuracy specification. Conflict of Interest: Work supported by Calypso Medical Technologies.