Intracoronary irradiation: dose response for the prevention of restenosis in swine

External beam irradiation inhibits neointimal hyperplasia after injury-induced arterial smooth muscle cell proliferation

PURPOSE

Restenosis after catheter-based revascularization has been demonstrated to be primarily caused by smooth muscle cell proliferation. This study examines the effects of external beam irradiation on neointimal proliferation after external injury to the central artery of the rabbit ear.

METHODS AND MATERIALS

Thirty male New Zealand White rabbits were used in this study. Crush lesions were performed on each ear under general anesthesia and bilateral auricular nerve blockade. A single dose of 1200 cGy (n = 10), 1600 cGy (n = 10), or 2000 cGy (n = 10) gamma radiation was delivered to the left or right central artery of the ear 24 hours after injury; the contralateral central artery served as control. All rabbits were sacrificed after 21 days and the central arteries of both ears were fixed for morphometric measurements.

RESULTS

Mean (+/-SD) neointimal area was 0.062 +/- 0.005 mm2 (1200 cGy), 0.022 +/- 0.005 mm2 (1600 cGy), and 0.028 +/- 0.006 mm2 in irradiated arteries compared with 0.081 +/- 0.009 mm2 in the control group. Mean (+/-SD) luminal area was 0.049 +/- 0.004 mm2 (1200 cGy), 0.059 +/- 0.002 mm2 (1600 cGy), and 0.072 +/- 0.006 mm2 (2000 cGy) in irradiated arteries compared with 0.043 +/- 0.008 mm2 in the control group. The differences in neointimal and luminal area between control and irradiated arteries were significant (p < 0.05) for the 1600 and 2000 cGy group only.

CONCLUSION

We conclude that in this model, external beam X-ray irradiation was successful in reducing neointimal proliferation after injury of the central artery of the rabbit ear. Marked reductions in neointimal proliferation were demonstrated in vessels subject to 1600 and 2000 cGy radiation; a less prominent effect was noted for 1200 cGy. Whether this approach can be used successfully to inhibit restenosis in the clinical setting requires further investigation.

Endovascular brachytherapy--treatment planning and radiation protection

The risk of restenosis, main late effect limiting the success of percutaneous transluminal coronary artery angioplasty, can be reduced significantly by vascular radiotherapy, subsequent to PTCA. This discovery lead to the development of new irradiation techniques. Endovascular brachytherapy is the choice in treatment of coronary artery stenosis. Successful irradiation, however, requires precise treatment planning. This review addresses the physical possibilities and problems of intravascular brachytherapy planning, and the radiobiologically based definition of the target volume and of structures at risk. Recommendations for dose specification, recording and reporting are given. The criteria for selecting a vascular radiotherapy technique are discussed as well as the possibilities of dosimetric treatment planning and quality assurance based on precise plastic scintillator dosimetry and intravascular ultrasound. Radiation protection and safety must be reconsidered prior to the usage of therapeutic radiation sources in the catheter laboratory and for the decision about emergency plans. Finally, the design of clinical trials, the role of medical physicists, and the future of irradiation treatment of stenosis is discussed.

Intracoronary radiation using radioisotope solution-filled balloons

A substantial body of experimental evidence suggests that local application of radiation at sites of angioplasty or stent implantation prevents neointima formation. Recent initial clinical studies report a significant reduction in clinical restenosis rates when gamma emitting radiation sources are used to treat the site of coronary intervention. Because of the energy of therapeutic gamma sources and the shielding requirements, it would be far preferable to deliver ionizing radiation using a local beta emitter. This report discusses a number of the physical, biological, and technical parameters involved in radiation from a balloon angioplasty catheter filled with a beta emitting radioisotope solution.

Irradiation for the treatment of intimal hyperplasia

Intimal hyperplasia represents a serious complication limiting the long-term benefits of vascular interventions such as balloon angioplasty and stent placement. Although pharmacological interventions have attempted to curtail restenosis, they have not been shown to be effective to date. Radiotherapy is one alternative that has shown promise as an inhibitor of intimal hyperplasia in several animal models. Irradiation causes cell death by producing irreparable damage to DNA. This is believed to be the mechanism of inhibition of VSMC proliferation. Delivery of irradiation can be either intraluminal via an angiographically directed catheter or extraluminal using an external radiation source such as an x-ray device. Intraluminal irradiation has generally utilized either gamma or beta-emitting sources. Both have been effective in producing a dose response, although some studies advocate the use of beta-type irradiation as a safer, more efficient means of delivery. Extraluminal irradiation also has been an effective inhibitor of intimal hyperplasia. Studies suggest that this form of irradiation provides a more even-dose distribution to vessel walls than an intravascular delivery system. The use of radiotherapy has more recently been extended to clinical trials, and initial studies have shown promising results. The success of irradiation must be balanced with its potential complications including radiation-induced arteritis, coronary artery stenosis, and secondary development of malignancy. Although these have been associated with irradiation, the dose used in these cases was often considerably higher than those used in the treatment of intimal hyperplasia. Finally, with the advent of gene therapy, irradiation may provide an additional means of supplementing this new type of therapy through radiation-inducible gene therapy.

[Role of intravascular brachytherapy in the prevention of vascular restenosis after angioplasty]

About 30% of patients who underwent percutaneous transluminal coronary angioplasty show evidence of restenosis, which appears to be independent of the angioplasty method used. The restenosis is due of two factors, firstly migration of smooth vascular muscle cells of the vascular media to the intima and multiplication which lead to the formation of a neo-intima. Irradiation limits the proliferation by acting of the cells in the mitotic stage. The vascular target volume is not very thick and is difficult to define it, that why brachytherapy seems to be the best procedure to prevent restenosis. However, the development of this treatment present many difficulties. Different irradiation techniques have been studied. Such techniques include catheter containing radioactive sealed source, radioactive stent, or balloon containing radioactive liquid inside. Each of these methods have their own advantages, inconveniences, problems and risks. Radioisotope may be either beta or gamma emitters. Gamma emitter presents problems for radioprotection but the satisfactory dose distribution may be difficult to obtain using beta emitter. Choice of dose, dose rate and delay between the end of angioplasty and the beginning of brachytherapy is subject to some discuss. Animal experiments using radioisotope have shown reduction in cell proliferation. Human trials showed feasibility, safety of the method and real impact on restenosis prevention. However, long-term efficacy has not been proved because the follow-up of the patients is too short. A randomized trial of 192Ir brachytherapy for prevention of restenosis has recently shown the efficacy in short and median term. However, long term efficiency and secondary effects have not yet been established as the follow up time of this study is still too short. That is why, collaboration between cardiologists and radiotherapists and physicists is indispensable to enable the development of an optimal technique.

Endovascular beta-radiation to reduce restenosis after coronary balloon angioplasty: results of the beta energy restenosis trial (BERT)

BACKGROUND

In the porcine overstretch injury model of restenosis, endovascular beta-radiation reduces neointima formation. To determine whether this therapy could be applied to patients with coronary artery disease, a special device was developed to allow delivery of 12 encapsulated 90Sr/Y sources, measuring a total of 30 mm, to various sites within the coronary arterial tree. This study was designed to evaluate the feasibility of the delivery of 12, 14, or 16 Gy at 2 mm after balloon angioplasty of stenoses of native coronary vessels.

METHODS AND RESULTS

Delivery of beta-radiation was attempted in 23 patients after successful balloon angioplasty. Source delivery was successful in 21 of the 23 patients (91%). There was no in-hospital or 30-day morbidity or mortality. Follow-up quantitative coronary arteriography in 20 patients demonstrated a late loss of 0.05 mm, a late loss index of 4%, and a restenosis rate of 15%. The use of the beta-emitter 90Sr/Y significantly reduced treatment time and operator exposure compared with previous trials with the gamma-emitter 192Ir.

CONCLUSIONS

In this study, the administration of endovascular beta-radiation after angioplasty was safe and feasible and substantially altered the postangioplasty late lumen loss, resulting in a lower-than-expected rate of restenosis. On the basis of these encouraging results, a multicenter, randomized trial with operators and patients blinded to treatment assignment is planned.

[Role of radiotherapy in benign diseases]

Radiation therapy of benign diseases represent a wide panel of indications. Some indications are clearly identified as treatment of arteriovenous malformations (AVM), hyperthyroid ophtalmopathy, postoperative heterotopic bone formations or keloid scars. Some indications are under evaluation as complications induced by neo-vessels of age-related macular degeneration or coronary restenosis after angioplasty. Some indications remain controversial with poor evidence of efficiency as treatment of bursitis, tendinitis or Dupuytren's disease. Some indications are now obsolete such as warts, or contra-indicated as treatment of infant and children.