Intracoronary artery radiation.

Abstract

This very successful meeting, commissioned by the Radiation Protection Committee of The British Institute of Radiology, was the ®rst UK symposium on this new clinical topic. Over 100 delegates from departments of cardiology, clinical oncology, radiation physics and industry attended this fascinating meeting, which provided not only important information about standard practices in each discipline but advanced insights into the physical (mechanical and radiation), biological and medical factors that interact in the use of radiotherapy to prevent intracoronary artery restenosis after angioplasty. A lucid description of the present management of atheromatous coronary narrowing by percutaneous angioplasty and stent insertion was given by Dr AH Gershlick (Leicester). The angioplasty balloon transmits 8±10 atmospheres of hydraulic pressure to split the atheromatous plaque and simultaneously dilate the arterial wall. This results in oedema and an in ammatory response controlled by speci®c growth factors that induce (via fos and c-myc receptors) a proliferation of smooth muscle cells derived from the adventitia. In approximately 40±50% of patients this process invades the media and breaches the disrupted internal elastic lamina, resulting in narrowing of the arterial lumen owing to a thickened neo-intima (initimal hyperplasia), a process called negative remodelling [1]. The alternative therapeutic approach is to perform a stent insertion, which in favourable situations termed benestent (short, proximal lesions) reduces restenosis to 10±15%. A larger restenosis rate (up to 40%) occurs in complex, distal lesions and in diabetic patients. It is apparent that the stent itself does not inhibit intimal hyperplasia but maintains vessel patency by its physical presence. Restenosis, particularly if diffuse, is dif®cult to treat. Reballooning restenosis rates are around 60%. Debulking by mechanical devices (rotablator or directional atherectomy) may help, but the most promising technique is brachytherapy (short range radiation therapy using sealed radioactive isotopes). Otherwise, coronary artery bypass grafts may be necessary. As yet, no bene®t has been detected in 38 clinical trials of drug therapy designed to reduce this process. The physics of radiotherapy in this situation were considered by Mr S Bachelor (Guy's Hospital, London). Brachytherapy by temporary intracoronary insertion of a radioactive source must be completed in less than 15 min because of the risks of protracted vascular occlusion: such treatments need high activity source(s) and must be delivered at a high dose rate. In contrast, radioactive-coated stents may deliver low dose rate treatment (with a reduction in biological effect so that higher doses may need to be given). Beta emitters (P and Y) and gamma emitters (Ir and I) can be used, although the prescribed dose will depend on the class of radiation owing to fall off characteristics. Gamma emitters are indicated for larger vessels. Several commercially produced narrow gauge afterloading catheters and intralumenal centralization devices are now available for intracoronary and peripheral artery brachytherapy. Dr WG Bass (Washington, USA) provided further physics information. Beta radiation duration is usually shorter, but there may be dose inhomogeneities due to remaining calcium plaques in the vessel wall. In order to treat the entire vessel media, the radiation dose should be adequate as far as the adventitia, which is approximately 3 mm from the central lumen in most affected coronary arteries. The dose is most frequently prescribed at 2 mm, but the target may be non-uniform. Table 1 shows the dose±distance relationships for Y and Ir. The use of b or c radiation emitters for brachytherapy requires additional radiation protection in cardiac catheter theatres/laboratories. Additional shielding around the patient requires a different geometry to that used for uoroscopy. Personnel shielding devices can be used but are of limited bene®t with penetrating gamma radiations such as Ir. Additional personnel, such as a radiation physicist and radiation oncologist, are required to be present at the procedure in the USA. Personnel are not allowed in the treatment room during therapy with Ir and additional wall shielding is required for safety in adjacent Received 14 May 1999 and accepted 8 July 1999.