Radiation dose in interventional fluoroscopic procedures

Abstract

Vascular interventional procedures carried out under fluoroscopic guidance often involve high radiation doses. Above certain thresholds, radiation can cause significant damage to the skin including hair loss and severe necrosis. Such damage has been reported by several investigators. Many attempts have been made to quantitate the radiation doses to the skin involved with these procedures, but dosimetry methods are often flawed. To improve the situation better monitoring of radiation doses, fluoroscopist education, and changes in technology and methods are needed.

Introduction

Vascular interventional procedures carried out under fluoroscopic guidance have become an increasingly prominent tool in treatment of vascular and cardiac diseases. These procedures encompass the specialties of Cardiology, Electrophysiology, Peripheral Vascular Radiology and Neurovascular Radiology. The most commonly known type of procedure in this area is probably coronary arterial angioplasty, in which occluded or stenosed coronary arteries are reopened by various means of removal or displacement of arterial plaque. Cardiac arrhythmia is often dealt with by the procedure known as radio frequency cardiac ablation. In this procedure an electrophysiologist applies a radio frequency (RF) heating probe to the muscle wall of the heart while monitoring electrocardiographic signals to determine if tissue providing an anomalous cardiac electrical path is destroyed. Peripheral vascular procedures often involve reopening vessels in arteries other than the coronary arteries, but similar angioplasty methods are used as in the heart. One of the more complicated types of peripheral vascular interventions are the placement of a shunt between the portal and hepatic veins in a compromised liver to stop gastroesophageal bleeding due to venous hypertension. This procedure is known as a transjugular intrahepatic portasystemic shunt (or TIPS) placement. In neurovascular radiology the most delicate fluoroscopically monitored interventional procedure is the blocking or embolization of a malformation in the cerebral vascular system. Typically a small vessel leading to an aneurysm is blocked by placement of a plug made of materials ranging from bits of plastic to glue. Similar embolization procedures are performed in the peripheral vasculature, but the intricacy of the neurovascular system and its variations and the potential for brain damage make the neurological embolization procedure particularly complex.

High radiation doses occur in these procedures due to one or more of the following aspects. The procedures are usually intricate, requiring in some cases many hours of fluoroscopic guidance (e.g., RF ablations). Most of these procedures require high quality, low noise images such as cine' fluorography or digital subtraction angiography to properly see the vessels. They are always performed on patients who have been diagnosed with a severe medical problem for whom few, or no, alternate treatments can be offered. The risks associated with ionizing radiation in these procedures have to be viewed in the context of the potential benefit of the procedure, the likelihood of greater trauma associated with surgery in the brain or heart, and perhaps with immanent death if the procedure is not performed.

For this reason, most physicians that perform such procedures seem to be relatively unconcerned with the long term risks associated with ionizing radiation, such as cancer and genetic effects. However, the potential for deterministic effects such as burns of the skin and loss of hair is of greater concern.

Deterministic effects of radiation are those which require a minimum number of cells to be involved before the effect can be detected. Thus there is a threshold radiation dose below which the effect can not be seen. Additionally, an increase in the dose above the threshold will lead to greater damage. The potential for deterministic effects from the type of procedures described above have been reviewed in detail recently by Wagner et al. (1994). The major organ affected by localized high doses of radiation is the skin. The threshold for radiation damage to skin varies from individual to individual, but is generally estimated to be in the range of two sieverts (Sv). Skin doses in this range can also cause temporary loss of hair. More severe consequences occur at higher doses. At doses of about six sieverts the basal cells of the skin are sufficiently damaged to cause a long term erythema lasting several weeks. Hair loss from doses in this range and above is likely to be permanent. If a sufficient number of basal cells are destroyed the skin will slough off or peel. This dry desquamation typically occurs at around ten sieverts. Desquamation can be delayed after the main erythema effect by a few weeks during which time the erythema appears to be subsiding. At doses above about 18 Sv the destruction of basal cells becomes complete. When desquamation occurs, it is accompanied by oozing of lymphatic fluids. Doses in this range and above lead to severe skin damage. Small regions of destroyed skin, less than about 10 cm diameter, can grow back from the outer edges, but larger lesions generally will require skin transplantation to repair them.

The skin is not the only tissue with a potential for damage from interventional procedures. The eyes are close to the region examined during neurological procedures. The bone near the skin surface, particularly the posterior aspect of the skull also receives a high dose. Cardiac and coronary artery procedures can involve comparatively high exposure of the posterior portions of the lungs.

The threshold dose for causing cataracts in the eye is similar to that of erythema, with a dose of two sieverts leading to sub-clinical cataract induction. Vision impairment is not likely to occur below doses of six sievert to the lens. Bone changes might occur in young patients in whom bone dose exceeds 30 Sv. Although this is a seemingly high number, one has to bear in mind that the relative dose per unit exposure (f-factor) in bone is about four times as high as that for soft tissue at the X-ray energies used for imaging. Pneumonitis induced by radiation requires a rather large area of the lung to be exposed to a dose of about seven sieverts or more.