Technique and preliminary results of interstitial irradiation for primary brain tumors

Abstract

We have conducted a phase-I clinical trial of CT guided stereotactic implantation of Ir¹⁹² in the treatment of malignant astrocytomas. During the past year, 16 patients have been implanted with two to four catheters in the residual enhancing portion of their tumor. These patients represent 50% of our total experience with the CT compatible Leksell frame. Each catheter contains three to six high intensity (2.0 to 2.5 mg Ra equivalent) seeds with 0.5 cm separation between the sources. The total activity of Ir¹⁹² per implant has been 30–65 mg radium equivalent. In the 16 patients, 49 catheters have been placed, an average of three targets calculated per patient and no targets have been missed. The radiation exposure to personnel has been surveyed in detail and drops off to less than 2 mr/h six feet from the patient when our custombuilt radiation shield is employed. We have reserved permanent implantation of I¹²⁵ for patients with tumors in unusual locations (e.g. pineal) or for individuals with slowly growing non-gliomatous lesions (i.e. meningioma). The tumor volumes have ranged from 12–120 cm³. Unique aspects of our implant procedure include the use of a Leksell frame already adapted to the GE-8800 scanner, the use of pre- and post-implant computerized treatment planning programs to determine the dose distribution profiles and the use of adjustable metal collars crimped to the outer catheters to provide ease of insertion, uniform pre-implant catheter length, and protection against source migration. Two of our patients have suffered from subacute radiation reactions, primarily due to delayed cerebral edema and both of these cases have largely resolved. We believe that it is technologically feasible to deliver 5000 rads to the tumor periphery in 50 h without undue patient toxicity or environmental radiation hazard. The system we have been studying is fully compatible with the simultaneous use of mechanical cytoreduction, chemotherapy and interstitial hyperthermia.