Clinical Experience with Long Half-Life and Low-Energy Encapsulated Radioactive Sources in Cancer Radiation Therapy

Abstract

THE TREATMENT of localized cancer by-small radioactive sources permanently placed in the tissue is generally recognized as an effective modality of radiation therapy. In the past ten years, we had the opportunity at Memorial Hospital to use various radioisotopes for permanent interstitial implants in cancer patients. These radioisotopes may be divided into two groups: those with short half-lives, radon 222 (3.8 days) and gold 198 (2.7 days); and those with long half-lives, iridium 192 (74 days) and iodine 125 (60 days). The radioisotopes with long half-lives, iridium 192 and iodine 125, present a number of practical advantages for permanent implantation. The first advantage is the greater availability of the seeds. With radon-222 and gold-198 seeds, the activity decreases so rapidly that the seeds must be ordered for a specific day and usually have to be discarded if they cannot be used within a day or two. This fact limits permanent implants with radon-222 and gold-198 seeds in most hospitals to tumors which can be measured accurately before implantation and requires a specific order for each implant. In contrast, the decrease in activity of iridium-192 and iodine-125 seeds is so slow that in practice one shipment of seeds of suitable activity per month is satisfactory. The second advantage of the seeds with a longer half-life is the greater economy. All long-life seeds which have been received can usually be used for implants, while many short-life seeds go to waste. The third advantage of the long-life seeds is the better radiation protection of the hospital personnel. This is possible because the long-life seeds are of much lower activity at the time of implantation than the short-life seeds. Table I summarizes our clinical experience with the long-life iridium-192 seeds. From October 1956 till January 1962, we used these seeds in 360 permanent implants in 327 patients. Of the 327 patients, 22 or 6.7 per cent survived five or more years. This was quite unexpected since the local disease in most of these patients was unresectable and often unsuitable for external irradiation. In 219 patients (65 per cent of all patients) the permanent implants were performed at thoracotomy or laparotomy. The implanted malignant tumors were considered unresectable because of location near vital organs or vessels, large size, or massive involvement of the regional lymph nodes. In approximately half of these 219 patients, the cancer had recurred after adequate x-ray therapy. A partial resection of the tumor was possible in only 11 patients. In Table II, the results of the permanent implantation with iridium-192 seeds of malignant unresectable intrathoracic and intra-abdominal tumors are compared with those of radon-222 seeds.2 Of the 219 patients treated by interstitial implantation of iridium-192 seeds from 1956 to 1962, 10 (4.6 per cent ± 1.4 per cent) survived five years or more.