The utilization of europium 152-154 in a cervical carcinoma applicator.

Abstract

In previous communications (1, 2), a specially designed tripartite applicator has been described for treatment of carcinoma of the cervix utilizing cobalt 60. The advantages of this applicator are: (a) ease of insertion; (b) fixed geometric relationships to facilitate calculation of gamma-ray dosage to the pelvis; (c) carefully evaluated relative intensities of various capsular sources to permit an optimum dose pattern in the pelvis; (d) selective relative diminution of radiation directed to the bladder and rectum. The chief disadvantage is the half-life of cobalt 60, namely 5.3 years, necessitating relatively frequent decay corrections and rather long treatment times after about two years. With this disadvantage in mind, the applicator has been modified to utilize europium 152-154, which has a half-life of 12.4 years (3). Basic Physical Concepts: Unlike cobalt 60, which in its decay emits gamma rays of only two energies—1.17 and 1.33 MEV—europium 152 and 154, which are always admixed, emit a complex array of about eleven gamma-ray energies, ranging from 0.040 to 1.40 MEV, with an “average” emission not too dissimilar from that of cobalt 60, when relatively small amounts of filtration are applied. Europium 152 and 154 are derived by neutron capture and gamma emission, from stable europium 151 and 153, the latter having high neutron capture cross sections. The sources utilized in the above applicator required irradiation for two weeks in a reactor at a flux density of about 1013 neutrons/cm. Vsecond. Very little europium 154 will be produced in such a short neutron irradiation. The radioactive strengths of the sources were expressed as equivalent amounts of cobalt 60, by direct gamma-ray comparison at 1 meter with a Lauritzen electroscope. The actual strength of the source could not be predicted with greater accuracy than ± 15 per cent. It is not proportional to the mass of Eu2O3 in the capsule, due presumably to the shielding of the inner parts of a pellet and the high cross section for neutron absorption of the various isotopes and their daughters. Source Design and Molding: Since europium for irradiation in the pile is available only as Eu2 O3 powder, which has first to be pelleted and sealed hermetically, the individual sources are pellets of Eu203 powder, admixed with aluminum. Aluminum pellets of appropriate size are used as inert separators, and all are enclosed in thin-walled aluminum capsules.4 Each source capsule, designed to have an active length of 1 ern. (Fig. 1), consisted of seven pellets of 2.38 mm. diameter and 1.43 mm. length, within a cylindrical aluminum can of 3.97 mm. overall diameter and 13.0 mm. length. After loading, the end of the aluminum cylinder was sealed by spinning.