AMERICIUM-241 SOURCES FOR INTRACAVITARY IRRADIATION

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Sealed sources of americium-241 emit primarily 60 keV photons which because of multiple Compton scattering produce dose distributions in water that are comparable to those from Ra-226 or Cs-137. However, americium gamma rays can be shielded by thin layers of high atomic number materials since the half value layer thickness is only 1/8th of a mm of lead for americium gamma rays as compared to a value of 12 mm for Ra-226 gamma rays. This allows effective in-vivo shielding of critical organs, for example, the bladder can be partially shielded by hypaque solution, and the rectum and sigmoid colon by barium sulfate. In addition, the exposure to medical personnel involved in intracavitary application and patient care can be reduced substantially by the use of relatively thin lead aprons and light weight, portable shields. This project seeks to continue an on-going investigation into the feasibility and development of americium sources. Dosimetry measurements around a prototype gynecological plaque applicator confirm that the dose distributions in water very similar to those produced by radium or cesium applicators can be obtained, and that highly effective shielding can be achieved by small amounts of high atomic number materials. In this project, we propose to design and fabricate a clinical gynecological tandem and plaque applicator, to perform dosimetry measurements on this applicator, to develop methods for computerized treatment planning for applications with and without shields, and to develop and test a probe for in-vivo dosimetry. Following these physics studies, we propose to introduce a first-generation americium applicator into clinical practice for treatment of gynecological malignancies, initially carrying out Phase I-II trials in patients with recurrent carcinoma of the cervix, late stage carcinoma of the cervix, post surgical carcinoma of the endometrium, etc. Based on the dose distributions and clinical experience with the first-generation applicator, a second-generation americium applicator will be designed and fabricated to carry out Phase II-III clinical studies on early and intermediate stage carcinoma of the cervix and endometrium.