Precise practical measurements of 90Y used in radiopharmaceuticals using re-entrant ionization chambers.

For diagnostic nuclear medicine, radiopharmaceuticals are labeled with γ-ray emitting nuclides. Routine activity measurements for these drugs are mainly performed with re-entrant ionization chambers. For radiotherapeutic applications, β-ray emitting nuclides are also used. Since very few gamma transitions are involved in these nuclides, an ionization chamber responds only to weak bremsstrahlung. In order to apply the re-entrant chamber method to routine activity assay for these β-ray emitting nuclides, detailed studies were carried out. It was determined that the response was strongly affected by the choice of thickness of the inner wall of the well in the measurement of high energy beta particle emitting nuclides. The geometrical dependences were successfully reduced. Accordingly, routine assay of radioactivity of (90)Y therapeutic pharmaceuticals can be performed within an acceptable uncertainty with ordinary re-entrant dose calibrators that are commercially available and equipped in most hospitals.