A 252Cf-based instrument for in vivo body protein monitoring in cancer patients.

A hospital-based facility for in vivo prompt gamma neutron activation analysis of nitrogen for body protein determination is described. The patient is laid on a movable couch and is scanned with a vertically collimated neutron beam from a 252Cf neutron source (the amount of Cf varying from 120 to 40 microg due to the physical decay) positioned below the patient. Four large NaI(Tl) detectors are used to measure the 10.8 MeV gamma-rays from nitrogen. To check the long-term stability of the system, a solid phantom simulating the geometry of the adult human trunk, having similar elemental composition as tissue, was constructed. Repeated phantom measurements over 6 months gave a reproducibility in nitrogen determination of 2.9% (1 SD). Duplicate patient measurements carried out within a week showed a reproducibility of 5% (1 SD). A calibration method for absolute protein measurements in patients is presented. Patients are normally measured for 40 min; giving a mean whole-body equivalent dose of 0.25 mSv. Results from measurements on 13 cancer patients are presented.