Spectrum analysis and counting efficiency of Lu-177 with specific counting systems

Objectives Lu-177-DOTA-Octreotate, a mixed beta and gamma emitter, is a new therapeutic radiopharmaceutical used to treat neuroendocrine tumors. In this study, the gamma energy spectrum, counting efficiency, lower level of detection (LLD) and trigger levels for Lu-177 were obtained on three specific counting instruments.

Methods A sample of Lu-177-DOTA-Octreotate (200 µl) was obtained and activity measured in a dose calibrator (201 µCi). Following serial dilutions, counting and spot samples were prepared (0.5405 µCi). Using the appropriate counting sample, gamma energy spectrums were obtained with an AtomLab 500 and 960 well detector counting system, an appropriate counting window established and specific instrument efficiency, LLD and trigger levels calculated. In addition, a spot sample was placed in the Inspector beta counter (S.E. International), counts obtained and detector efficiency, LLD and trigger levels calculated.

Results Gamma energy spectrums were obtained on both AtomLab 500 and 960 instruments and a counting window established (37-360 KeV), consisting of 5 energy peaks with the major peak at 208 KeV. The counting efficiencies for AtomLab 500, 960 and Inspector instruments were 7.6%, 7.5%, and 0.3%, respectively. LLD for AtomLab 500, 960 and Inspector instruments were 640 dpm, 852 dpm, and 11,552 dpm. Due to the low counting efficiency, the Inspector counting instrument was not acceptable for removable contamination detection. Calculated trigger levels for removable contamination in restricted areas (2000 dpm) were 152 net cpm and 150 net cpm for the AtomLab 500 and 960 counting instruments, respectively.

Conclusions Appropriate gamma energy spectrums, counting windows and counting efficiencies were established for specific counting instruments. Counting efficiencies were utilized to calculate LLD and trigger levels. Calculated LLD were acceptable for removable contamination detection in an AtomLab 500 and 960 counting system and not acceptable in an Inspector counting system.