Individual dosimetry planning using the conjugate view mode with reduced scan time for improved patient compliance

Full text of publication follows. Aim: subject-specific dosimetry planning (DP) for radionuclide therapy (RNT) is mandatory. The conjugate view method (CVM) is a well-established method for DP whereby dynamic data is derived from a series of whole-body, planar scintigraphies to determine pharmacokinetics of target tissue and organs at risk. However, many patients experience pain during these scans and cannot remain still for extended imaging times. Therefore, our goal is to reduce the scan duration by shortening the scan length during CVM without compromising data integrity. Materials and Methods: this is a retrospective study based on 10 patients subject to RNT with [Lu-177]-DOTATATE. All patients underwent routine DP following the injection of 3,7 GBq [Lu-177]-DOTATATE. Six whole body scans were performed from 30 min to 140 hours p.i. with a speed of 10 cm/min. Our assumption is that the activity in distal legs can be extrapolated from the activity in the upper thighs covered in the upper 70% of the scan (head, torso and upper thighs only). We therefore compare the measured total body counts for each of the scans (anterior and posterior view) to the counts calculated from the upper 70% of the scan (simulating a partial body scan of head, torso and upper thighs only), while extrapolating for the lower 30%. This was performed by multiplying the counts in the upper thighs for all scans following the initial scan by the ratio of counts for total legs to upper thighs derived from the first scan. Results: preliminary evaluation of 10 patients indicated the extrapolation method to reproduce the counts in the 'disregarded' extremities with a mean error of 12% (max. 16%). The resulting difference in whole-body counts was less then 2%. This small impact is based on the fact that the counts in the legs only contribute 5% to 16% to the total body counts for this tracer. Conclusion: Our data indicate that the total scan length in the CVM can be reduced by covering the torso only, thereby reducing the total scan duration by about 30%, or 5 min, respectively. The resulting difference in DP was less then 2% for the determination of total body counts in later scans. (authors)