Selective CT for PET/CT dose reduction in Langerhans cell histiocytosis

Objectives In Langerhans cell histiocytosis (LCH), FDG PET demonstrates active disease in bone, while x-ray, bone scan, MRI and CT show effects of bone destruction. We describe a selective CT method for reducing effective dose (ED) from FDG PET/CT in LCH, using whole body attenuation correction (AC) only CT with selective regional CT at localization (loc) settings.

Methods 51 PET/CT scans were performed in 23 LCH patients (pt). Median pt age was 8.5 yrs (range 1-25 yrs). 17 scans were performed with loc CT settings. 34 were performed with AC only CT settings; bed positions were repeated at loc CT settings in regions with abnormal PET findings.

Results Of 34 AC only PET/CT scans, 10 required repeat loc CT of limited body regions (non-head), with 1-3 bed positions re-scanned for each region (total 17 bed positions). The head was re-scanned with loc CT 18 times, but further analysis indicated that, with minor changes in AC only CT settings, lytic bone lesions were easily recognized at modified AC only settings. Calculated average ED for AC only PET/CT scans at modified settings was 0.60 mSv. Average ED for the 10 repeat regional body CT scans at loc settings was 1.16 mSv. Average total ED for all 34 scans performed at AC CT settings (including the 10 loc CT body re-scans) was 0.94 mSv. High quality PET scans were consistently obtained with FDG activities of 0.10 mCi/kg; average total ED for FDG+CT for the 34 scans was 5.45 mSv. For routine FDG+CT using a typical 0.14 mCi/kg FDG dose and whole body loc CT, average ED for the 34 scans is calculated to be 12.2 mSv. Use of the selective CT method reduced ED from CT by 70%; reduction in total FDG+CT ED was 55% for selective CT and a 0.1 mCi/kg FDG dose. PET was abnormal in 25/51 scans in 12/23 pt. In active LCH, abnormal FDG uptake was seen in all lytic bone lesions >1cm, including the skull. FDG uptake in skull lesions <1cm was inconsistently detected.

Conclusions Significant reductions in ED can be achieved with use of selective CT techniques for FDG PET/CT imaging in LCH