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

Abstract

In Langerhans cell histiocytosis (LCH), FDG PET demonstrates active disease in bone. Other imaging modalities show the effects of bone destruction by LCH. To evaluate a selective CT method for reducing effective dose from FDG PET/CT in LCH, using whole-body modified attenuation correction CT at extremely low exposure settings, with repeat selective limited-volume CT at typical localization settings. Fifty-one PET/CT scans were performed in 23 LCH patients, median patient age 8.5 years (range: 1-25 years). Thirty-four were performed with modified attenuation correction CT settings, with bed positions (excluding head and neck) repeated at localization CT settings in regions with abnormal or difficult to interpret PET findings. Of 34 modified attenuation correction PET/CT scans, 10 required repeat localization CT of 1 to 3 bed positions (total: 17 bed positions). Lytic bone lesions were easily recognized at modified attenuation correction settings. Calculated average effective dose for the 34 whole-body CT scans at modified attenuation correction settings was 1.65 mSv. Average effective dose per patient for repeat imaging of 17 bed positions at localization settings was 1.19 mSv. Average total effective dose from CT for all 34 scans performed at the modified attenuation correction CT settings, including the 10 repeat localization CT scans, was 2.0 mSv. High-quality PET scans were consistently obtained with reduced FDG-administered activities of 3.7 MBq/kg (0.10 mCi/kg). In active LCH, abnormal FDG uptake was seen in all lytic bone lesions ≥9 mm, including cranial vault lesions. Substantial reduction in effective dose is possible using selective CT techniques for FDG PET/CT.