Purpose or Case Report: To compare the detection of lesions between DW-MRI and ¹⁸F-FDG PET/MR for staging and restaging of Langerhans Cell Histiocytosis (LCH), using all clinical outcomes and imaging data as the reference standard. Also, this study will compare the differences between LCH chemotherapy responders and non-responders.
Methods & Materials: In a prospective clinical trial, we enrolled 11 children (2-16 years old, 4 female and 7 male) with LCH, who underwent an ¹⁸F-FDG PET and DW-MRI scan on an integrated 3T PET/MR scanner at baseline (n=11) and after chemotherapy (n=7). We determined the presence or absence of tumor lesions in 5 anatomical areas per patient on ¹⁸F-FDG PET and DW-MRI. We calculated sensitivities, specificities and diagnostic accuracies of the two imaging modalities, with biopsy results and follow up imaging as the reference standard. In addition, we measured the SUVratio as the maximum standardized uptake value (SUVmax) of each lesion, normalized to the SUVmean of the liver and the apparent diffusion coefficients (ADCmean) of all lesions. Quantitative data before and after chemotherapy were compared with a Mann-Whitney U test. SUVratio and ADCmean were correlated with a linear regression analysis.
Results: On baseline scans, our patients had 37 LCH lesions according to the reference standard. ¹⁸F-FDG PET detected 37 of 37 lesions and DW-MRI detected 35 of 37 lesions. The sensitivity and specificity for baseline scans were 100% and 100%, respectively for ¹⁸F-FDG PET and 94.59% and 100% for DW-MRI. DW-MRI missed two lesions in the head and thorax as there was no restricted diffusion of the lesion in the head and the thorax had motion artifacts. Chemotherapy responders demonstrated significant decline in SUVratio (p=0.002) and significant increase in ADCmean (p<0.001). Non-responders did not show significant changes in SUVratio (p=0.122) or ADCmean (p=0.156). The regression analysis showed that resolution of a lesion leads to a decrease in SUVratio and an increase in ADCmean.
Conclusions: Our data provides preliminary evidence that DW-MRI can be used for staging and treatment monitoring of LCH. While we found slightly lower sensitivities of DW-MRI compared to ¹⁸F-FDG PET, this difference did not impact our ability to determine overall therapy response. Since patients with multifocal LCH need multiple follow-up scans throughout their lifetime, using DW-MRI could substantially decrease the radiation exposure of these young patients.