Measuring diffuse metabolic activity on PET/CT as a new method for evaluating pulmonary Langerhans cell histiocytosis activity



Year of publication 2011
Type Conference abstract
Description Introduction: Langerhans cell histiocytosis (LCH) infiltrating pulmonary parenchyma is characterized by formation of nodules in active phase of the disease that evolve into non-active cystic lesions later on. A non-invasive evaluation of pulmonary LCH activity is routinely performed by means of high-resolution computed tomography (HRCT) imaging or pulmonary function tests. However, determining the number of nodules and cystic formations on HRCT scans is extremely difficult and time-consuming and pulmonary function tests cannot always distinguish between end-stage cystic disease and active LCH. Therefore, we developed a new method for measuring diffuse metabolic activity on fluorine-18-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) using a lung-to-liver activity ratio. Materials and Methods: Nodules in pulmonary LCH often measure several millimeters (4 - 6 mm) and are below detectable levels of used PET scanners (about 7 mm), which precludes direct measurement of their activity. Thus, we tried to find these active lesions in a larger area on PET/CT scans. Considering the anatomical conditions and locations of predominant pulmonary LCH involvement, we chose the volume-of-interest shaped into spheres of 6 - 8 cm in diameter in the right upper and middle lung fields. To decrease the variability between single examinations as well as to compare examinations from different scanners, lung-to-liver activity ratio was established. In the liver parenchyma we chose the volume-of-interest shaped into spheres of 9 - 10 cm in diameter. For semiquantitative analysis of radiopharmaceutical (fluorodeoxyglucose) accumulation in a lesion, maximum Standardized Uptake Values (SUVmax) were calculated. SUVmax in the spherical volume in the right lung and SUVmax in the spherical volume in the reference hepatic parenchyma were measured and put into relation to set up the SUVmaxPULMO/SUVmaxHEPAR index. In our work we retrospectively studied a series of 4 FDG-PET and 23 FDG-PET/CT scans from 7 patients with pulmonary LCH, where index values were compared to the disease course in each patient. We also analyzed a sample of 100 randomly chosen FDG-PET/CT studies of patients free from any known lung or hepatic diseases. Results: In patients with pulmonary LCH, a close correlation between the index values and the disease course was found in 7/7 subjects, where the increasing index values indicated disease activity, while the decreasing index values were observed after therapy administration. In the group of 100 healthy control subjects we found the index values lower than 0.3 in 80% and lower than 0.4 in 96% (median: 0.22, range: 0.14-0.43), which is in accordance with our observation that values about 0.5 bear high probability of pulmonary involvement in patients with Langerhans cell histiocytosis. Conclusions: Measuring SUVmaxPULMO/SUVmaxHEPAR values represents a simple, non-invasive screening tool allowing an early diagnosis and monitoring of therapy effect in patients with pulmonary LCH.

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