The thymus in Langerhans cell histiocytosis ( LCH) : evaluation with F18-FDG PET/CT

Objectives Langerhans cell histiocytosis (LCH) is a rare condition. The aim of our study was to clarify, using F18-FDG PET/CT, the prevalence and etiology (neoplastic infiltration or autoimmune response) of thymic anomalies in LCH.

Methods We conducted a retrospective study of 28 LCH patients imaged with F18-FDG PET/CT at time of presentation. Those patients were divided in four groups:non active and active unifocal bone disease (NAD and AD), multisystemic (MS) and at risk bone (SS) disease. Comparaison was made with 28 normal patients (N). Mean, standard deviation, variance, correlation and p values (Student t-test ) were calculated for thymic uptake (thymus), unaffected bone marrow activity (marrow) and spleen to unaffected liver ratio (ratio). The qualitative aspect -nodular/smooth- of the thymus was reviewed.

Results Thymic activity was higher in MS and SS group when compared to the AD, NAD and N (p= 0.06 to 0.09). Using 2.5 SUVmax as the limit for disease, 11 patients would be considered as having thymic disease and using 3 SUVmax, 6 patients . Nodular thymus that could indicate disease was found in 11 patients. No correlation was found between ``thymus`` , ``ratio`` and ``marrow`` in either groups (r= -0.41- 0.84). In all groups, bone marrow activity was similar to the normal group and there was no difference within the groups (p=0.1 to 0.48). Spleen to liver ratio was superior to 1 only in the MS group, when compared to SS, N, AD,NAD (p ranging from 0.003 to 0.05).

Conclusions Nodular and/or increased uptake at the level of the thymus suggestive of disease was found in about 40% of our patients. Some patients classified SS may, in fact, have MS disease with thymus involvement. There was no sign of autoimmune activation at time of presentation in our 28 patients. Non diseased bone marrow was normal. Increased spleen to liver ratio was found only in the MS group and could represent either diffuse infiltration of the spleen by LCH or an index of severity.