Diffuse diseases and comprehensive tests- PET/CT in Multisystem Langerhans celll Histiocytosis.

Introduction: LCH is a clonal proliferative disorder of the histiocyte, noted to have multiple organ system involvement. LCH is considered a multisystemic disease when 2 or more organ systems are seen to be involved. It predominantly involves bones, skin, lymph nodes, liver, spleen, lungs, and CNS. Staging and response evaluation of LCH requires the utilization of multiple modalities ranging from clinical history to invasive radiological investigations. 18F-FDG PET/CT being a metabolic marker of disease activity could serve as an objective tool in this pursuit. The study was conducted to understand the utility of 18F-FDG in this regard.

Methods: Methods: Twenty-three children (median age =3.01 years, range - 3months- 9 years, M: F=15:8 ) diagnosed with multisystem LCH who underwent baseline with at least 1 post-induction chemotherapy PET/CT and a year of follow up were analyzed. Initial staging and follow-up were done by conventional methods (i.e. history, clinical examination, USG abdomen, chest X-Ray, skeletal survey, liver function tests, hemogram, biopsy and bone marrow examination, and CT/ MRI when indicated). PET/CT done at the time of initial staging and post-induction chemotherapy were reviewed and organs systems affected as diagnosed by conventional modalities and PET/CT were compared.

Results: Results: Forty-six FDG PET/CTs of twenty-three patients with multisystem LCH were analyzed, skeletal system involvement was noted in 19 patients (82.6%), skin in 15 (65.2%), the liver in fourteen patients (60.8%), lung in thirteen patients (56.5%), lymph node in thirteen patients (56.5%), spleen in twelve patients (52.2%), pituitary involvement in 6 patients (26.1%) and marrow involvement in 4 patients (17.3%). Organ wise sensitivity and specificity were calculated and are as follows respectively - Skeletal system- 89.47%, 100%, Liver -85.71%, 88.89%, Spleen- 91.7%,72.7% and Lung-84.6%, 100%. PET/CT also detected additional and uncommon sites of disease in 4 patients- which included thyroid involvement in 2 patients, thymus in 1, and pancreatic involvement in 1 patient. Response evaluation post-induction chemotherapy showed a clinical response in 13 patients and no clinical response in 10 patients. PET/ CT was concordant with conventional staging in all 23 patients. Further, among the 10 non-responders, PET/CT identified progressive disease in four patients, leading to initiation of second-line chemotherapy.

Conclusions: Conclusion: 18F-FDG PET/CT, with its single examination -whole body assessment can serve as a valuable tool for initial extent determination, assess response post-chemotherapy, and identification of a subgroup of patients requiring treatment escalation.