Assessing early therapeutic response in extrapulmonary tuberculosis using [18F]FDG-PET/CT imaging

Monitoring treatment response in extrapulmonary tuberculosis (EPTB) remains challenging due to limited microbiological accessibility and the lack of validated biomarkers. [18F]FDG positron emission tomography/computed tomography [(18F)FDG-PET/CT] enables assessment of metabolic disease activity and has been explored in infectious diseases, but evidence supporting its use in EPTB is limited.

We conducted a prospective observational study including adults diagnosed with EPTB at a tertiary referral center in Denmark between 2016 and 2018. Participants underwent baseline and follow-up [18F]FDG-PET/CT imaging after completion of the intensive treatment phase. Clinical data and laboratory parameters, including C-reactive protein (CRP), hemoglobin, leukocytes, albumin, and alanine aminotransferase were collected at baseline and during follow-up. Changes in standardized uptake values (SUVmax) and laboratory markers were analyzed descriptively.

Seventeen patients were included. Metabolically active disease was detected in all participants at baseline. Most patients (11 out of 14) demonstrated a reduction in lesion number and SUVmax at follow-up, indicating a metabolic response to therapy. However, persistent [18F]FDG uptake was observed in a subset of patients (3 out of 14). Median CRP levels declined from 12 mg/L (range: 0.3-190) at baseline to 3.6 mg/L (range: 0.9-20) at week 8 and remained low thereafter, broadly paralleling PET/CT findings. In several cases, residual metabolic activity did not result in treatment prolongation.

Serial [18F]FDG-PET/CT demonstrated changes in metabolic activity during treatment with reductions in lesion number and SUVmax in most patients. However, SUVmax reflects focal metabolic activity rather than total disease burden, and persistent uptake after the intensive treatment phase was observed in some patients. Larger prospective studies incorporating standardized response criteria and volumetric PET measures are needed before [18F]FDG-PET/CT can be implemented as a routine biomarker for treatment response in EPTB.