Predictive value of semi-quantitative index from F-18-fluoride PET/CT for treatment response in patients with ankylosing spondylitis.

Purpose: We aimed to investigate whether the lesion-to-background ratio (LBR) of the whole spinal column in ankylosing spondylitis (AS) patients based on baseline F-18-fluoride positron emission tomography/computed tomography (PET/CT) is associated with treatment response.

Methods: Twenty-eight male patients with a wide range of disease activity were included in this study. Abnormal bone metabolic lesions on baseline F-18-fluoride PET/CT were visually assessed in three areas: anterior discovertebral units (DVUs) and posterior joints (cervical, thoracic, and lumbar facet joints, costovertebral joints, and costotransverse joints) of vertebrae and sacroiliac joints of the sacrum. The highest maximum standardized uptake value (SUV) of lesions within each area was measured. The LBR as a semi-quantitative index was defined as SUV of the lesion divided by mean SUV of the L5 vertebral body. Clinical disease activity was assessed using the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), and treatment response was evaluated based on follow-up BASDAI score.

Results: The median follow-up period was 4.5 years (range, 0.9-5.5). The BASDAI score significantly decreased from median 6.6 (range, 1.0-9.5) at baseline to 2.0 (range, 0.6-8.1) at follow-up (P <  0.001), and the median change in BASDAI score was -3.4 (range, -8.2-4.4). The LBR of posterior joints in the spinal column had a significant positive correlation with follow-up BASDAI score (r = 0.394, P =  0.042).

Conclusion: In patients with AS, the LBR of posterior joints of the spinal column on baseline PET/CT could be considered one of the potential surrogate markers for predicting treatment response.