Comparison of PET/CT versus CT only in the assessment of new heterotopic ossification bone lesions in patients with fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare disorder characterized by the deposition of bone in soft tissues, known as heterotopic ossification (HO). This post hoc analysis compared the performance of two imaging modalities for the detection and volumetric measurement of new HO lesions. LUMINA-1, a phase 2, randomized, double-blind study (NCT03188666), evaluated the safety and efficacy of garetosmab, an anti-activin A antibody, versus placebo in adult patients with FOP. From baseline through to week 28, ¹⁸F-labeled sodium fluoride positron emission tomography (PET)/X-ray computed tomography (CT) and CT-only scans prospectively acquired during the initial placebo-controlled period of the study were independently reviewed by two sets of fixed blinded readers plus an adjudicator for the presence and volume of new HO lesions. The number of patients with new lesions was 14/44 (31.8 %) and 12/44 (27.3 %) as detected by PET/CT and CT only, respectively. The aggregate number/volume of new lesions were very similar both for the placebo and the garetosmab group between PET/CT (27/245.0 cm³ and 3/21.3 cm³, respectively) and CT only (37/261.8 cm³ and 1/0.1 cm³, respectively). The mean (standard deviation) number of new lesions per patient by PET/CT through week 28 was 0.68 (1.57) versus 0.86 (1.95) as detected by CT only. Through week 28, the mean (standard deviation) volume of new lesions per patient detected by PET/CT was 6.05 (14.88) cm³ versus 5.94 (21.13) cm³ by CT only. Moderate agreement between PET/CT and CT-only detection was observed when identifying patients with new lesions, with a kappa coefficient of 0.46 (standard error, 0.146; 95 % confidence interval, 0.17–0.74). CT-only imaging showed similar performance to PET/CT in the detection and characterization of new HO lesions. CT-only imaging therefore is a viable option for the assessment of therapies on new HO in patients with FOP.