Fusion of positron emission tomography/computed tomography with magnetic resonance imaging reveals hamstring peritendonitis in polymyalgia rheumatica

Objectives. To characterize F-18-fluorodeoxyglucose (F-18-FDG) uptake on whole-body PET/CT in PMR, and identify its precise anatomic correlate using MRI. Methods. Patients with newly diagnosed PMR according to the 2012 EULAR/ACR classification criteria were prospectively recruited. Participants with GCA were excluded. A whole-body 18 F-FDG PET/CT scan was performed in all untreated patients. Qualitative and semiquantitative [standardized uptake value maximum (SUVmax)] scoring of abnormal F-18-FDG uptake was undertaken. MRI of the pelvis, knee and wrist and hand was performed in three representative patients with anatomical correlation of FDG-avid sites carried out using Medview fusion software. Results. Twenty-two patients with PMR were recruited. Their mean age was 68.3 years (S.D. 6.3) and 13/22 were male. On whole-body PET/CT, F-18-FDG uptake adjacent to the ischial tuberosities was observed in 21 participants (95.4%) and recorded the highest mean SUVmax value [3.6 (S.D. 1.7)]. A high frequency of posteromedial knee (61.9%) and wrist and/or hand involvement (66.7%) was also appreciated. MRI of the pelvis revealed high T2 signal surrounding the proximal hamstring tendon origins of both semimembranosus and the conjoint tendon of the semitendinosus and biceps femoris. At the knee, peritendonitis at the distal insertion of the semimembranosus was observed. PET/MRI fusion at the pelvis and knee confirmed semimembranosus peritendonitis as the anatomical correlate of F-18-FDG uptake adjacent to the ischial tuberosities and of posteromedial knee structures. Conclusion. Hamstring peritendonitis is a common and distinctive manifestation of PMR on whole-body PET/CT.