Metabolic Activity of Neurogenic Heterotopic Ossification on ¹⁸F-FDG PET/CT Matching with Ongoing Osteoblastic Activity on Bone Scan

Abstract

A 54-year-old man, with previous history of neurogenic heterotopic ossification (HO) in muscles around the left hip following a spinal cord injury ten months earlier, was referred to our nuclear medicine center for an ¹⁸fluorine-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT) to rule out a spondylodiscitis. No sign of spondylodiscitis was found on ¹⁸F-FDG PET/CT, but images revealed an increased ¹⁸F-FDG uptake in HO areas, matching with ongoing osteoblastic activity on a following bone scan.

Figure 1.

A 54-year-old man was referred to our nuclear medicine center for a combined ¹⁸fluorine-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT) to rule out a spondylodiscitis. He had a previous history ten months earlier of a thoracolumbar fracture complicated with paraplegia, treated with anterolateral decompression and fixation. He presented unexplained persistent lumbar pain and C-reactive protein level above the normal range after two successfully treated obstructive pyelonephritis, and a suspicion of spondylodiscitis was waived. The patient was known to present heterotopic ossification (HO) associated with the spinal cord injury with multiple areas of dense calcifications in the muscles around the left hip, as the anterior compartment of the thigh and iliopsoas muscle. No sign of spondylodiscitis was found on ¹⁸F-FDG PET/CT (maximum intensity projection ¹⁸F-FDG PET/CT image A, axial fused PET/CT image B), but images revealed a diffuse increased ¹⁸F-FDG uptake in a few of HO areas (axial and sagittal fused PET/CT images C and D).

Figure 2.

A complementary 3-phase bone scan was secondly performed. The bone scan showed extra-osseous extensive accumulations of technetium-99m (Tc-99m) hydroxymethylene diphosphonate matching with increased ¹⁸F-FDG uptake areas. Coronal fused PET/CT image (A), coronal fused single photon emission computed tomography/CT images on soft-tissue phase (B) and on delayed phase (C), showing a matching tracer uptake between the two modalities in HO areas around the left hip (asterisks). On CT images, multiple areas of dense calcifications were seen (arrows, coronal CT images D, E, and F). HO is a diverse pathologic process, defined as the formation of extraskeletal bone in muscle and soft tissues, usually between the muscle and the joint capsule. The reported incidence of HO in patients with spinal cord injury is approximately 20% (1). Such development of neurogenic HO generally occurs within a few months after spinal cord injury and progresses over a period of years (2). Clinically, neurogenic HO may cause severe pain during the process of formation and adversely affecting quality of life (3). Three-phase bone scanning is a highly sensitive method for the assessment of progression, maturation process and extent of HO. An increased vascularity on angiographic flow and soft-tissue phases is the earlier sign. Then occurs Tc-99m methyl diphosphonate (MDP) accumulation on delayed-phase, which is often seen 4 to 6 weeks earlier than ossification (4). Threephase bone scanning could be required for the clinical management of HO. Indeed, conservative treatment such as medication or local radiotherapy could be performed over surgical excision in case of high Tc-99m MDP accumulation (5). ¹⁸F-FDG uptake in HO, suggestive of active inflammation, is known as a pitfall of ¹⁸F-FDG PET/CT imaging. The concern, especially in early cases is that the imaging findings on FDG PET/CT could mimic that of aggressive sarcomas (6,7,8). Elucidating the antecedent history becomes important in making the current diagnosis in these patients.