^99mTc MDP SPECT/CT imaging in diagnosing Erdheim-Chester disease

As noted in Figure 2, whole-body planar images show intense but symmetrical hypermetabolism at the proximal ends of the bilateral wrist joints, the long bones of the lower extremities, especially metaphyseal region of the bilateral distal femur and femoral condyles, as well as the proximal tibial shaft and tibial plateau. Focal areas of intense uptake were also visualized in the sternum, several thoracic vertebrae, and ribs.

In Figures 3 and 4, the SPECT/CT shows diffuse sclerosis in the shaft of the distal femur and proximal tibia, which corresponds to intense skeletal hypermetabolism correlating with the hypermetabolic zones defined on the planar images. Focal sclerotic lesions in the thoracic vertebrae, sternum, and ribs also reflect the diffuse nature of the disease process shown on planar images.

The pattern of skeletal involvement and the presence of bilateral orbital infiltration as well as perinephric and pericardial exudates together with histopathological confirmation from the retro-ocular left orbital mass excised (Figure 5) lead to a diagnosis of Erdheim-Chester disease (ECD). 

ECD is a non-Langerhans-cell histiocytosis, originally defined as lipogranuloma lesions characterized by infiltration of tissues by foamy histerismos, multinucleated giant cells, and lipid-laden macrophages commonly associated with exophthalmos, symmetrical long-bone involvement, and extra-skeletal involvement involving the kidneys, skin, and heart. In 2016, the World Health Organization reclassified this disease as histiocytic and dendritic cell tumors.¹ Due to the rarity of ECD and the diversity of clinical manifestations, diagnosis is extremely challenging. An ECD diagnosis depends on the histopathology of the biopsy specimen and immunohistochemistry results.² Radionuclide bone imaging, CT, MRI, and other imaging methods also have important diagnostic value in ECD. The standard diagnosis procedure should be based on the clinical and imaging studies to determine the biopsy site and further histopathological confirmation.

More than 95% of ECD patients have cortical sclerosis in the diaphysis and metaphysis of long bones with the distal femur, proximal tibia, and distal tibia being most typically involved.³ Radionuclide bone imaging is highly sensitive to the presence of ECD-related changes in affected bones. ^99mTc MDP SPECT/CT of the lesion can be used to observe the sclerosis of the shaft and metaphyses of long bones of the limbs. The whole-body planar bone images of this patient showed the typical characteristics of ECD, including symmetrical concentration of radionuclides in the shaft and metaphyses of the long bones of the lower extremities as well as the distal ulna and radius. Involvement of the shaft of the long bones is typically limited to the distal and proximal end of the shaft as seen in the bilateral distal femur, distal and proximal tibial shaft, and distal radius. Hypermetabolism and corresponding sclerosis were denser towards the metaphyseal end compared to rest of the shaft.

ECD is a rare disease and can be easily missed. In this case, symptoms such as exophthalmos, dizziness, and dysarthria occurred successively, but the diagnosis of ECD was not considered. Therefore, there is a need for pathologists, radiologists, and clinicians to increase their awareness of ECD and its manifestations. The patient reported here had predominant bone involvement along with the central nervous system, peripheral kidney, and retroperitoneum. Retroperitoneal and pelvic involvement in ECD is rare.

^99mTc MDP SPECT/CT imaging has high diagnostic sensitivity for early skeletal involvement. Fusion imaging enables further evaluation of bone sclerosis and has important clinical value for the diagnosis of skeletal involvement in ECD patients. Symmetrical concentration of radionuclides and osteosclerosis in the long bones of the extremities is a typical imaging manifestation of ^99mTc MDP SPECT/CT imaging of bone involvement in ECD patients.⁴ When ^99mTc MDP SPECT/CT imaging shows the long bones of the limbs are symmetrically concentrated with radionuclides and osteosclerosis, and other lesions involve corresponding symptoms, the possibility of ECD should be considered.