Bone scintigraphy in osseous sarcoidosis

INTRODUCTION

Sarcoidosis is a multisystem granulomatous disorder of unknown etiology. It commonly affects young and middle-aged patients and shows a predilection for adults under age 40 years.[1] The incidence of radiographically evident osseous involvement is between 1% and 13%, with an average of 5% on conventional imaging.[2] The majority of osseous lesions occur in the short tubular bones of the hands and feet and the abnormalities can be either unilateral or bilateral.[2] Bony involvement is often associated with cutaneous lesions.[3] Technetium-99m labeled pyrophosphate or diphosphonate compounds, gallium-67 citrate and F-18 FDG are the agents which are used for imaging of osseous sarcoidosis.[4–6] We present a case of sarcoidosis with unusual skeletal bone involvement and review the literature.

CASE REPORT

A 52-year-old man with back pain and right hip pain was referred to our hospital. He had a history of lung sarcoidosis for 21 years. The patient was treated with corticosteroids. As his disease was considered inactive, corticosteroids were discontinued approximately 2 years after diagnosis. Physical examination was normal. Neither lymphadenopathy nor skin lesions were detected. Laboratory tests were normal except alkaline phosphatase which was elevated (448 U/L, normal 40-150 U/L).

Pelvic radiographs showed sclerosis and radiolucency on multiple areas mimicking bone metastases [Figure 1].

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Figure 1 Anterior view of the pelvis and proximal femurs showing widespread lesions mimicking sclerotic bone metastasis

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A computed tomography (CT) of thorax showed sclerotic and lytic lesions on 1^st and 10^th thoracic vertebras in addition to the findings of stage 4 sarcoidosis in both lungs [Figure 2].

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Figure 2 (a) Sagittal section of computed tomography images show osteolytic lesions and sclerosis on 1^st and 10^th thoracic vertebras and osteolytic lesion on 7^th cervical vertebra with corresponding increased tracer uptake in bone scan; (b) Transaxial images of 10^th thoracic vertebra show multipl sclerotic and lytic lesions; (c) Lung parenchyma image shows diffuse fibrosis consistent with stage 4 sarcoidosis

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Furthermore, cranial CT and radiographs were obtained to investigate the involvement of cranium. Although radiographs were normal, CT of the cranium revealed lytic lesions in addition to extensive sclerosis [Figure 3]. There was no involvement of the brain parenchyma.

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Figure 3 (a) No lesion on the lateral radiograph of the skull; (b) Although cranial computed tomography showed sclerotic and lytic lesions on bones, brain parenchyma was normal

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The patient was referred for bone scintigraphy to scan the whole skeleton. Bone scintigraphy was performed 3 h after intravenous injection of 20 mCi (740 MBq) Tc99m methylene diphosphonate. Bone scan demonstrated multiple areas of increased tracer accumulation at the calvarium, upper-to-lower thoracic spine, pelvic bones, sacrum, both sacroiliac joints, and both proximal femurs [Figure 4]. After scintigraphy, bone marrow biopsy was done at 10^th thoracic vertebra and demonstrated non-caseating granulomata consistent with skeletal sarcoidosis.

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Figure 4 Bone scintigraphy showed diffusely increased tracer accumulation at the calvarium, pelvic bones, both sacroiliac joints and proximal femurs and posterior left 7. rib. Additional focal uptakes were observed upper and lower thoracic spine, 7^th cervical vertebra and sacrum

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DISCUSSION

Bone lesions are uncommon in sarcoidosis and the incidence varies from 1% to 13%.[2] Osseous sarcoidosis is most often seen in the short tubular bones of hands and feet but has been reported in the spine, sternum, calvarium, pelvis, ribs, nasal bones, tibia, and femur.[27–12] Our patient had no involvement of his hands or feet bones. However, he had involvement of his calvarium, thoracic spine, pelvis, and femurs which are rare sites. Spinal lesions may be seen at any level, but the thoracic spine usually affected.[13] Bone involvement is reported to indicate a worse prognosis with a mortality rate 4 times higher in patients who are detected to have abnormalities on bone radiography than in those with normal findings.[14]

Sarcoid bone involvement is generally asymptomatic but symptoms may be severe in some patients. Our patient also had severe back and hip pain. The radiographic, CT, and magnetic resonance (MR) findings of the bone lesions are non-specific. Lesions may be osteolytic or osteosclerotic on plain radiography and CT.[14] Our patient's lesions were sclerotic on pelvis radiography and were lytic-sclerotic on thoracic and cranial CT. The MR usually demonstrates multifocal lesions within the vertebrae that are hypointense on T1-weighted images, hyperintense on T2-weighted images, and enhance following contrast medium administration.[1415] The differential diagnosis includes metastatic lesions, lymphoma, myeloma, infectious processes including tuberculosis. Biopsy is generally recommended in all cases to exclude other diagnoses.[214]

Nuclear medicine studies including Technetium-99m labeled pyrophosphate or diphosphonate compounds, Ga-67, and F-18 FDG have been used for imaging of osseous sarcoidosis.[4–6] Bone involvement is much more commonly detected with the use of bone scan than with routine radiography.[16] Bone scan findings of osseous sarcoidosis are non-specific such as radiological methods. Bone scan findings may not be distinguished from other pathologies such as bone metastasis and osteomyelitis.[1718] The sensitivity of Ga-67 for detecting osseous sarcoidosis is less than that of bone scintigraphy.[16] However, the advantage of gallium scan over diphosphonate bone scan is that besides demonstrating bone involvement, it also detects pulmonary and extrapulmonary foci of sarcoidosis. Nishiyama, et al., suggested that FDG PET is more accurate and contributes to a better evaluation of extrapulmonary involvement in the patients with sarcoidosis.[19] Although FDG PET imaging is highly sensitive for skeletal involvement of sarcoidosis, it can mimic widespread skeletal metastases.[6]

In conclusion, bone scintigraphy is a sensitive imaging modality in order to demonstrate the sarcoid lesions of bones including the uncommon axial skeletal involvement. But, because of the low specificity of the imaging techniques, radiological correlation and histopathological confirmation should be done.