Extraosseous accumulation of bone scan tracer ^99mTc-methylene diphosphonate in a phlebolith

INTRODUCTION

Bone scintigraphy is an important modality for the examination of various pathological conditions of the skeletal system. Radionuclide bone imaging is quick, relatively inexpensive, widely available, and exquisitely sensitive and is invaluable in the diagnostic evaluation of numerous pathological conditions. The procedure is performed with ^99mTechnetium (^99mTc)-labeled diphosphonates. These compounds accumulate rapidly in bone, and by 2-6 h after injection, about 50% of the injected dose is in the skeletal system.[1] The extraosseous uptake is often an unexpected finding on the bone scan.[2] ^99mTc-methylene diphosphonate (^99mTc-MDP) imaging, customarily directed toward identifying altered skeletal metabolism, has been used extensively to image a variety of extraosseous lesions, especially heterotopic ossification[3] and metabolic, metastatic calcification.[4]

We report a rare case of accumulation of bone scintigraphy tracer in a phlebolith in a patient of adenocarcinoma of lung.

CASE REPORT

A 43-year-old female patient, known case of adenocarcinoma of right lung was referred for a ^99mTc-MDP bone scan with complaint of persistent low backache since 3 months. A whole body ^99mTc-MDP bone scan was performed. The scan showed abnormal, focal increased tracer uptake in left pelvic cavity, not localizing to any bone of the skeleton [Figures 1a and b, arrows]. All the bones of the axial and appendicular skeleton showed normal tracer uptake. The uptake was persistent even in post-void imaging done after adequate hydration, giving us a clue that it was not due to tracer hold up in the ureter. On single photon emission computed tomography and computed tomography (SPECT-CT) imaging, the focal tracer uptake localized to a small focus of calcification in the left pelvic cavity away from the ureter [Figure 2, arrow]. An additional ultrasonography of the pelvis confirmed that there was no ureteric calculus [Figure 3]. Review of the patient's recent contrast-enhanced computed tomography revealed a phlebolith in left iliac vein (CT Hounsfield unit: 338). Thus, we concluded the extraosseous ^99mTc-MDP uptake was due to phlebolith in the left iliac vein.

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Figure 1 Planar bone scan (a and b) of a 43-year-old female patient, a known case of adenocarcinoma of right lung. Focal, intense tracer uptake seen in left pelvic cavity (arrow)

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Figure 2 Correlative single photon emission computed tomography and computed tomography fusion imaging localized this uptake to a phlebolith in left iliac vein (arrow)

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Figure 3 Ultrasonography of the pelvis showing ureteric jets bilaterally (arrows), ruling out obstruction (calculus) in the ureter

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DISCUSSION

Bone scintigraphy is one of the most commonly performed scintigraphic procedures. ^99mTc-MDP, a bone scan tracer, has rapid blood clearance, excellent in vivo chemical stability, and a high bone-to-soft tissue ratio, which are ideal for bone imaging. Although ^99mTc-MDP is customarily used to evaluate skeletal pathological conditions, its excellent clearance from normal soft tissues allows the detection of abnormal extraskeletal ^99mTc-MDP accumulation.[5]

It is hypothesized that ^99mTc-MDP and its analogues bind to bone by adsorption to the surface of the hydroxyapatite crystal.[6] While in soft tissue, calcium localizes through a limited number of common pathways. These pathways result in uptake of ^99mTc-MDP through what is believed to be chemisorption of the radiopharmaceutical onto the surface of the calcium salts.[7]

Phleboliths are deposits which develop in the wall of a vein and are composed of calcium or lime. These are most commonly seen in the lower pelvis and sometime occur in large numbers. They are of no particular pathological significance. These are usually symptomless and are usually incidentally detected. They are often confused with ureteric stones when seen around the urinary bladder in the pelvic cavity. However, on plain radiographs, these can be distinguished from a calculus due to the presence of a translucent center and circular or oval shape.[8–10]

Bone scan tracer uptake in a phlebolith has been described in past.[11] Uptake in phlebolith is an example of tracer localization in dystrophic calcification. Dystrophic calcification occurs in patients with normal Ca² + and PO₄ levels and refers to Ca² + deposition in tissues secondary to histological disruption caused by trauma, ischemia, or cellular necrosis or in the enzymatic necrosis of fat. It is thought that calcium ion binds to phospholipids present in membrane bound vesicles, phosphatases generate phosphate groups which in turn bind to the calcium, and the cycle is repeated until local concentrations are elevated and crystals begin to form. Hyaline collagen degeneration, a consequence of tissue damage, is particularly associated with Ca² + deposition. Similar mechanism operates in other known causes of extraskeletal bone scan tracer uptake in tissue infarction, deep vein thrombosis, scars, and dermatomyositis.[12]

To conclude, we report a rare case of extraosseous uptake of bone scan tracer in a phlebolith. With growing use of integration of CT with SPECT, Nuclear Medicine physicians should become aware of such anatomical changes and variants so as to avoid misinterpretation. The awareness of the underlying pathophysiological basis of these changes and uptake of MDP in extraosseous soft tissue can significantly enhance the interpretation and diagnostic value of bone scintigraphy.