Incidental Bilateral Thighs Muscular Uptake of Technetium-99m MDP on Bone Scan in a Patient with Left Condylar Hyperplasia

Introduction

Bone scans with technetium-99m methylene diphosphonate (99 mTc MDP) are commonly used for the evaluation of various musculoskeletal conditions, including bone tumors, fractures, and infections. Incidental findings of abnormal radiotracer uptake in nonosseous structures can occasionally be observed. Herein, we describe a case of incidental bilateral thigh muscle muscular uptake of 99 mTc MDP in a patient undergoing a bone scan for left condylar hyperplasia.

Case Report

A 22-year-old male presented to our department with a history of progressive left mandibular swelling and pain. Clinical examination and imaging findings were consistent with left condylar hyperplasia, and a bone scan was performed for further evaluation .

Bone scan was performed using 25 mCi of 99 mTc MDP, which is a radiotracer that localizes to areas of increased osteoblastic activity. Whole-body planar images were obtained at 2 h postinjection. Unexpectedly, the images revealed bilateral symmetric uptake of 99 mTc MDP in the muscles of the posterior compartment of the thigh [Figure 1].

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Figure 1 Planar whole body MDP bone scan of a 22 year old male, a) anterior view and b) posterior view showing normal skeletal uptake with extra-osseous muscular uptake noted in bilateral thigh muscles which is prominent in the posterior view. (Image source: Department of Nuclear Medicine, AIIMS, New Delhi)

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To determine the cause of this abnormal uptake, a thorough review of the patient’s history was undertaken. The patient reported engaging in a high-intensity exercise routine involving lower limb strength training and cycling the day before the bone scan. Based on this information, it was hypothesized that the muscular uptake observed on the bone scan could be related to postexercise effects rather than any pathological process.

Discussion

Normally, the uptake of bone scanning agents is seen in osseous structures, kidney, and bladder. Occasionally, extra-osseous uptake in soft tissue can be seen on delayed images, due to local tissue abnormalities, blood flow changes, and metabolic conditions.[1] The pathogenesis of uptake of bone scanning agents in soft tissue is multi-factorial; one of the primary underlying factors is excess calcium in the soft tissue.[2] Scintigraphy with Tc-99 m MDP delineates a wide spectrum of non-osseous disorders, i.e. neoplastic, hormonal, inflammatory, ischemic, traumatic, excretory, and artefactual entities demonstrate abnormal soft-tissue uptake of Tc-99 m MDP.[3]

Damaged muscle tissue has an increased avidity to calcium, like accounting for the labeling of recent myocardial infarct with 99 mTc polyphosphate.[14] Soft-tissue uptake in MDP bone scintigraphy has been reported in inflammatory conditions like polymyositis.[5] Calcium deposition in the soft tissue can be found in a variety of disease processes (such as ischemia, necrosis, metastatic calcification in renal failure, or hypercalcemia of any cause), it is conceivable to find uptake of the bone radiotracer in any organ in the body. The uptake patterns occurring in individual organs usually point to a specific pathology. For example, cardiac uptake might be due to a recent myocardial infarction or to the presence of amyloid deposits.[6] Pleural effusions may be delineated on the bone scan by diffuse increased uptake in a hemithorax. Such pleural uptake indicates a malignant effusion and is an ominous sign in patients scanned for skeletal metastases.[7]

Mechanisms leading to increased extra-osseous Tc-99 m MDP uptake include extracellular fluid expansion, enhanced regional vascularity and permeability, and elevated tissue calcium concentration. In this case, the MDP scan shows muscular uptake in most of the muscles which is most prominent in medial thigh muscles, and on a detailed history, it has come to knowledge that the patient had exercised regularly in the recent past, which shows the reason for muscular MDP uptake due to damaged tissue.

Incidental muscular uptake of 99 mTc MDP on bone scans is a rare but recognized phenomenon. Several factors can contribute to such uptake, including muscle injury, inflammation, muscle hypertrophy, and exercise-related changes.[8] In the present case, postexercise effects were identified as the cause of the bilateral thigh muscle muscular uptake observed on the bone scan.

High-intensity exercise can lead to increased vascularity and metabolic activity in muscles, which can result in increased radiotracer uptake on bone scans.[9] This phenomenon is likely related to increased blood flow and capillary recruitment in response to exercise-induced demand. The uptake is typically symmetric and resolves with time and cessation of exercise.

Clinicians and nuclear medicine specialists should be aware of exercise-related muscular uptake to avoid misinterpretation and unnecessary further investigations. Correlation with the patient’s history, including recent physical activity, can provide valuable insights and guide the appropriate management of such cases.

Conclusion

Incidental findings on bone scans can occasionally lead to diagnostic dilemmas. This case report highlights the importance of considering benign causes of abnormal radiotracer uptake, such as postexercise effects, to avoid unnecessary diagnostic interventions. Correlation with the patient’s clinical history, including recent physical activity, is crucial for accurate interpretation of bone scan findings. Awareness of exercise-related muscular uptake can aid in the appropriate management and prevent potential patient anxiety and unnecessary investigations.