Extraosseous 99mTc-MDP Uptake Guiding Intraoperative Sampling in Severe Inflammatory Myopathy: A Case Report and Literature Review

Background/Objectives: We report a case of severe dermatomyositis demonstrating characteristic widespread extraosseous uptake on 99mTc-methylene diphosphonate (99mTc-MDP) bone scintigraphy. This study highlights the diagnostic value of this modality in detecting active inflammatory myopathy when conventional muscle biopsy is inconclusive and introduces its novel use for intraoperative gamma-probe-guided biopsy to precisely target metabolically active muscle. This approach may help target metabolically active muscle in heterogeneous idiopathic inflammatory myopathies (IIMs). Case Presentation: A 49-year-old man developed progressive proximal muscle weakness (Medical Research Council grade 2/5 proximally, 5/5 distally) beginning in June 2025 following influenza infection, accompanied by dysphagia, classic dermatomyositis cutaneous manifestations, back pain, and difficulty standing. Laboratory evaluation revealed elevated inflammatory markers (ESR 55 mm/hr, CRP 20 mg/L), leukocytosis (16.58 × 109/L), markedly raised creatine kinase (19,937 IU/L), and troponin T levels. An initial quadriceps muscle biopsy performed on 29 July 2025 was non-diagnostic. Three-phase 99mTc-MDP scintigraphy (~1110 MBq) demonstrated intense, diffuse extraosseous uptake involving bilateral deltoids (symmetric), biceps and triceps (patchy), paraspinal muscles (longitudinal), gluteal muscles, thighs (quadriceps and hamstrings), and gastrocnemius muscles, with relative suppression of appendicular skeletal uptake on delayed images due to soft-tissue tracer dominance—findings consistent with severe inflammatory myopathy. Following reinjection (~1100 MBq), intraoperative gamma-probe-guided biopsy targeted areas of highest uptake (left quadriceps femoris and distal triceps brachii; intraoperative counts 1300–1400 versus background ~500). Histopathology revealed histiocyte-predominant inflammation with myofibre necrosis and regeneration, sparse CD4+ T-cell infiltrates, and absence of fibrosis, consistent with necrotising myopathy. Positive antinuclear antibodies and strong anti-Mi-2 antibodies confirmed the diagnosis of dermatomyositis. Treatment included pulse methylprednisolone followed by oral prednisone taper, methotrexate, azathioprine, intravenous immunoglobulin, and planned rituximab therapy. Discussion: Whole-body 99mTc-MDP scintigraphy provided a complementary whole-body functional assessment of disease extent, revealing widespread muscular involvement. The novel application of intraoperative gamma-probe-guided biopsy enabled real-time targeting of metabolically active muscle, facilitating targeted sampling after an initial non-diagnostic biopsy and yielding supportive histopathological findings. This dual diagnostic and interventional role demonstrates the technical feasibility of gamma-probe guidance in a diagnostically challenging case of dermatomyositis. Conclusions: In our case, the integration of 99mTc-MDP scintigraphy with gamma-probe-guided biopsy enabled precise targeting of metabolically active muscle following an initial non-diagnostic biopsy. This multimodal approach may be useful in selected diagnostically challenging cases of severe inflammatory myopathy. Larger studies are needed to evaluate its reproducibility and added value.