1187 Core needle biopsy for diagnosis of immune-mediated myositis

<h3>Background</h3> Immune checkpoint therapies (ICTs) are increasingly used to treat different cancers. A rare but potentially life-threatening immune-related adverse event (irAE) associated with ICT use is myositis, an inflammatory process affecting skeletal muscle. The incidence is between 0.5–1%. Open surgical muscle biopsy has traditionally been used for diagnosis but is associated with side effects related to sedation and increased recovery time. This study evaluated the diagnostic utility of core needle biopsy for the diagnosis of immune-related myositis. <h3>Methods</h3> We conducted a single-center, retrospective cohort study of patients with genitourinary cancers hospitalized for clinically diagnosed immune-related myositis at MD Anderson Cancer Center between December 1, 2016 to July 1, 2023. Diagnosis of severe (CTCAE grade 3 or higher) immune-related myositis required all three criteria to be met: (1) symptoms requiring hospitalization; (2) elevated serum markers (creatine kinase [CK, muscle enzyme] ≥5x upper limit of normal; (3) positive autoantibodies or EMG result.¹ Clinically diagnosed cases of myositis were then correlated with biopsy results to determine diagnostic yield. In all patients, 14G core biopsies were obtained under ultrasound guidance from the rectus femoris muscle with local anesthesia only. <h3>Results</h3> Thirty-six patients were identified with clinically diagnosed, severe immune-mediated myositis, and who also underwent core needle skeletal muscle biopsy. Twenty-eight (77.8%) had pathological findings on biopsy suggestive of myositis, 2 (5.6%) had negative findings, 5 (13.9%) had inconclusive findings, and 1 (2.8%) was nondiagnostic. Most patients had mixed immune infiltrates on biopsy with T cells (CD3+, CD4+, and CD8+) being seen in 18 biopsies, macrophages (CD163+) in 14 biopsies, and B cells (CD20+) in 5 biopsies. Of the 28 individuals with positive muscle biopsy results, 13 (44.8%) had detectable anti-striated muscle antibodies and 23 (79.3%) had EMG findings consistent with myopathy. Five patients had positive antibody, EMG, and biopsy results suggestive of myositis. <h3>Conclusions</h3> Core needle skeletal muscle biopsy is a reasonable alternative to open surgical muscle biopsy and may provide further insight into the immunological mechanisms underpinning this severe toxicity. Future studies will be needed to directly compare the utility of core needle biopsy to open surgical biopsy. <h3>Reference</h3> Siddiqui BA, Palaskas NL, Basu S, <i>et al</i>. Molecular pathways and cellular subsets associated with adverse clinical outcomes in overlapping immune-related myocarditis and myositis. <i>Cancer Immunol Res</i>. Published online May 20, 2024. doi:10.1158/2326-6066.CIR-24-0011. <h3>Ethics Approval</h3> This study was conducted under an Institutional Review Board approved protocol (PA16-0736) covering retrospective chart reviews in the Department of Genitourinary Medical Oncology at the University of Texas MD Anderson Cancer Center (MDACC).