Clinical Value of ¹⁸F-FDG PET in Paraneoplastic and Pembrolizumab-Associated Myositis

Abstract

Fluorine-18 fluorodeoxyglucose positron emission tomography (¹⁸F-FDG PET) is widely used to evaluate patients with cancer and to detect various inflammatory processes. Here, we report a rare case of a 53-year-old woman with breast cancer who developed generalized pain, weakness, rash, and edema over the trunk after the first cycle of neoadjuvant pembrolizumab and cytotoxic chemotherapeutic agents. ¹⁸F-FDG PET revealed diffusely increased uptake in the muscles without skeletal or visceral metastasis, aiding in the diagnosis of paraneoplastic and pembrolizumab-associated myositis and subsequent monitoring. This case underscores the value of ¹⁸F-FDG PET in differentiating myositis from tumor progression and monitoring treatment response in such cases.

Figure 1. A 53-year-old woman with triple-negative breast cancer (TNBC), classified as cT2N1M0 [1], received neoadjuvant therapy with pembrolizumab and cytotoxic chemotherapeutic agents [2]. After the first cycle, she developed generalized pain, weakness, rash, and edema over the trunk and limbs, eventually becoming bedridden. Fluorine-18 fluorodeoxyglucose positron emission tomography (¹⁸F-FDG PET) was performed to rule out disease progression and skeletal metastasis. The patient fasted for at least 4 h before intravenous ¹⁸F-FDG injection (400.00 MBq). The patient did not exercise before the injection. We performed the ¹⁸F-FDG PET scan from head to thigh 60 min after the radiotracer injection. (A): Maximum Intensity Projection showing diffusely increased uptake in the muscles without skeletal or visceral metastasis. Coronal (B,C) and axial PET and fused PET/CT images showed the scapular (D,E) and gluteal (F,G) muscle-to-liver mean standardized uptake value (SUV_mean) ratios of 1.51 and 1.69, respectively. Laboratory tests showed elevated creatine kinase at 1945 U/L (reference range: 30–223 U/L) [3] and positive anti-transcription intermediary factor 1-γ autoantibodies. Taken together, these findings confirmed the diagnosis of paraneoplastic and pembrolizumab-induced myositis [4,5,6].

Figure 1. A 53-year-old woman with triple-negative breast cancer (TNBC), classified as cT2N1M0 [1], received neoadjuvant therapy with pembrolizumab and cytotoxic chemotherapeutic agents [2]. After the first cycle, she developed generalized pain, weakness, rash, and edema over the trunk and limbs, eventually becoming bedridden. Fluorine-18 fluorodeoxyglucose positron emission tomography (¹⁸F-FDG PET) was performed to rule out disease progression and skeletal metastasis. The patient fasted for at least 4 h before intravenous ¹⁸F-FDG injection (400.00 MBq). The patient did not exercise before the injection. We performed the ¹⁸F-FDG PET scan from head to thigh 60 min after the radiotracer injection. (A): Maximum Intensity Projection showing diffusely increased uptake in the muscles without skeletal or visceral metastasis. Coronal (B,C) and axial PET and fused PET/CT images showed the scapular (D,E) and gluteal (F,G) muscle-to-liver mean standardized uptake value (SUV_mean) ratios of 1.51 and 1.69, respectively. Laboratory tests showed elevated creatine kinase at 1945 U/L (reference range: 30–223 U/L) [3] and positive anti-transcription intermediary factor 1-γ autoantibodies. Taken together, these findings confirmed the diagnosis of paraneoplastic and pembrolizumab-induced myositis [4,5,6].

Figure 2. The paraneoplastic and pembrolizumab-induced myositis were treated with intravenous immunoglobulin (2 g/kg) and prednisolone at an equivalent daily dose of 50 mg [7,8,9]. The prednisolone dose was gradually tapered to 5 mg daily over a two-month period. The patient subsequently underwent a modified radical mastectomy, with a final pathological stage of ypT2N1a. Follow-up ¹⁸F-FDG PET performed 5 months after the initial scan revealed reduced muscular ¹⁸F-FDG uptake (A–C), with scapular (D,E) and gluteal (F,G) muscle-to-liver SUV_mean ratios of 0.85 and 0.98, respectively. The creatine kinase level normalized, and her symptoms gradually subsided. She could walk using a walker and required minimal assistance with self-care. Myositis is a rare comorbidity among patients with TNBC, whether as an immune-related adverse event of pembrolizumab or as a paraneoplastic manifestation. In this case, the onset occurred immediately after the first cycle of pembrolizumab, suggesting that the pembrolizumab may trigger a latent paraneoplastic phenomenon. While serum creatine kinase levels are routinely used as a biomarker of muscle injury, ¹⁸F-FDG PET enables direct visualization of inflammatory activities, allows assessment of myositis severity, and assists in distinguishing myositis from cancer metastasis. This case demonstrates the value of ¹⁸F-FDG PET as a molecular imaging modality to evaluate patients with cancer with suspected paraneoplastic or immune checkpoint inhibitor-associated myositis.

Figure 2. The paraneoplastic and pembrolizumab-induced myositis were treated with intravenous immunoglobulin (2 g/kg) and prednisolone at an equivalent daily dose of 50 mg [7,8,9]. The prednisolone dose was gradually tapered to 5 mg daily over a two-month period. The patient subsequently underwent a modified radical mastectomy, with a final pathological stage of ypT2N1a. Follow-up ¹⁸F-FDG PET performed 5 months after the initial scan revealed reduced muscular ¹⁸F-FDG uptake (A–C), with scapular (D,E) and gluteal (F,G) muscle-to-liver SUV_mean ratios of 0.85 and 0.98, respectively. The creatine kinase level normalized, and her symptoms gradually subsided. She could walk using a walker and required minimal assistance with self-care. Myositis is a rare comorbidity among patients with TNBC, whether as an immune-related adverse event of pembrolizumab or as a paraneoplastic manifestation. In this case, the onset occurred immediately after the first cycle of pembrolizumab, suggesting that the pembrolizumab may trigger a latent paraneoplastic phenomenon. While serum creatine kinase levels are routinely used as a biomarker of muscle injury, ¹⁸F-FDG PET enables direct visualization of inflammatory activities, allows assessment of myositis severity, and assists in distinguishing myositis from cancer metastasis. This case demonstrates the value of ¹⁸F-FDG PET as a molecular imaging modality to evaluate patients with cancer with suspected paraneoplastic or immune checkpoint inhibitor-associated myositis.