Paraneoplastic Syndromes Mimicking Dermatomyositis and Remitting Seronegative Symmetrical Synovitis With Pitting Edema (RS3PE) Syndrome in Cancer of Unknown Primary Origin: A Case Report

Abstract

A 70-year-old male patient presented with fever, polyarthritis, systemic muscle weakness and pain, and skin rash, initially suspected to be an autoimmune disorder. Imaging revealed right supraclavicular and paratracheal lymphadenopathy, and a right supraclavicular lymph node biopsy confirmed squamous cell carcinoma. Still, the primary site remained unidentified, leading to a diagnosis of cancer of unknown primary origin (CUP). Laboratory tests showed no positive autoantibodies such as anti-Jo-1, anti-ribonucleoprotein (RNP), anti-Smith (Sm), and anti-SS-A antibodies, and a skin biopsy of the back indicated panniculitis with neutrophilic infiltration. Given the absence of infectious or autoimmune causes, the symptoms were attributed to paraneoplastic syndrome (PNS) associated with CUP, mimicking dermatomyositis and remitting seronegative symmetrical synovitis with pitting edema (RS3PE) syndrome. Treatment with prednisolone (15 mg/day) led to the rapid resolution of joint pain, rash, and fever. Chemotherapy with carboplatin and paclitaxel for CUP was initiated with minimal adverse effects, allowing for continued outpatient management. This case highlights the importance of considering PNS when collagen disease-like symptoms are present in malignancy, particularly in CUP. Early recognition and corticosteroid therapy can improve performance status, enabling timely cancer treatment. Identifying atypical PNS presentations in CUP remains challenging, but a multidisciplinary approach can aid in diagnosis and management.

Keywords: arthritis; carcinoma; dermatomyositis; neoplasms; paraneoplastic syndromes; remitting seronegative symmetrical synovitis with pitting edema syndrome; rheumatoid; squamous cell; steroids; unknown primary.

Figure 1. Positron emission tomography-computed tomography demonstrating the uptake in the right supraclavicular (A) and right paratracheal (B) lymph nodes (white arrows).

Figure 2. A right supraclavicular lymph node biopsy showing squamous cell carcinoma with keratinization and intercellular bridges, supporting the diagnosis of metastatic squamous cell carcinoma.

Figure 3. Excoriated-like rashes on the back (white arrows)

Figure 4. Contrast-enhanced computed tomography showing fluid accumulation in the left acromioclavicular joint (white arrows)

Figure 5. A skin biopsy revealing lobular and septal panniculitis with perivascular lymphocytic infiltration and neutrophilic predominance (white arrows)