# Rapidly Progressive Joint Destruction in Idiopathic Multicentric Castleman’s Disease: A Case Report

## TL;DR

A patient with idiopathic multicentric Castleman's disease experienced rapid joint destruction similar to rheumatoid arthritis, highlighting overlapping disease mechanisms.

## Contribution

This case report highlights a unique instance of rapid joint destruction in iMCD, suggesting shared mechanisms with RA-like synovitis.

## Key findings

- The patient required bilateral total hip arthroplasty due to progressive joint destruction.
- Rheumatoid arthritis-like synovitis was observed despite treatment with tocilizumab.
- mTOR activation in iMCD may contribute to synovitis and joint erosion.

## Abstract

Castleman's disease (CD) is a group of lymphoproliferative disorders characterized by common morphological features on lymph node biopsy, with idiopathic multicentric Castleman's disease (iMCD) being a notable subtype. Here, we report a 69-year-old Japanese iMCD patient complicated by rapidly progressive joint destruction. Her joint destruction progressed rapidly around the time of her iMCD diagnosis, and she underwent right total hip arthroplasty (THA). Synovial tissue revealed rheumatoid arthritis (RA)-like synovitis. Joint destruction continued to progress even after tocilizumab (TCZ) was initiated for iMCD. Before long, she underwent left THA. Joint destruction due to cytokines other than interleukin-6 (IL-6), or other inflammatory pathologies, was suspected. Mechanistic target of rapamycin (mTOR) activation in iMCD may promote synovitis and joint erosion by regulating immune cell function and osteoclast differentiation. Although there are common pathological mechanisms between iMCD and destructive synovitis (RA-like synovitis), a case of rapidly progressive bone destruction has not been previously reported. Therefore, we report a unique case of iMCD complicated by destructive synovitis (RA-like synovitis) to contribute to the understanding of these overlapping disease mechanisms and potential therapeutic strategies.