The study is a pilot, observational, retrospective, and (italian) multicenter study.

The study will involve the collection of patient data from medical records for patients with Multicentric Castelman Disease Relapsed/Refractory who received treatment with at least one dose of siltuximab, as part of standard of care in a real-life context, from July 2016 till April 2022 in 31selected italian centres.

The aim of the present study is to provide useful information about use, effectiveness, and safety profile of siltuximab given to relapsed or refractory (R/R) MCD in a real-life context since approval as more data are needed to improve knowledge in this rare disease. The observational, non-interventional nature of the study is based on the retrospective observation of current clinical practice without the application of any kind of ad hoc 'intervention' for the study itself. Infact, patients participating in the study will not be subjected to any procedures outside the normal clinical practice; likewise, the clinical variables that will be collected for the study are those that are already commonly collected by the physicians in clinical practice.

Inclusion Criteria:

1. Histologically confirmed diagnosis of R/R MCD HIV and HHV-8 negative patients who underwent siltuximab in a real-life context from July 2016 till April 2022.
2. Age ≥ 18 yearsatenrollment
3. Signature of written informed consent (where applicable)

Exclusion Criteria:

1. R/R MCD patients who underwent siltuximab in a clinical trial context

Patients with R/R MCD who receive at least one dose of siltuximab in a real-life context from July 2016 till April 2022.