This study will prospectively investigate the feasibility and safety of administering LDRT in patients with immunotherapy-induced inflammatory arthritis.

Inclusion Criteria:

• Patients must have histologically confirmed malignancy that is metastatic or unresectable and for which they are receiving single agent or combination immunotherapy or within the last 6 months.
• Rheumatologist-established diagnosis of new onset CTCAE grade 2-3 immunotherapy-induced inflammatory arthritis or activated osteoarthritis, involving one or more joints (up to 3) including but not limited to hands, shoulders, elbows, hips, knees and ankles.
• Age ≥ 40 years.
• ECOG performance status ≤2 (Karnofsky ≥60%).
• Patients with ability and willingness to sign consent.

Exclusion Criteria:

• Contraindications to radiation treatment, such as pregnancy, breast-feeding women and active collagen-vascular disease.
• Women of child-bearing potential who have positive urine or serum pregnancy test or refuse to do pregnancy test unless last menstrual cycle was >1 year prior to consent and/or clear documentation states that the patient is peri- or post-menopausal or there was recent supporting objective evidence of 'no pregnancy' status (e.g. blood or imaging) within 30 days prior to initiation of study treatment.
• Have not received systemic steroids within the last 3 months for the treatment of immune related adverse events.
• History of prior autoimmune conditions, or concurrent immune related adverse events requiring systemic immunosuppressants.
• Estimated life expectancy less than 3 months as assessed by investigators.
• Any condition at the discretion of investigator that will preclude participation in the study.
• History of secondary malignancy as assessed by investigators.
• History of radiation to affected joints as assessed by investigators.