The STELLAR trial will assess the effect of acalabrutinib taken in combination with CHOP-R compared to taking CHOP-R alone in patients with newly diagnosed Richter's Syndrome (RS). It will also be a platform to test other new drugs that show potential for treating RS. Chronic lymphocytic Leukaemia (CLL) is the most common blood cancer in adults, usually in their 70s or older. In a few patients, CLL can transform from a slow-growing cancer into an aggressive lymphoma called Richter's Syndrome. RS is very difficult to treat and patients have a short life-expectancy - usually a few months after diagnosis. Treatment for Richter's Syndrome in the UK is CHOP (four chemotherapy drugs) plus rituximab ('R' - an antibody treatment). The CHOP-R treatment is given as a standard of care for RS but has limited benefit - it is often temporary to extend life. Richter's Syndrome returns in most patients who then die from this disease. The STELLAR trial will investigate if a new drug called acalabrutinib, which is effective used by itself in patients with relapsed CLL and also some with Richter's Syndrome, will improve outcomes for newly diagnosed patients with RS. Acalabrutinib blocks a protein in CLL which can stop the cancer growing. Participants who have Richter's Syndrome and are suitable for CHOP-R will be recruited by specialised hospitals across the UK. People with another cancer, heart problems, or recent stroke cannot take part. Participants will have a lymph node biopsy, 3-4 bone marrow biopsies, blood samples, and PET-CT and CT scans. CHOP-R is given in a hospital every three weeks up to 6 times. All participants will receive CHOP-R; half will also receive acalabrutinib. When treatment with CHOP-R ends the patients who had acalabrutinib can continue to take it; patients who had CHOP-R alone may have acalabrutinib if their Richter's Syndrome returns after CHOP-R.