Neo-adjuvant Pembrolizumab and Radiotherapy in Localised MSS Rectal Cancer

This study is testing whether adding the immunotherapy drug pembrolizumab (Keytruda) to short-course radiation therapy before surgery can improve outcomes for people with rectal cancer that has specific biological features (called microsatellite stable, or MSS). This type of cancer does not usually respond well to immunotherapy on its own. **You may be eligible if...** - You are 18 years old or older - You have been diagnosed with rectal cancer (adenocarcinoma) that has not been treated yet - Your tumour is a locally advanced stage (T3-T4 or with lymph node involvement) but has not spread to distant organs - Your tumour has been confirmed to be microsatellite stable (MSS) - A specialist team has recommended short-course radiation and surgery as your treatment **You may NOT be eligible if...** - Your tumour has high microsatellite instability (MSI-High) - You have previously received any immunotherapy drugs (such as anti-PD-1 or anti-CTLA-4) - You have received prior systemic cancer treatment for rectal cancer Talk to your doctor to see if this trial is right for you.