Oncolytic Virotherapy to Enhance PReoperative IMmunotherapy Efficacy in Patients With Proficient Mismatch Repair (pMMR) Rectal Cancer

Inclusion Criteria: * Histological diagnosis of primary, localised rectal adenocarcinoma (cT2N0M0 to cT3bN2M0, TNM classification version 8 * Diagnosis of Proficient Mismatch Repair (pMMR) rectal adenocarcinoma (using biopsy from the initial diagnostic endoscopy) * Suitable for potentially curative surgical resection * No contraindications for treatment with pembrolizumab * Not requiring neoadjuvant therapy * Aged \> 18 years at the time of inclusion * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * Have baseline laboratory results as follows: * Absolute neutrophil count (ANC) ≥ 1.0 x 109/L * Platelets ≥ 100 ×109/L (without platelet transfusion) * Haemoglobin ≥ 6.2 mmol/L or 10.0 g/dL (with or without red blood cell (RBC) transfusion) * Serum creatinine ≤ 1.5 × upper limit of normal (ULN) * Bilirubin \< 1.5 × ULN (or \< 2.5 x ULN in patients with Gilbert's syndrome) * ALT, AST and alkaline phosphatase \< 3 × ULN * Provide written informed consent in accordance with all applicable regulations and follow the study procedures. Subjects must be capable of understanding the investigational nature, potential risks, and benefits of the study. Exclusion Criteria: * Have impending bowel obstruction or other indications for acute surgical intervention * Have had concurrent immunotherapy in the 3 months before the start of the study therapy. * Have acute or chronic hepatitis B or hepatitis C infection * Evidence of immunosuppression for any reaso…