Combined radiotherapy and immunotherapy in urothelial bladder cancer: harnessing the full potential of the anti-tumor immune response

PURPOSE: Radiotherapy (RT), as part of trimodal therapy, is an attractive alternative treatment in patients with urothelial muscle-invasive bladder cancer (MIBC). There is accumulating evidence suggesting the immunomodulatory effects of RT and its potential synergy when combined with immunotherapy. The aim of this review was to report on the most recent advances on this combination, including the mechanisms of RT immunomodulation, practical approach to combining RT and immunotherapy, and ongoing clinical trials in bladder cancer. METHODS: Using the PubMed database, we identified articles published between March 2004 and April 2020 on the combination of RT with immunotherapy in localized or metastatic MIBC. A search of the Clinicaltrials.gov and Clinicaltrialsregister.eu/ retrieved ongoing clinical trials on the topic as well. RESULTS: Combination of RT with immunotherapy leads to immunogenic cell death and an increase in immune markers thus leading to improved tumor control. For localized MIBC, there are safety concerns related to the use of concurrent immunotherapy with hypofractionated RT, thus neoadjuvant or adjuvant immunotherapy is preferred. In the metastatic setting, the combination of multi-site RT with SBRT-like doses (≥ 6 Gy per fraction) and concurrent immunotherapy seems most efficacious at harnessing the abscopal effect. At least 25 clinical trials combining immunotherapy and RT in MIBC are currently ongoing and will answer pending questions on safety, efficacy, and practical considerations on RT scheduling, fractionation, and targets volumes. CONCLUSION: RT has the potential to synergize with immunotherapy to improve oncological outcomes in patient with localized or metastatic MIBC. Clinical trials results are eagerly awaited.