A Reminder App to Optimize Bladder Filling During Radiotherapy for Patients With Prostate Cancer (REFILL-PAC): Protocol for a Prospective Trial - Dirk Rades; Jan-Dirk Küter; Michael von Staden; Ahmed Al-Salool; Stefan Janssen; Carmen Timke; Marciana Nona Duma; Tobias Bartscht; Christine Vestergård Madsen; Charlotte Kristiansen; Florian Cremers (JMIR Research Protocols)