Developing and Integrating Digital Sources in an Accessible and Sustainable Online Platform for Adolescents and Young Adult Cancer Survivors: Collaborative Design Approach - Carla Vlooswijk; Sophia HE Sleeman; Jonas Pluis; Daphne Bakker; Lisanne de Groot; Eveliene Manten; Peter Heine; Olga Husson; Mies C van Eenbergen; Pieter Vandekerckhove (JMIR Formative Research)