Self-monitoring of Physical Activity After Hospital Discharge in Patients Who Have Undergone Gastrointestinal or Lung Cancer Surgery: Mixed Methods Feasibility Study - Marijke Elizabeth de Leeuwerk; Martine Botjes; Vincent van Vliet; Edwin Geleijn; Vincent de Groot; Erwin van Wegen; Marike van der Schaaf; Jurriaan Tuynman; Chris Dickhoff; Marike van der Leeden (JMIR Cancer, 2022)