Rationale: Cancer treatment is increasingly successful, resulting in a rising number of cancer survivors. A substantial number of survivors may experience long-term and late side effects from their cancer treatment. In addition, evidence is accumulating that an active lifestyle positively influences cancer treatment outcome and changes the recurrence rates of the disease. Therefore, physical activity (PA) programs are urgently needed and should be incorporated in current treatment regimens. It is noted though that cancer patients and survivors experience difficulties in remaining physically active. A 'smartphone application' (app) may be an accessible way to counteract these problems. The app 'RunKeeper', founded by Mr. Jason Jacobs, 2008, FitnessKeeper Inc. (RunKeeper) is a free, widely spread and well-known app for self-monitoring PA. Convenient features of RunKeeper are self-monitoring PA with GPS or stopwatch, recording progress, goal setting, and personal records. In this study the investigators aim to determine if the RunKeeper app use improves PA by self-monitoring and empowering PA during or after cancer treatment in comparison with usual care in a 12-week follow-up.

Objective: The primary objective is to identify an improvement in PA as measured by the PASE questionnaire when using the RunKeeper app in comparison with usual care for 12 weeks during or after cancer treatment. The investigators hypothesize that the RunKeeper app might assist in stimulating and improving PA behavior. Secondary objectives are to explore the usability of the RunKeeper app.

Study design: The present study is a single-centre prospective two-armed randomized controlled feasibility study.

Study population: Adult patients diagnosed with cancer currently being treated or under surveillance at the department of Medical Oncology at the UMCG.

Intervention: Patients who give informed consent will be randomized in one of the two study arms; 'Group A', usual care (N=15) or 'Group B', usual care + the RunKeeper app (N=15). Directly after randomization, Group B will be sent a brief user's manual for RunKeeper and requested to install the RunKeeper app. Group B will use the RunKeeper app for 12 weeks to self-monitor PA.

Main study parameters/endpoints: The primary endpoint is to calculate the effect-size of the RunKeeper app use for 12 weeks on PA as compared to usual care as measured by the Physical Activity Scale for the Elderly (PASE) questionnaire. The investigators hypothesize that the RunKeeper app might assist in stimulating and improving PA behavior.

Inclusion Criteria:

• Patients >18 years of age diagnosed with cancer (regardless of type/taxonomy) at the medical oncology department, UMCG
• Patients undergoing cancer treatment or surveillance
• WHO-performance score ≤1
• Signed informed consent

Exclusion Criteria:

• Inability to read or understand the Dutch language
• Inability to handle or not in possession of a smartphone
• Active user of the RunKeeper app
• Severe cardiac (recent cardiovascular event) or psychiatric disease
• Severe kidney or liver impairment
• Pancytopenia
• Patients following supervised oncologic rehabilitation