Here is the design of a clinical trial:

Official Title: A Prospective Multi-center Randomized Clinical Trial to Compare Survival Rates and Quality of Life According to Follow-up Period in Patients Who Underwent Radical Gastrectomy for Advanced Gastric Cancer (STOFOLUP Trial)

Included conditions:
- Gastric Cancer
- Recurrence
- Quality of Life

Study Armgroups:
- {'label': '3 months follow-up', 'type': 'EXPERIMENTAL', 'description': 'Patients will be follow-up every 3 months after gastrectomy.', 'interventionNames': ['Diagnostic Test: Computed tomography, Chest X-ray, and blood test']}
- {'label': '6 months follow-up', 'type': 'ACTIVE_COMPARATOR', 'description': 'Patients will be follow-up every 6 months after gastrectomy.', 'interventionNames': ['Diagnostic Test: Computed tomography, Chest X-ray, and blood test']}

Interventions:
- {'type': 'DIAGNOSTIC_TEST', 'name': 'Computed tomography, Chest X-ray, and blood test', 'description': 'CT will be performed every 3 months after gastrectomy in the 3-months follow-up group. CT will be performed every 6 months after gastrectomy in the 6 months follow-up group.', 'armGroupLabels': ['3 months follow-up', '6 months follow-up']}

Primary Outcomes:
- {'measure': '3-year overall survival', 'description': 'Death from any cause is defined as an event.', 'timeFrame': '3 years after surgery'}

Please estimate the sample size based on the assumption: 
The sample size calculation assumes an alpha error of 0.05, a statistical power of 0.80, and a drop-out rate of 10%.
Sample size calculation
        The hypothesis of the poSTOperative FOLlow UP of gastric cancer patients for improved survival and quality of life (STOFOLUP) trial is that the 3-year OS will be different between the 3-month and 6-month groups. We assumed that the 3-year OS rate in the 6-month group will be 83% based on the results of the Capecitabine and Oxaliplatin Adjuvant Study in Stomach Cancer (CLASSIC) trial.10 Moreover, a survival difference of 6% has been considered between the two groups based on the COLOFOL trial.11 The sample size is planned for an accrual period of 24 months and a follow-up duration of 36 months. A total of 796 events are needed to detect this difference with an alpha error of 0.05 and a statistical power of 0.80. A drop-out rate of 10% has been considered and the final sample size is estimated to be 443 patients in each group (886 patients in total).