Here is the design of a clinical trial:

Official Title: MAintenance Therapy With Aromatase Inhibitor in Epithelial Ovarian Cancer: a Randomized Double-blinded Placebo-controlled Multi-centre Phase III Trial (ENGOT-ov54/Swiss-GO-2/MATAO), Including LOGOS (Low Grade Ovarian Cancer Sub-study).

Included conditions:
- Ovarian Neoplasm Epithelial
- Fallopian Tube Neoplasms
- Peritoneal Neoplasms
- High-grade Serous Ovarian Carcinoma (HGSOC)
- Low-grade Serous Ovarian Carcinoma (LGSOC)
- Ovarian Endometrioid Carcinoma

Study Armgroups:
- {'label': 'Letrozole (aromatase inhibitor)', 'type': 'EXPERIMENTAL', 'description': 'Letrozole, 2.5 mg Femara tablet, administered once daily for 5 years or until symptoms of toxicity or progression of underlying disease', 'interventionNames': ['Drug: Letrozole 2.5mg']}
- {'label': 'Placebo', 'type': 'PLACEBO_COMPARATOR', 'description': 'Placebo tablet of Femara (without aromatase inhibitor), 0 mg Femara tablet, administered once daily for 5 years or progression of underlying disease', 'interventionNames': ['Other: Placebo']}

Interventions:
- {'type': 'DRUG', 'name': 'Letrozole 2.5mg', 'description': 'Aromatase inhibitor', 'armGroupLabels': ['Letrozole (aromatase inhibitor)'], 'otherNames': ['Femara']}
- {'type': 'OTHER', 'name': 'Placebo', 'description': 'Placebo tablet of Femara', 'armGroupLabels': ['Placebo']}

Primary Outcomes:
- {'measure': 'Progression-free survival (PFS) for each study group', 'description': "PFS defined as the time from the date of first IMP administration until the date of progression (recurrence) or death by any cause in the absence of progression.\n\nAssessment of progression (recurrence) is generally indicated by SYMPTOMS and will be assessed by the investigator most commonly on the basis of CT scans of the pelvis, abdomen and thorax, according to RECIST v1.1 criteria recommended and mostly presented by an elevated CA-125 level. Elevated CA-125 levels alone shouldn't be considered as progression. Progression assessment according to local standard of care, however, is similarly acceptable.", 'timeFrame': 'Up to approximately 12 years'}

Please estimate the sample size based on the assumption: 
For the whole cohort, a two-sided alpha level of 5%, 90% power, and a 10% dropout rate are assumed. For the LOGOS cohort, a two-sided alpha level of 0.05, 80% power, a 15% dropout rate, and an O'Brien-Fleming group sequential design with one interim and one final look are assumed.
Sample size considerations of the whole cohort
          We estimated how many events are needed to achieve a 90% power to detect a hazard ratio (HR) of 0.70 between the two treatment arms (hazard letrozole/hazard placebo, assuming an exponential distribution of PFS). From the ICON7 adjuvant trial [10], which investigated bevacizumab in the adjuvant maintenance setting, it is expected that PFS is approximately exponentially distributed and the median PFS under standard care is 18Â months. The assumed HR of 0.70 would increase the median PFS in the treatment arm to 25.7Â months.
          Calculations were done assuming a two-sided alpha level of 5% and a log-rank test for comparison. The approximation described by Machin et al., (2009) resulted in a required sample size of 330 events, i.e., cases with progression over both study arms [45]. The recruitment will continue until this number of events is observed and the power of 90% has been achieved. When assuming, in addition to the aforementioned conditions, a uniform recruitment rate during three and a half years and a dropout rate of 10%, a total number of 540 patients would suffice to achieve the target number of 330 events five years after study start (calculated using the method by Lachin & Foulkes, 1986) [46].