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 consideration and statistical analyses
        
          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].
        
        
          Sample size considerations specific for the low grade cohort, LOGOS
          Assumptions about efficacy for the power calculations are based on the retrospective analysis of Gershenson et al. [17]. We assume exponential PFS distributions, the analysis by a two-sided log-rank test with alpha level 0.05, and a power of 80%. With an accrual duration of 7Â years and a follow-up phase of 5Â years after accrual of the last patient, a dropout rate of 15%, and a median PFS of 36Â months in the control arm and 60Â months in the experimental arm (i.e. a hazard ratio of 0.6 between arms). Because the main protocol of the MATAO trial schedules its final PFS analysis at approximately 5Â years after inclusion of the last MATAO patient (while recruitment into the LOGOS stratum will still be ongoing) and states grade as a factor for subgrouping, technically, this constitutes an interim analysis of the LOGOS data. To account for this interim analysis, we plan an OâBrien-Fleming group sequential design with one interim look and one final look. The sample size / power calculations were performed using ADDPLAN version 6.1. Accounting for 15% dropout, 186 patients (i.e. approximately 2.2 patients per month) should be recruited into LOGOS in order to have the 158 evaluable patients, necessary to achieve a power of 80% with this OâBrien-Fleming group sequential design. The boundaries for the group sequential design at the interim look will be based on the actual number of observed events and will be calculated using the alpha-spending function for an OâBrien-Fleming design. The final analysis will be performed after observation of 121 events.