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Quote from a clinical trial report (JGOG3022 trial):

Patients who were scheduled to receive bevacizumab concomitantly with platinum-based combination chemotherapy after PDS or IDS were included in the observation cohort of this study before enrollment.

This observation cohort is not mentioned anywhere else in the document. What is its purpose?

Higher in the text, there's a mention of PDS/IDS in the inclusion criteria paragraph:

(3) patients in whom the period from primary debulking surgery (PDS) or interval debulking surgery (IDS) to the initiation of bevacizumab was at least 28 days.

Does it mean that the "observation cohort" was meant for keeping patients under observation until the required perios of 28 days has elapsed, and then to decide whether they are fit to be enrolled in the trial and given additional bevacizumab with their chemo treatment?

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This entire study was observational; they refer to the entire group as the "observation cohort" but they selected a subset of cases who received a certain treatment to complete their further analyses. Since they don't talk further about the "observational cohort" you can mostly ignore it for interpreting their results, it is only important in the context of how they selected their subjects, which I think is fairly clear if you just ignore those two words.

I think it's likely, though this is just a guess because I don't see clear description in the paper and I don't intend to dig further, that this 'study' they report is part of a larger observational study that will include other treatments, perhaps other cancers, etc. However, they are reporting on just one piece of it and treating it as a self-contained study. There's nothing really wrong with that as long as the 'sub-study' is planned in advance.

"Observational", in this context, means that the researchers made no treatment decisions: they assess the results of treatment only by how routine practice is applied and the resulting outcomes. This sort of study design is common for studying standards of care and establishing baselines, but it is less informative than a randomized controlled trial because subjects are selected for treatment on non-random criteria. For example, sicker patients might be more likely to receive a certain treatment, which might make that treatment look less effective or more dangerous than it really is.

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