code: def get_cancer_studies(study_filter=None):
      
      data = {'cmd': 'getCancerStudies'}
      df = send_request(**data)
      res = _filter_data_frame(df, ['cancer_study_id'],
                               'cancer_study_id', study_filter)
      study_ids = list(res['cancer_study_id'].values())
      return study_ids
docs: Return a list of cancer study identifiers, optionally filtered.
  
      There are typically multiple studies for a given type of cancer and
      a filter can be used to constrain the returned list.
  
      Parameters
      ----------
      study_filter : Optional[str]
          A string used to filter the study IDs to return. Example: "paad"
  
      Returns
      -------
      study_ids : list[str]
          A list of study IDs.
          For instance "paad" as a filter would result in a list
          of study IDs with paad in their name like "paad_icgc", "paad_tcga",
          etc.