De-identification of dates for this dataset uses the DICOM Part 3.15 Annex E standard “Retain Longitudinal With Modified Dates Option” which allows dates to be retained as long as they are modified from the original date. TCIA implements this using a technique which de-identifies the dates while preserving the longitudinal relationship between them. Original dates will be first normalized to 01 January, 1960 and then offset relative to the date of registration for each patient. This normalized date system was chosen in order to make it obvious that the dates are not real, and to make it easy to quickly determine how much time has passed between the date of registration and the patients' related imaging studies.
For example, if the real date of a patient's registration was 03/27/2018 and the original imaging Study Date was 03/29/2018 then the "Days from registration" would be +2 and the anonymized TCIA Study Date would become 01/03/1960.
In addition to modifying the actual date fields in the DICOM header, the "days from registration" values are calculated and stored in the DICOM tag (0012,0052) Longitudinal Temporal Offset from Event with the associated tag (0012,0053) Longitudinal Temporal Event Type set to “REGISTRATION”.
Note: If these DICOM tags are not present, DICOM tag (0012,0050) Clinical Trial Time Point ID with the associated tag (0012,0051) Clinical Trial Time Point Description provides this same information. This inconsistency is due to a change in how dates were handled in the first NCTN trials that were published on TCIA.