De-identification of DICOM dates
The resulting DICOM dates are meaningless yet preserve the relative temporal distance between studies for a patient
De-identification of dates uses the DICOM standard “Retain Longitudinal With Modified Dates Option” which allows dates to be retained as long as they are modified from the original date. Date and Date-Time fields in TCIA DICOM image headers are de-identified by normalizing to a base date of January 1, 1960 and then shifted by the number of days between the original Study Date and an "anchor date". The anchor date for NCTN IROC data is the Date of Registration; this is the date the participant was registered for the study. The choice of '1960' was arbitrary, but it allows one to ensure that the dates in de-identified DICOM files have been properly de-identified as anything not around that year would be suspect.
TCIA Study Date = 01/01/1960 + (Original Study Date – Date of registration).
For example, if the original Study Date was 03/29/2018 and the Date of registration was 03/27/2018 then the Days from registration would be +2 and the TCIA Study Date would become 01/03/1960.
This technique de-identifies the dates while preserving the longitudinal relationship between dates. Therefore, a researcher won’t know the precise date the scan occurred, but if a follow up scan was performed 120 days later, that same 120 day difference between scans of a subject will exist in the TCIA images. Dates that occur in DICOM tags other than Date or Date-Time fields are removed. An example of this would be a date entered into the Series Description field. If the date is associated with a library for Code Meaning then that date is preserved as the date would be required to look up the meaning in the correct version of the library. To show that the dates have been modified, the term “MODIFIED” is written into DICOM tag (0028,0303) “LongitudinalTemporalInformationModified”.
Original dates will be first normalized to 01 January, 1960 and then offset relative to the date of registration.
Insertion of computed "REGISTRATION"/Days offset from registration" value
The inserted "REGISTRATION" or "Days offset from registration" value can be compared with similar values in the NCTN clinical data to understand the clinical context of the imaging study
The number of days the study occurred relative to the date of registration is calculated (using the registration date as identified for each patient) 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”. The value in the (0012,0052) DICOM tags is given in days as related to the date of registration which links the imaging data to the clinical data for a given subject. 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 Attribute set to “Days offset from registration” provides this same information.
The normalized date workflow described above requires that the registration date be present and this workflow does not handle the example where the registration date is not present.