@Edent via what method did they give the file set to you?

It looks like DICOM mandates use of the ISO9660 standard at various points[0] and it also defines a "Secure DICOM File Format"[1] - perhaps that "Compact File Set" specification is a convenient patent-free and royalty-free way to acheive compatibility with DICOM?

[0] https://dicom.nema.org/medical/dicom/2018d/output/html/part12.html#sect_3.1
[1] https://dicom.nema.org/medical/dicom/2018d/output/html/part10.html#sect_7.4