Lars is right, this indeed seems to be a problem of you using old SNOMED RT codes. And Dicom has a mapping table, but it is only a subset. The real RT has 28k T- codes and 5k M-codes. I’ll attach the zip file of SNOMED RT to CT (in OMOP ID notation), so you don’t have to do any more gymnastics.
SNOMED RT to OMOP.txt (557.9 KB)