diff --git a/gdcdictionary/schemas/imaging_study.yaml b/gdcdictionary/schemas/imaging_study.yaml
index fc8b025..eed8357 100644
--- a/gdcdictionary/schemas/imaging_study.yaml
+++ b/gdcdictionary/schemas/imaging_study.yaml
@@ -94,7 +94,9 @@ properties:
 
   PatientSex: # use subject.gender as source of truth, but this can store DICOM value
     description: (0010,0040) the patient's sex; the biologic character or quality that distinguishes male and female from one another as expressed by analysis of the person's gonadal, morphologic  (internal and external), chromosomal, and hormonal characteristics
-    type: string
+    enum:
+      - "Female"
+      - "Male"
 
   StudyDescription:
     description: (0008, 1030) Study Description.