My experience is due to my wife's pancan treatment which started upon diagnosis in July 2019. A significant drop in CA19-9 as you've had could be a good sign. There are no absolutes with the meaning of the number and one person's 900 can mean something different for another patient. In my wife's case she was at 2000 when diagnosed at stage 3 and dropped to 28 after chemo, sbrt and surgery. She hit 3000 when metastasis was identified then after more surgery and chemo hit a low of 180. Without verification of a scan, we don't know what the CA19-9 is telling us yet. Good luck and prayers for you.