Sorry to hear about your Dad's situation. It's awful that you will be responsible for so much of the cost.
There are never any guarantees that even with adjuvant chemotherapy that the cancer won't come back, but it's used because there is evidence that it reduces the chances that it does.
The likelihood that there are cancer cells floating around now that the tumor burst seems high to me. I'm no doctor.
It's worth a good discussion with your father's oncologists .
I'm sure others will chime in with some more useful advice.
Female age 39- ,2 teens.
Colon Cancer - DX March 2013
Age 34 at DX - Stage III B
Resection surgery -May 2013
FOLFOX - June, 2013 to Sept, 2013
5FU plus leukavorin Sept, 2013 to Dec, 2013
METs liver and lungs discovered Sept, 2015
KRAS - MSS
FOLFIRI plus Avastin - Sept, 2015 - July 2017
Durvalumab and Cediranib Sept 2017 Dec 17
FOLFOX with desensitization protocol - current