My 2p FWIW:
Metastatic cancer is cancer that has metastasised/metastasized (spread away from the original tumour).
Metastatic is an adjective. Metastasised/metastasized is the past participle of the verb (to metastasise/metastasize) that is sometimes also used as an adjective.
They basically mean the same thing but I would say there is a slight difference in usage.
As a general rule, I would use “metastatic” in technical scientific/medical texts, e.g. “metastatic bladder/breast/prostate/renal cancer” “metastatic sarcoma” or “metastatic cancer in the lungs”.
I might use “metastasised cancer” if I wanted to emphasise the metastases themselves rather than the type of cancer, or if I was aiming at a lay readership, e.g.: “If you have previously been diagnosed with breast cancer, the biology of the original breast cancer will be compared to the biology of the new metastasized cancer identified in the distant organ.”
There are plenty of reputable sources where you can check out how “metastasized cancer” is used. As in the link above, many of them are intended for a lay readership:
"Understanding Metastasized Cancer"