I have a left ovarian 12cm cystic mass with ascites found in ct. is septated with minimal peripheral enhancement. no calcification/fat component. & dilated appendix 13mm, subtle stranding, small local nodes. is this likely to be cancer?

Difficult to say: The simple answer is yes, this COULD be cancer BUT there are several other possibilities, including a range of benign (not life-threatening) conditions. The answer to your question most likely will come from blood tests and/or a tissue sample. A 12cm is large and in most situations you would have a surgery to remove this. You need to discuss this further with your specialist.