I'm so sorry you are dealing with this. I developed a 9 cm complex ovarian cyst when I was 49. I was told that it had a chance, albeit small, of being malignant. I regret that I let the fear (more so expressed by others than I personally felt) drive me to follow my gyn's recommendation to have everything removed. I have deeply regretted it ever since for all the problems it has caused.
My gyn (whom I had trusted for 20 years) was dishonest about my treatment options and the short and long term effects of having the uterus and ovaries removed.
The ovaries don't shut down at menopause. They continue producing hormones throughout the life span especially androgens, some of which is converted into estrogen. Castrated women have 50% less testosterone than naturally menopausal women.
I wish I had sought out a surgeon who would do a cystectomy (cyst removal) if the cyst was benign or, worst case, remove JUST the one ovary if the ovary couldn't be saved. (Surgeons with good cystectomy skills can almost always save the ovary or enough of it for hormone production. I wish I had known to revise my surgical consent form and have the surgeon initial the changes. I would probably still have my organs except possibly the one ovary.
Dermoid cysts are benign. It is also my understanding that they are usually pretty well defined on imaging / high certainty of being dermoid. You may want to get a copy of the CT report as well as the ultrasound report. That will tell you the size (if you don't already know) as well as how it was defined by the radiologist (its traits / characteristics).
As far as blood work, if CA 125 is one of the tests they want to do, it is not very accurate at diagnosing ovarian cancer. Benign conditions can cause an elevated result. There are also false negatives.
I wish you the best in advocating for proper treatment. I know it can be hard especially since female organ removal is often unwarranted.
Please keep me posted.