I'm curious why you think it's cancer and what type (endometrial, ovarian)? What has your doctor told you?
There are benign causes for inhomogenous (heterogenous) endometrium. And most of them are benign. Uterine fibroids are one example. Polyps are another. Fibroids are extremely common and 99% of them are benign. Depending on the type of fibroids, they can make the lining appear thicker than it is.
In women who are still cycling (pre-menopausal), the lining typically gets as thick as 1.6cm (16mm). After menopause and if not on hormones, they like to see it less than .5cm. But a thickened lining doesn't necessarily mean cancer either. It can take awhile for a thickened lining to cause abnormal cells that have the potential to become cancerous (endometrial hyperplasia). Most cases of hyperplasia are treatable with medication (progestins).
It sounds like you have a small ovarian cyst. Most cysts are benign and resolve on their own within a few cycles / months, no intervention needed. Imaging is not exact (MRI may be more so than u/s) so it can be difficult to tell the type of cyst. Some cysts that resolve on their own can appear solid on imaging such as hemorrhagic. This site may be helpful - http://ovaryresearch.com/ovarian_cysts.htm.
Let us know what more you find out.