Could you please post the op notes completely. There are some improtant points to be known in this.
It could be a fibroid with cystic changes by degeneration- like RED DEGENERATION which shows cystic character. If so, it could only be considered as yet another sub mucous or intramural myoma and it is bundled in the 58146.
To validate for 49203, a tumor/cyst endometrioma , should be in the abdominal cavity-side, like, serosal side of the uterus, or the peritoneum, broad ligament , uterine adenexa, utro-sacral septum or to other pelvic ligament or structure.
This cyst is intramural/ luminal/ submucous of the uterine cavity side. So does not merit the code 49203.
I feel that you would be fine with 58146.
If more work and time consuming and extra efforts taken, you could append with Modifier-22.
Thank you