Can A TVS Test Be Undetectable For A Tumor Or Mass In Endometrium?

Is it possible for a transvaginal ultrasound to miss a tumor or mass in the endometrium? I have Atypical Endometrial Hyperplasia, and have a surgery already scheduled to remove everything, ovaries, fallopian tubes, and uterus. A recent Ultrasound did not detect a mass or tumor. That said, my GYN Oncologist said that due to my medical history, weight and family history or colon cancer, my odds remain high for cancer already being present. So, my question is how reliable are Ultrasounds in helping to locate a mass or tumor. I was told that my endometrium was enlarged and significantly thickened