What is the smallest tumor ultrasound can detect?

No clear number: There is no specific size that is required for an ultrasound to be accurate. An ultrasound looks both at the size and shape of a nodule, but also looks at what kind of shadow is cast by the nodule when the ultrasound directs the waves toward the area. Ultrasound can be very helpful to assess lymph nodes as small as 1 cm for women with breast cancer.