Is single hypoechoic 2.5cm liver lesion in lateral segment of left lobe always cancer?

Not at all: A hypoechoic liver lesion, which is something seen on ultrasound, can arise from a number of things, both benign and malignant. The most common cause of this finding is a benign (non-cancerous) lesion called a hepatic hemangioma. It can also be a benign adenoma, fat in the liver, or a lipoma. However, it can also be a finding in cancer. You should have further testing to know for sure.

More details: about the appearance and clinical setting would be necessary to assign a probability of malignancy, but in general, as an incidental finding, this almost always is NOT cancer. There are a number of benign liver lesions that commonly occur. However, additional imaging studies and/or interval follow up may be necessary to make a determination. Talk to your doc.