Soreness and discomfort in the left shoulder is not necessarily a sign of cancer. For example, frozen shoulder, muscle strain or other injuries to the shoulder joint may also cause this soreness. Of course, if it is a manifestation of cancer, it is also possible, for example, if a cancer patient has bone metastasis in the shoulder joint, or if, for example, lung cancer or breast cancer on the left side causes discomfort in the shoulder joint. Cancer cannot be diagnosed based on the symptom of soreness alone. There are many different types of cancer in different areas, and the clinical manifestations are all different. They may mainly manifest as local lumps, pain, bleeding, ulcers and other lesions. Preliminary diagnosis can be made through imaging examination, but surgical biopsy pathology is usually required to determine whether it is cancer.