Peau d'orange is a classic clinical sign of advanced breast cancer caused by lymphatic obstruction due to tumor infiltration of the dermal lymphatics. This results in edema of the skin, producing a characteristic thickening and dimpling that resembles the surface of an orange. It indicates locally advanced disease and often suggests poor prognosis. This finding helps differentiate advanced malignancy from benign breast lumps, which are usually mobile and not associated with skin changes. Hence, peau d'orange is a critical marker for inflammatory breast cancer or other aggressive cancers invading dermal lymphatics.

Reference: Bailey & Love's Short Practice of Surgery, 27th Edition.