Vandetanib hydrochloride is primarily used in the treatment of medullary thyroid cancer, particularly in cases where the cancer is advanced or cannot be surgically removed. It works by inhibiting specific enzymes and receptors involved in the growth and spread of cancer cells, such as vascular endothelial growth factor receptor (VEGFR) and epidermal growth factor receptor (EGFR). This targeted approach helps to slow down tumor progression and manage symptoms associated with the disease. It is typically prescribed for patients who are not candidates for surgery or other localized treatments. Due to its potential side effects, its use is closely monitored by healthcare professionals.