Well, cancer do not cause hair loss but the treatment ( chemotherapy and radiation on the head) does.
Hair loss occurs because chemotherapy targets all rapidly dividing cells including the healthy hair cells. Chemotherapy is designed to target rapidly dividing cancer cells but in targeting these cells, other rapidly dividing healthy cells are destroyed. Hair follicles is one of the fastest-growing cells in the body, so is among the healthy cells that is destroyed during chemotherapy.
During cancer treatment “chemotherapy’, hair loss begins gradually or dramatically. The extent of hair loss depends the timing of treatment and type of drug or treatment used. Some chemotherapy drugs causes thining of hair, hair loss on the head, while others cause complete hair loss on head, pubic, brows, lashes, arms and legs.
Radiation also cause hair loss but only on the particular part of the body treated. Radiation cause hair loss on the head when it is used to treat cancer on the head such as the metastatic cancer in the brain. There might also be loss of hair around the nipple when it is used to treat breast cancer for women who have hair in that location.
Tamoxifen, hormonal treatment drug can also cause thinning of hair but not baldness.
Knowing these facts, most cancer patients choose to shave their hair before the treatment.