Radiation therapy is a precise cancer treatment that uses high-energy rays to destroy cancer cells and shrink tumors. A common concern for patients undergoing this treatment is hair loss, a side effect that occurs when the treated area contains hair follicles. Unlike chemotherapy, which causes widespread hair loss because the drug travels throughout the body, radiation-induced hair loss is highly localized.
How Radiation Affects Hair Follicles
Radiation targets and damages rapidly dividing cells, a characteristic shared by both cancer cells and the active cells within hair follicles. The hair follicle matrix cells, which are responsible for hair shaft production during the active growth phase, are extremely sensitive to this damage. This acute injury causes the hair shaft to narrow and break off near the scalp surface.
This type of sudden and significant hair shedding is medically termed anagen effluvium. Since the effect is limited to the direct path of the radiation beam, only the hair in the treated area is affected, creating a distinct patch of hair loss. Hair loss may begin to become noticeable as early as two to four weeks after the first treatment session.
Factors Influencing Hair Loss Severity
The degree of hair loss from radiation therapy can range from minor thinning to complete baldness in the treated area. This severity is primarily determined by the total cumulative radiation dose received, which is measured in units called Gray (Gy). Higher total doses increase the likelihood and extent of hair loss; for example, hair thinning can occur with doses as low as 2 Gy.
The method of delivery, known as fractionation, also plays a role. Delivering the total dose in smaller, multiple fractions allows healthy cells a chance to repair themselves between treatments. However, a higher dose per fraction or a higher total dose increases the risk of irreversible damage to the hair follicle stem cells.
Regrowth Expectations After Treatment
If the total radiation dose delivered to the hair follicle was low or moderate, hair growth usually resumes within a few months after treatment completion. Most patients will begin to see initial, fine hair emerging about two to three months post-treatment, with more noticeable coverage developing between three and six months. This regrowth can sometimes result in hair that is slightly different from before, such as a finer texture, a different color, or a tendency to be wavier or curlier.
The potential for permanent hair loss is directly linked to the maximum radiation dose absorbed by the hair follicle stem cells. Permanent hair loss is expected when the total dose exceeds a certain threshold. Studies suggest that a dose of approximately 43 Gy has a 50% chance of causing permanent hair loss, and doses higher than 36 Gy are often associated with irreversible damage. This severe damage destroys the stem cells required for future hair cycles, meaning the follicle cannot regenerate new hair.