Radiation causes hair loss, a side effect highly dependent on the type of exposure and the dose received. Hair loss, medically termed alopecia, occurs when high-energy radiation interacts directly with the rapidly dividing cells responsible for hair growth. This reaction is localized, meaning hair loss is generally confined to the exact area where the radiation beam is aimed.
How Radiation Targets Hair Follicles
The hair follicle is a complex structure that contains some of the fastest-dividing cells in the human body, making them particularly vulnerable to radiation damage. Hair growth occurs in a cycle with three main phases, and radiation primarily targets the anagen phase, which is the active growth stage. During this phase, the hair matrix cells within the follicle bulb are replicating constantly to form the hair shaft.
Radiation disrupts the DNA of these highly proliferative cells, effectively halting their division and function. This damage prevents the cells from maturing and forming a healthy hair shaft. The resulting hair loss is known as anagen effluvium, where the growing hair suddenly narrows at the base before breaking off.
When the hair matrix cells are damaged, the hair shaft weakens and detaches from the follicle, leading to hair shedding that typically begins two to three weeks after the initial exposure. The degree of damage to the hair follicles determines whether the hair loss is temporary or permanent.
The Role of Dose and Delivery
The probability and severity of hair loss are directly tied to the quantity of radiation administered, measured in units called Gray (Gy). Low-dose radiation, such as that used in diagnostic imaging like X-rays or CT scans, rarely causes hair loss because the dose is well below the threshold for follicle damage. The hair follicles can usually repair the minimal DNA damage from these low exposures without noticeable shedding.
Hair loss becomes a concern when the radiation dose reaches therapeutic levels, which are designed to destroy cancer cells. For temporary hair loss, the threshold is relatively low, with doses above 2 Gray to the scalp often causing some shedding in the treatment area. The location of the radiation delivery is equally important, as only hair in the direct path of the radiation beam is affected.
The factor determining permanent damage is a higher cumulative dose. For the entire scalp, transient hair loss generally occurs when the dose reaches around 22 Gray, while the threshold for permanent hair loss is often cited near 36 to 40 Gray. These high doses cause irreversible damage to the stem cells within the follicle, preventing the hair from ever growing back in that specific area.
Hair Loss in Therapeutic Radiation Contexts
In the context of external beam radiation therapy for cancer, hair loss is a predictable side effect when the treatment field includes a hair-bearing area. The goal of radiation therapy is to target rapidly dividing cells, which unfortunately includes both cancer cells and the healthy cells of the hair follicle.
The hair loss is strictly localized to the area receiving the high-energy beams. A patient receiving treatment to the chest will lose hair on the chest but not the scalp. For patients undergoing radiation for brain tumors or head and neck cancers, hair loss on the scalp is a common expectation. The hair may also thin or fall out where the radiation beam exits the body, which can be on the opposite side of the treatment target.
Treatment variables, such as the total accumulated dose and the fractionation (dividing the total dose into smaller, daily treatments), influence the extent of hair loss. While fractionation allows healthy tissue to repair between sessions, the hair follicles still accumulate damage over the course of the treatment period. Hair shedding typically begins two to four weeks after the start of a course of therapeutic radiation.
Recovery and Regrowth Outlook
The prognosis for hair regrowth after radiation is highly dependent on the total radiation dose delivered to the hair follicles. For most people who experience hair loss from lower therapeutic doses, the damage to the hair follicles is temporary. Once the treatment concludes, the follicles often recover and re-enter the active growth cycle.
Regrowth usually begins within three to six months after the final radiation treatment. The texture and color of the new hair may initially differ from the original hair; it may grow back finer, curlier, or even a different shade, though these changes are often not permanent.
If the cumulative radiation dose exceeds the threshold for permanent damage, irreversible scarring or fibrosis of the follicle occurs. When very high doses of radiation are used, the capacity for the stem cells in the follicle to regenerate is destroyed. In these cases, the hair loss is permanent, and the skin in the treated area will remain bald. Permanent hair loss is more likely with cumulative doses above 40 Gray.