Radiation-induced alopecia (RIA) commonly follows x-ray exposure from therapeutic radiation for head and neck cancers and sometimes follows diagnostic and interventional radiology examinations. Host biologic factors such as age, tissue hypoxemia, genetic factors, and nutrition factors interplay with exposure-related factors such as dose fractionation, dose rate, and the size of the radiation window to determine the severity of the tissue injury.

RIA is typically an intense anagen effluvium due to radiation interrupting rapid mitotic activity during the anagen hair growth stage. Despite advancement in dose fractionation and tissue-sparing delivery of radiotherapy, rapidly proliferating hair follicles are still at elevated risk for damage.

RIA can be categorized as either temporary (tRIA), characterized by hair restoration within 6 months, or persistent (pRIA), indicating hair loss that extends beyond 6 months from the completion of the final treatment. pRIA can have persistent miniaturization of up to 1.5 years, but in some individuals, hair regrowth takes up to 6 years.

Permanent cicatricial alopecia may occur if high-dose radiation has completely obliterated hair follicles.

Alopecia, whether temporary or permanent, may manifest when exposed to radiation doses exceeding approximately 3 Gy and 7 Gy, respectively. There is a dose-dependent phenomenon where a 1-unit increase in radiation is correlated with increased severity of alopecia. Distribution is also correlated with the treatment area.

Proton therapy, compared to traditional photon therapy, is thought to spare healthy tissues located beyond a tumor. While it is rarer to experience RIA with this therapy, it is still possible.