PURPOSE: Effects of clinical radiotherapy are often studied between or after irradiations. The current study's aim was to monitor an immediate irradiation response in cerebral water and hemodynamics in patients treated with whole brain radiotherapy (WBRT) and to assess the response's individuality. METHODS: We used functional near-infrared spectroscopy (fNIRS) to monitor changes in cerebral water, oxyhemoglobin (HbO), and deoxyhemoglobin (HbR) during the irradiation of 31 patients (age 69.3 ± 12.5 years, 16 females) receiving WBRT. The radiation dose delivered to a patient during a single measurement was 4 Gy (total dose of 20 Gy in five fractions) for most patients and 3 Gy (total dose of 30 Gy in ten fractions) for three patients. RESULTS: 106 patient recordings were analyzed. They showed an immediate irradiation induced increase in HbO and HbR, and decrease in cerebral water content (P <
.001) as soon as 5 s after the start of irradiation. The radiation dose, age, and gender affected recorded signals. A smaller dose resulted in a steeper change in HbR (P <
.01), but larger total change in HbO (P <
.01). Younger age was associated with a more significant decrease in the water signal (P <
.05). In contrast, female gender was associated with a greater total increase in HbO (P <
.01) and HbR (P <
.001) signals. CONCLUSION: There is an immediate cerebral water and hemodynamic response to irradiation and this response shows dependency on the radiation dose, age, and gender. Better understanding about the immediate radiation response may help improve the patient outcome in clinical radiotherapy.