A dosimeter based on a gas electron multiplier and method of use thereof for measurement of doses of therapeutic radiation to which a tissue-phantom is exposed. Subsequent to the in-phantom measurement and verification of radiation beam delivery, radiation can be effectively delivered to a human target organ, based on the verification of radiation quantities to which the phantom was exposed. Use of a gas electron multiplier-based dosimeter facilitates precise and accurate verification of the radiation dose within a phantom by taking measurements in real time, with no need for subsequent film processing.