Superheated drop detector for determination of neutron dose equivalent to patients undergoing high-energy x-ray and electron radiotherapy.

The superheated drop detector (SDD) consists of thousands of superheated drops dispersed in a small vial of gel, which vaporize upon exposure to high LET radiation, thereby providing a directly observable indication of neutron dose. This detector possesses high sensitivity to neutrons and insensitivity to high-energy photons and electrons, making it suitable for the determination of neutron dose equivalent rates around high-energy photon and electron radiotherapy beams. In the present work, the SDD was used to measure the neutron dose equivalent in and around the radiotherapy beams produced by a 32-MeV linear accelerator. For both x-ray and electron beams, the neutron dose profiles were observed to follow the photon/electron radiotherapy beam profiles. For 25-MV x rays, the neutron dose equivalent per photon dose on the central axis increased by a factor of about 3 as field size increased from 5 x 5 to 30 x 30 cm. However, the neutron dose equivalent rate at 50 cm off-axis in the patient plane was essentially independent of field size. The neutron dose equivalent per electron dose was essentially zero for electron beams with energies below 15 MeV, but increased rapidly above 15 MeV. For 25-MeV electrons, neutron dose equivalent on the central axis was about 1/5 that for 25-MV x rays. Analogous to the data for 25-MV x rays, the neutron dose equivalent rate on the central axis of a 25-MeV electron beam exhibited a similar field size dependence and outside the beam it was essentially independent of field size.(ABSTRACT TRUNCATED AT 250 WORDS)