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 superheat ed drops dispersed in a small vial of gel, which vaporize upon exposur e to high LET radiation, thereby providing a directly observable indic ation of neutron dose. This detector possesses high sensitivity to neu trons and insensitivity to high-energy photons and electrons, making i t suitable for the determination of neutron dose equivalent rates arou nd high-energy photon and electron radiotherapy beams. In the present work, the SDD was used to measure the neutron dose equivalent in and a round the radiotherapy beams produced by a 32-MeV linear accelerator. For both x-ray and electron beams, the neutron dose profiles were obse rved 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 o ff-axis in the patient plane was essentially independent of field size . The neutron dose equivalent per electron dose was essentially zero f or electron beams with energies below 15 MeV, but increased rapidly ab ove 15 MeV. For 25-MeV electrons, neutron dose equivalent on the centr al 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. Compare d with P2O5 phosphorous activation detector data [Med. Phys. 7, 545-54 8 (1980)], the SDD neutron dose equivalent rate reading is about 20% h igher on the central axis of a 25-MV x-ray beam. At points off-axis in the patient plane, agreement between SDDs and phosphorous activation detectors is excellent. The SDD provides an easy-to-use and accurate m ethod for the determination of neutron dose in the presence of high-en ergy photons and electrons generated by medical accelerators.