Measurements of secondary neutron dose from 18 MV IMRT

Accelerators used for medical radiotherapy produce electrons and x-rays in the energy range from 4 to 25 MV. Electrons and photons with energies greater than 7 MeV can generate neutrons through interactions with accelerator structures and treatment room, as well as within the patient. Many studies have been performed to determine the energy spectra, the effect of unwanted dose to the patient and shielding requirements for these neutrons for conventional radiation therapy. Recently, a new radiation treatment delivery technique, Intensity Modulated Radiation Therapy (IMRT), has been implemented in radiotherapy centers around the world. IMRT is based on the concept that the radiation beam intensity can be varied across the treatment field while delivering the radiation to the target volume using multileaf collimators. IMRT has the potential to improve tumor control without compromising the surrounding normal tissue by delivering a higher, more conformal target volume dose relative to conventional radiation therapy. Although, the prescribed photon dose is consistent for both types of therapy, the beam-on time required to deliver a typical IMRT treatment can be substantially greater than for conventional radiation treatment. Thus, for IMRT, the secondary neutron production is presumed to be greater. In the past, the whole-body neutron dose has been clinically acceptable for conventional therapy. However, for IMRT, this secondary neutron dose may no longer be acceptable. TLD measurements were performed at Emory University to evaluate the secondary neutron dose from conventional and IMRT treatments with 18 MV X-ray beams. Results reveal a higher secondary neutron dose from the IMRT treatment versus the conventional treatment.