Purpose: Patients treated with protons are subjected to secondary neutrons originating from the beam delivery system and inside the patient. Treatment plans featuring vertex beams, i.e., proton beams oriented in the superior-inferior direction, raised concerns of increased neutron exposure to the patients. A study was performed to assess the neutron dose equivalent to patients treated with such beam configurations.
Methods: A Monte Carlo model of the complete treatment room, including all relevant beam line equipment, was produced for IMPT and PSPT treatments, each equipped with a simplified representation of an adult and a 10 year old pediatric patient. Proton beams of about 15 cm range in water were simulated to the heads of the phantoms in the vertex beam orientation and from a lateral direction. The neutron fluences in the phantom’s arms, legs, torso, as well as head and neck were scored as a function of neutron energy and converted to neutron dose equivalent.
Results: The IMPT treatments yielded moderately reduced neutron dose equivalent values for Vertex Beams, mostly to the neck of the phantoms. Using PSPT, there was a noticeable systematic reduction in neutron dose equivalent in the Vertex Beam configuration.
Conclusion: We found that the neutron dose equivalent to the phantoms was more dependent on the distance to the beam line/nozzle than on beam direction, and that the application of Vertex Beams does not pose additional hazards to a proton therapy patient due to neutron exposure.
Not Applicable / None Entered.