title: Measurements of neutron dose equivalent for a proton therapy center using uniform scanning proton beams.
abstract: OBJECTIVE
  Neutron exposure is of concern in proton therapy, and varies with beam delivery technique, nozzle design, and treatment conditions. Uniform scanning is an emerging treatment technique in proton therapy, but neutron exposure for this technique has not been fully studied. The purpose of this study is to investigate the neutron dose equivalent per therapeutic dose, H/D, under various treatment conditions for uniform scanning beams employed at our proton therapy center.
  
  
  METHODS
  Using a wide energy neutron dose equivalent detector (SWENDI-II, ThermoScientific, MA), the authors measured H/D at 50 cm lateral to the isocenter as a function of proton range, modulation width, beam scanning area, collimated field size, and snout position. They also studied the influence of other factors on neutron dose equivalent, such as aperture material, the presence of a compensator, and measurement locations. They measured H/D for various treatment sites using patient-specific treatment parameters. Finally, they compared H/D values for various beam delivery techniques at various facilities under similar conditions.
  
  
  RESULTS
  H/D increased rapidly with proton range and modulation width, varying from about 0.2 mSv/Gy for a 5 cm range and 2 cm modulation width beam to 2.7 mSv/Gy for a 30 cm range and 30 cm modulation width beam when 18 × 18 cm(2) uniform scanning beams were used. H/D increased linearly with the beam scanning area, and decreased slowly with aperture size and snout retraction. The presence of a compensator reduced the H/D slightly compared with that without a compensator present. Aperture material and compensator material also have an influence on neutron dose equivalent, but the influence is relatively small. H/D varied from about 0.5 mSv/Gy for a brain tumor treatment to about 3.5 mSv/Gy for a pelvic case.
  
  
  CONCLUSIONS
  This study presents H/D as a function of various treatment parameters for uniform scanning proton beams. For similar treatment conditions, the H/D value per uncollimated beam size for uniform scanning beams was slightly lower than that from a passive scattering beam and higher than that from a pencil beam scanning beam, within a factor of 2. Minimizing beam scanning area could effectively reduce neutron dose equivalent for uniform scanning beams, down to the level close to pencil beam scanning.
authors: ProCure Proton Therapy Center, 5901 West Memorial Road, Oklahoma City, OK 73142, USA. yuanshui.zheng@okc.procure.com
authors: Yuanshui
authors: Y
authors: Zheng
authors: Yaxi
authors: Y
authors: Liu
authors: Omar
authors: O
authors: Zeidan
authors: Andries Niek
authors: AN
authors: Schreuder
authors: Sameer
authors: S
authors: Keole
mesh_terms: D006801:Humans; D009502:Neutrons / Q000009:adverse effects*; D061766:Proton Therapy; D011829:Radiation Dosage; D011874:Radiometry; D011522:Protons
chemical_list: D011522:Protons