Secondary neutron doses for several beam configurations for proton therapy.
OBJECTIVE
To compare possible neutron doses produced in scanning and scattering modes, with the latter assessed using a newly built passive-scattering proton beam line.


METHODS
A 40 x 30.5 x 30-cm water phantom was irradiated with 230-MeV proton beams using a gantry angle of 270 degrees , a 10-cm-diameter snout, and a brass aperture with a diameter of 7 cm and a thickness of 6.5 cm. The secondary neutron doses during irradiation were measured at various points using CR-39 detectors, and these measurements were cross-checked using a neutron survey meter with a 22-cm range and a 5-cm spread-out Bragg peak.


RESULTS
The maximum doses due to secondary neutrons produced by a scattering beam-delivery system were on the order of 0.152 mSv/Gy and 1.17 mSv/Gy at 50 cm from the beam isocenter in the longitudinal (0 degrees ) and perpendicular (90 degrees ) directions, respectively. The neutron dose equivalent to the proton absorbed dose, measured from 10 cm to 100 cm from the isocenter, ranged from 0.071 mSv/Gy to 1.96 mSv/Gy in the direction of the beam line (i.e., phi = 0 degrees ). The largest neutron dose, of 3.88 mSv/Gy, was observed at 135 degrees and 25 cm from the isocenter.


CONCLUSIONS
Although the secondary neutron doses in proton therapy were higher when a scattering mode rather than a scanning mode was used, they did not exceed the scattered photon dose in typical photon treatments.
Proton Therapy Center, National Cancer Center, Goyang, Korea.
Dongho
D
Shin
Myonggeun
M
Yoon
Jungwon
J
Kwak
Jungwook
J
Shin
Se Byeong
SB
Lee
Sung Yong
SY
Park
Soah
S
Park
Dae Yong
DY
Kim
Kwan Ho
KH
Cho
D002138:Calibration; D019471:Elementary Particle Interactions; D006801:Humans; D009502:Neutrons / Q000009:adverse effects; D019047:Phantoms, Imaging; D061766:Proton Therapy; D011874:Radiometry / Q000295:instrumentation; D011879:Radiotherapy Dosage; D012062:Relative Biological Effectiveness; D012542:Scattering, Radiation; D011522:Protons
D011522:Protons