Dose and LET distributions due to neutrons and photons emitted from stopped negative pions

Abstract

Computer calculations are made of the dose and LET distributions due to neutrons and photons produced when negative pions are stopped in a phantom. When negative pions are stopped in a material they undergo nuclear capture, resulting in the disintegration of the nucleus and the emission of short range charged particles and longer range neutrons and photons. The uncharged radiation constitutes a potentially large source of dose outside the treatment volume. A simple phantom consisting of a 0-25 m cube of either tissue or bone-equivalent material is set up with a 0-05 m cube in the centre to represent the treatment volume. Neutrons and photons are started in this central volume and transported across the phantom using Monte Carlo transport codes. Several different initial energy spectra for the neutrons are used, taken from experimental and theoretical data. These different spectra are found to give significant differences in dose, though the distance to the 80% dose level is always about 0-015 m. Order of magnitude differences in some LET regions are also found. The dose deposited by neutrons in bone is about 24% less than in soft tissue, the photon dose being small compared with the neutron dose.

Charged particle emission from the capture of negative pions: energy spectra, LET distributions and W-value

“…This result would suggest that the change in the LET spectrum on going across a bone-tissue interface would be very small. Calculations by Brenner and Smith (1977) on the LET spectral differences in soft tissue and bone-equivalent material due t o neutrons emitted from stopped pions similarly show no significant difference. , 4 comparison of LET distributions between plateau and peak shows only a small reduction in the fractional dose that is delivered above 10 keV pm-l and 30 keJ7 pm-1.…”

“…Our results are in agreement 7 ( b ) for muscle-equivalent solution at the peak shows that about 46% of the total dose due to charged particles is delivered with an LET > 10 keV pm-1 and about 29% with an LET > 30 keV p -l . The calculations of Brenner and Smith (1977)) when converted to a cumulative fractional dose : LET distribution, show that the peak neutron dose in a tissue-equivalent medium delivered above the same LET values is 65% at LET > 10 keV pm-1 and 40% at LET > 30 keV pm-l. Reading (1076) has calculated the contributions of charged particles and neutrons t o the total peak dose in the x 11 NIMROD beam to be 430/& from charged particles and 8yo from neutrons. The remaining 49%, comprising electrons, muons and pions still in flight, will provide low LET components ( < 10 keV pm-l) t o the peak dose.…”

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“…In order to estimate the neutron dose per specific treatment dose, the peak dose per pion must be known experimentally. In the absence of an experimental determination, we followed Brenner and Smith (1977) and estimated the peak dose from the calculation of Turner et a1 (1976). Turner et a1 calculated a peak dose of 1.71 X 10"' Gy per incident pion for a 67 MeV pion beam with a momentum spread of 2%.…”

“…These measurements on the highenergy portion of the spectra generally agree with the measurements by Hartmann er a1 (1978) and Klein et a1 (1979) for light targets and reveal that the neutrons emitted per pion capture carry away about 76 MeV of kinetic energy. Roeder (1973,1974) and Brenner and Smith (1977) reported calculations of the neutron dose to be expected during pion radiotherapy based on the calculated neutron spectrum of Guthrie et a1 (1968) with a value of about 61 MeV for the kinetic energy carried away by the neutrons. The result that neutrons carry away more kinetic energy than was assumed in previous estimates of the dose means that a higher dose is to be expected in the region farther away from the treatment volume.…”

“…Also they found that the dose expected from gamma rays originating from the capture of negative pions is negligible compared to the neutron dose. Brenner and Smith (1977) used the neutron transport code 05R, described by Irving et al (1965), to calculate the absorbed dose of neutrons outside of a cubic treatment volume of 125 cm3. Using the spectrum calculated by Guthrie et al, Brenner andSmith (1977, and1978 private communication) found the dose to be about half of that reported by Schillaci and Roeder. The purpose of this paper is to assess the neutron dose by using a more reliable starting neutron spectrum and a more realistic model representation of interactions of high-energy neutrons.…”