Range-shifter effects on the stray field in proton therapy measured with the variance–covariance method

Abstract

Measurements in the stray radiation field from a proton therapy pencil beam at energies 70 and 146 MeV were performed using microdosimetric tissue-equivalent proportional counters (TEPCs). The detector volumes were filled with a propane-based tissue-equivalent gas at low pressure simulating a mean chord length of 2 μm in tissue. Investigations were performed with and without a beam range shifter, and with different air gaps between the range shifter and a solid water phantom. The absorbed dose, the dose-mean lineal energy, and the dose equivalent were determined for different detector positions using the variance–covariance method. The influence from beam energy, detector- and range-shifter positions on absorbed dose, LET, and dose equivalent were investigated. Monte Carlo simulations of the fluence, detector response, and absorbed dose contribution from different particles were performed with MCNP 6.2. The simulated dose response for protons, neutrons, and photons were compared with, and showed good agreement with, previously published experimental data. The simulations also showed that the TEPC absorbed dose agrees well with the ambient absorbed dose for neutron energies above 20 MeV. The results illustrate that changes in both dose and LET variations in the stray radiation field can be identified from TEPC measurements using the variance–covariance method. The results are in line with the changes seen in the simulated relative dose contributions from different particles associated with different proton energies and range-shifter settings. It is shown that the proton contribution scattered directly from the range shifter dominates in some situations, and although the LET of the radiation is decreased, the ambient dose equivalent is increased up to a factor of 3.

Simulation and experimental verification of ambient neutron doses in a pencil beam scanning proton therapy room as a function of treatment plan parameters

“…This is probably due to the fact that the range shifter is positioned within line of sight from position D. The absorbed dose measured at position A with the Sievert was also shown to increase up to a factor of nine when introducing a range shifter. This increase is probably not due to neutrons but due to protons from the range shifter as discussed in detail in ( 23 ).…”

“…The absorbed dose measured at position A with the Sievert was shown to increase with increasing air gap. This increase is probably not due to neutrons but due to protons from the range shifter as discussed in detail in ( 23 ).…”

Range shifter contribution to neutron exposure of patients undergoing proton pencil beam scanning

“…Previous studies on the increased neutron exposure from the use of a RS have demonstrated a two-or threefold increase in doses compared to similar irradiations without the RS. 7,8 However, it is important to note that these studies were based on measurements of H* (10) in the treatment room, and this scenario may not be representative for what happens inside the patient.…”

“…Nevertheless, the RS becomes an additional external source of neutrons during the proton treatment, increasing the out‐of‐field doses in proton patients, especially when taking into account the high radiobiological effectiveness of neutrons. Even though in PBS the out‐of‐field neutron dose is relatively small, 6 measurements in air and inside an anthropomorphic phantom during irradiations with a RS showed that this element could increase the production of neutrons by a factor of around 2 or 3, 7,8,9 while the overall neutron production in PBS is lower than in passive scattering delivery. Therefore, the evaluation of the contribution of RS neutrons in the patient is warranted.…”