Collimator scatter and 2D dosimetry in small proton beams

Abstract

Monte Carlo simulations have been performed to determine the influence of collimator-scattered protons from a 150 MeV proton beam on the dose distribution behind a collimator. Slit-shaped collimators with apertures between 2 and 20 mm have been simulated. The Monte Carlo code GEANT 3.21 has been validated against one-dimensional dose measurements with a scintillating screen, observed by a CCD camera. In order to account for the effects of the spatial response of the CCD/scintillator system, the line-spread function was determined by comparison with measurements made with a diamond detector. The line-spread function of the CCD/scintillator system is described by a Gaussian distribution with a standard deviation of 0.22 mm. The Monte Carlo simulations show that protons that hit the collimator on the entrance face and leave it through the wall of the aperture make the largest scatter contribution. Scatter on air is the major contribution to the extent of the penumbra. From the energy spectra it is derived that protons with a relative biological effectiveness greater than 1 cause at most 1% more damage in tissue than what would be expected from the physical dose.

2D dosimetry in a proton beam with a scintillating GEM detector

“…According to the literature (van Luijk et al 2001), the Lanex screen has a spatial resolution (σ ) of about 0.2 mm. Consequently, the light intensity profile measured with the screen is used as the beam profile for calculating the spatial response of the scintillating GEM detector.…”

Volume-Dependent Expression of In-Field and Out-of-Field Effects in the Proton-Irradiated Rat Lung

“…injection of xylazine (Rompun; Bayer, Leverkusen, Germany) plus S-ketamine (Ketalar; Pfizer, Capelle aan de IJssel, The Netherlands) and placed in a holder hanging on a positioning rod by their upper incisors (19) for CT scanning or irradiation. The use of 150-MeV protons in a fixed beam line facilitated the irradiation of subvolumes of the lung with sharply demarcated (20-80% penumbra of approximately 1 mm) radiation fields (27,28). Radiation portals were designed using planning CT images of 5 age-matched rats, as described previously (19,20).…”

“…The numbers of animals used per dose/volume group are depicted in Table 1. Lung tissue samples were taken from both inside and outside the radiation field, with sufficient margins to ensure that the tissue was either irradiated (''in field''; i.e., tissue receives more than 97% of the prescribed dose) or shielded (''out-of-field''; tissue receives less than 7% of the prescribed dose) (27,28). Details of the procedure and scoring have been published previously (16,20).…”

Extended collimator model for pencil-beam dose calculation in proton radiotherapy

“…A Monte Carlo study showed that particles scattered at the edge would lose large energy in the collimator (van Luijk et al 2001), to cause large range shortening. Therefore, the edgescattered particles may be reasonably neglected since we are especially interested in the dose distribution in the treatment target located near the original beam range.…”