Monte Carlo simulations of a low energy proton beamline for radiobiological experiments

<p><b>Background:</b> In order to determine the relative biological effectiveness (RBE) of protons with high accuracy, radiobiological experiments with detailed knowledge of the linear energy transfer (LET) are needed. Cell survival data from high LET protons are sparse and experiments with low energy protons to achieve high LET values are therefore required. The aim of this study was to quantify LET distributions from a low energy proton beam by using Monte Carlo (MC) simulations, and to further compare to a proton beam representing a typical minimum energy available at clinical facilities.</p> <p><b>Materials and methods:</b> A Markus ionization chamber and Gafchromic films were employed in dose measurements in the proton beam at Oslo Cyclotron Laboratory. Dose profiles were also calculated using the FLUKA MC code, with the MC beam parameters optimized based on comparisons with the measurements. LET spectra and dose-averaged LET (LET_d) were then estimated in FLUKA, and compared with LET calculated from an 80 MeV proton beam.</p> <p><b>Results:</b> The initial proton energy was determined to be 15.5 MeV, with a Gaussian energy distribution of 0.2% full width at half maximum (FWHM) and a Gaussian lateral spread of 2 mm FWHM. The LET_d increased with depth, from approximately 5 keV/μm in the entrance to approximately 40 keV/μm in the distal dose fall-off. The LET_d values were considerably higher and the LET spectra were much narrower than the corresponding spectra from the 80 MeV beam.</p> <p><b>Conclusions:</b> MC simulations accurately modeled the dose distribution from the proton beam and could be used to estimate the LET at any position in the setup. The setup can be used to study the RBE for protons at high LET_d, which is not achievable in clinical proton therapy facilities.</p

Linear energy transfer distributions in the brainstem depending on tumour location in intensity-modulated proton therapy of paediatric cancer

<p><b>Background:</b> For tumours near organs at risk, there is concern about unintended increase in biological dose from elevated linear energy transfer (LET) at the distal end of treatment fields. The objective of this study was therefore to investigate how different paediatric posterior fossa tumour locations impact LET and biological dose to the brainstem during intensity-modulated proton therapy (IMPT).</p> <p><b>Material and methods:</b> Multiple IMPT plans were generated for four different simulated tumour locations relative to the brainstem for a five-year-old male patient. A prescribed dose of 59.4 Gy(RBE) was applied to the planning target volumes (PTVs). Plans with two lateral and one posterior non-coplanar fields were created, along with plans with modified field arrangements. The dose-averaged LET (LET_d) and the physical dose × RBE_LET (<i>D</i> × RBE_LET), where RBE_LET=1+<i>c</i> × LET_d, were calculated using the FLUKA Monte Carlo code. A scaling parameter <i>c</i> was applied to make the RBE_LET represent variations in the biological effect due to LET.</p> <p><b>Results:</b> High LET_d values surrounded parts of the PTV and encompassed portions of the brainstem. Mean LET_d values in the brainstem were 3.2–6.6 keV/μm. The highest absolute brainstem LET_d values were seen with the tumour located most distant from the brainstem, whereas lower and more homogeneous LET_d values were seen when the tumour invaded the brainstem. In contrast, the highest mean <i>D</i> × RBE_LET values were found in the latter case (54.0 Gy(RBE)), while the case with largest distance between tumour and brainstem had a mean <i>D</i> × RBE_LET of 1.8 Gy(RBE).</p> <p><b>Conclusions:</b> Using IMPT to treat posterior fossa tumours may result in high LET_d values within the brainstem, particularly if the tumour volume is separated from the brainstem. However, the <i>D</i> × RBE_LET was greater for tumours that approached or invaded the brainstem. Changing field angles showed a reduction of LET_d and <i>D</i> × RBE_LET in the brainstem.</p