SU‐E‐T‐671: Range‐Modulation Effects of Carbon Ion Beams in Lung Tissue

Abstract

Purpose:When particles traversing inhomogeneous materials like lung they show a characteristic range modulation which cannot be observed in homogeneous materials. It is possible to describe the range modulation by a convolution of an unperturbed Bragg‐Curve and a normal distribution. The sigma of the normal distribution is a parameter for the strength of the modulation effect. A new material parameter (modulation power, P_mod) is introduced which is independent of the material thickness. It is defined as the square of sigma divided by the mean water equivalent thickness of the target (µ).Methods:The modulation power of lung tissue was determined by actual Bragg‐peak measurements after traversing an ex‐vivo porcine lung and by Monte‐Carlo simulations with micro‐CT data of human lung tissue. The determined modulation powers were used to show the effect of range modulation effects in a simplified treatment situation. A four centimeter spread‐out Bragg‐peak after traversing eight centimeter of lung tissue was simulated in FLUKA. The SOBP with and without consideration of range modulation effects were compared.Results:As well in the measurements as in the MC simulations range modulation effects of lung tissue were observed. The determined modulation powers showed a great range from 0.05 mm, in the micro‐CT data, to 0.7 mm in the lung measurements. The SOBP comparison showed that range modulation effects Result in over‐ and underdosages at the distal and proximal edge of the SOBP. In the investigated case, the last 0.5 cm of the SOBP showed an underdosage of up to 50% at the distal edge, while 0.5 cm distal to the SOBP an overdosage of up to 50% was observed.Conclusion:Range modulation effects occur in inhomogeneous materials like lung. These modulation effects may Result in clinically relevant over‐ and underdosages but are currently not considered in commercially available treatment planning systems.