Assessment of the dosimetric impact when modelling complex-shaped multi-leaf collimators in Monte Carlo simulations

Abstract

Introduction We present a code, developed with the Geant 4 Monte Carlo (MC) toolkit, to model automatically complex-shaped multi-leaf collimators (MLC) by means of geometrical parameters provided by the vendor. In this work, we reproduce the Siemens 160-MLC, which leaves present a complex curved shape, and assess the dose impact found in small fields when using geometry models of different levels of detail. Purpose To evaluate with MC simulations the dosimetric impact of using a simplified geometry model of the 160-MLC and of certain geometry parameters of this MLC design. Materials and methods Dose measurements were carried out with radiochromic films inserted between solid water slabs. The dosimetric tests were: (I) Leaf ends field, to verify the penumbra shape sensitivity against the discretization level set to simulate the curved leaf ends. (II) Transmission, to adjust leaf density and real distance between opposite leaves from intra- and interleaf radiation leakage profiles. (III) Picket fence, to fit the leaf tilt angle with respect of the divergent ray emerging from the source. Results We found a variation in penumbra tails of up to 0.5 mm for varying leaf end discretization levels. Picket fence dose distributions showed that a 0.1°variation of the leaf tilt angle can lead to an underdosage variation on the dose distribution larger than 3%. Conclusion We present a versatile code which can reproduce accurately the singularities of complex MLC designs, which is of key importance for dose calculation of small fields with likely clinical implications. Disclosure Authors have no conflicts of interest.