On the determination of planning target margins due to motion for mice lung tumours using a four-dimensional MOBY phantom.

Abstract

This work aims to analyse the effect of respiratory motion on optimal irradiation margins for murine lung tumour models. Four-dimensional mathematical phantoms with different lung tumour locations affected by respiratory motion were created. Two extreme breathing curves were adopted and divided into time-points. Each time-point was loaded in a treatment planning system and Monte Carlo (MC) dose calculations were performed for a 360° arc plan. A time-resolved dose was derived, considering the gantry rotation and the breathing motion. Radiotherapy metrics were derived to assess the final treatment plans. An interpolation function was investigated to reduce calculation cost. The effect of respiratory motion on the treatment plan quality is strongly dependent on the breathing pattern and the tumour position. Tumours located closer to the diaphragm required a compromise between tumour conformity and healthy tissue damage. A recipe, which considers collimator size, was proposed to derive tumour margins and spare the organs at risk (OARs) by respecting constraints on user-defined metrics. It is recommended to add a target margin, especially on sites where movement is substantial. A simple recipe to derive tumour margins was developed. This work is a first step towards a standard planning target volume concept in pre-clinical radiotherapy.