Poster - Thur Eve - 62: Assessing the clinical application of the van Herk margin formula for lung radiotherapy.

According to a margin recipe developed by van Herk et al. the Planning Target Volume (PTV) margin to ensure the Clinical Target Volume is covered by at least 95% of the prescribed dose can be calculated by applying the following formula: M = 2.5Σ + 1.64σ2 - 1.64σp. In the van Herk Margin formula (VHMF), Σ is the standard deviation (SD) of all systematic errors; σ is the SD of random errors and σ is the width of the penumbra. This formula is based on an idealized dose profile model that may not account for factors that vary significantly in lung radiotherapy such as tumour size and tissue density. The purpose of this study was to use accurate dose calculation algorithms and respiratory motion modeling to investigate the validity of the VHMF for lung radiotherapy. Random and systematic errors were simulated in treatment planning software using dose accumulation techniques for clinically relevant 3DCRT and IMRT treatment plans constructed on virtual phantoms. Phantom parameters such as target size, peak-to-peak motion amplitude and tissue density were varied to investigate their impact on the systematic and random error components of the margin formula. The VHMF was found to provide adequate dose coverage for all plans generated on different target sizes and motion amplitudes. Although discrepancies existed between idealized and realistic dose profiles in water and lung, the dose coverage defined by the V was not affected. The margin formula was found to be robust; however, further investigation of the influence of plan conformity is needed.