Optimisation of Margin Adaptation for Respiratory Motion in Lung Cancer Stereotactic Body Radiation Therapy Using...

This study applies a previously developed four-dimensional (4D) dose calculation method using virtual four-dimensional computed tomography (v4DCT) to evaluate and optimise the use of an optimal margin (OM) in lung stereotactic body radiation therapy (SBRT). Specifically, we assess the clinical feasibility of an OM derived from different prescription isodose levels and its impact on target coverage and normal tissue sparing.

Volumetric modulated arc therapy (VMAT) plans were created using a whole-body phantom with a virtual lung tumour. Treatment planning ensured that 95% of the planning target volume (PTV) was covered by the prescription isodose lines for 70%, 75%, and 80%. v4DCT images were generated assuming free breathing. The four-dimensional radiotherapy (v4DRT) dose represented the accumulated dose across all respiratory phases. The dosimetric internal margin (DIM) was defined as the maximum amplitude ensuring D_100% and D_99% coverage of the gross tumour volume (GTV). The OM was determined based on the average dose to the GTV and the dose to organs at risk (OARs) for the 70% to 80% isodose plans.

The DIM was determined to be 2.0 to 2.6 times larger than the conventional margin accounting for respiratory motion. Implementation of the optimised margin resulted in a significant reduction of the V_20Gy for normal lung tissue under respiratory motion conditions. Additionally, the maximum dose at distances of 0.5 to 2.0 cm from the PTV showed a reduction with larger amplitudes. The OM with 70% and 75% isodose plan achieved a 55% to 62% reduction compared to the conventional PTV margin with 80% isodose plan.

This study proposes an OM for lung SBRT with marginal prescription. This approach reduced the respiratory motion margin by 55% to 62% while maintaining target dose coverage and lowering OAR exposure.