Margin reduction and optimal prescription isodose model for liver stereotactic radiotherapy with respiratory motion

This study aimed to establish a framework for optimizing planning target volume (PTV) margins in liver SBRT using virtual 4DCT (v4DCT), by introducing the concept of the Dosimetric Coverage Amplitude (DCA) to quantify motion tolerance and deriving an Optimal Margin (OM) that balances tumor coverage with normal-tissue sparing.

VMAT plans were developed using a whole-body phantom with a virtual tumor, ensuring that the prescription dose corresponded to D_95% of the PTV. The 60%-80% isodose levels were defined relative to the maximum dose to represent alternative prescription surfaces. v4DCT simulated free-breathing conditions across 10 respiratory phases to generate virtual four-dimensional radiotherapy (v4DRT) dose distributions. The DCA was defined as the maximum respiratory amplitude at which GTV dose coverage (D100% or D99%) was maintained. Based on DCA analysis, the OM was determined as the clinically applicable margin derived from the isodose plan that satisfied the DCA condition and minimized normal tissue dose.

DCA analysis revealed that GTV dose coverage was maintained for respiratory motion amplitudes up to 1.9-2.2 times larger than the conventional PTV margin of 10 mm. Taking into account GTV dose coverage, average GTV dose relative to the 80%-static reference, and normal-tissue dose, the 60% isodose plan was identified as the optimal prescription level. From this plan, the OM was calculated as 5.6 mm, representing a 44% reduction compared with the conventional 10-mm PTV, while maintaining GTV coverage and minimizing liver dose. This margin reduction was attributed to the steeper dose gradient associated with prescribing to a lower isodose level. The v4DCT approach enabled the generation of multiple respiratory phase images, allowing quantitative evaluation of motion-induced dose distribution and optimization of margins based on respiratory variability.

The proposed v4DCT-based framework demonstrated that the 60% isodose plan provided the optimal balance between tumor coverage and normal-tissue sparing, yielding an OM of 5.6 mm (44% reduction compared with the conventional 10-mm PTV). This approach offers a clinically applicable strategy for margin reduction in liver SBRT while maintaining robust dosimetric coverage.