Lung SBRT: Dose gradient optimization based on target size.

Area of Science:

• Radiation Oncology
• Medical Physics
• Radiotherapy Planning

Background:

• Stereotactic body radiation therapy (SBRT) for lung cancer requires precise dose delivery.
• Optimizing the dose gradient is crucial for sparing normal tissues while effectively treating the target.
• The relationship between target size and dose gradient optimization in lung SBRT needs further investigation.

Purpose of the Study:

• To investigate how varying normal tissue objective (NTO) fall-off values impact dose gradients for different planning target volume (PTV) sizes in lung SBRT.
• To determine the optimal NTO settings for achieving steep dose gradients as a function of target size.

Main Methods:

• Sixty-eight lung SBRT patients with PTVs categorized into small, medium, and large groups were analyzed.
• Volumetric Modulated Arc Therapy (VMAT) plans were generated using progressively steeper NTO fall-off values (0.1-0.5 mm⁻¹).
• Dose calculations were performed using the AcurosXB algorithm, and statistical analyses assessed differences in Conformity Index (CI50%), maximum dose (Dmax), and monitor units (MU).

Main Results:

• Steeper NTO fall-off values significantly increased Dmax and MU across all target sizes (p < 0.05).
• Improved CI50% was observed for small (0.3 mm⁻¹) and medium (0.2 mm⁻¹) targets with steeper fall-off values (p < 0.05).
• Large targets did not show significant CI50% differences with varying fall-off values, indicating diminishing importance as target size increases.

Conclusions:

• Smaller lung targets benefit from steeper NTO fall-off values in SBRT, leading to improved dose conformity despite increased Dmax and MU.
• For larger targets, the impact of NTO fall-off values on dose conformity is less significant.
• Tailoring NTO fall-off values to target size is essential for optimizing lung SBRT plans and minimizing radiation dose to surrounding healthy tissues.