Cleaning the dose falloff with low modulation in SBRT lung plans

Area of Science:

• Radiation Oncology
• Medical Physics
• Dosimetry

Background:

• Highly modulated lung stereotactic body radiation therapy (SBRT) plans can be susceptible to the interplay effect.
• Reducing the modulation factor is crucial for improving plan quality and robustness in lung SBRT.
• Current planning techniques may not sufficiently address modulation reduction while adhering to clinical constraints.

Purpose of the Study:

• To develop and evaluate a novel planning strategy to significantly lower the modulation factor in lung SBRT plans.
• To generate lung SBRT plans that replace highly modulated plans prone to the interplay effect.
• To ensure generated plans meet established clinical constraints, such as those from RTOG 0813 and 0915.

Main Methods:

• Twenty clinical lung SBRT plans with high modulation factors (≥4) were replanned using Varian Eclipse TPS.
• A novel optimization methodology incorporating a unique shell structure (OptiForR50) for R50% optimization was employed.
• Light optimization with specific block margins and adherence to RTOG dose objectives (PTV D95%=Rx, PTV Dmax<140%Rx) were utilized.

Main Results:

• The novel strategy significantly reduced modulation factors (3.65 vs. 4.59, p < 0.001) and CI_RTOG (0.97 vs. 1.02, p = 0.001).
• Higher homogeneity index (1.35 vs. 1.14, p < 0.001) and lower R50% (4.09 vs. 4.56, p < 0.001) were achieved.
• Lung V8-12.8Gy (Timmerman Constraint) was significantly reduced (4.61% vs. 4.92%, p < 0.001), with borderline significant reduction in V105% (0.44% vs. 1.10%, p = 0.051).

Conclusions:

• Lung SBRT plans with significantly lower modulation factors can be generated using the proposed planning strategy.
• This approach meets established RTOG constraints while improving plan quality metrics.
• The method offers a viable alternative to highly modulated plans, potentially mitigating the interplay effect.