Fine-tuning the normal tissue objective in eclipse for lung stereotactic body radiation therapy

Area of Science:

• Radiation Oncology
• Medical Physics
• Radiotherapy Planning

Background:

• Lung stereotactic body radiation therapy (SBRT) requires precise dose distribution to maximize tumor control and minimize normal tissue toxicity.
• The Normal Tissue Objective (NTO) is a planning tool used in Eclipse to constrain dose to organs at risk by steepening the dose gradient.
• Optimizing NTO settings is challenging due to the numerous parameter combinations, impacting treatment plan quality.

Purpose of the Study:

• To characterize the effects of various Normal Tissue Objective (NTO) settings on lung stereotactic body radiation therapy (SBRT) dose distributions.
• To identify optimal NTO parameters for volumetric modulated arc therapy (VMAT) lung SBRT treatment planning.
• To compare the quality of NTO-optimized plans against previously generated treatment plans.

Main Methods:

• Ten retrospective lung SBRT cases were replanned using a wide range of NTO priorities (1-999) and fall-off values (0.01-5.00 mm⁻¹).
• Volumetric modulated arc therapy (VMAT) was employed for all treatment plans.
• Plan quality was assessed using metrics including conformity index (CI), R50%, D2cm, V20Gy, PTVmax, and monitor units (MUs).

Main Results:

• NTO setting combinations produced significant variations in plan quality metrics (CI: 1.00-1.54, R50%: 3.95-7.57, D2cm: 33.4%-67.9%, V20Gy: 1.66%-2.75%, PTVmax: 118%-175%).
• A fall-off of 0.15 mm⁻¹ and priority of 500 best met institutional criteria.
• NTO-optimized plans demonstrated significantly lower R50% (4.00 vs 4.35) and V20Gy (1.22% vs 1.32%) compared to prior plans, with higher PTVmax (138% vs 122%).

Conclusions:

• The study successfully characterized the impact of diverse NTO settings on lung SBRT dose distributions.
• Optimized NTO plans, particularly with a fall-off of 0.15 mm⁻¹ and priority of 500, favorably compared to prior plans and met RTOG 0813 guidelines.
• The findings provide valuable insights for refining NTO tuning to achieve superior lung SBRT treatment plans.