SU-E-T-567: Improve Dose Conformity with IMRT Using Shorter Source to Tumor Distance

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Planning

Background:

• Late toxicity, including tissue necrosis and cognitive impairment, is a concern in brain radiotherapy.
• Improving radiation dose conformity is crucial for minimizing damage to healthy brain tissue.

Purpose of the Study:

• To investigate a novel planning technique using variable source-to-tumor distances (SADs) on linear accelerators (LINACs).
• To enhance radiation dose conformity and reduce high doses delivered outside the planning target volume (PTV) for brain radiotherapy.

Main Methods:

• Feasibility study on brain CT datasets with single and multiple lesions of varying volumes.
• Creation of 5-field intensity-modulated radiation therapy (IMRT) plans using SADs of 100 cm, 80 cm, and 65 cm.
• Quantification of dose spillage using R50 (50% isodose volume to PTV ratio) and V12 (volume receiving ≥12 Gy).

Main Results:

• Shorter SADs consistently reduced R50, with average reductions of 13% (80 cm SAD) and 19% (65 cm SAD) compared to 100 cm SAD.
• Improved dose conformity was more pronounced for smaller and multiple lesions.
• Significant decreases in V12 were observed with shorter SADs.

Conclusions:

• Reducing treatment distance for IMRT fields on existing LINACs enhances dose conformity.
• Sharper beam penumbra and smaller multi-leaf collimator (MLC) leaf width contribute to improved conformity.
• Precise delivery of non-isocentric beams is manageable on LINACs with advanced robotic gantry and couch capabilities.