Sci-Thur PM: YIS - 04: Forcing lateral electron disequilibrium to spare lung tissue: A novel technique for SBRT of small lung tumours.

Stereotactic body radiation therapy(SBRT), a technique that uses tightly conformed Megavoltage(MV) x-ray fields, improves local control of lung cancer. However, small MV x-ray fields can cause lateral electron disequilibrium(LED), which reduces the dose within lung. These effects are difficult to predict and are presently a cause of alarm for the radiotherapy community. Previously, we developed The Relative Depth Dose Factor(RDDF), which is an indicator of the extent of LED (RDDF < 1). We propose a positive application of LED for lung sparing in SBRT: LED can be exploited to irradiate a small tumor while greatly reducing the dose in surrounding lung tissue. The Monte Carlo code, DOSXYZnrc, was employed to calculate dose within a cylindrical lung phantom. The phantom's diameter and height were set to 25 cm, and consisted of water and lung (density = 0.25g/cm ) shells surrounding a small water tumor (volume = 0.8 cm ). Two 180° 6MV arcs were focused onto the tumor with field sizes of 1×1cm (RDDF∼0.5) and 3×3cm (RDDF∼1). Analyzing dose results, the 1×1cm arc reduced dose within lung and water tissues by 70% and 80% compared to the 3×3cm arc. Although, central tumor dose was also reduced by 15% using the 1×1cm arc, these reductions can be offset by escalating the prescription dose appropriately. Using the RDDF as a guideline, it's possible to design a SBRT treatment plan that reduces lung dose while maintaining relatively high tumor dose levels. Clinical application requires an accurate dose algorithm and may lower SBRT dose-induced toxicity levels in patients.