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

Abstract

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/cm3) shells surrounding a small water tumor (volume = 0.8 cm3). Two 180° 6MV arcs were focused onto the tumor with field sizes of 1×1cm2(RDDF∼0.5) and 3×3cm2(RDDF∼1). Analyzing dose results, the 1×1cm2 arc reduced dose within lung and water tissues by 70% and 80% compared to the 3×3cm2 arc. Although, central tumor dose was also reduced by 15% using the 1×1cm2 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. © 2012, American Association of Physicists in Medicine. All rights reserved.