Blomquist M, Sätherberg A, Karlsson M, Zackrisson B
Department of Radiation Physics, Umeå University, Sweden.
Phys Med Biol. 1998 May;43(5):1185-97. doi: 10.1088/0031-9155/43/5/010.
Optimization of the dose distributions by individual beam compensation is a useful tool in conformal radiation therapy. Intensity modulation by electromagnetic scanning of a narrow elementary beam allows fast dose delivery and causes little change in beam quality compared with other methods, especially for high energies such as 50 MV. Intensity modulated beams from the MM50 accelerator were measured and compared with calculations based on Monte Carlo simulations. Good agreement between measurements and calculations were found, typically within 1% for central dose profiles. The steepest wedge angle that was produced with the scanning beam technique was of 45 degrees or 3.5% cm(-1) for a 20 cm x 20 cm field, slightly varying with depth. The elementary 50 MV photon 'pencil beam' for a full range, high-z bremsstrahlung target, is a wide dose distribution at 10 cm depth in water which limits the modulation gradient and hence the complexity of the modulation by the scanning of a photon pencil beam only. Scanned wedge beam distributions were modelled in the treatment planning system and a pelvic treatment with three fields was used to illustrate a clinical application. The resulting dose volume data were compared for different radiation qualities but with similar beam portals. 'Energy modulation' by field matching with lower photon energies was performed to sharpen the penumbra towards organs at risk.
通过个体射束补偿优化剂量分布是适形放射治疗中的一种有用工具。与其他方法相比，通过对窄基本射束进行电磁扫描实现强度调制能够快速输送剂量，并且对射束质量的改变很小，特别是对于50 MV这样的高能量。对来自MM50加速器的强度调制射束进行了测量，并与基于蒙特卡罗模拟的计算结果进行了比较。测量结果与计算结果吻合良好，中心剂量分布通常在1%以内。扫描射束技术产生的最陡楔形角在20 cm×20 cm射野时为45度或3.5% cm⁻¹，随深度略有变化。对于全范围、高原子序数轫致辐射靶的基本50 MV光子“笔形束”，在水中10 cm深度处是一个宽剂量分布，这限制了调制梯度，从而也限制了仅通过光子笔形束扫描进行调制的复杂性。在治疗计划系统中对扫描楔形束分布进行了建模，并使用一个三野盆腔治疗来说明临床应用。对不同辐射质但射野入口相似的情况比较了所得的剂量体积数据。通过与较低光子能量进行射野匹配来进行“能量调制”，以锐化朝向危及器官的半影。
