Reese Adam S, Das Shiva K, Curie Charles, Marks Lawrence B
Department of Radiation Oncology, Duke University Medical Center, Durham, North Carolina 27710, USA.
Med Phys. 2009 Mar;36(3):734-40. doi: 10.1118/1.3070585.
Treatment planners frequently modify beam arrangements and use IMRT to improve target dose coverage while satisfying dose constraints on normal tissues. The authors herein analyze the limitations of these strategies and quantitatively assess the extent to which dose can be redistributed within the patient volume. Specifically, the authors hypothesize that (1) the normalized integral dose is constant across concentric shells of normal tissue surrounding the target (normalized to the average integral shell dose), (2) the normalized integral shell dose is constant across plans with different numbers and orientations of beams, and (3) the normalized integral shell dose is constant across plans when reducing the dose to a critical structure. Using the images of seven patients previously irradiated for cancer of brain or prostate cancer and one idealized scenario, competing three-dimensional conformal and IMRT plans were generated using different beam configurations. Within a given plan and for competing plans with a constant mean target dose, the normalized integral doses within concentric "shells" of surrounding normal tissue were quantitatively compared. Within each patient, the normalized integral dose to shells of normal tissue surrounding the target was relatively constant (1). Similarly, for each clinical scenario, the normalized integral dose for a given shell was also relatively constant regardless of the number and orientation of beams (2) or degree of sparing of a critical structure (3). 3D and IMRT planning tools can redistribute, rather than eliminate dose to the surrounding normal tissues (intuitively known by planners). More specifically, dose cannot be moved between shells surrounding the target but only within a shell. This implies that there are limitations in the extent to which a critical structure can be spared based on the location and geometry of the critical structure relative to the target.
治疗计划制定者经常修改射束排列并使用调强放射治疗（IMRT）来改善靶区剂量覆盖，同时满足对正常组织的剂量限制。本文作者分析了这些策略的局限性，并定量评估了剂量在患者体内可重新分布的程度。具体而言，作者假设：（1）在围绕靶区的正常组织的同心壳层中，归一化积分剂量是恒定的（相对于平均积分壳层剂量进行归一化）；（2）在具有不同射束数量和方向的计划中，归一化积分壳层剂量是恒定的；（3）在降低对关键结构的剂量时，不同计划中的归一化积分壳层剂量是恒定的。利用先前因脑癌或前列腺癌接受过照射的7名患者的图像以及一个理想化场景，使用不同的射束配置生成了相互竞争的三维适形放疗和IMRT计划。在给定计划内以及对于具有恒定平均靶区剂量的相互竞争计划，对周围正常组织的同心“壳层”内的归一化积分剂量进行了定量比较。在每名患者体内，围绕靶区的正常组织壳层的归一化积分剂量相对恒定（1）。同样，对于每种临床场景，给定壳层的归一化积分剂量也相对恒定，无论射束的数量和方向（2）或关键结构的 sparing 程度（3）如何。三维放疗和IMRT计划工具可以重新分布，而不是消除对周围正常组织的剂量（计划制定者凭直觉知道）。更具体地说，剂量不能在围绕靶区的壳层之间移动，而只能在一个壳层内移动。这意味着基于关键结构相对于靶区的位置和几何形状，在 sparing 关键结构的程度方面存在局限性。
