Lind B K, Källman P, Sundelin B, Brahme A
Department of Radiation Physics, Karolinska Institute, Stockholm, Sweden.
Acta Oncol. 1993;32(3):331-42. doi: 10.3109/02841869309093605.
The often large uncertainties that exist in beam patient alignment during radiation therapy may require modification of the incident beams to ensure an optimal delivered dose distribution to the target volume. This problem becomes increasingly severe when the required dose distribution of the incident beams becomes more heterogeneous. A simple analytical formula is derived for the case when the fraction number is high, and the desired relative dose variations are small. This formula adjusts the fluence distribution of the incident beam so that the resultant dose distribution will be as close as possible to the desired one considering the uncertainties in beam patient alignment. When sharp dose gradients are important, for instance at the border of the target volume, the problem is much more difficult. It is shown here that, if the tumor is surrounded by organs at risk, it is generally best to open up the field by about one standard deviation of the positional uncertainty--that is sigma/2 on each side of the target volume. In principle it is simultaneously desirable to increase the prescribed dose by a few per cent compared to the case where the positional uncertainty is negligible, in order to compensate for the rounded shoulders of the delivered dose distribution. When the tissues surrounding the tumor no longer are dose limiting even larger increases in field size may be advantageous. For more critical clinical situations the positional uncertainty may even limit the success of radiotherapy. In such cases one generally wants to create a steeper dose distribution than the underlying random Gaussian displacement process allows. The problem is then best handled by quantifying the treatment outcome under the influence of the stochastic process of patient misalignment. Either the coincidence with the desired dose distribution, or the expectation value of the probability of achieving complication-free tumor control is maximized under the influence of this stochastic process. It is shown that the most advantageous treatment is to apply beams that are either considerably widened or slightly widened and over flattened near the field edges for small and large fraction numbers respectively.
在放射治疗期间，射束与患者的对准往往存在很大的不确定性，这可能需要对入射射束进行调整，以确保向靶区提供最佳的剂量分布。当入射射束所需的剂量分布变得更加不均匀时，这个问题就变得越来越严重。针对分次照射次数较多且期望的相对剂量变化较小的情况，推导了一个简单的解析公式。该公式可调整入射射束的注量分布，使得在考虑射束与患者对准不确定性的情况下，最终的剂量分布尽可能接近期望的剂量分布。当尖锐的剂量梯度很重要时，例如在靶区边界处，问题就困难得多。本文表明，如果肿瘤被危及器官包围，通常最好将射野扩大约一个位置不确定性的标准差，即在靶区两侧各扩大σ/2。原则上，与位置不确定性可忽略不计的情况相比，同时将处方剂量提高百分之几是可取的，以便补偿所给予剂量分布的圆肩部分。当肿瘤周围的组织不再是剂量限制因素时，更大程度地扩大射野尺寸可能是有利的。对于更关键的临床情况，位置不确定性甚至可能限制放射治疗的成功。在这种情况下，人们通常希望创建比潜在的随机高斯位移过程所允许的更陡峭的剂量分布。然后，通过量化患者错位的随机过程影响下的治疗结果，能最好地处理这个问题。在这个随机过程的影响下，要么使与期望剂量分布的符合度最大化，要么使实现无并发症肿瘤控制概率的期望值最大化。结果表明，最有利的治疗方法是分别对小分次照射次数和大分次照射次数应用在射野边缘处显著加宽或略微加宽并过度平坦化的射束。
