Wadi-Ramahi S J, Naqvi S A, Chu J C
Medical Physics Department, Rush-Presbyterian-St. Luke's Medical Center, Chicago, Illinois 60612, USA.
Med Phys. 2001 Aug;28(8):1711-7. doi: 10.1118/1.1386780.
The problem of underdosing lesions adjacent to upper respiratory cavities and a proposal to correct it are presented in this work. The EGS4 Monte Carlo code was used to simulate a 6 MV x-ray beam passing through a block of tissues with air cavities 2, 4, and 6 cm wide. The geometry used approximates the tracheal geometry used by previous researchers who investigated the underdosing phenomenon. A uniform longitudinal magnetic field of 0.5 T strength is used to reduce secondary electron outscatter caused by the presence of an air gap, and thus improving the dose at the distal surface of air cavities. We introduce the term "percent dose reduction" (PDR), which is defined as the difference between the dose after the air cavity and the dose at the same depth in soft-tissue phantom normalized to the dose in the tissue phantom, to quantify the reduction in dose after an air gap. We also introduce the term dose improvement ratio (DIR), which is defined as the dose ratio with magnetic field to the dose, at the same point, without magnetic field, to quantify the improvement in dose when the magnetic field is applied. For 2 x 2 x 20 cm3 and 4 x 4 x 20 cm3 air cavities irradiated by 2 x 2 cm2 beams, we found PDRs of 38% and 52%, respectively. This means that for these cavities, there is a 38% and a 52% reduction in dose at the cavity edge compared to the same dose in tissue at the same depth for each cavity. The dose improved by 30% (DIR= 1.3) and 87% (DIR= 1.87), respectively, when applying the magnetic field. The worst effect on dose at the distal side came from larger cavities irradiated with small fields. In these situations, the improvement in dose due to the presence of magnetic field was the largest. This article deals with "ideal" head and neck geometries with a uniform magnetic field. In a paper to follow we will use a CT-based phantom to study the effect in realistic geometries with the presence of a magnetic field from a Helmholtz coil pair.
本文提出了上呼吸道腔邻近病变剂量不足的问题及纠正该问题的建议。使用EGS4蒙特卡罗代码模拟6兆伏X射线束穿过宽度为2厘米、4厘米和6厘米的带气腔组织块。所采用的几何结构近似于先前研究剂量不足现象的研究人员所使用的气管几何结构。使用强度为0.5特斯拉的均匀纵向磁场来减少由气隙导致的二次电子外散射，从而提高气腔远端面的剂量。我们引入了“剂量降低百分比”（PDR）这一术语，其定义为气腔后的剂量与软组织模体中相同深度处的剂量之差，并归一化为组织模体中的剂量，以量化气隙后的剂量降低情况。我们还引入了剂量改善率（DIR）这一术语，其定义为有磁场时的剂量与同一点无磁场时的剂量之比，以量化施加磁场时剂量的改善情况。对于由2×2平方厘米射束照射的2×2×20立方厘米和4×4×20立方厘米气腔，我们分别发现PDR为38%和52%。这意味着对于这些气腔，与每个气腔相同深度处组织中的相同剂量相比，气腔边缘的剂量分别降低了38%和52%。施加磁场时，剂量分别提高了30%（DIR = 1.3）和87%（DIR = 1.87）。对远端面剂量的最不利影响来自小射束照射大尺寸气腔的情况。在这些情况下，由于磁场的存在，剂量改善最为显著。本文探讨了具有均匀磁场的“理想”头颈部几何结构。在后续论文中，我们将使用基于CT的模体来研究在存在亥姆霍兹线圈对磁场的实际几何结构中的效果。
