Tangboonduangjit P, Metcalfe P, Butson M, Quach K Y, Rosenfeld A
Department of Engineering Physics, University of Wollongong, NSW, Australia.
Phys Med Biol. 2004 Sep 7;49(17):N287-92. doi: 10.1088/0031-9155/49/17/n03.
The Varian millennium 120 multileaf collimator has curved leaf ends. Transmission through the leaf ends generates a small asymmetric penumbral dose effect. This design can lead to hot spots between neighbouring beam segments during step and shoot IMRT dose delivery. We have observed some matchlines with film for clinical beams optimized using the pinnacle radiotherapy treatment planning system; hence we sought to verify the optimum leaf offset required to minimize the matchline effect. An in-house program was created to control the MLC leaf banks in 2 cm steps with a 2 cm gap. The gap was varied by the following offset values from 0.0 to 0.1 cm. Two types of radiographic films (Kodak EDR and XV films) and a radiochromic film (Gafchromic MD-55-2) were used to measure the optical density maps. The films were positioned in a solid water phantom perpendicular to the beam axis and irradiated at d(max) using a 6 MV photon beam. An ion chamber (IC4) was used to measure point doses for normalization in a beam umbral minima position. The relative mean peak to valley dose ratios measured with no leaf offset were 1.31, 1.30 and 1.31 for the XV, EDR2 and Gafchromic films, respectively. For a 0.07 cm gap per leaf and a performance of end leaf repeatability of 0.01 cm, the central matchline was reduced to about 1.0 for all dosimeters, with two mini-peaks measured as 1.05, 1.05 and 1.08 each side of the matchline, for XV, EDR2 and Gafchromic, respectively. The average relative dose across the umbra for this offset was XO-mat V = 1.01, EDR = 1.01 and radiochromic film = 1.02, respectively. While we expected the beam penumbral tails from segment neighbours to cause overprediction of the dose in the central valley regions due to the energy response of radiographic films, by normalizing all dosimeters to an ion chamber reading in the minimum we could not observe any major shape distortion between the radiographic film and radiochromic film results. In conclusion, relative doses measured by radiographic and radiochromic films agree well with IC4 within +/-2%.
瓦里安千年120多叶准直器的叶片末端呈弯曲状。射线穿过叶片末端会产生微小的不对称半影剂量效应。这种设计在步进式静态调强放疗剂量输送过程中，可能会在相邻射野段之间产生热点。我们使用Pinnacle放射治疗计划系统对临床射野进行优化后，用胶片观察到了一些匹配线；因此，我们试图验证将匹配线效应降至最低所需的最佳叶片偏移量。我们编写了一个内部程序，以2厘米的步长控制多叶准直器的叶片组，叶片组之间有2厘米的间隙。间隙通过从0.0到0.1厘米的以下偏移值进行变化。使用两种类型的射线照相胶片（柯达EDR和XV胶片）和一种辐射变色胶片（Gafchromic MD - 55 - 2）来测量光密度图。将胶片垂直于射束轴放置在固体水模体中，并使用6兆伏光子束在最大剂量深度（d(max)）处进行照射。使用一个电离室（IC4）测量射束半影最小值位置的点剂量，用于归一化。在无叶片偏移情况下，XV、EDR2和Gafchromic胶片测得的相对平均峰谷剂量比分别为1.31、1.30和1.31。对于每片叶片0.07厘米的间隙以及叶片末端重复性为0.01厘米的情况，所有剂量计的中心匹配线均降至约1.0，在匹配线两侧，XV、EDR2和Gafchromic胶片测得的两个小峰值分别为1.05、1.05和1.08。对于此偏移量，在半影范围内的平均相对剂量分别为：XO - mat V = 1.01，EDR = 1.01，辐射变色胶片 = 1.02。虽然我们预计相邻射野段的射束半影尾部会因射线照相胶片的能量响应而导致中心谷区剂量预测过高，但通过将所有剂量计归一化为电离室在最小值处的读数，我们未观察到射线照相胶片和辐射变色胶片结果之间有任何明显的形状畸变。总之，射线照相胶片和辐射变色胶片测得的相对剂量与IC4的测量结果在±2%的范围内吻合良好。
