Photon beam dosimetry in the superficial buildup region using radiochromic EBT film stack
Photon beam dosimetry in the superficial buildup region using radiochromic EBT film stack
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DOI：
10.1118/1.3125134
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发表时间：
2009-06-01
期刊：
影响因子：
3.8
通讯作者：
Chan, Maria F.
中科院分区：
文献类型：
--
作者：
Chiu-Tsao, Sou-Tung;Chan, Maria F.
在具有陡峭剂量梯度的表面区域对兆伏级光子束进行精确的二维或三维剂量测量一直是一个挑战。我们开发了一种剂量测定方法，在表面建立区域的6和15 MV的光子束使用辐射变色EBT膜堆栈。八个辐射变色EBT膜条（3 × 20 × 0.024 cm（3））堆叠在一起形成3D剂量计。将膜堆放置在聚苯乙烯体模上方，并由固体水板（0.2 cm）包围，顶部膜层为100 cm SSD。使用10 × 10 cm（2）的开放场以1000 MU照射膜叠堆。使用Epson 4870平板扫描仪以透射模式、48位颜色和150 dpi（0.017 cm像素分辨率）扫描所有胶片。使用已建立的校准曲线将像素值转换为剂量。该方法允许以良好的空间分辨率（水平0.017厘米和垂直0.024厘米）测量深度为0.012至0.18厘米的剂量。对于每种能量模态，我们获得了中心轴深度剂量百分比和沿着中心线的射束轮廓，该中心线覆盖每个深度处的初级场和外围区域。初级场剂量随深度急剧变化，而在外围区域的剂量弱依赖于深度。对于6 MV和15 MV光子束，（1）8个膜层中的中心轴百分比深度剂量范围分别为22%至66%和15%至44%;（2）d=0时的外推百分比深度剂量分别为15%和14%。用平行板薄窗电离室和平行板电离室测量了该区域的中心轴深度剂量率，与文献报道的一致。可以将百分比深度剂量和射束轮廓数据并入治疗计划系统中，以更准确地评估皮肤和浅表肿瘤的剂量，从而在临床使用中实现更准确的计算结果。
It has been a challenge to perform accurate 2D or 3D dosimetry in the surface region with steep dose gradient for megavoltage photon beams. We developed a dosimetry method in the superficial buildup region for the 6 and 15 MV photon beams using a radiochromic EBT film stack. Eight radiochromic EBT film strips (3x20x0.024 cm(3)) stacked together formed a 3D dosimeter. The film stack was positioned above a polystyrene phantom and surrounded by Solid Water slabs (0.2 cm) with the top film layer at 100 cm SSD. A 10x10 cm(2) open field was used to irradiate the film stack with 1000 MU. All films were scanned using Epson 4870 flatbed scanner with transmission mode, 48 bit color, and 150 dpi (0.017 cm pixel resolution). The pixel values were converted to doses using an established calibration curve. This method allowed dose measurement for depths from 0.012 to 0.18 cm with fine spatial resolution (0.017 cm horizontally and 0.024 cm vertically). For each energy modality, we obtained both the central axis percent depth doses and the beam profiles along the central line covering the primary field and peripheral region at each depth. The primary field doses varied steeply with depth, while those in the peripheral region were weakly dependent on depth. For the 6 MV and 15 MV photon beams, (1) the central axis percent depth doses in the eight film layers ranged from 22% to 66% and from 15% to 44%, respectively; (2) the extrapolated percent depth doses at d=0 were 15% and 14%, respectively. Agreement with the previously reported central axis percent depth doses in this region using parallel plate thin window ion chamber and ultrathin TLD was observed. The percent depth doses and beam profiles data can be incorporated in the treatment planning system for more accurate assessment of the doses to skin and shallow tumors to accomplish more accurate calculation results in the clinical usage.