Yadav Girigesh, Yadav R S, Kumar Alok
Department of Physics, D.A.V. (P.G.) College, Kanpur, U.P, India.
J Med Phys. 2010 Oct;35(4):202-6. doi: 10.4103/0971-6203.71761.
The purpose of this study was to find out the effect of various physical parameters on the skin and build-up doses of 15-MV photon beams. The effects of field dimensions, acrylic shadow tray, focus to-skin distance (FSD) on surface and buildup dose were determined for open, motorized 60° wedge (MW) and blocked fields. A 'Markus' plane parallel plate chamber was used for these measurements in an Elekta (6-15MV) linear accelerator. The surface dose for MW fields was lower than the dose for an open field, but the trend reversed for large fields and higher degree wedges. With the use of an acrylic shadow tray, the surface dose increased for all field sizes, but the increase was dominant for large fields. The surface dose for blocked fields was lower than the dose for open fields. The percentage depth dose of 10 × 10 cm(2) field at surface (PDD(0)) for open beam were 13.89%, 11.71%, and 10.74% at 80 cm, 100 cm, and 120 cm FSD, respectively. The blocking tray increased PDD(0) of 10 × 10 cm(2) field to 26.29%, 14.01%, and 11.53%, while the motorized 60° wedge decreased PDD(0) to 11.32%, 9.7%, and 8.9 % at these FSDs. The maximum PDD difference seen at surface (i.e., skin) for 5 × 5 cm(2), 15 × 15 cm(2), and 30 × 30 cm(2) are 0.5%, 4.6%, and 5.6% for open field and 0.9%, 4.7%, and 7.2% for motorized 60° wedge field, when FSDs varied from 80 cm to 120 cm. The maximum PDD difference seen at surface for 5 × 5 cm(2), 15 × 15 cm(2), and 30 × 30 cm(2) fields are 5.6%, 22.8%, and 29.6%, respectively, for a 1.0-cm perspex-blocking tray as the FSD is changed. The maximum PDD difference was seen at the surface (i.e., skin) and this decreased with increasing depth.
本研究的目的是探究各种物理参数对15兆伏光子束皮肤剂量和建成剂量的影响。针对开放野、电动60°楔形野（MW）和挡铅野，确定了射野尺寸、丙烯酸阴影托盘、源皮距（FSD）对表面剂量和建成剂量的影响。在医科达（6 - 15兆伏）直线加速器中，使用“马库斯”平行板电离室进行这些测量。MW野的表面剂量低于开放野，但对于大射野和更高角度楔形野，这种趋势会反转。使用丙烯酸阴影托盘时，所有射野尺寸的表面剂量均增加，但大射野增加更为显著。挡铅野的表面剂量低于开放野。在80厘米、100厘米和120厘米源皮距下，开放野10×10平方厘米射野在表面的百分深度剂量（PDD(0)）分别为13.89%、11.71%和10.74%。挡铅托盘使10×10平方厘米射野的PDD(())增加到26.29%、14.01%和11.53%，而电动60°楔形野在这些源皮距下使PDD(0)降至11.32%、9.7%和8.9%。当源皮距从80厘米变化到120厘米时，对于5×5平方厘米、15×15平方厘米和30×30平方厘米的开放野，表面（即皮肤）处最大PDD差值分别为0.5%、4.6%和5.6%，对于电动60°楔形野分别为0.9%、4.7%和7.2%。当源皮距改变时，对于1.0厘米厚有机玻璃挡铅托盘，5×5平方厘米、15×15平方厘米和30×30平方厘米射野在表面处最大PDD差值分别为5.6%、22.8%和29.6%。最大PDD差值出现在表面（即皮肤），且随深度增加而减小。
