Arunkumar T, Supe Sanjay S, Ravikumar M, Sathiyan S, Ganesh M
Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Hosur Road, Bangalore, India.
J Med Phys. 2010 Oct;35(4):207-14. doi: 10.4103/0971-6203.71763.
Electron beam therapy is widely used in the management of cancers. The rapid dose fall-off and the short range of an electron beam enable the treatment of lesions close to the surface, while sparing the underlying tissues. In an extended source-to-surface (SSD) treatment with irregular field sizes defined by cerrobend cutouts, underdosage of the lateral tissue may occur due to reduced beam flatness and uniformity. To study the changes in the beam characteristics, the depth dose, beam profile, and isodose distributions were measured at different SSDs for regular 10 × 10 cm(2) and 15 × 15 cm(2) cone, and for irregular cutouts of field size 6.5 × 9 cm(2) and 11.5 × 15 cm(2) for beam energies ranging from 6 to 20 MeV. The PDD, beam flatness, symmetry and uniformity index were compared. For lower energy (6 MeV), there was no change in the depth of maximum dose (R100) as SSD increased, but for higher energy (20 MeV), the R(100) depth increased from 2 cm to 3 cm as SSD increased. This shows that as SSD increases there is an increase in the depth of the maximum dose for higher energy beams. There is a +7 mm shift in the R(100) depth when compared with regular and irregular field sizes. The symmetry was found to be within limits for all the field sizes as the treatment distance extended as per International Electro technical Commision (IEC) protocol. There was a loss of beam flatness for irregular fields and it was more pronounced for lower energies as compared with higher energies, so that the clinically useful isodose level (80% and 90%) width decreases with increase in SSD. This suggests that target coverage at extended SSD with irregular cut-outs may be inadequate unless relatively large fields are used.
电子束疗法在癌症治疗中被广泛应用。电子束剂量快速下降且射程较短，这使得靠近体表的病变能够得到治疗，同时可保护深层组织。在用铈镧合金挡块定义不规则射野尺寸的扩展源皮距（SSD）治疗中，由于射束平坦度和均匀性降低，外侧组织可能会出现剂量不足。为研究射束特性的变化，针对能量范围为6至20 MeV的情况，在不同SSD下测量了常规10×10 cm²和15×15 cm²圆锥野以及野尺寸为6.5×9 cm²和11.5×15 cm²的不规则挡块野的深度剂量、射束轮廓和等剂量分布。比较了百分深度剂量（PDD）、射束平坦度、对称性和均匀性指数。对于较低能量（6 MeV），随着SSD增加，最大剂量深度（R100）没有变化，但对于较高能量（20 MeV），随着SSD增加，R100深度从2 cm增加到3 cm。这表明随着SSD增加，较高能量射束的最大剂量深度会增加。与常规和不规则野尺寸相比，R100深度有+7 mm的偏移。根据国际电工委员会（IEC）协议，随着治疗距离延长，所有野尺寸的对称性均在允许范围内。不规则野存在射束平坦度损失，与较高能量相比，较低能量时更明显，因此随着SSD增加，临床有用等剂量水平（80%和90%）宽度减小。这表明除非使用相对较大的射野，否则在扩展SSD下使用不规则挡块时靶区覆盖可能不足。
