Das I J
Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111, USA.
Med Phys. 1997 Nov;24(11):1781-7. doi: 10.1118/1.597943.
High atomic number (Z) materials such as lead, used for field shaping and shielding normal tissues in kilovoltage beams could produce significant dose enhancement in the forward direction contrary to our normal belief with respect to the attenuation of photon beams. Such a dose enhancement has not been studied in kilovoltage beams, which is investigated in this study. Using a Siemens ortho-voltage unit (60-240 kVp) and a thin window (5 microns) parallel plate ion chamber, forward dose perturbation factor (FDPF) was measured at interfaces created by high- and low-Z materials. The FDPF is defined as the ratio of doses with and without an interface (FDPF = Di/Dh; where Di is the dose at an interface and Dh is the dose in a homogeneous medium). Results indicate that dose enhancement (FDPF > 1) as high as 20-fold can be observed for a thin (> or = 0.02 mm) Pb sheet in contact with soft tissue. The magnitude of FDPF is relatively independent of field size and falls off exponentially with Pb thickness. The typical photon beam attenuation takes at a thickness > 1 mm. This intense dose enhancement is localized within 250 microns of the interface. The FDPF is energy dependent but saturates above 140 kVp, unlike the backscatter dose perturbation that peaks around 200 kVp. The FDPF varies inversely with the thickness of high Z and distance between the surface and high-Z medium. The FDPF falls off rapidly to a level of photon transmission usually predicted by exponential attenuation when distance is increased. In conclusion, with kilovoltage beam, a high-Z medium placed in contact with soft tissue may not attenuate radiation dose unless adequate thickness and proper distance between the surface and high-Z medium is used. The localized intense dose enhancement (approximately 20-fold) created by the high-Z interface could be exploited for clinical use.
高原子序数（Z）材料，如铅，用于千伏级射束的射野塑形和正常组织屏蔽，与我们对光子束衰减的通常认知相反，它可能在正向产生显著的剂量增强。千伏级射束中尚未对这种剂量增强进行过研究，本研究对此展开了调查。使用西门子正交电压单元（60 - 240 kVp）和薄窗（5微米）平行板电离室，在由高Z和低Z材料形成的界面处测量正向剂量扰动因子（FDPF）。FDPF定义为有界面和无界面时的剂量之比（FDPF = Di/Dh；其中Di是界面处的剂量，Dh是均匀介质中的剂量）。结果表明，与软组织接触的薄（≥0.02毫米）铅板可观察到高达20倍的剂量增强（FDPF > 1）。FDPF的大小相对与射野大小无关，并随铅厚度呈指数下降。典型的光子束衰减发生在厚度>1毫米时。这种强烈的剂量增强局限于界面250微米范围内。FDPF与能量有关，但在140 kVp以上饱和，这与在200 kVp左右达到峰值的反向散射剂量扰动不同。FDPF与高Z材料的厚度以及表面与高Z介质之间的距离成反比。当距离增加时，FDPF迅速下降到通常由指数衰减预测的光子传输水平。总之，对于千伏级射束，除非使用足够的厚度以及表面与高Z介质之间有适当的距离，否则与软组织接触的高Z介质可能不会衰减辐射剂量。高Z界面产生的局部强烈剂量增强（约20倍）可用于临床。
