Forward dose perturbation at high atomic number interfaces in kilovoltage x-ray beams.

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.