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 signif icant dose enhancement in the forward direction contrary to our normal belief with respect to the attenuation of photon beams. Such a dose e nhancement has not been studied in kilovoltage beams, which is investi gated in this study. Using a Siemens orthovoltage unit (60-240 kVp) an d a thin window (5 mu m) parallel plate ion chamber, forward dose pert urbation factor (FDPF) was measured at interfaces created by high-and low-Z materials. The FDPF is defined as the ratio of doses with and wi thout an interface (FDPF = D-i/D-h; where D-i is the dose at an interf ace and D-h is the dose in a homogeneous medium). Results indicate tha t dose enhancement (FDPF>1) as high as 20-fold can be observed for a t hin (greater than or equal to 0.02 mm) Pb sheet in contact with soft t issue. The magnitude of FDPF is relatively independent of field size a nd falls off exponentially with Pb thickness. The typical photon beam attenuation takes place at a thickness >1 mm. This intense dose enhanc ement is localized within 250 mu m of the interface. The FDPF is energ y dependent but saturates above 140 kVp, unlike the backscatter dose p erturbation that peaks around 200 kVp. The FDPF varies inversely with the thickness of high Z and distance between the surface and high-Z me dium. The FDPF falls off rapidly to a level of photon transmission usu ally predicted by exponential attenuation when distance is increased. In conclusion, with kilovoltage beam, a high-Z medium placed in contac t with soft tissue may not attenuate radiation dose unless adequate th ickness and proper distance between the surface and high-Z medium is u sed. The localized intense dose enhancement (approximate to 20-fold) c reated by the high-Z interface could be exploited for clinical use. (C ) 1997 American Association of Physicists in Medicine.