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

Abstract

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.

Increased apoptotic potential and dose‐enhancing effect of gold nanoparticles in combination with single‐dose clinical electron beams on tumor‐bearing mice

“…C57BL/6 mice were inoculated subcutaneously (s.c.) in the thigh with syngeneic mouse melanoma B16F10 cells (1 × 10 6 ) suspended in 0.1 mL of PBS at day 0, and at day 7, visible nodules developed at all injection sites with approximate tumor volumes of 50-90 mm 3 . Groups of four to seven tumorbearing mice were injected intravenously (i.v.)…”

“…In theory, loading high Z materials into the tumor could result in greater photoelectric absorption within the tumor than in surrounding tissues, and thereby enhance the dose delivered to a tumor during radiation therapy. At least 20 years ago, it was noted in vitro that this effect might be employed to enhance radiotherapy for cancer.(1) Accumulating studies have demonstrated the dose enhancement caused by high Z materials in kilovoltage beams (2)(3)(4) and in megavoltage beams.(5-8) Moreover, enhanced cell killing was also observed when cells were irradiated adjacent to high Z materials by kilovoltage X-rays. (9)(10)(11)(12) In clinical practice, electron beams from linear accelerators have increasingly taken the place of kilovoltage X-ray beams for skin and subcutaneous tumors because they offer distinct advantages in terms of dose uniformity in the target volume and in minimizing the dosage to deeper tissues.…”

“…(1) Accumulating studies have demonstrated the dose enhancement caused by high Z materials in kilovoltage beams (2)(3)(4) and in megavoltage beams.…”

“…At least 20 years ago, it was noted in vitro that this effect might be employed to enhance radiotherapy for cancer. (1) Accumulating studies have demonstrated the dose enhancement caused by high Z materials in kilovoltage beams (2)(3)(4) and in megavoltage beams. (5)(6)(7)(8) Moreover, enhanced cell killing was also observed when cells were irradiated adjacent to high Z materials by kilovoltage X-rays.…”

Dosimetric evaluation of internal shielding in a high-dose-rate skin applicator

“…When there is a critical structure located only at a few millimeters under the tumor, like for example in case of the eyelid, an internal shielding is very useful to reduce the dose to this structure. Dose perturbation at different material interfaces has been extensively studied for many years for different beam energies and modalities: electron beams [10], kV photon beams [11][12][13], and high-energy and gamma photon beams [14][15][16]. The perturbation depends on beam energy, field size, thickness, width, position, and atomic number of the heterogeneity creating the interface.…”