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Volume 17, Number 1, 2012

Effect of the bone heterogeneity on the dose prescription in orthovoltage radiotherapy: A Monte Carlo study

James C.L. Chow, Grigor N. Grigorov


Background: In orthovoltage radiotherapy, since the dose prescription at the patient’s surface
is based on the absolute dose calibration using water phantom, deviation of delivered dose
is found as the heterogeneity such as bone present under the patient’s surface.
Aim: This study investigated the dosimetric impact due to the bone heterogeneity on the
surface dose in orthovoltage radiotherapy.
Materials and methods: A 220 kVp photon beam with field size of 5 cm diameter, produced
by a Gulmay D3225 orthovoltage X-ray machine was modeled by the BEAMnrc. Phantom
containing water (thickness = 1–5 mm) on top of a bone (thickness = 1 cm) was irradiated by
the 220 kVp photon beam. Percentage depth dose (PDD), surface dose and photon energy
spectrum were determined using Monte Carlo simulations (the BEAMnrc code).
Results: PDD results showed that the maximum bone dose was about 210% higher than the
surface dose in the phantoms with different thicknesses of water. Surface dose was found
to be increased in the range of 2.5–3.7%, when the distance between the phantom surface
and bone was increased in the range of 1–5 mm. The increase of surface dose was found not
to follow the increase of water thickness, and the maximum increase of surface dose was
found at the thickness of water equal to 3 mm.
Conclusions: For the accepted total orthovoltage radiation treatment uncertainty of 5%, a
neglected consideration of the bone heterogeneity during the dose prescription in the sites
of forehead, chest wall and kneecap with soft tissue thickness = 1–5 mm would cause more
than two times of the bone dose, and contribute an uncertainty of about 2.5–3.7% to the
total uncertainty in the dose delivery.

Signature: Rep Pract Oncol Radiother, 2012; 17(1) : 38-43

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