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

Abstract

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[[ce:hsp sp="0.25"/]]kVp photon beam with field size of 5[[ce:hsp sp="0.25"/]]cm diameter, produced by a Gulmay D3225 orthovoltage X-ray machine was modeled by the BEAMnrc. Phantom containing water (thickness[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]1–5[[ce:hsp sp="0.25"/]]mm) on top of a bone (thickness[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]1[[ce:hsp sp="0.25"/]]cm) was irradiated by the 220[[ce:hsp sp="0.25"/]]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[[ce:hsp sp="0.25"/]]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[[ce:hsp sp="0.25"/]]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[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]1–5[[ce:hsp sp="0.25"/]]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.