Purpose: This study evaluated the dosimetric impact of surface dose reduction due to the loss of backscatter from the bone interface in superficial x‐ray radiation therapy. Monte Carlo simulation was carried out using the EGSnrc code. Methods: An inhomogeneous phantom containing a thin layer of tissue (1,3 and 5 mm) on top of a bone (thickness = 1 cm) was irradiated by a clinical 105 kVp photon beam produced by a Gulmay D3225 kV x‐ray machine. The field size and SSD was equal to 5 cm diameter and 20 cm, respectively. Surface doses for different phantom configurations were calculated using the DOSXYZnrc code. Photon energy spectra at the phantom and bone surface were determined according to phase‐space files at the particle scoring planes. For comparison, all Monte Carlo simulations were repeated in a phantom with the bone replaced by soft tissue. Results: Surface dose reduction was found when a bone was underneath the tissue layer. When the tissue thickness = 1 mm, a surface dose reduction of 3.5% was found. The dose reduction decreased to 2.1% and 1.8% when the tissue thickness increased to 3 and 5 mm, respectively. This shows that the impact of the surface dose uncertainty decreased while the tissue thickness on top of the bone increased. This result was supported by the decrease of intensity in the photon energy spectrum, when the tissue layer was with and over the bone, compared to without the bone. Conclusions: Surface dose reduction of 3.5%–1.8% was found when the tissue layer increased from 1 to 5 mm. This decrease of surface dose results in an overestimation of prescribed dose at the patient's surface, and should be a concern when using superficial x‐ray to treat skin tumours in sites such as forehead, chest wall and kneecap.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging