Surface Dosimetry in the Presence of a Bone in Skin Radiotherapy: A Monte Carlo Study Using KV Photon Beams
J Chow1*, A Owrangi2, G Grigorov3, (1) Princess Margaret Hospital, Toronto, ON, (2) Robarts Research Laboratories, London, ON, (3) Grand River Regional Cancer Center, Kitchener, ONSU-E-T-533 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
This study examined the variation of surface dose in the presence of a bone located under the surface of patient (e.g. sites of forehead, chest wall and kneecap) in skin radiotherapy. A 105 kVp photon beam was used, and simulated by Monte Carlo method using the EGSnrc code.
Heterogeneous phantoms containing a water layer (thickness = 0.5-5 mm) over a bone with thickness equal to 1 cm were irradiated by a clinical 105 kVp photon beam (Gulmay D3225 x-ray machine). The field size was 5 cm diameter and the source-to-surface distance of the beam was 20 cm. Phase-space file of the photon beam was generated using the BEAMnrc code and verified by measurements. Surface doses for different phantom geometries were calculated using the DOSXYZnrc code. For comparison, all Monte Carlo simulations were repeated in a water phantom with the same dimension of the corresponding heterogeneous phantom.
With the presence of a bone under a water layer of 1 mm thickness, surface dose was found decreased 6.3% using the 105 kVp photon beam. This is due to the loss of backscatter from the bone in the beam irradiation. When the water thickness increased to 3 and 5 mm, the surface dose reduction decreased to 4.7% and 3.4%, respectively. This shows that the dosimetric impact due to the presence of bone decreased, when the bone was located further away from the phantom surface.
Surface dose reduction was found in the presence of a bone under the skin tissue. Since such dose reduction is not considered in the dose calculation based on the absolute dose calibration using a homogeneous water phantom, an overestimation of dose occurs when it is prescribed. This results in a dosimetric uncertainty with a variation of tissue thickness in skin radiotherapy.