Abstract:
Introduction
The primary role of breast cancer treatment with radiation is to deliver a sufficient
radiation dose to the cancer cells without unduly causing biological damage to the
healthy tissues. For over 50 years, electron beam therapy has been an important modality
for providing an accurate dose of radiation to superficial cancers and disease and for
limiting the dose to underlying normal tissues and structures in particular to boost the
dose to the tumour bed and surgical scars after mastectomy. The Monte Carlo code
MCNP5 was used to determine the effect of silicone gel breast prosthesis on the electron
beam dose distribution.
Materials and Method
Percentage depth dose curves (PDD) for 6, 9, 12, and 15 MeV electron energies along the
electron central axis depth dose distributions in a water phantom and with silicone
prosthesis immersed in a water phantom were simulated using MCNP5.
In order to establish the accuracy of the MCNP5 code, the depth dose curves obtained
using MCNP5 were compared against the measured depth dose curves obtained from the
Varian 2100C linear accelerator. The simulated depth dose curves with silicone
prosthesis immersed in water were compared to the measured depth dose curves with the
vi
silicone prosthesis in water. The dose at the interface of the prosthesis with water was
measured using thermoluminiscent dosimeters.
Results
The simulated and measured depth dose curve and the investigated dosimetric parameters
are within 2%. Simulations in the presence of silicone showed a decrease in dose as the at
the interface as the beam passes from the prosthesis to water for most energies however,
for 15 MeV beam there is an increase in dose at the interface between the prosthesis and
water and this was verified by physical measurements.
Conclusion
There were good correlations between the measured and MCNP simulated depth dose
curve. Differences were in order of 2%. Small deviations occurred due to the fact that the
simulations assumed a monoenergetic beam that exits the accelerator head, while in the
measured results the beam exiting from the accelerator head includes scatted radiation
from the collimators and the applicator. The presence of the prosthesis does not perturb
the electron beam central axis depth dose curve however, the 15 MeV beam enhanced the
dose in front of the interface between the prosthesis and water. Despite the limitations
mentioned above MCNP5 results agree reasonably with the measured results. Hence,
MCNP5 can be very useful in simulating electron percentage depth dose data.
