Patient-specific skin dose evaluation in breast cancer radiotherapy: A Monte Carlo study

Abstract

Introduction

Accurate assessment of skin dose in radiation treatment of breast cancer is very important. The analytical algorithms of treatment planning systems are limited in calculating the surface dose where electron equilibrium is not established. Measuring the skin dose requires a special dosimeter, which is time-consuming. Monte Carlo simulation has been developed as a golden method for calculating the skin dose. This study aimed to assess the factors affecting the skin dose in radiotherapy of intact breast cancer by developing a computational Monte Carlo framework.

Methods

The BEAMnrc-based model of 6 MV beam of a Siemens Primus linac was developed and validated. The skin dose in tangential breast fields was calculated and measured in the CIRS phantom by simulation and film dosimetry. The effect of treatment parameters, including field size, obliquity, skin-to-surface distance and the wedge angle, on surface dose was investigated.

Results

The calculated beam model with electron energy 6.7 MeV and FWHM 3.0 mm were validated. The mean relative skin dose differences between calculated and measured for inside the field was 17.24%, and for out-of-field points on the contralateral breast, was 21.05%. Increasing the open field size, increased the skin dose. By increasing the gantry angle from 50° to 65°, the skin dose increased by 178% only along the contralateral breast. The in-field and over the contralateral breast skin dose changed, by increasing the SSD from 95 cm to 105 cm. By changing the wedge angle from 15° to 45°, the in-field skin dose was reduced by 9.21%.

Conclusion

Several limitations make surface dosimetry challenging in radiotherapy of breast cancer. The computational Monte Carlo framework developed in this study, including the Primus linear accelerator head model with the patient's treatment plan specifications and the patient's tomographic phantom, can be used to measure the skin dose in radiation therapy for various cancers.