Calculation of organ doses from breast cancer radiotherapy: a Monte Carlo study

Abstract

The current study aimed to: a) utilize Monte Carlo simulation methods for the assessment of radiation doses imparted to all organs at risk to develop secondary radiation induced cancer, for patients undergoing radiotherapy for breast cancer; and b) evaluate the effect of breast size on dose to organs outside the irradiation field. A simulated linear accelerator model was generated. The in-field accuracy of the simulated photon beam properties was verified against percentage depth dose (PDD) and dose profile measurements on an actual water phantom. Off-axis dose calculations were verified with thermoluminescent dosimetry (TLD) measurements on a humanoid physical phantom. An anthropomorphic mathematical phantom was used to simulate breast cancer radiotherapy with medial and lateral fields. The effect of breast size on the calculated organ dose was investigated. Local differences between measured and calculated PDDs and dose profiles did not exceed 2% for the points at depths beyond the depth of maximum dose and the plateau region of the profile, respectively. For the penumbral regions of the dose profiles, the distance to agreement (DTA) did not exceed 2 mm. The mean difference between calculated out-of-field doses and TLD measurements was 11.4% ± 5.9%. The calculated doses to peripheral organs ranged from 2.32 cGy up to 161.41 cGy depending on breast size and thus the field dimensions applied, as well as the proximity of the organs to the primary beam. An increase to the therapeutic field area by 50% to account for the large breast led to a mean organ dose elevation by up to 85.2% for lateral exposure. The contralateral breast dose ranged between 1.4% and 1.6% of the prescribed dose to the tumor. Breast size affects dose deposition substantially.

Figure 1

Descriptive representation of the linear accelerator (linac) simulation. The dashed line divides the geometry conceptually and specifies the linac parts included in each simulation step.

Figure 2

Photon spectra calculated by the central circular and the outmost annular detector. The photon spectrum was “softer” towards the edges of the field.

Figure 3

The mean energy of the photon spectra calculated by each one of the annular detectors just below the flattening filter holder.

Figure 4

Superimposed measured and calculated percentage depth dose (PDD) curves for the $20\times 20\hspace{0.17em}{\text{cm}}^2$ photon field. The inset shows the local differences of the calculated values from the corresponding measured PDD values. The horizontal dashed lines mark the $\pm 2\%$ limit of acceptance. The vertical dashed line marks the depth of maximum dose.

Figure 5

Superimposed measured and calculated lateral dose profiles for the $20\times 20\hspace{0.17em}{\text{cm}}^2$ photon field at depth of maximum dose (${\mathrm{d}}_{max}$). The inset shows the local differences of the calculated values from the corresponding measured dose profile values. The horizontal dashed lines mark the $\pm 2\%$ limit of acceptance. The vertical dashed lines limit the plateau region of the profiles.

Figure 6

Superimposed measured and calculated lateral dose profiles for the $20\times 20\hspace{0.17em}{\text{cm}}^2$ photon field at 10 cm depth (${\mathrm{d}}_{10}$). The inset shows the local differences of the calculated values from the corresponding measured dose profile values. The horizontal dashed lines mark the $\pm 2\%$ limit of acceptance. The vertical dashed lines limit the plateau region of the profiles.