Validation of total skin electron irradiation (TSEI) technique dosimetry data by Monte Carlo simulation

Abstract

Total skin electron irradiation (TSEI) is a complex technique which requires many nonstandard measurements and dosimetric procedures. The purpose of this work was to validate measured dosimetry data by Monte Carlo (MC) simulations using EGSnrc-based codes (BEAMnrc and DOSXYZnrc). Our MC simulations consisted of two major steps. In the first step, the incident electron beam parameters (energy spectrum, FWHM, mean angular spread) were adjusted to match the measured data (PDD and profile) at SSD = 100 cm for an open field. In the second step, these parameters were used to calculate dose distributions at the treatment distance of 400 cm. MC simulations of dose distributions from single and dual fields at the treatment distance were performed in a water phantom. Dose distribution from the full treatment with six dual fields was simulated in a CT-based anthropomorphic phantom. MC calculations were compared to the available set of measurements used in clinical practice. For one direct field, MC calculated PDDs agreed within 3%/1 mm with the measurements, and lateral profiles agreed within 3% with the measured data. For the OF, the measured and calculated results were within 2% agreement. The optimal angle of 17° was confirmed for the dual field setup. Dose distribution from the full treatment with six dual fields was simulated in a CT-based anthropomorphic phantom. The MC-calculated multiplication factor (B12-factor), which relates the skin dose for the whole treatment to the dose from one calibration field, for setups with and without degrader was 2.9 and 2.8, respectively. The measured B12-factor was 2.8 for both setups. The difference between calculated and measured values was within 3.5%. It was found that a degrader provides more homogeneous dose distribution. The measured X-ray contamination for the full treatment was 0.4%; this is compared to the 0.5% X-ray contamination obtained with the MC calculation. Feasibility of MC simulation in an anthropomorphic phantom for a full TSEI treatment was proved and is reported for the first time in the literature. The results of our MC calculations were found to be in general agreement with the measurements, providing a promising tool for further studies of dose distribution calculations in TSEI.

Figure 1

Modeling geometry at $\text{SSD}=100\hspace{0.17em}\text{cm}$ including Elekta Precise components, water phantom, and the phase‐space location.

Figure 2

Modelling geometry at $\text{SSD}=400\hspace{0.17em}\text{cm}$. Linac components and location of the degrader, solid water phantom, and the phase‐space file are shown.

Figure 3

The measured and calculated PDD curves for a $40\times 40\hspace{0.17em}{\text{cm}}^2$ field at $\text{SSD}=100\hspace{0.17em}\text{cm}$.

Figure 4

The measured and calculated lateral profiles at depth = 1.2 cm for a $40\times 40\hspace{0.17em}{\text{cm}}^2$ field at $\text{SSD}=100\hspace{0.17em}\text{cm}$.

Figure 5

The measured and calculated PDD curves (with and without degrader) at $\text{SSD}=400\hspace{0.17em}\text{cm}$.

Figure 6

The measured and calculated profiles at depth 0.25 cm at $\text{SSD}=400\hspace{0.17em}\text{cm}$.

Figure 7

The calculated profiles at depth 0.25 cm at $\text{SSD}=400\hspace{0.17em}\text{cm}$ for: (a) one field for different tilt angles, and (b) different dual‐field angles. For the 17° angle, measurements at depth 0.25 cm are also shown.

Figure 8

The calculated and measured profiles at depth 0.25 cm for one field with 17° tilt, with and without degrader, at $\text{SSD}=400\hspace{0.17em}\text{cm}$.

Figure 9

The MC‐calculated transversal dose distribution from the whole treatment in the anthropomorphic phantom: (a) no degrader is used; (b) the degrader is used. Treatment fields are shown with the arrows.

Figure 10

MC‐calculated and measured PDDs along anterior and lateral directions in the anthropomorphic phantom: (a) no degrader; (b) with degrader.