A Monte Carlo evaluation of beam characteristics for total body irradiation at extended treatment distances

Abstract

The aim is to study beam characteristics at large distances when focusing on the electron component. In particular, to investigate the utility of spoilers with various thicknesses as an electron source, as well as the effect of different spoiler-to-surface distances (STSD) on the beam characteristics and, consequently, on the dose in the superficial region. A MC model of a 15 MV Varian accelerator, validated earlier by experimental data at isocenter and extended distances used in large-field total body irradiation, is applied to evaluate beam characteristics at distances larger than 400 cm. Calculations are carried out using BEAMnrc/DOSXYZnrc code packages and phase space data are analyzed by the beam data processor BEAMdp. The electron component of the beam is analyzed at isocenter and extended distances, with and without spoilers as beam modifiers, assuming vacuum or air surrounding the accelerator head. Spoiler thickness of 1.6 cm is found to be optimal compared to thicknesses of 0.8 cm and 2.4 cm. The STSD variations should be taken into account when treating patients, in particular when the treatment protocols are based on a fixed distance to the patient central sagittal plane, and also, in order to maintain high dose in the superficial region.

Figure 1

Cross profiles in water for $10\times 10\hspace{0.17em}{\text{cm}}^2$, $20\times 20\hspace{0.17em}{\text{cm}}^2$, and $40\times 40\hspace{0.17em}{\text{cm}}^2$ fields at $\text{SSD}=90\hspace{0.17em}\text{cm}$. Lines: measured data; symbols: MC data.

Figure 2

MC geometry for calculation of central depth dose profile for different spoiler‐to‐surface distances (STSD). Phase space is produced at 445 cm from the accelerator target. Source‐to‐water surface distance is fixed to 480 cm; STSD varies.

Figure 3

Characteristics of the electron component of the beam (estimated real fluence) at (a) $\text{SSD}=90\hspace{0.17em}\text{cm}$ and (b) $\text{SSD}=445\hspace{0.17em}\text{cm}$. Black histograms: vacuum surrounding the accelerator head; red histograms : air surrounding the accelerator head; green histogram: air surrounding the accelerator head and presence of 1.6 cm PMMA spoiler.

Figure 4

Angular distribution of the electron component of the beam. Black histogram: vacuum surrounding the accelerator head, $\text{SSD}=90\hspace{0.17em}\text{cm}$; red histogram: vacuum surrounding the accelerator head, $\text{SSD}=445\hspace{0.17em}\text{cm}$; green histogram: air surrounding the accelerator head, $\text{SSD}=90\hspace{0.17em}\text{cm}$; blue histogram: air surrounding accelerator head, $\text{SSD}=445\hspace{0.17em}\text{cm}$.

Figure 5

Energy fluence distribution (a) of electrons at 445 cm distance (black histogram: without spoiler; red histogram: 0.8 cm PMMA spoiler; green histogram: 1.6 cm PMMA spoiler; blue histogram: 2.4 cm PMMA spoiler); (b) depth dose distribution of electrons at 445 cm distance (black line: without spoiler; red line: 0.8 cm MMA spoiler; green line: 1.6 cm PMMA spoiler; blue line: 2.4 cm PMMA spoiler).

Figure 6

Depth dose distributions in water at $\text{SSD}=480\hspace{0.17em}\text{cm}$ behind 1.6 cm PMMA spoiler (red circles: $\text{STSD}=5\hspace{0.17em}\text{cm}$; blue squares: $\text{STSD}=10\hspace{0.17em}\text{cm}$, black triangles: $\text{STSD}=30\hspace{0.17em}\text{cm}$).

Figure 7

Energy fluence distribution for electrons at 480 cm distance behind 1.6 cm PMMA; black, red, and green lines for $\text{STSD}=5$, 10, and 30 cm, correspondingly.