Evaluation of the eclipse electron Monte Carlo dose calculation for small fields

Abstract

Varian Medical Systems (Palo Alto, CA) has implemented the Monte Carlo electron dose calculation algorithm (eMC) in the Eclipse treatment planning system. Previous algorithms for electron treatment planning were limited in their calculation ability for small field depth doses and monitor units. An old rule of thumb to approximate the limiting cutout size for an electron field was determined by the lateral scatter equilibrium and approximated by E (MeV)/2.5 in centimeters of water. In this study we compared eMC calculations and measurements of depth doses, isodose distributions and monitor units for several different energy and small field cutout size combinations at different SSDs. Measurements were made using EBT film (International Specialty Products, Wayne, NJ) and a PinPoint Ion Chamber (PTW, Hicksville, NY). Our results indicate that the eMC algorithm can accurately predict depth doses, isodose distributions and monitor units (within 2.5%) for field sizes as small as 3.0 cm diameter for energies in the 6 to 20 MeV range at 100 cm SSD. Therefore, the previous energy dependent rule of thumb does not apply to the Eclipse electron Monte Carlo code. However, at extended SSDs (105-110 cm), the results show good agreement (within 4 %) only for higher energies (12, 16, and 20 MeV) for a field size of 3 cm.

Figure 1

Water tank setup used for film measurements. Gafchromic EBT film was lined up to the surface of the water set to different SSDs and held in place by adjustable clamps (not shown). Film was aligned to bisect the electron cutout placed in the electron applicator.

Figure 2

Percent depth dose curve comparisons between eMC calculations and EBT in water measurements for $10\times 10{\text{cm}}^2$ open field at 100 cm SSD using $10\times 10{\text{cm}}^2$ cone. All curves are normalized to 100%. The Eclipse measurements are represented by solid lines while the EBT measurements are represented by dotted lines as follows: 6 MeV (dark blue), 9 MeV (brown), 12 MeV (purple), 16 MeV (magenta), 20 MeV (light blue).

Figure 3

(a) – 3(e). Percent depth dose curve comparisons between eMC calculations and EBT in water measurements for 5, 4, 3, 2 and 1 cm cutout, respectively, at 100 cm SSD using $10\times 10{\text{cm}}^2$ cone. All curves are normalized to 100%. The Eclipse measurements are represented by solid lines while the EBT measurements are represented by dotted lines as follows: 6 MeV (dark blue), 9 MeV (brown), 12 MeV (purple), 16 MeV (magenta), 20 MeV (light blue).

Figure 4

Sagittal isodose comparisons of a 12 MeV beam along the central axis for 5 cm cutout at 100 cm SSD using $10\times 10{\text{cm}}^2$ cone. Thick lines represent eMC calculations and thin lines represent EBT film measurements. The 30, 50, 70, and 90 percentages are shown.

Figure 5

Gamma analysis results for beams of energy 6, 9, 12, 16 and 20 MeV for cutout sizes of 5, 4, 3, 2 and 1 cm in water at $\text{SSD}=100\text{cm}$ using $10\times 10{\text{cm}}^2$ cone.

Figure 6

Comparison of calculated and measured (a) MU% difference, and (b) gamma analysis results for 3 cm cutout size at extended SSDs for all tested energies using $10\times 10{\text{cm}}^2$ cone.

Figure 7

Comparison of calculated and measured (a) MU% difference, and (b) gamma analysis results for 3 cm cutout size at 100 cm SSD for all tested energies using $10\times 10{\text{cm}}^2$ and $6\times 6{\text{cm}}^2$ cones.

Figure 8

Comparison of calculated and measured (a) MU% difference, and (b) gamma analysis results for 2 cm cutout size at 100 cm SSD for all tested energies using $10\times 10{\text{cm}}^2$ and $6\times 6{\text{cm}}^2$ cones.

Figure 9

Comparison of calculated and measured (a) MU% difference, and (b) gamma analysis results for cutout sizes of 3 cm circle and $3\text{cm}\times 9\text{cm}$ at 100 cm SSD using $10\times 10{\text{cm}}^2$ cone for all tested energies.

Figure 10

Comparison of calculated and measured (a) MU% difference, and (b) gamma analysis results for cutout sizes of 2 cm circle and $2\text{cm}\times 9\text{cm}$ at 100 cm SSD using $10\times 10{\text{cm}}^2$ cone for all tested energies.