Verification of IMRT dose calculations using AAA and PBC algorithms in dose buildup regions

Abstract

The purpose of this comparative study was to test the accuracy of anisotropic analytical algorithm (AAA) and pencil beam convolution (PBC) algorithms of Eclipse treatment planning system (TPS) for dose calculations in the low- and high-dose buildup regions. AAA and PBC algorithms were used to create two intensity-modulated radiotherapy (IMRT) plans of the same optimal fluence generated from a clinically simulated oropharynx case in an in-house fabricated head and neck phantom. The TPS computed buildup doses were compared with the corresponding measured doses in the phantom using thermoluminescence dosimeters (TLD 100). Analysis of dose distribution calculated using PBC and AAA shows an increase in gamma value in the dose buildup region indicating large dose deviation. For the surface areas of 1, 50 and 100 cm2, PBC overestimates doses as compared to AAA calculated value in the range of 1.34%-3.62% at 0.6 cm depth, 1.74%-2.96% at 0.4 cm depth, and 1.96%-4.06% at 0.2 cm depth, respectively. In high-dose buildup region, AAA calculated doses were lower by an average of -7.56% (SD = 4.73%), while PBC was overestimated by 3.75% (SD = 5.70%) as compared to TLD measured doses at 0.2 cm depth. However, at 0.4 and 0.6 cm depth, PBC overestimated TLD measured doses by 5.84% (SD = 4.38%) and 2.40% (SD = 4.63%), respectively, while AAA underestimated the TLD measured doses by -0.82% (SD = 4.24%) and -1.10% (SD = 4.14%) at the same respective depth. In low-dose buildup region, both AAA and PBC overestimated the TLD measured doses at all depths except -2.05% (SD = 10.21%) by AAA at 0.2 cm depth. The differences between AAA and PBC at all depths were statistically significant (p < 0.05) in high-dose buildup region, whereas it is not statistically significant in low-dose buildup region. In conclusion, AAA calculated the dose more accurately than PBC in clinically important high-dose buildup region at 0.4 cm and 0.6 cm depths. The use of an orfit cast increases the dose buildup effect, and this buildup effect decreases with depth.

Figure 1

The head and neck wax phantom with registration points for TLD placement (holes of different depths: 2 mm, 4 mm and 6 mm perpendicular to the phantom surface and on the transverse axial positions of the phantom).

Figure 2

The organs contoured on the CT slice images at isocentre, 5 cm inferior and superior to isocenter, with registration points for TLD placements: spinal cord (magenta color), the PTV to be delivered with 66 Gy (red). The magenta color contour to the right side of the CT axial slice represents the contralateral parotid (left parotid) to be saved; dark blue contours represent three strips of 2 mm thickness at three different depths of 2 mm, 4 mm and 6 mm from the skin of the phantom; yellow contour represents the region of interest which is defined by 1.4 cm extra margin from PTV for the defining of points of high‐ and low‐dose buildup regions.

Figure 3

Sensitivity curves against TL chips identification number, generated by reading the TL output on four different dates (26th October 2009, 29th October 2009, 9th November 2009 and 28th January 2010) using UL 300 TLD reader.

Figure 4

Calibration curve of TLD 100 dosimeters.

Figure 5

The histogram (a) of gamma values (gamma evaluation parameters of 3% dose difference and 3 mm distance to dose agreement) between AAA and PBC on transverse plane at isocentre; (b) the increase of gamma values from blue color to red color showing the increase in dose difference in dose buildup region.

Figure 6

The DVH data (a) of 2 mm strips structures at three different depths of 2 mm (light black continuous lines), 4 mm (light black broken lines) and 6 mm (dark blue continuous lines) for PBC (triangle markers) and AAA (square markers) for low‐dose buildup region (far away from planning target volume, PTV), showing larger dose calculation of AAA over PBC; (b) the same DVH data for high‐dose buildup region (proximal to PTV), showing larger dose calculation by PBC over AAA.

Figure 7

The positions (a) (represented by white spots for TLD placement) and the comparison of dose distributions of PBC and AAA on the transverse axial slices at isocentre, with inner green and magenta isodose curves representing the 190 cGy isodose curves calculated by PBC and AAA respectively, and the outer green and magenta curves representing 110 cGy isodose curves calculated by PBC and AAA respectively. This shows the better homogeneous dose calculated by AAA than that of PBC. Figs. (b), (d) and (e) show the graphs of TLD without orfit (black $\text{line}=3\%$ error bar in dose), TLD with orfit (blue $\text{line}=3\%$ error bar in dose), PBC (black short discontinuous line), and AAA (black long discontinuous line) at 2 mm, 4 mm and 6 mm depths, respectively. Figs. (c), (f) and (g) show the graphs of the variation of AAA (black line) and PBC (black discontinuous line) from TLD doses at 2 mm, 4 mm and 6 mm depths, respectively.

Figure 8

Comparison (a) of calculated percent depth dose (PDD) of AAA (shorter discontinuous lines), PBC (continuous lines) and measured PDD (longer discontinuous lines) using ion chamber for field sizes (FS) of $2\times 2{\text{cm}}^2$ (no marker), $10\times 10{\text{cm}}^2$ (marker) and $30\times 30{\text{cm}}^2$ (marker). The difference of PDD (b), calculated by PBC (no marker) and AAA (marker) from those of ion chamber measured for field sizes of $2\times 2{\text{cm}}^2$ (continuous lines), $10\times 10{\text{cm}}^2$ (longer discontinuous lines) and $30\times 30{\text{cm}}^2$ (shorter discontinuous lines). The inset the enlarged PDD differences in the scale within $\pm 0.05\%$.

Figure 9

Comparison of calculated dose profiles (AAA and PBC) at depth of dose maximum $(d_{max})$, 5 cm, 10 cm, 20 cm and 30 cm depths and measured dose profiles using CC13 ion chamber at the same depths for field sizes of $2\times 2{\text{cm}}^2,10\times 10{\text{cm}}^2$ and $30\times 30{\text{cm}}^2$.

Figure 10

Gamma values calculated which pass the tolerance dose of 3% and distance to dose agreement of 1 mm.