Dose comparison between Gafchromic film, XiO, and Monaco treatment planning systems in a novel pelvic phantom that contains a titanium hip prosthesis

Abstract

The presence of metallic prostheses during external beam radiotherapy of malignancies in the pelvic region has the potential to strongly influence the dose distribution to the target and to tissue surrounded by the prostheses. This study systematically investigates the perturbation effects of unilateral titanium prosthesis on 6 and 15 MV photon beam dose distributions using Gafchromic EBT2 film measurements in a novel pelvic phantom made out of a stack of nylon slices. Comparisons were also made between the film data and dose calculations made on XiO and Monaco treatment planning systems. The collapsed cone algorithm was chosen for the XiO and the Monte Carlo algorithm used on Monaco is XVMC. Transmission measurements were taken using a narrow-beam geometry to determine the mass attenuation coefficient of nylon = 0.0458 cm² /g and for a water-equivalent RW3 phantom, it was 0.0465 cm² /g. The perturbation effects of the prosthesis on dose distributions were investigated by measuring and comparing dose maps and profiles. The magnitude of dose perturbations was quantified by calculating dose enhancement and reduction factors using field sizes of 3 × 3, 5 × 5, 10 × 10, and 15 × 15 cm² . For the studied beams and field sizes, dose enhancements between 21 and 30% and dose reductions between 15 and 21% were observed at the nylon-prosthesis interface on the proximal and distal sides of the prosthesis for film measurements. The dose escalation increases with beam energy, and the dose reduction due to attenuation decreases with increasing beam energy when compared to unattenuated beam data. A comparison of film and XiO depth doses for the studied fields gave relative errors between 1.1 and 23.2% at the proximal and distal interfaces of the Ti prosthesis. Also, relative errors < 4.0% were obtained between film and Monaco dose data outside the prosthesis for 6 and 15 MV lateral opposing fields.

Keywords: dose perturbations; pelvic phantom; photon beams; prosthesis.

Figure 1

The novel pelvic phantom (a) with unilateral Ti prosthesis on the left (b).

Figure 2

A schematic setup of narrow‐beam geometry for 6 MV transmission measurements

Figure 3

Beam setups/directions for dose perturbation measurements. A 10 × 10 cm² field was used for lateral beams and field sizes of 3 × 3, 5 × 5, 10 × 10, and 15 × 15 cm² were used for AP beams.

Figure 4

Transmission factors as a function of thickness for Nylon‐12 and RW3 slabs.

Figure 5

Calibration curve for Gafchromic EBT2 film using the red channel tiff image pixel data

Figure 6

Dose maps taken with the single right (a) and left (b) lateral beams for a 10 × 10 cm² field illustrating the effect of prosthesis on 6 MV photon beam dose distributions.

Figure 7

Dose maps taken with (left column) and without (right column) prosthesis obtained using AP beams for a 10 × 10 cm² field, illustrating the effect of prosthesis on (a) 6 MV and (b) 15 MV photon beams.

Figure 8

(a)–(d). Variations of 6 MV beam depth dose data with and without prosthesis. Dose perturbation factors with depth are indicated by the green line. Field sizes of 15 × 15, 10 × 10, 5 × 5, and 3 × 3 cm² were studied in Figs. 8(a)–8(d), respectively.

Figure 9

(a)–(d). Variations of 15 MV photon beam depth dose data with and without prosthesis. Dose perturbation factors with depth are indicated by the green line. Field sizes of 15 × 15, 10 × 10, 5 × 5, and 3 × 3 cm² were studied in Figs. 9(a)–9(d), respectively.

Figure 10

Variation of average values of dose perturbation factor (DPF) with distance from the nylon–prosthesis proximal interface for 6 and 15 MV beams. The error bars show the uncertainty (standard deviation of the mean values) in the data.

Figure 11

(a) and (b). Comparisons of photon beam dose distributions determined by film measurements and XiO TPS calculations for a field size of 5 × 5 cm². Data are shown for 6 MV (a) and 15 MV (b) beams.

Figure 12

(a) and (b). Comparisons of dose profiles determined by film measurements and Monaco TPS calculations for two opposing lateral fields. Data are shown for 6 MV (a) and 15 MV (b) photon beams. The left and right sides of the pelvic phantom are labeled on the graphs.