The dose distributions calculated from TPSs for the photon and proton beams suing phantom images with and without metal stents are shown in Figure 3. As shown in Figure 3, there were no dose perturbations in the area containing the metal stent in the photon and proton plans obtained from the TPS. This means that TPS calculations for photon and proton beams did not accurately calculate the dose perturbation in the area containing the metal stent. The differences between the axial dose distributions calculated by the MC simulations for the photon and proton beams, with and without the stent, are shown in Figure 4 and 5. As shown in these figures, it was visually confirmed that the dose was changed compared to the dose distribution calculated without the stent due to the change in the stent size depending on the various position angles (0°, 45°, and 90°) of the stent.
The differences in the values of the dosimetric parameters calculated in the MC simulation corresponding to the various stent positions and angles, compared to the values without a stent, are listed in Table 1 and 2. For the photon beam, the values of Dmax when the stent was placed into the body were higher, by 0~3.9%, than the value without the stent. The largest difference (3.9%) in Dmax was observed in the GB stent positioned at 0° compared with the absence of the stent. The values of Dmin were within 1.1% of the no-stent value for the GB stent and the GI stent when the stents were placed at 0° and 45°. The Dmin value obtained at an angle of 90° showed a relatively large difference, with decreases of 5.9% for the GB stent and 7.0% for the GI stent, respectively. The values of Dmean were not significantly different for the stents positioned at 0° and at 45°. For the stents positioned at 90°, there were small differences for both the GB stent (-1.7%) and the GI stent (0.7%). The differences in the dosimetric parameters (compared to not using the stent) were large for both the GB stent and the GI stent located at 90°.
Table 1
Percentage difference of dosimetric parameters for photon beam according to stent position and angle compared to that without a stent though MC simulation.
|
0°
|
45°
|
90°
|
|
GB stent
|
GI stent
|
GB stent
|
GI stent
|
GB stent
|
GI stent
|
Dmax (%)
|
3.9
|
2.4
|
0.4
|
0.0
|
1.9
|
1.7
|
Dmin (%)
|
-1.1
|
-1.0
|
0.2
|
-0.1
|
-5.9
|
-7.0
|
Dmean (%)
|
0.0
|
-0.1
|
-0.1
|
-0.1
|
-1.7
|
-0.7
|
GB stent: stent used in the bile duct, GI stent: stent used in gastrointestinal tract, Dmax: maximum dose, Dmin: minimum dose, Dmean: mean dose
Table 2
Percentage difference of dosimetric parameters for proton beam according to stent position and angle compared to that without a stent though MC simulation.
|
0°
|
45°
|
90°
|
|
GB stent
|
GI stent
|
GB stent
|
GI stent
|
GB stent
|
GI stent
|
Dmax (%)
|
0.7
|
0.6
|
1.4
|
1.6
|
4.3
|
2.7
|
Dmin (%)
|
-6.0
|
-2.7
|
-11.7
|
-14.7
|
-33.7
|
-21.9
|
Dmean (%)
|
-0.5
|
-0.1
|
-0.6
|
-0.6
|
-4.1
|
-1.2
|
GB stent: stent used in the bile duct, GI stent: stent used in gastrointestinal tract, Dmax: maximum dose, Dmin: minimum dose, Dmean: mean dose
For the proton beam, the values of Dmax were 0.7~4.3% higher than the value without the stent because of the interaction between the proton beam and the metal stent. Compared to not using the stent, Dmax showed the largest difference (4.3%) for the GB stent located at 90°. In terms of Dmin, all values showed lower doses; the differences were greater than 6% compared with the no-stent case, except for a difference of 2.7% for the GI stent positioned at 0°. The differences in the values of Dmin were relatively large, -33.7% and -21.9% in the GB and GI stents, respectively, for stent locations at 90°. The values of Dmean were within 1% of the no-stent values for the GI and GB stents located at 0° and 45° (as in the photon beam), whereas there was a slightly higher difference with the GB and GI stents located at 90°.
Comparisons of the DVHs at the various position angles (0°, 45°, and 90°) of the stent with those without the stent are shown in Figure 6 and 7. The analysis based on the DVHs enables straightforward discrimination of the dose difference with respect to the volume. In both the photon beam and the proton beam, it was observed that the dose varied with the angle of the stent as shown in Figure 6 and 7.
For the photon beam, the DVHs for the GB and GI stents positioned at 0° showed a difference with respect to the volume of the high-dose region. For the GB and GI stents positioned at 45°, the DVHs were similar to those without the stent. In contrast, the DVHs for the GI and GB stents positioned at 90° showed significant differences, indicating underdoses for the target volume.
For the proton beam, the DVHs indicated underdoses in the target volume for the GI and GB stents at all angles, as compared with the DVHs obtained without stents. The underestimated volume seen in the DVHs was exacerbated as the stent angle was increased. The GI and GB stents positioned at 90° were accompanied by differences in the high-dose region, indicating that high-dosing increased in accordance with the use of the stent. This trend was similar for the photon beam and the proton beam; however, the difference was much larger for the proton beam than for the photon beam.