The ED assessed for a total of 42 imaging datasets (CT and corresponding CHCT) of 21 patients who underwent liver metastases SBRT treatment at Unity MR-Linac were compared. The average ED evaluated for each structure (body, liver, heart, lungs, spinal cord, duodenum, great vessels, ribs, spinal cord, esophagus, small bowel, etc.) along with the values recommended in ICRU Report 46[30], are reported in Table 2. For the lungs, imaging datasets from three patients were excluded because the scans did not cover the entire lung volume due to the target’s location near the lower lobe of the liver, and because of significant differences in the acquisition phases between the pCT and CHCT scans[31]. The average ED obtained on the pCT and CHCT for each of the structure shows significant differences (p < 0.05) with the larger deviations registered for the lungs (11.73% and 12.15% for the left and right lung respectively), great vessels (8.35%), and heart (6.18%). The ED values suggested by ICRU 46 correspond to those obtained from pCT; a discrepancy exists in the value for the ribs and the lumbar region from the different way of delineation used[32].
Table 2
Average electronic density for each ROIs on the different imaging datasets; The International Commission on Radiation Units and Measurements (ICRU) 46 reports values are reported as reference.
OAR | No. of patients | ICRU46 | pCT | CHCT | Maximum Difference (%) | Average Difference (%) | p |
Liver-GTV | 21 | 1.05 | 1.06 ± 0.01 | 1.09 ± 0.01 | 3.93 | 2.42 ± 0.78 | <0.001* |
Spinal Cord | 21 | 1.04 | 1.04 ± 0.00 | 1.04 ± 0.01 | 0.77 | 0.31 ± 0.2 | <0.001* |
Heart | 21 | 1.04 | 1.03 ± 0.01 | 1.09 ± 0.01 | 9.03 | 6.18 ± 1.27 | <0.001* |
Great Vessels | 21 | 1.06 | 1.05 ± 0.00 | 1.14 ± 0.02 | 11.48 | 8.35 ± 1.61 | <0.001* |
Rib | 21 | 1.44 | 1.2 ± 0.04 | 1.21 ± 0.04 | 4.73 | 0.5 ± 1.19 | 0.069 |
Left Lung^ | 18 | 0.26 | 0.31 ± 0.06 | 0.34 ± 0.06 | 21.05 | 11.73 ± 5.46 | <0.001* |
Right Lung^ | 18 | 0.26 | 0.29 ± 0.06 | 0.33 ± 0.06 | 18.15 | 12.15 ± 5.41 | <0.001* |
Tissue | 21 | 1.02 | 0.95 ± 0.04 | 0.97 ± 0.04 | 4.78 | 2.21 ± 1.41 | <0.001* |
Esophagus | 21 | 1.04 | 1.02 ± 0.03 | 1.03 ± 0.03 | 3.41 | 1.52 ± 1.09 | <0.001* |
Duodenum | 21 | 1.02 | 1.03 ± 0.02 | 1.06 ± 0.02 | 5.47 | 3.46 ± 1.38 | <0.001* |
Lumbar | 21 | 1.44 | 1.17 ± 0.02 | 1.18 ± 0.02 | 0.85 | 0.41 ± 0.23 | <0.001* |
Small bowel | 21 | 1.02 | 0.99 ± 0.02 | 1.01 ± 0.03 | 3.09 | 1.67 ± 0.91 | <0.001* |
KidneyL | 21 | 1.04 | 1.03 ± 0.01 | 1.1 ± 0.01 | 8.40 | 6.57 ± 0.91 | <0.001* |
KidneyR | 21 | 1.04 | 1.03 ± 0.01 | 1.1 ± 0.01 | 8.59 | 6.57 ± 1.13 | <0.001* |
Stomach | 21 | 1.02 | 1.02 ± 0.03 | 1.05 ± 0.02 | 9.97 | 2.47 ± 2.06 | <0.001* |
Note: * p < 0.05 indicating significant difference |
^ scans from 3 patients were excluded because the imaging did not cover the entire lung volume, and because of the large difference on the acquisition phases of the pCT and the CHCT scans.
Of the 2\(1 {\text{T}\text{P}}_{\text{p}\text{C}\text{T}}\) delivered plans, 16 met the clinical requirements for both target coverage and OARs constraints, while for 5 patients, due to the proximity of the target with the ribs (4 patients) and stomach (1 patients), a lower target coverage (81%<\({\text{V}}_{\text{D}\text{p}\text{r}\text{e}}\)<91%) was accepted in order to meet the OARs constraints; for the corresponding \({\text{T}\text{P}}_{\text{s}\text{C}\text{T}}\) the results show a variation of \({\text{V}}_{\text{D}\text{p}\text{r}\text{e}}\) with an median value of 0.83% (range 0.15% -2.03%), reducing the patients achieving the target goal (\({\text{V}}_{\text{D}\text{p}\text{r}\text{e}}\)> 95%) to 2 patients; moreover significant differences (p < 0.05) were found for the target \({\text{V}}_{\text{D}\text{p}\text{r}\text{e}}\), CI, \({\text{D}}_{2\text{%}}\), \({\text{D}}_{98\text{%}}\) of \({\text{T}\text{P}}_{\text{p}\text{C}\text{T}}\) and \({\text{T}\text{P}}_{\text{s}\text{C}\text{T}}\). Nevertheless, the deviation of \({\text{V}}_{\text{D}\text{p}\text{r}\text{e}}\) between \({\text{T}\text{P}}_{\text{s}\text{C}\text{T}}\) and \({\text{T}\text{P}}_{\text{p}\text{C}\text{T}}\) is higher than 1.5% for only 2 out of 21 patients (9.5%).
With regard to OARs, their position respect to the target, with the exception of the 5 patients where the target was close to the rib/stomach, was far enough away not to affect their dosimetry, resulting in doses well below the limits indicated by international guideline. Therefore, even though significant differences (p < 0.05) were found between \({\text{T}\text{P}}_{\text{s}\text{C}\text{T}}\) and \({\text{T}\text{P}}_{\text{p}\text{C}\text{T}}\) for the liver-GT \({\text{D}}_{30\text{%} }\), spinal cord \({\text{D}}_{0.35\text{c}\text{c} }\)and \({\text{D}}_{1.2\text{c}\text{c} }\), heart \({\text{D}}_{0.5\text{c}\text{c} }\), and stomach \({\text{D}}_{10\text{c}\text{c} }\), the doses received remain so far from the limits that these discrepancies are not clinically appreciable.
The OARs and target dosimetric parameters obtained with \({\text{T}\text{P}}_{\text{p}\text{C}\text{T}}\) and \({\text{T}\text{P}}_{\text{s}\text{C}\text{T}}\) are shown in Table 3 and Fig. 4.; for each patient studied, the left lung and the right lung and duodenum received doses below 2.5 Gy (5%\({\text{D}}_{\text{p}\text{r}\text{e}}\)) in both treatment plans compared and have not been included.
Table 3
Comparison of \({\text{T}\text{P}}_{\text{p}\text{C}\text{T}}\) and \({\text{T}\text{P}}_{\text{s}\text{C}\text{T}}\) DVH dosimetric parameters; median values and in parenthesis minimum and maximum; p values in bold represent a significant difference.
| N° | Parameter | \({\mathbf{T}\mathbf{P}}_{\mathbf{p}\mathbf{C}\mathbf{T}}\) | \({\mathbf{T}\mathbf{P}}_{\mathbf{s}\mathbf{C}\mathbf{T}}\) | Difference (%) | p |
PTV | 21 | a\({\text{V}}_{\text{D}\text{p}\text{r}\text{e} }\)(Gy) | 93.2(81.00 ~ 95.11) | 92.56(80.24 ~ 95.94) | 0.83(0.15 ~ 2.03) | <0.001 |
21 | b\({\text{D}}_{2\text{%} }\)(Gy) | 66.75(62.26 ~ 68.91) | 66.55(61.55 ~ 68.75) | 0.52(0.01 ~ 1.37) | 0.035 |
21 | b\({\text{D}}_{98\text{%} }\)(Gy) | 46.32(36.46 ~ 48.79) | 46.06(36.30 ~ 48.91) | 0.73(0.18 ~ 1.44) | <0.001 |
21 | c\({\text{D}}_{\text{m}\text{a}\text{x} }\)(Gy) | 68.18(63.50 ~ 70.87) | 68.01(62.38 ~ 71.23) | 0.58(0.02 ~ 1.76) | 0.136 |
21 | d\({\text{D}}_{\text{m}\text{i}\text{n} }\)(Gy) | 39.40(27.39 ~ 45.74) | 39.23(27.48 ~ 46.00) | 1.02(0.07 ~ 5.57) | 0.253 |
21 | eCI | 0.83(0.71 ~ 0.92) | 0.82(0.71 ~ 0.91) | 0.54(0.00 ~ 2.71) | 0.055 |
21 | fGI | 3.97(3.27 ~ 4.95) | 3.99(3.24 ~ 4.86) | 0.97(0.16 ~ 2.15) | 0.051 |
Liver-GTV | 5 | g\({\text{D}}_{700\text{c}\text{c} }\)(Gy) | 5.81(2.90 ~ 9.38) | 5.78(2.89 ~ 9.33) | 0.73(0.01 ~ 3.27) | * |
14 | b\({\text{D}}_{30\text{%} }\)(Gy) | 8.04(3.94 ~ 15.09) | 7.99(3.92 ~ 15.01) | 0.67(0.12 ~ 1.65) | <0.001 |
Spinal Cord | 18 | c\({\text{D}}_{\text{m}\text{a}\text{x} }\)(Gy) | 8.58(3.81 ~ 15.22) | 8.59(3.89 ~ 15.62) | 1.29(0.16 ~ 3.16) | 0.689 |
15 | g\({\text{D}}_{0.35\text{c}\text{c} }\)(Gy) | 8.36(2.84 ~ 14.44) | 8.33(2.90 ~ 14.41) | 0.8(0.14 ~ 2.28) | 0.044 |
14 | g\({\text{D}}_{1.2\text{c}\text{c} }\)(Gy) | 7.53(3.95 ~ 13.32) | 7.47(3.89 ~ 13.36) | 1.08(0.18 ~ 2.34) | 0.002 |
Heart | 13 | c\({\text{D}}_{\text{m}\text{a}\text{x} }\)(Gy) | 13.06(3.47 ~ 35.22) | 12.90(3.54 ~ 34.75) | 1.32(0.28 ~ 2.69) | 0.056 |
13 | g\({\text{D}}_{0.5\text{c}\text{c} }\)(Gy) | 11.28(2.98 ~ 29.82) | 11.11(3.00 ~ 29.11) | 1.35(0.11 ~ 5.45) | 0.039 |
Esophagus | 6 | g\({\text{D}}_{5\text{c}\text{c} }\)(Gy) | 6.15(2.83 ~ 12.32) | 6.13(2.84 ~ 12.35) | 0.63(0.07 ~ 1.43) | * |
Stomach | 20 | c\({\text{D}}_{\text{m}\text{a}\text{x} }\)(Gy) | 12.12(3.67 ~ 29.84) | 12.04(3.49 ~ 29.42) | 1.59(0.04 ~ 4.95) | 0.091 |
16 | g\({\text{D}}_{10\text{c}\text{c} }\)(Gy) | 8.93(4.07 ~ 16.29) | 8.87(4.01 ~ 16.37) | 0.98(0.10 ~ 1.77) | 0.024 |
Rib | 21 | c\({\text{D}}_{\text{m}\text{a}\text{x} }\)(Gy) | 28.71(15.24 ~ 40.95) | 28.68(15.42 ~ 41.25) | 1.23(0.05 ~ 4.82) | 0.665 |
21 | g\({\text{D}}_{1\text{c}\text{c} }\)(Gy) | 23.14(12.47 ~ 34.82) | 23.08(12.56 ~ 34.85) | 0.66(0.08 ~ 4.04) | 0.194 |
Small Bowel | 13 | c\({\text{D}}_{\text{m}\text{a}\text{x} }\)(Gy) | 15.07(4.01 ~ 28.06) | 15.02(3.93 ~ 27.33) | 1.83(0.43 ~ 4.35) | 0.582 |
10 | g\({\text{D}}_{5\text{c}\text{c} }\)(Gy) | 10.58(3.00 ~ 18.29) | 10.57(2.94 ~ 18.52) | 1.04(0.16 ~ 2.15) | 0.990 |
a \({\text{V}}_{\text{D}\text{p}\text{r}\text{e}}\) , volume covered by the prescription dose; b\({\text{D}}_{\text{z} \text{%} },\)dose received z% of the volume; c\({\text{D}}_{\text{m}\text{a}\text{x} },\) point of maximum dose; d\({\text{D}}_{\text{m}\text{i}\text{n}}\), minimum dose; eCI, conformity index; fGI, gradient index, g\({\text{D}}_{ycc}\): dose received y cube centimeters; *statistical analysis was not performed due to the small number of patients.
Note
dose lower than 2.50 Gy were not considered in the analysis;
Figure 4. Dose distribution in a pCT (a) and sCT (b) transversal image and DVH comparison for a representative patient.
The differences in dose distribution of \({\text{T}\text{P}}_{\text{s}\text{C}\text{T}}\) vs \({\text{T}\text{P}}_{\text{p}\text{C}\text{T}}\) assessed with the global gamma analysis for the heart, stomach, liver-GTV and target, confirm the discrepancies found using the comparison of the DVH dosimetric parameters and the statistical analysis. For the target the pass rate range between 82.22% and 99.65%, for the heart between 58.00% and 100.0%, and for the stomach between 95.0% and 100%, while the gamma analysis for the spinal cord and the liver minus GTV do not highlight discrepancies. The details of the gamma analysis are shown in the Table 4.
Table 4
\({\text{T}\text{P}}_{\text{s}\text{C}\text{T}}\) vs \({\text{T}\text{P}}_{\text{p}\text{C}\text{T}}\) target and OARs gamma analysis with 2%-2mm criteria.
ROI | No. of cases | γ(2%-2mm)% |
PTV | 21 | 95.64 (82.22 ~ 99.65) |
Liver-GTV | 21 | 99.71 (98.97 ~ 99.97) |
Spinal Cord | 20 | 100.00 (100.00 ~ 100.00) |
Heart | 13 | 96.69 (58.00 ~ 100.00) |
Esophagus | 16 | 99.98 (99.96 ~ 100.00) |
Stomach | 20 | 99.61 (95.00 ~ 100.00) |
Rib | 21 | 99.87 (98.69 ~ 100.00) |
Small bowel | 13 | 99.33 (94.53 ~ 100.00) |
Body | 21 | 99.15 (95.53 ~ 99.96) |
Note: organs with dose lower than 2.50 Gy were excluded from the analysis; |