Phantom configuration
The 062M electronic density phantom (CIRS, Norfolk, VA, USA) with various tissue-equivalent inserts was used to test the accuracy of the relative ED value of the ED map. It should be declared that the relative ED value is defined as the percentage of the ED of a substance relative to the ED of water (3.343 × 1023 m− 3). The expected relative ED values of these inserts were provided by the phantom vendor. The detailed information and arrangements for these inserts are shown in Table 1; Fig. 1.
Insert arrangements of 062 M phantom for measurements of the accuracy of the relative ED value
Scanning and measurement
The specific scanning parameters for these four default scanning protocols utilized in clinical practice are as follows. The scan parameters for the head & neck protocol were 120 kVp, 350mAs, 55mGy CTDIvol, 0.5 pitch, 500-mm field of view (FOV), 64 × 0.625 mm collimation, and 3-mm slice thickness. For the chest protocol, the scan parameters were 120 kVp, 250 mAs, 19.4 mGy CTDIvol, 1 pitch, 128 × 0.625 mm collimation, 500-mm FOV, and 5-mm slice thickness. For the abdomen and pelvis protocol, the scan parameters were 120 kVp, 300 mAs, 23.2 mGy CTDIvol, 1 pitch, 128 × 0.625 mm collimation, 500-mm FOV, and 5 mm slice thickness. For the pediatrics protocol, the scan parameters were 100 kVp, 300 mAs, 15.4 mGy CTDIvol, 0.8 pitch, 64 × 0.625 mm collimation, 350 mm FOV, and 3 mm slice thickness.
All image series reconstructed conventional CT images and corresponding ED maps, and all of them were exported to Eclipse treatment planning system V15.6 (Varian Medical Systems, Palo Alto, CA).
Regions of interest (ROIs) were selected on the ED maps by using TPS’s contour module, which identifies the phantom center and orientation and performs data analysis at predefined locations. The diameter of the ROIs was 75% of the diameter of the cylindrical inserts. The measured values were compared with the corresponding expected values, including the absolute difference (measured—expected) and percentage deviation (100*[measured — expected]/expected).
Treatment plans based on conventional CT images
In this study, the data of 30 patients from different treatment sites were selected retrospectively, including CT images and treatment plans. Among these conventional plans, 10 were head & neck plans, 10 were chest plans, and 10 were pelvic plans. The techniques used in the treatment plans include fixed-field intensity modulated radiation therapy (FF-IMRT) and volumetric modulated arc therapy (VMAT).
The prescribed dose for head & neck plans was 5040 cGy with 28 fractions. For chest plans, the prescribed dose was 4240 cGy with 16 fractions. For pelvic plans, the prescribed dose was 4500 cGy with 25 fractions. All selected treatment plans were designed and optimized by using the Eclipse TPS V15.6 and delivered by Halcyon2.0 and Truebeam linac. Anisotropic analytic algorithm (AAA) and phonon optimizer (PO) were used for dose calculation and plan optimization.
Treatment plans based on ED maps and plan comparison
We transplanted the original treatment plans based on conventional CT images to ED maps with the same plan parameters and recalculated the dose distribution. The transplanted plans were referred to as ED plans. It should be noted that when transplanting the treatment plans, there is no need to register the two types of images beforehand because the ED maps and conventional CT images were homologous and acquired at the same time. In this study, we evaluated and compared the following DVH parameters of the planning target volume and OARs for both types of plans, respectively. For planning target volume (PTV), Paddick’s conformity index (CI), gradient index (GI) [15], and ICRU 83 homogeneity index (HI) [16] were defined as follows:
Conformity index (CI):
$$CI=\frac{{\text{(PTV volume receiving the prescription isodose)}}^{\text{2}}}{\text{(}\text{PTV volume * Prescription isodose volume)}}$$
represents the adequacy between the dose distribution and the shape of the target volume treated. The ideal value is 1.
Gradient index (GI):
$$GI=\frac{\text{Volume of the isodose } 50\% \text{ of the prescribed dose }}{\text{The volume of the isodose of the prescribed dose}}$$
represents the dose gradient between the prescribed dose level and 50% of the prescribed dose. The lowest possible value is ideal.
$$HI=\frac{{\text{D}}_{2\%}\text{-}{\text{D}}_{98\%}}{{\text{D}}_{50\%}}$$
corresponds to the homogeneity of the dose distribution of the target volume. The ideal value is 0.
For OARs of the head & neck treatment plans, the dose to the spinal cord (\(({{\rm{D}}_{0.1c{m^3}}})\)), parotids (Dmean and D50%), brain stem (\(({{\rm{D}}_{0.1c{m^3}}})\)) were scored. For the chest plans, both plans were evaluated the dose to the double lungs (V20Gy, Dmean), lateral lung (Dmean), contralateral lung (Dmean), and heart (V5Gy and Dmean), and spinal cord (\(({{\rm{D}}_{0.1c{m^3}}})\)). For the pelvic plans, the dose to the small intestine (Dmean, D50% and \({\text{D}}_{{\text{2}\text{cm}}^{\text{3}}}\)), Bladder (Dmean, D50%), Rectum (Dmean, D50%), and spinal cord (\(({{\rm{D}}_{0.1c{m^3}}})\)) were recorded.
Besides, we exported these two kinds of treatment plans’ RT-Dose files in DICOM format and used MEPHYSTO Navigator software (PTW, Freiburg, Germany) for Gamma analyses. All gamma analyses were performed under absolute dose mode, and the gamma criteria were 1%/1 mm, 2%/2 mm, and 3%/2 mm with a 10% threshold of the maximum dose.
Data analysis
Paired t-test was used to examine the significance of all DVH parameters’ differences via the SPSS 26, and a significance level of P < 0.05 was considered statistically significant.
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