We evaluated the changes in the dose distribution of radiation during volumetric arc radiotherapy (VMAT), to determine the right time for adaptive replanning in prostate cancer patients with progressive weight (WT) changes. Five prostate cancer patients treated with VMAT were selected for dosimetric analysis. On the original computed tomography images, nine artificial body contours were created to reflect progressive WT changes. Combined with three different photon energies (6, 10, and 15-MV), 27 comparable virtual VMAT plans were created per patient. The dosimetric analysis included evaluation of target coverage
The goal of radiotherapy (RT) is to maximize tumor control while minimizing damage to the surrounding normal tissue [
Prostate cancer is the most common cancer in men, and RT is often used for the treatment of this cancer [
Therefore, in this study, we calculated and analyzed the dosimetric changes for the targets and organs at risk (OAR) as the patient’s WT changed. With the addition of the gamma index, which is usually used to quantify the quality of the delivered calculated plans, we have suggested criteria which will help in deciding the optimal time to consider ART in prostate cancer patients with WT changes.
Five prostate cancer patients treated at the Gyeongsang National University Changwon Hospital between July 2016 and May 2017 were selected for the study. All patients were treated by VMAT due to low- to intermediate-risk (on the basis of the National Comprehensive Cancer Network) prostate cancer. All patients were immobilized in the supine position with an empty rectum and full bladder. CT images were taken and imported into the Eclipse treatment planning system Version 13.7 (Varian Inc., Sunnyvale, CA, USA). The targets and OAR were contoured on these CT images. The prostate and seminal vesicle were delineated as the gross tumor volume (GTV). The clinical target volume (CTV) was defined to be the same as the GTV. The planning target volume (PTV) was created by expanding the CTV by 5 mm except 3 mm posteriorly. The rectum and bladder were delineated as OAR. The VMAT plans were made using double 360-degree photon arcs with high definition 120-leaf multileaf collimator from the Varian TrueBeam (Varian Medical Systems, Palo Alto, CA, USA). All patients were prescribed a dose of 78 Gy in 39 fractions. The VMAT plan was normalized so that 95% of the PTV received more than 100% of the prescribed dose. Based on the QUANTEC (Quantitative Analyses of Normal Tissue Effects in the Clinic) guidelines radiation exposure of the rectum was limited so that
The body contours were artificially contracted and expanded in the conventional CT images from −2.0 cm to 2.0 cm, by 0.5 cm increments (Figure
The changes in body contour and target contour. (a) Axial image and (b) sagittal image. The planning target volume (PTV) is shown in red, bladder in blue, and rectum in orange. (c) Dose distribution: the isodose lines are 100% (pink), 90% (yellow), and 50% (sky blue), respectively.
By analyzing the dose-volume histogram (DVH), each VMAT plan was compared in terms of PTV coverage, conformity index (CI), homogeneity index (HI), and doses to the rectum and bladder. PTV coverage was analyzed using the terms
The conformity and homogeneity indices for all the plans were calculated using the following formula [
Gamma index is a concept to calculate the difference between the calculated plan doses and the measured plan doses by specific quality assurance (QA) devices and is usually used in RT clinics for QA of IMRT and VMAT plans [
For this measurement, we used 0.5 and 1 cm thick plate phantoms to reflect the patients’ body contour changes (−2.0 cm to 2.0 cm). A VMAT plan with the same monitor unit (MU) and gantry rotation was employed to irradiate these phantoms with varied thicknesses (Figure
A simplified illustration to show the gamma index evaluation method.
A gamma index analysis with the 3 mm/3% criterion for comparing the original plan and experimental plan using a −2.0 cm contracted body contour. The number of points falling beyond our gamma criteria is represented by the red spots in the middle image.
The results of the dosimetric analysis of target coverage are shown in Table
The changes in dosimetric parameters for (a) planning target volume coverage and (b) conformity and homogeneity indices with changes in body contour from −2.0 cm to 2.0 cm.
Planning target volume coverage (%)
Body contour changes (cm) | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
−2.0 | −1.5 | −1.0 | −0.5 | 0 | 0.5 | 1.0 | 1.5 | 2.0 | ||
6-MV |
|
103.08 | 102.33 | 101.49 | 100.66 | 100.00 | 99.28 | 98.31 |
|
|
|
|
|
|
106.06 | 105.36 | 104.72 | 104.06 | 103.52 | 103.00 | |
10-MV |
|
102.83 | 101.83 | 101.13 | 100.51 | 100.00 | 99.48 | 98.81 | 98.04 |
|
|
|
|
106.32 | 105.48 | 105.02 | 104.68 | 104.06 | 103.56 | 103.12 | |
15-MV |
|
102.22 | 101.66 | 101.05 | 100.46 | 100.00 | 99.52 | 98.93 | 98.30 |
|
|
|
|
106.50 | 105.62 | 105.12 | 104.60 | 104.02 | 103.54 | 103.12 |
Conformity and homogeneity indices
Body contour changes (cm) | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
−2.0 | −1.5 | −1.0 | −0.5 | 0 | 0.5 | 1.0 | 1.5 | 2.0 | ||
6-MV | CI | 1.18 | 1.14 | 1.10 | 1.05 | 1.00 | 0.91 | 0.62 | 0.33 | 0.19 |
HI | 1.047 | 1.037 | 1.032 | 1.029 | 1.028 | 1.029 | 1.033 | 1.037 | 1.043 | |
10-MV | CI | 1.15 | 1.11 | 1.08 | 1.03 | 1.00 | 0.93 | 0.74 | 0.50 | 0.31 |
HI | 1.040 | 1.039 | 1.029 | 1.028 | 1.027 | 1.028 | 1.030 | 1.033 | 1.036 | |
15-MV | CI | 1.14 | 1.11 | 1.07 | 1.02 | 0.99 | 0.94 | 0.78 | 0.55 | 0.39 |
HI | 1.042 | 1.037 | 1.033 | 1.031 | 1.030 | 1.026 | 1.031 | 1.033 | 1.034 |
CI, conformity index; HI, homogeneity index.
The changes in dosimetric parameters (a)
The dose to the rectum and bladder increased gradually as the body contour was contracted and decreased as the body contour was expanded. Among the three different photon energy plans, the 6-MV plan showed the most prominent decrease. The rectum
The changes in dosimetric parameters for the rectum (R) and bladder (B) with the changes of body contour from −2.0 cm to 2.0 cm.
Body contour changes (cm) | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
−2.0 | −1.5 | −1.0 | −0.5 | 0 | 0.5 | 1.0 | 1.5 | 2.0 | ||
6-MV |
|
10.87 | 10.45 | 10.06 | 9.73 | 9.44 | 9.24 | 8.91 | 8.57 | 8.30 |
|
14.66 | 14.34 | 13.98 | 13.65 | 13.39 | 13.16 | 13.09 | 12.54 | 12.48 | |
10-MV |
|
10.67 | 10.36 | 10.08 | 9.82 | 9.57 | 9.36 | 9.12 | 8.88 | 8.64 |
|
14.61 | 14.33 | 14.07 | 13.76 | 13.59 | 13.36 | 13.08 | 12.79 | 12.56 | |
15-MV |
|
10.65 | 10.35 | 10.06 | 9.71 | 9.63 | 9.40 | 9.13 | 8.93 | 8.70 |
|
14.61 | 14.34 | 14.08 | 13.80 | 13.63 | 13.43 | 13.22 | 12.91 | 12.77 |
The changes in dosimetric parameters (a) rectum
An example of a dose-volume histograms (DVH) of the volumetric modulated arc radiotherapy plan using 10-MV photon energy for a prostate cancer patient. The dose-volume histograms are indicated using the different colors (PTV, red; rectum, orange; bladder, blue) and types of lines (−2.0 cm body contour, square; original body contour, triangle; 2.0 cm body contour, small circle).
Table
The changes in gamma index with changes in body contour from −2.0 cm to 2.0 cm.
Body contour changes (cm) | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
−2.0 | −1.5 | −1.0 | −0.5 | 0 | 0.5 | 1.0 | 1.5 | 2.0 | ||
6-MV | GPR |
|
|
|
98.00 | 100.00 | 98.70 | 95.50 |
|
|
MG |
|
|
|
3.21 | 0.00 | 1.85 | 2.88 |
|
|
|
10-MV | GPR |
|
|
95.70 | 99.40 | 100.00 | 99.80 | 96.90 | 95.10 |
|
MG |
|
|
3.19 | 1.75 | 0.00 | 1.39 | 2.19 | 2.85 | 3.30 | |
15-MV | GPR |
|
|
96.50 | 99.70 | 100.00 | 99.80 | 98.00 | 95.80 |
|
MG |
|
|
3.01 | 1.58 | 0.00 | 1.06 | 1.68 | 2.08 | 2.38 |
GPR, gamma pass rate; MG, maximum gamma values.
In conclusion, we have found that WT changes during prostate VMAT can cause considerable change in the target dose distribution. We suggest that when using 10-MV VMAT plans for patients with prostate cancer, the appropriate time to consider adaptive replanning is when the body contour becomes 1.5 cm smaller (WT loss) or 2.0 cm larger (WT gain) than the original value.
The authors declare that they have no conflicts of interest regarding the publication of this paper.
Hoon Sik Choi and Guang Sub Jo contributed equally to this work.