Abstract
Due to image quality limitations, online Megavoltage cone beam CT (MV CBCT), which represents real online patient anatomy, cannot be used to perform adaptive radiotherapy (ART). In this study, we used a deep learning method, the cycle-consistent adversarial network (CycleGAN), to improve the MV CBCT image quality and Hounsfield-unit (HU) accuracy for rectal cancer patients to make the generated synthetic CT (sCT) eligible for ART. Forty rectal cancer patients treated with the intensity modulated radiotherapy (IMRT) were involved in this study. The CT and MV CBCT images of 30 patients were used for model training, and the images of the remaining 10 patients were used for evaluation. Image quality, autosegmentation capability and dose calculation capability using the autoplanning technique of the generated sCT were evaluated. The mean absolute error (MAE) was reduced from 135.84 ± 41.59 HU for the CT and CBCT comparison to 52.99 ± 12.09 HU for the CT and sCT comparison. The structural similarity (SSIM) index for the CT and sCT comparison was 0.81 ± 0.03, which is a great improvement over the 0.44 ± 0.07 for the CT and CBCT comparison. The autosegmentation model performance on sCT for femoral heads was accurate and required almost no manual modification. For the CTV and bladder, although modification was needed for autocontouring, the Dice similarity coefficient (DSC) indices were high, at 0.93 and 0.94 for the CTV and bladder, respectively. For dose evaluation, the sCT-based plan has a much smaller dose deviation from the CT-based plan than that of the CBCT-based plan. The proposed method solved a key problem for rectal cancer ART realization based on MV CBCT. The generated sCT enables ART based on the actual patient anatomy at the treatment position.
Highlights
Neoadjuvant chemoradiotherapy, which can improve the local control rates, is a standard of care for locally advanced rectal cancer [1, 2]
In the image guide process, FBCT was acquired for position correction, and the Megavoltage cone beam CT (MV cone beam CT (CBCT)) was acquired for position verification
A CycleGAN method was used to improve MV CBCT image quality to make it eligible for adaptive radiotherapy (ART). This method relies on unpaired CT and CBCT images, making it easier to apply them in clinical situations
Summary
Neoadjuvant chemoradiotherapy, which can improve the local control rates, is a standard of care for locally advanced rectal cancer [1, 2]. Another study compared an online adaptive radiotherapy strategy for planning the selection with respect to the dose to the organ at risk for rectal cancer [9], and they found that the adaptive treatment maintained target coverage and reduced the doses to the organs at risk (OARs) Both of these strategies are superior to using one plan throughout the whole course, but they all have limitations in that the calculation of the dose distribution was based on the planning CT rather than on the online patient anatomy; rather, they take into account the delineation on the online CBCT. To fulfill the ART process, we should directly use the images with actual online anatomy for dose calculation
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