Background: Accurate segmentation of tumor targets is critical for maximizing tumor control and minimizing normal tissue toxicity. We proposed a sequential and iterative U-Net (SI-Net) deep learning method to auto-segment the high-risk primary tumor clinical target volume (CTVp1) for treatment planning of nasopharyngeal carcinoma (NPC) radiotherapy. Methods: The SI-Net is a variant of the U-Net architecture. The input of SI-Net includes one CT image, the CTVp1 contour on this image, and the next CT image. The output is the predicted CTVp1 contour on the next CT image. We designed the SI-Net, using the left side to learn the volumetric features and the right to localize the contour on the next image. Two prediction directions, one from inferior to superior (forward direction) and the other from superior to inferior (backward direction), were tested. The performance was compared between the SI-Net and the U-Net using Dice similarity coefficient (DSC), Jaccard index (JI), average surface distance (ASD), and Hausdorff distance (HD) metrics. Results: The DSC and JI values from the forward direction SI-Net model were 5 and 6% higher than those from the U-Net model (0.84 ± 0.04 vs. 0.80 ± 0.05 and 0.74 ± 0.05 vs. 0.69 ± 0.05, p < 0.001). The smaller ASD and HD values also indicated a better performance (2.8 ± 1.0 vs. 3.3 ± 1.0 mm and 8.7 ± 2.5 vs. 9.7 ± 2.7 mm, p < 0.01) for the SI-Net model. For the backward direction SI-Net model, the DSC and JI values were still better than those from the U-Net model ( p < 0.01), although there were no significant differences in ASD and HD. Conclusions: The SI-Net model preserved the continuity between adjacent images and thus improved the segmentation accuracy compared with the conventional U-Net model. This model has potential of improving the efficiency and consistence of CTVp1 contouring for NPC patients.
To compare the dosimetric influence of applicator displacement on two-dimensional brachytherapy (2D-BT) and three-dimensional brachytherapy (3D-BT) for cervical cancer. Nineteen patients who received computed tomography-guided tandem-and-ovoid (T&O) brachytherapy were retrospectively selected. Both 2D (point-based) and 3D (volume-based) plans with and without virtual applicator displacement in the 3 axes were created for each patient. Dose changes at point A, D90 of the high-risk clinical target volume (HR-CTV) and intermediate-risk CTV (IR-CTV), and the D0.1cc, D1cc, D2cc, and D5cc of organs-at-risk (OARs) caused by applicator displacement were evaluated. Both 2D-BT and 3D-BT plans were sensitive to T&O applicator displacement. The D90 of the CTV and the dose at point A were very sensitive to applicator displacement in the right–left direction ( X-axis). An applicator shift of >2 mm in the X-axis resulted in a change of >5% in the dose at point A and D90 of HR-CTV and IR-CTV. In addition, the doses to the OARs were mostly affected by applicator displacement in the anterior–posterior direction ( Z-axis). A displacement of <1.5 mm in the Z-axis was required to avoid a dose change of >10% for OARs. For both 2D-BT and 3D-BT plans, T&O displacement greater than ± 2 mm in the X-axis or T&O applicator displacement ± 1.5 mm in the Z-axis resulted in significant dose changes to the tumor and OARs. In comparison with 3D-BT plans, 2D-BT plans delivered a higher dose to the tumor, and the OARs received more undesirable doses when applicator displacement occurred. The influence of applicator displacement on the doses to the tumor and OARs differed between 2D-BT and 3D-BT. Physicians should take individual patient differences into account when selecting a brachytherapy plan to mitigate the influence of applicator displacement.
locations; Dose to the prostate and pelvic node CTVs, rectum, bladder and femoral heads were calculated and presented for the total 20 plan scenarios of each patient. Results: For bony matching, prostate CTV dose degradation was prominent for interfraction prostate motion beyond prostate PTV margins. Potential prostate intrafraction motion would further degrade the prostate CTV coverage. Lymph node CTV dose coverages were not affected; for seed matching, dose degradation of pelvic lymph node CTV was prominent for interfraction prostate motion beyond pelvic node PTV margins. Dose coverage to prostate CTV were comparable to original treatment plans. For the seed matching image guidance, there were large dose variations to the rectum, bladder and femoral head among different prostate interfraction motion datasets. For the bony matching image guidance, there were large dose variations only to the rectum among different prostate interfraction motion datasets. Conclusion: Prostate interfraction motion from pelvic lymph node has significant impact on prostate and lymph node CTVs coverage. Due to this motion, different image guidance approach affect the target coverage and dose to OARs in a different manner.
Background and purposeMultiple patient transfers have a nonnegligible impact on the accuracy of dose delivery for cervical cancer brachytherapy. We consider using on-site cone-beam CT (CBCT) to resolve this problem. However, CBCT clinical applications are limited due to inadequate image quality. This paper implements a scatter correction method using planning CT (pCT) prior to obtaining high-quality CBCT images and evaluates the dose calculation accuracy of CBCT-guided brachytherapy for cervical cancer.Materials and methodsThe CBCT of a self-developed female pelvis phantom and five patients was first corrected using empirical uniform scatter correction in the projection domain and further corrected in the image domain. In both phantom and patient studies, the CBCT image quality before and after scatter correction was evaluated with registered pCT (rCT). Model-based dose calculation was performed using the commercial package Acuros®BV. The dose distributions of rCT-based plans and corrected CBCT-based plans in the phantom and patients were compared using 3D local gamma analysis. A statistical analysis of the differences in dosimetric parameters of five patients was also performed.ResultsIn both phantom and patient studies, the HU error of selected ROIs was reduced to less than 15 HU. Using the dose distribution of the rCT-based plan as the baseline, the γ pass rate (2%, 2 mm) of the corrected CBCT-based plan in phantom and patients all exceeded 98% and 93%, respectively, with the threshold dose set to 3, 6, 9, and 12 Gy. The average percentage deviation (APD) of D90 of HRCTV and D2cc of OARs was less than 1% between rCT-based and corrected CBCT-based plans.ConclusionScatter correction using a pCT prior can effectively improve the CBCT image quality and CBCT-based cervical brachytherapy dose calculation accuracy, indicating promising prospects in both simplified brachytherapy processes and accurate brachytherapy dose delivery.
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