Megavoltage computed tomography: an emerging tool for image-guided radiotherapy.

Megavoltage Computed Tomography Imaging: A Potential Tool to Guide and Improve the Delivery of Thoracic Radiation Therapy

Purpose: Intensity modulated radiation therapy (IMRT) planning using pre‐treatment megavoltage computed tomography (MVCT) provides a viable solution to adaptive radiotherapy. A strategy was developed to assess the impact of MVCT image quality on treatment plans using Helical TomoTherapy. Methods: The Octavius phantom (PTW) was scanned with a Philips CT‐scanner and the Helical TomoTherapy unit for kilovoltage CT (kVCT) and MVCT, respectively. MVCT images were acquired with three modes (slice thickness): coarse (6‐mm), normal (4‐mm) and fine (2‐mm). Contours from existing head‐and‐neck plans were transferred from the Pinnacle treatment planning system to phantom images and IMRT plans were generated using the TomoTherapy planning station. Comparison between the kVCT‐based plan (benchmark plan) and MVCT‐based plans were conducted to evaluate dosimetric variations. Dose volume histograms (DVH) were examined for plan quality. Plans using MVCTs acquired with three modes were also evaluated for dosimetric differences. Additional plan evaluations were performed with retrospective dose reconstruction based on the delivered leaf sequences. Results: Isodose distributions of four plans are comparable despite the intrinsic difference between MVCT and kVCT images. No significant variations on the target coverage were found from DVHs. However, plan using coarse MVCT images gave largest dose deviations (> 50%) for critical structures from that of benchmark plan while the best agreement was observed between fine MVCT and kVCT. Plan parameters were found to be consistent among four plans. Comparisons between planning doses and reconstructed treatment doses revealed high fidelity delivery. Conclusions: IMRT planning using MVCTs proved to be equivalent with that using standard kVCT when fine imaging mode was selected. Target coverage on MVCT plans is comparable to that on the standard kVCT plan. MVCT thicknesses were shown to have impact on plan quality. Caution is needed when performing treatment planning with MVCTs.

Purpose: Verify the accuracy of the dose calculation algorithm in the presence of a hip prosthesis when planning with megavoltage CT (MVCT) images of the TomoTherapy Hi‐Art II planning system. Methods and Materials: Artifacts from imaging high‐Z materials are greatly reduced in MVCT compared to kilovoltage CT (kVCT). This may allow more accurate treatment planning for patients with metallic implants. The Radiological Physics Center's (RPC) pelvic phantom was modified to accommodate a commercial Co‐Cr‐Mo hip prosthesis and imaged with both kVCT and MVCT. On the TomoTherapy Hi‐Art II planning system, the MVCT‐to‐density table was extended to high‐Z materials by including stainless steel and lead in the calibration. Contours were drawn on kVCT images without the prosthesis present and fused to the MVCT image set. Helical tomotherapy plans were created using the MVCT images with no constraints on beam entry or exit location. The delivered dose distribution was measured using TLDs in the PTV and EBT radiochromic film in the coronal and sagittal planes. Criteria based on TG‐53 recommendations were used for comparison between measurements and calculations. Results: Dose calculation with MVCT images resulted in minor variations between calculated and measured dose. It was found that on average 97% of the dose distribution measured with film agreed with the treatment planning system to within +/−5% or 3mm. Profiles across the PTV also showed good agreement between measurement and calculation. The average TLD dose at the center of the PTV was 96.0% of the expected dose. Conclusion: Using dose calculations based on MVCT images, it is possible to accurately calculate the dose distribution in the pelvic region while treating through a high‐Z prosthetic implant. Work was supported by PHS grant CA10953 and CA081647 awarded by NCI.

Objective. In helical tomotherapy, image-guided radiotherapy employs megavoltage computed tomography (MVCT) for precise targeting. However, the high voltage of megavoltage radiation introduces substantial noise, significantly compromising MVCT image clarity. This study aims to enhance MVCT image quality using a deep learning-based denoising method. Approach. We propose an unpaired MVCT denoising network using a coupled generative adversarial network framework (DeCoGAN). Our approach assumes that a universal latent code within a shared latent space can reconstruct any given pair of images. By employing an encoder, we enforce this shared-latent space constraint, facilitating the conversion of low-quality (noisy) MVCT images into high-quality (denoised) counterparts. The network learns the joint distribution of images from both domains by leveraging samples from their respective marginal distributions, enhanced by adversarial training for effective denoising. Main Results. Compared to an analytical algorithm (BM3D) and three deep learning-based methods (RED-CNN, WGAN-VGG and CycleGAN), the proposed method excels in preserving image details and enhancing human visual perception by removing most noise and retaining structural features. Quantitative analysis demonstrates that our method achieves the highest peak signal-to-noise ratio and Structural Similarity Index Measurement values, indicating superior denoising performance. Significance. The proposed DeCoGAN method shows remarkable MVCT denoising performance, making it a promising tool in the field of radiation therapy.

Megavoltage computed tomography (MVCT) is an essential imaging modality for verifying patient positioning in helical tomotherapy. However, its clinical application in daily anatomical monitoring and adaptive radiotherapy is hindered by inherent image artifacts and poor soft-tissue contrast. This issue is particularly pronounced in pelvic radiotherapy, where the intra- and interfraction anatomical variations necessitate high-quality image guidance to ensure precise dose delivery. We developed a deep-learning-based framework for MVCT enhancement to improve anatomicalvisualizationand facilitate accurate adaptive treatment planning for cervical cancer. This study analyzed a retrospective cohort of 170 patients with cervical cancer who underwent helical tomotherapy. The proposed deep-learning-based algorithm employed a generative adversarial network (GAN) that integrated deformable convolution and a self-attention mechanism (SADC-EGAN) to improve MVCT image quality. Comparative analyses were conducted against representative baseline methods, including U-Net, Attention U-Net, U-Net++, Swin-UNet, CycleGAN, and Pix2Pix. Model performance was assessed using quantitative metrics, including mean absolute error (MAE), peak signal-to-noise ratio (PSNR), structural similarity index measure (SSIM), and Fréchet inception distance (FID). The synthetic computed tomography (sCT) images generated by the proposed SADC-EGAN method demonstrated superior Hounsfield unit (HU) accuracy and structural similarity comparedto the original MVCT. Specifically, the MAE between the sCT and kilovoltage computed tomography (kVCT) was reduced to 36.32±6.69 HU, compared with 56.72±9.09 HU for MVCT. In terms of image quality, the sCT images exhibited notable enhancements over MVCT images, with higher PSNR (32.54±2.31 vs. 29.40±1.56dB), improved SSIM (0.93±0.01vs. 0.89±0.02), and substantially lower FID (66.68±22.31vs. 153.52±28.77). The proposed SADC-EGAN framework, integrating deformable convolutions and self-attention, effectively generated high-quality kVCT-like images from MVCT, improving both HU accuracy and image quality. This approachhas clinical potential to enable online adaptive helical tomotherapy for cervical cancer.

Initial results of megavoltage computed tomography (MVCT) brachytherapy treatment planning are presented, using a commercially available helical tomotherapy treatment unit and standard low dose rate (LDR) brachytherapy applicators used for treatment of cervical carcinoma. The accuracy of MVCT imaging techniques, and dosimetric accuracy of the CT based plans were tested with in-house and commercially-available phantoms. Three dimensional (3D) dose distributions were computed and compared to the two dimensional (2D) dosimetry results. Minimal doses received by the 2 cm3 of bladder and rectum receiving the highest doses (D(B2cc) and D(R2cc), respectively) were computed from dose-volume histograms and compared to the doses computed for the standard ICRU bladder and rectal reference dose points. Phantom test objects in MVCT image sets were localized with sub-millimetric accuracy, and the accuracy of the MVCT-based dose calculation was verified. Fifteen brachytherapy insertions were also analyzed. The ICRU rectal point dose did not differ significantly from D(R2cc) (p=0.749, mean difference was 24 cGy +/- 283 cGy). The ICRU bladder point dose was significantly lower than the D(B2cc) (p=0.024, mean difference was 291 cGy +/- 444 cGy). The median volumes of bladder and rectum receiving at least the corresponding ICRU reference point dose were 6.1 cm(3) and 2.0 cm(3), respectively. Our initial experience in using MVCT imaging for clinical LDR gynecological brachytherapy indicates that the MVCT images are of sufficient quality for use in 3D, MVCT-based dose planning.

TomoTherapy involves image-guided radiation therapy (IGRT) using Mega-voltage CT (MVCT) for each treatment session. The acquired MVCT images can be utilized for the retrospective assessment of dose distribution. The TomoTherapy provides 18 distinct imaging conditions that can be selected based on a combination of algorithms, acquisition pitch, and slice interval. We investigated the accuracy of dose calculation and deformable image registration (DIR) depending on MVCT scan parameters and their effects on adaptive radiation therapy (ART). We acquired image values for density calibration tables (IVDTs) under 18 different MVCT conditions and compared them. The planning CT (pCT) was performed using a thoracic phantom, and an esophageal intensity-modulated radiation therapy (IMRT) plan was created. MVCT images of the thoracic phantom were acquired under each of the 18 conditions, and dose recalculation was performed. DIR was performed on the MVCT images acquired under each condition. The accuracy of DIR, depending on the MVCT scan parameters, was compared using the mean distance to agreement (MDA) and Dice similarity coefficient (DSC). The dose distribution calculated on the MVCT images was deformed using deformed vector fields (DVF). No significant differences were observed in the results of the 18 IVDTs. The esophageal IMRT plan also showed a small dose difference. Regarding verifying the DIR accuracy, the MDA increased, and the DSC decreased as the acquisition pitch and slice interval increased. The difference between the dose distributions after dose mapping was comparable to that before DIR. The MVCT scan parameters had little effect on ART.

The purpose of this investigation was the creation of a new protocol allowing more precise dose calculations on megavoltage CT (MVCT) images for tomotherapy, both for adaptive and StatRT planning. Daily MVCT images offer, next to positioning purposes, the possibility for daily dose check and adaptive planning. Dose calculations use the image value to density table (IVDT) to calculate physical densities from Hounsfield Units (HUs). These measured HUs change over time, leading to a dose calculation error. We noticed dose calculation variations due to IVDT changes of: 0.2% dose during a day, up to 1.6% dose from long‐term variations, and up to 1.5% dose due to technical interventions. An analysis was performed applying the general methodology of a calibration problem. A model HU=bρc‐1020 was obtained using a weighted least squares inverse prediction method (HU as function of density) taking into account the heteroscedasticity. The b parameter is the major variable and depends also on the dose rate (DR). We demonstrate the correction for DR variations and the constance of the c parameter. Instead of scanning the whole tissue characterization phantom daily, we propose a simplified daily protocol: (a) morning airscan‐like procedure with only two inserts on the table (defining b and thus the IVDT curve), (b) DR variations throughout the day can be corrected for using the DR model. A patient‐specific protocol for which two inserts next to the patient are scanned could also be used, but results in equal uncertainties and is less practical. Therefore we recommend the morning procedure with dose rate variation correction. Applying the proposed transformations and the model, the correct IVDT of the moment can be reconstructed, with a simple measurement in the morning, and corrected with DR changes during the day. This corresponds with a linear mapping in time of the proposed IVDT function. The dosimetric variation is hereby reduced from up to 3% to 0.4 % for the tested pelvic and head‐and‐neck cases. In practice, several IVDT curves corresponding to “b” values can be entered. The correct IVDT curve of that moment can then be chosen from the list. Instead of the two high‐density inserts on table, any calibrated single density phantom could be used in order to create the IVDT curve of the day, but it should have a larger size than the current inserts.PACS number: 87.55.Gh

High-dose-rate prostate brachytherapy in a patient with bilateral hip prostheses planned using megavoltage computed tomography images acquired with a helical tomotherapy unit

Modeling set-up error by daily MVCT for prostate adjuvant treatment delivered in 20 fractions: Implications for the assessment of the optimal correction strategies

Purpose: Correcting anterior‐posterior, lateral, superior‐inferior, pitch, roll, and yaw with Image‐Guided Radiation Therapy (IGRT) will not completely correct for anatomical deformation. The purpose of this work was to identify anatomical deformation error and develop multiple strategies for addressing this error using adaptive IGRT. Method and Materials: Ten H&N patients have been treated with helical tomotherapy. Megavoltage CT (MVCT) images were acquired prior to treatment The daily setup were then used to calculate the random positional setup errors between the setup marks on the patient's mask and the internal anatomy. Level I lymph nodes (LV1) and the cord were contoured on each MVCT image. DVHs and LV1 EUDs were then calculated for each fraction. If the 2 Gy per fraction isodose line did not cover the LV1 nodes, then the treatment delivery was re‐planned and adapted to account for anatomical deformation. Results: The total shifts were measured for over 400 treatment fractions. The standard deviation was ±2.8 mm in the AP direction, ±2.9 mm in the lateral direction, and ±3.2 mm in SI direction. Dose reconstruction results indicated that the LV 1 nodes were frequently being under‐dosed (even with IGRT). Using IGRT to align the patient relative to the spinal cord resulted in a reduction in the LV1 nodal EUD by 2 to 6 Gy. This is of particular concern because the risk (5 to 50 percent) of LV1 involvement for node‐positive patients. By utilizing adaptive therapy, the LV1 were completely covered by the prescribed dose (50 Gy) in all cases. In addition, dose to the cord and mandibles were minimized. Conclusion: Adapting the treatment delivery by modifying the anterior border of the field can correct for the decrease in coverage due to anatomical deformation. This technique is an alternative to increasing PTV margins that allows increased sparing of normal tissues.

Purpose: To investigate the effectiveness of highly conformal radiation modalities in conjunction with a patient setup correction method, Helical Tomotherapy (HT) was employed using the Helical Megavoltage CT (MVCT) scanner of the tomotherapy unit. Any discrepancy between the planned and delivered dose distributions would likely affect the clinical outcome. Method and Materials: The case of a head and neck cancer was used to evaluate the accuracy of dose delivery. The right parotid, left parotid and spinal cord are the primary organs at risk proximal to the target volume. MVCT images were taken before and after the patient setup correction. MVCT images of the patient were acquired daily. The three dose distributions (planned, with and without patient setup correction) were compared by using the complication‐free tumor control probability (P +) and the biologically effective uniform dose (BEUD) concepts. Results: For the computerized treatment plan, the maximum value of P + is 88.3%, the total control probability, P B is 97.3% and the total risk for complications, P I is 9.3%. For the delivered dose distribution without patient setup correction, the value of P + is 79.8% and the probabilities P B and P I are 83.6% and 3.8%, respectively. Finally, for the delivered dose distribution with patient setup correction in every fraction the value of P + is 87.2% and the probabilities P B and P I are 92.7% and 5.5%, respectively. The expected clinical effectiveness dropped by a ΔP + of 1.1% and 8.5% in the cases with and without patient setup correction. Conclusion: Highly conformal treatment plans can deliver higher doses to the target allowing for increased control rates for the same or a reduced risk for complications. The results show that the effectiveness of the applied dose distribution drops between the treatment plan and the dose delivery with and without patient setup correction.

To evaluate the performance of a model based image reconstruction method in reducing metal artifacts in the megavoltage computed tomography (MVCT) images of a phantom representing bilateral hip prostheses and to compare with the filtered-backprojection (FBP) technique. An iterative maximum likelihood polychromatic algorithm for CT (IMPACT) is used with an additional model for the pair∕triplet production process and the energy dependent response of the detectors. The beam spectra for an in-house bench-top and TomoTherapy™ MVCTs are modeled for use in IMPACT. The empirical energy dependent response of detectors is calculated using a constrained optimization technique that predicts the measured attenuation of the beam by various thicknesses (0-24 cm) of solid water slabs. A cylindrical (19.1 cm diameter) plexiglass phantom containing various cylindrical inserts of relative electron densities 0.295-1.695 positioned between two steel rods (2.7 cm diameter) is scanned in the bench-top MVCT that utilizes the bremsstrahlung radiation from a 6 MeV electron beam passed through 4 cm solid water on the Varian Clinac 2300C and in the imaging beam of the TomoTherapy™ MVCT. The FBP technique in bench-top MVCT reconstructs images from raw signal normalized to air scan and corrected for beam hardening using a uniform plexiglass cylinder (20 cm diameter). The IMPACT starts with a FBP reconstructed seed image and reconstructs the final image in 150 iterations. In both MVCTs, FBP produces visible dark shading in the image connecting the steel rods. In the IMPACT reconstructed images this shading is nearly removed and the uniform background is restored. The average attenuation coefficients of the inserts and the background are very close to the corresponding values in the absence of the steel inserts. In the FBP images of the bench-top MVCT, the shading causes 4%-9.5% underestimation of electron density at the central inserts with an average of (6.3 ± 1.8)% for the range of electron densities studied. In the uniform plexiglass background, the shadow creates 0.8%-4.7% underestimation of electron density with an average of (2.9 ± 1.2)%. In the corresponding IMPACT images, the underestimation in the shaded plexiglass background is 0.3%-1.8% with an average of (0.9 ± 0.5)% and 1.4%-6.8% with an average of (2.8 ± 2.7)% in the central insert region. In the FBP images of the TomoTherapy™ MVCT, this shading creates 2.6%-6.7% underestimation of electron density with an average of (3.7 ± 1.4)% at the central inserts and 5.9%-7.2% underestimation in the background with an average of (6.4 ± 0.5)%. In the IMPACT images, the uniform background between the steel rods is restored with 0.3%-1.0% underestimation of electron density with an average of (0.7 ± 0.3)%. The corresponding underestimation at the central inserts of the IMPACT images is -0.4%-0.1% with an average of (-0.1 ± 0.2)%. The shading metal artifact has been nearly removed in MVCT images using the IMPACT algorithm with the accurate geometry of the system, proper modeling of energy dependent response of detectors, and all relevant photon interaction processes. This results less than 1% difference in electron density in the background plexiglass and less than 3% averaged over the range of electron densities investigated.

The aim of this study was to analyse the possibility of using stent in pretreatment megavoltage computed tomography (MVCT) images with respect to that on planning kilovoltage computed tomography as tumour surrogate during matching for daily registration in cases of pancreatic and periampullary cancer treated on a TomoTherapy Hi-Art system. Planning CT and pretreatment MVCT of the first and then after every three fractions were transferred to a FocalSim workstation for ten patients. Planning CT of each patient was independently fused with each of the seven MVCT images of that patient. The stent was contoured on all of the eight images for each patient. The difference between the three co-ordinates of centre of mass (CM) of the stent on the planning CT and seven MVCT images was found. The difference between CM of the liver and stents on the planning CT as well as on the MVCT for all seven fractions was also calculated. The mean of these differences across all patients was calculated and analysed. The mean difference in planning and MVCT CMs for stents in the X, Y and Z directions was 0.13cm (±0.4), 0.16cm (±2.2) and 0.35cm (±0.7), respectively. Average difference between CM of the liver and stent on the planning CT in the X, Y and Z directions was found to be 1.832cm (±1.64), 5.34cm (±1.33) and 0.54cm (±0.26), respectively. Average difference between CM of the liver and CM of stent on the MVCT for that day in the X, Y and Z directions was found to be 1.93cm (±1.5), 4.6cm (±1.03) and 0.654cm (±0.35), respectively. Endobiliary stents are stable tumour localisation surrogates and can be used to correct for interfraction target motion.

Helical tomotherapy represents both a novel radiation treatment device and an innovative means of delivering radiotherapy. The helical tomotherapy unit itself is essentially a hybrid between a linear accelerator and a helical CT scanner for the purpose of delivering intensity-modulated radiation therapy (IMRT). The imaging capacity conferred by the CT component allows targeted regions to be visualized prior to, during, and immediately after each treatment. The megavoltage CT (MVCT) images supplant the port-films used in conventional radiotherapy, providing unprecedented anatomical detail. Image-guidance through MVCT will allow the development and refinement of the concept of "adaptive radiotherapy", the reconstruction of the actual daily delivered dose (as opposed to planned dose) accompanied by prescription and delivery adjustments when appropriate. In addition to this unique feature, helical tomotherapy appears capable of further improvements over 3-dimensional conformal radiation therapy and non-helical IMRT in the specific avoidance of critical normal structures, i.e "conformal avoidance", the counterpart of conformal radiation therapy. Based on radiobiological principles that exploit the physical advantages of helical tomotherapy, several dosimetric and clinical investigations are underway.