Target Volume Margins Research Articles

Prostate Cancer Stereotactic Body Radiation Therapy with Image-guided Radiation Therapy (IGRT) Technique with Three-Dimensional Transperineal Ultrasound for IGRT Tracking of the Prostate

Abstract Introduction Ultrahypofractionated radical radiotherapy for prostate cancer, also known as stereotactic body radiation therapy (SBRT), is an unprecedented modality of prostate radiotherapy and is still used very sparsely in Brazil. The use of fiducial implants and spacers may make it even more difficult to implement prostate SBRT on a large scale in Brazil. We describe a completely non-invasive technique for the treatment of localized prostate cancer with stereotactic body radiotherapy. Materials and Methods This is a case series study with 9 patients and 48 SBRT fractions analyzed. In our treatment protocol, we use the Elekta Versa-HD linear accelerator (Elekta, Stokholm, Sweden), a transperineal ultrasound (TPUS) with tracking and synchronization capabilities (Clarity 4D ultrasound system, Elekta), and the Monaco Planning System (Elekta). Displacements were measured in the three axes (lateral, longitudinal, and vertical). The discrepancy between the initial ultrasound location of the prostate and the location of the cone beam has also been documented. Results The mean displacements were 2.02 mm, 3.12 mm, and 2.93 mm for the lateral, longitudinal, and vertical directions, respectively. The data show that treatment was interrupted in 14 of the 48 treatment fractions (29.17%), 8 (57%) with displacements greater than 5 mm and 6 (43%) with displacements between 3 and 5 mm.The initial TPUS image-guided radiation therapy (IGRT) and its mean displacements for localization with the cone-beam computed tomography (CBCT) were 1.3 mm, 1.9 mm, and 1.5 mm, for the lateral, longitudinal, and vertical directions, respectively. Using van Herk formula, a margin of 7.3 mm in the lateral directions, 9.35 mm longitudinally, and 7.74 mm vertically, would be required Conclusion Here we describe an SBRT technique for prostate cancer that is completely non-invasive and allows for a high level of accuracy. Transperineal 3D ultrasound provides real-time position data to the prostate that can be used to gate the SBRT treatment, allowing for smaller, more personalized planning target volume (PTV) margins even without fiducial markers or spacers, which may be more applicable for low- and middle-income countries.

Spatial accuracy of dose delivery significantly impacts the planning target volume margin in linear accelerator-based intracranial stereotactic radiosurgery

The impact of three-dimensional (3D) dose delivery accuracy of C-arm linacs on the planning target volume (PTV) margin was evaluated for non-coplanar intracranial stereotactic radiosurgery (SRS). A multi-institutional 3D starshot test using beams from seven directions was conducted at 22 clinics using Varian and Elekta linacs with X-ray CT-based polymer gel dosimeters. Variability in dose delivery accuracy was observed, with the distance between the imaging isocenter and each beam exceeding 1 mm at one institution for Varian and nine institutions for Elekta. The calculated PTV margins for Varian and Elekta linacs that could cover the gross tumor volume with 95% probability at 95% of the institutions were 2.3 and 3.5 mm, respectively, in the superior–inferior direction. However, with multifactorial system management (i.e., high-accuracy 3D dose delivery with rigorous linac quality assurance, strict patient immobilization, and high intra-fractional positioning accuracy), these margins could be reduced to 1.0 mm and 1.5 mm, respectively. The findings indicate significant millimeter-level variability in 3D dose delivery accuracy among linacs installed in clinical settings. Thus, maximizing a linac’s 3D dose delivery accuracy is essential to achieve the required PTV margin in intracranial SRS.

Prostate motion in magnetic resonance imaging-guided radiotherapy and its impact on margins.

This study focused on reducing the margin for prostate cancer treatment using magnetic resonance imaging-guided radiotherapy by investigating the intrafractional motion of the prostate and different motion-mitigation strategies. We retrospectively analyzed intrafractional prostate motion in 77patients with low- to intermediate-risk prostate cancer treated with five fractions of 7.25 Gy on a1.5 T magnetic resonance linear accelerator. Systematic drift motion was observed and described by an intrafractional motion model. The planning target volume (PTV) margin was calculated in acohort of 77patients and prospectively evaluated for geometric coverage in aseparate cohort of 24patients. The intrafractional model showed that the prostate position starts out of equilibrium for the anterior-posterior (-1.8 ± 3.1mm) and superior-inferior (1.7 ± 2.6mm) directions, with relaxation times of12 and 15min, respectively. Position verification scans are acquired at 30min on average. At that time, the transient drift motion becomes indistinguishable from the residual random intrafractional motion. PTV margins can be reduced to 1.8 mm (left-right), 3.2 mm (anterior-posterior), and 2.9 mm (superior-inferior). Evaluation of the overlap with the clinical target volume (CTV) was performed for atotal of 120 fractions of 24patients. The overlap range between the CTV and the PTV was 93-100% and the applied 3‑mm PTV margin for the CTV had a99.5% averaged geometric overlap for all patients. APTV margin reduction to 3 mm is feasible. Apatient-specific approach could reduce the margins further.

Optimal Planning Target Volume Margins to Account for Intra-Fractional Prostate Motion Relative to Treatment Duration: A Study Using Real-Time Transperineal Ultrasound Guidance.

Prostate motion during external beam radiotherapy (EBRT) is common and typically managed using fiducial markers and cone beam CT (CBCT) scans for inter-fractional motion correction. However, real-time intra-fractional motion management is less commonly implemented. This study evaluated the extent of intra-fractional prostate motion using transperineal ultrasound (TPUS) and examined the impact of treatment time on prostate motion. Patients undergoing prostate EBRT with TPUS at a single institution from August 2016 to August 2021 were analysed. Pre-treatment daily CBCT corrected inter-fractional prostate shift. Continuous intra-fractional prostate motion was recorded at two frames per second in three dimensions, with three-dimensional (3D) displacement calculated as a vector. Motion data were modelled to determine the probability of the prostate remaining within pre-specified PTV margins relative to treatment delivery time. The study analysed 3364 fractions delivered to 122 patients. The mean treatment delivery time was 3.8 min. The prostate remained within a 5 mm margin with high frequencies in the superior-inferior (SI) and left-right (LR) directions, 97.8% and 98.4% of fractions respectively while 5.5% of fractions had deviations greater than 5 mm in the anterior-posterior (AP) direction. By contrast, the 3D vector exceeded a 5 mm margin in 14.5% of fractions. Drift motion modelling indicated a 99% probability of the vector staying within a 3 mm margin for 2 min, while for a 5 mm margin, the duration extended to 3.4 min. Intra-fractional prostate motion monitoring is increasingly important as SABR with reduced PTV margins are utilised in prostate radiotherapy. Smaller PTV margins and longer treatment time require real-time monitoring to avoid geographical miss.

Influence of planning target volume margins using various prescription isodoses in gamma knife radiosurgery for single brain metastasis: a phantom study

Abstract Objective: The study seeks to evaluate the influence of planning target volume (PTV) margins on plan parameters during inverse planning of brain metastases with the Gamma Knife treatment unit, considering various prescription isodose levels (PIL). Material & Method: CT scan images of a STEEV anthropomorphic phantom were transferred into the GAMMA PLAN Treatment Planning System. A target measuring a volume of 4·9cc was centrally contoured. Plans with a 0 mm volume margin at five prescription isodose levels from 50% to 70% at 5% increment were created. With 0.5 mm, 1 mm, 1.5 mm and 2 mm PTV margins, identical plans were regenerated. Adjustments were made to each plan when necessary to achieve same target coverage. One-way ANOVA test was used to analyse the influence of PTV margins on parameters including Selectivity[S], Gradient index [GI], V12, Paddick’s conformal index [PCI] and Treatment time [TI]. Results: Margin addition resulted in PTV volume increase. The findings indicated that the PTV margin of 2.0 mm exhibited the highest mean selectivity of (0.93 ± 0.00), PCI (0.92 ± 0.01), GI (2.50 ± 0.04), V12 (16.17 ± 0.38) and treatment time (118.32 ± 2.91 min). The 0.0 mm PTV margin had the lowest mean value for all the parameters except for the treatment time (105.58 ± 3.48 min) which was slightly higher compared to the 0.5 mm PTV margin (M = 86.36 ± 4.13 min). Conclusion: Incremental increases in PTV margins for Gamma knife radiosurgery though a relatively controversial concept influence all dosimetric parameters, which may pose potential detrimental effects and thus need to be carefully evaluated for brain metastasis treatment.

Adaptive radiotherapy in locally advanced head and neck cancer: The importance of reduced margins.

Adaptive radiotherapy (ART) involves treatment re-planning based on anatomical changes, which may improve target coverage and sparing of organs-at-risk (OARs). This study retrospectively assessed the technical feasibility and potential benefits of daily ART in combination with reduced planning target volume (PTV) margins for head and neck squamous cell carcinoma (HNSCC). Thirty-one patients, encompassing 902 treatment fractions, treated with radiotherapy to 60.0-68.0Gy in 2Gy/fraction were studied. Synthetic CTs (sCT) from daily kVCT images were created and contours propagated using deformable image registration (DIR). Target contours were reviewed and corrected. On the sCT, non-adapted delivered doses and ART-plans with 5mm (clinical standard) and 2mm PTV-margin were evaluated. All daily dose distributions were then accumulated. Target contours required correction in 48% of the fractions. Daily non-adapted D98%,CTV was>95% in 890 (5mm) and 825 (2mm) out of 902 fractions. All adapted plans achieved D98%,CTV>95%. Significant reductions in mean doses to OARs were observed for PTV=2 mm ART-plans: 4.1Gy for parotid, 2.6Gy for submandibular, 3.3Gy for oral cavity, 4.0Gy for esophagus, and 3.8Gy for larynx. ART-planning on sCT and DIR propagated contours was feasible and promising for further clinical testing. To obtain a potential clinical benefit of ART, a synchronous reduction of the PTV-margin was warranted. Daily ART can be used to maintain adequate target dosimetry for every fraction, though for the accumulated treatment, insufficient target coverage without ART is unlikely to occur.

Electrocardiogram-gated cardiac computed tomography-based patient- and segment-specific cardiac motion estimation method in stereotactic arrhythmia radioablation for ventricular tachycardia.

Motion management strategies such as gating under breath-hold can reduce breathing-induced motion during stereotactic arrhythmia radioablation (STAR) for refractory ventricular tachycardia. However, heartbeat-induced motion is essential to define an appropriate cardiac internal target volume (ITV) margin. In this study, we introduce a patient- and segment-specific cardiac motion estimation method and cardiac motion data of the clinical target volume (CTV), ICD lead tips and left ventricle (LV) segments. Data from 10 STAR-treated patients were retrospectively analyzed. The LV was semi-automatically segmented according to the 17-segment model. Electrocardiogram-gated contrast-enhanced breath-hold cardiac CTs were automatically non-rigidly registered for motion estimation. The correlation and significant differences between ICD tip motion and CTV motion were assessed using the Pearson correlation coefficient (PCC) and Wilcoxon signed-rank test, while spatial discrepancies with both CTV and segment motion were quantified using the Euclidean distance. The CTVs (center of mass) moved 3.4±1.4mm and the ICD lead tips moved 4.9±2.2mm. The maximum motion per patient was observed in basal and mid-cavity LV segments in 3D. The PCC showed a strong positive motion correlation between the ICD tip and CTV in 3D (0.84), while the p-values indicated statistically significant differences in the right-left, anterior-posterior and 3D directions. The proposed methods enable patient- and segment-specific cardiac ITV margin estimation. The motion in most LV segments was limited, however, cardiac ITV margins may need adjustment in individual cases. The impact of cardiac motion on the dosimetry needs further investigation.

Determining the Correlation and Effect of Rectal and Bladder Volume Change in Shift of Planning Target Volume and Calculating Planning Target Volume Margin with Van Herk Formula in Prostate Cancer Tomotherapy

Determining the Correlation and Effect of Rectal and Bladder Volume Change in Shift of Planning Target Volume and Calculating Planning Target Volume Margin with Van Herk Formula in Prostate Cancer Tomotherapy

Comparable Performance Between Automatic and Manual Laryngeal and Hypopharyngeal Gross Tumor Volume Delineations Validated With Pathology.

Deep learning is a promising approach to increase reproducibility and time-efficiency of gross tumor volume (GTV) delineation in head and neck cancer, but model evaluation primarily relies on manual GTV delineations as reference annotation, which are subjective and tend to overestimate tumor volume. This study aimed to validate a deep learning model for laryngeal and hypopharyngeal GTV segmentation with pathology and to compare its performance with clinicians' manual delineations. A retrospective data set of 193 patients with laryngeal and hypopharyngeal cancer was used to train a deep learning model with clinical GTV delineations as reference. For validation, a data set comprising 18 patients who underwent imaging before total laryngectomy was used, with histopathology-based (n = 16) tumor delineations as ground truth. The performance of the automatic segmentations was compared with that of clinicians' manual delineations, both quantitatively and qualitatively. Median sensitivity (0.90 and 0.91) and largest required clinical target volume margin (6.4 and 6.6 mm) were comparable between automatic and manual GTV delineations. The positive predictive value yielded the only significant difference between automatic and manual GTV delineations, with medians of 0.52 and 0.61, respectively (P = .03). Clinical target volumes derived from automatic and manual GTVs exhibited similar sizes (median of 44.5 and 40.1 mL) and achieved a sensitivity of 1.00 in 13/16 and 14/16 tumors, respectively. Automatic segmentations were considered clinically acceptable in 67% of cases, compared with 63% of manual delineations. The proposed deep learning model for laryngeal and hypopharyngeal GTV segmentation achieved comparable results with clinicians' manual delineations, showing the potential for more consistency and efficiency in the radiation therapy workflow.

Assessing intra- and interfraction motion and its dosimetric impacts on cervical cancer adaptive radiotherapy based on 1.5T MR-Linac

PurposeThe purpose of this study was to quantify the intra- and interfraction motion of the target volume and organs at risk (OARs) during adaptive radiotherapy (ART) for uterine cervical cancer (UCC) using MR-Linac and to identify appropriate UCC target volume margins for adapt-to-shape (ATS) and adapt-to-position (ATP) workflows. Then, the dosimetric differences caused by motion were analyzed.MethodsThirty-two UCC patients were included. Magnetic resonance (MR) images were obtained before and after each treatment. The maximum and average shifts in the centroid of the target volume and OARs along the anterior/posterior (A/P: Y axes), cranial/caudal (Cr/C: Z axes), and right/left (R/L: X axes) directions were analyzed through image contours. The bladder wall deformation in six directions and the differences in the volume of the organs were also analyzed. Additionally, the motion of the upper, middle and lower rectum was quantified. The correlation between OAR displacement/deformation and target volume displacement was evaluated. The planning CT dose distribution was mapped to the MR image to generate a plan based on the new anatomy, and the dosimetric differences caused by motion were analyzed.ResultsFor intrafraction motion, the clinical tumor volume (CTV) range of motion along the XYZ axes was within 5 mm; for interfraction motion, the range of motion along the X axis was within 5 mm, and the maximum distances of motion along the Y axis and Z axis were 7.45 and 6.59 mm, respectively. Additionally, deformation of the superior and anterior walls of the bladder was most noticeable. The largest magnitude of motion was observed in the upper segment of the rectum. Posterior bladder wall displacement was correlated with rectal and CTV centroid Y-axis displacement (r = 0.63, r = 0.50, P < 0.05). Compared with the interfractional plan, a significant decrease in the planning target volume (PTV) D98 (7.5 Gy, 7.54 Gy) was observed. However, there were no significant differences within the intrafraction.ConclusionDuring ART for UCC patients using MR-Linac, we recommend an ATS workflow using isotropic PTV margins of 5 mm based on intrafraction motion. Based on interfraction motion, the recommended ATP workflow uses anisotropic PTV margins of 5 mm in the R/L direction, 8 mm in the A/P direction, and 7 mm in the Cr/C direction to compensate for dosimetric errors due to motion.

Efficacy of a thermoplastic mask and pneumatic abdominal compression device for immobilization in stereotactic ablative radiotherapy of spine metastases.

Stereotactic ablative radiotherapy (SABR) has become a key technique in management of spine metastases. With improved control over treatment plan dosimetry, there is a greater need for accurate patient positioning to guarantee agreement between the treatment plan and delivered dose. With serious potential complications such as fracture and myelopathy, the margins of error in SABR of the spine are minimal. In this study, we assessed the performance of two patient immobilization setups in SABR for spinal metastases. First, a Type-S head and shoulders mask (CQ Medical, Avondale, PA), and second, the BPL1 setup, which includes a wing board, vacuum bag, and the Respiratory Belt for the Body Pro-Lok ONE (CQ Medical, Avondale, PA). Immobilization was assessed using image-guided intrafraction repositioning shifts. Required planning target volume (PTV) margins were calculated based on repositioning data for 172 treated fractions using 2 standard deviation (2SD) and analytic approaches. Overall, 91.7% and 74.1% of fractions treated had total 3D repositioning shifts ≤3.0mm using the Type-S and BPL1 setups, respectively. In the thoracic spine, 43.2% and 46.5% of fractions had shifts ≤1.5mm for the respective setups. Suggested margins were under 3.5mm in all directions and use cases. In the posterior-anterior direction, the BPL1 setup had a 0.6mm smaller suggested margin for the thoracic spine compared to the Type-S setup, at 1.4mm, calculated using the analytic approach. Both the Type-S and BPL1 setups are effective for immobilization in spine SABR. The Type-S demonstrated superior immobilization in the upper spine and remains the clinical standard for cervical and upper thoracic spine positioning. The BPL1 setup showed effective immobilization in use cases treating the mid-to lower thoracic spine and lumbar spine and remains our clinical standard for those use cases. Results additionally demonstrate feasibility of potential PTV margin reduction.

Patient Positional Uncertainty and Margin Reduction in Lung Stereotactic Ablative Radiation Therapy Using Pneumatic Abdominal Compression.

Motion management presents a significant challenge in thoracic stereotactic ablative radiation therapy (SABR). Currently, a 5.0-mm standard planning target volume (PTV) margin is widely used to ensure adequate dose to the tumor. Considering recent advancements in tumor localization and motion management, there is merit to reassessing the necessary PTV margins for modern techniques. This work presents a large-scale analysis of intrafraction repositioning for lung SABR under forced shallow breathing to determine the margin requirements for modern delivery techniques. Treatment data for 124 lung SABR patients treated in 607 fractions on a linear accelerator were retrospectively collected for analysis. All patients were treated using pneumatic abdominal compression and intrafraction 4-dimensional (4D) cone beam computed tomography (4D CBCT)-guided repositioning halfway through treatment. Executed repositioning shifts were collected and used to calculate margin requirements using the 2-SD method and an analytical model which accounts for systematic and random errors in treatment. A total of 85.7% of treated fractions had 3-dimensional repositioning shifts under 5.0 mm. Fifty-three fractions (8.7%) had shifts ≥ 5.0 mm in at least 1 direction. Margins in the right-left, inferior-superior, and posterior-anterior directions were 3.62 mm, 4.34 mm, and 3.50 mm, respectively, calculated using the 2-SD method. The analytical approach estimated that 4.01 mm, 4.37 mm, and 3.95 mm margins were appropriate for our workflow. Executing intrafraction repositioning reduced margin requirements by 0.73 ± 0.07 mm. Clinical data suggest that the uniform 5.0-mm margin is conservative for our workflow. Using modern techniques such as 4D CT, 4D CBCT, and effective motion management can significantly reduce required margins, and therefore necessary healthy tissue dose. However, the limitations of margin calculation models must be considered, and margin reduction must be approached with caution. Users should conduct a formal risk assessment prior to adopting new standard PTV margins.

Assessment of Setup-errors in 3Dd-Conformal Radiotherapy for Head and Neck Cancer Patients Using an Electronic Portal Imaging Device

Patient setup is crucial in radiotherapy since treatment is delivered as fractionated treatment over a period of time. Using own institutional margins by considering the setup errors will provide better radiotherapy outcome. Therefore, this study aims to assess set up errors for head and neck cancer patients using an electronic portal imaging device at Apeksha Hospital, Maharagama, Sri Lanka. A total of 101 head and neck cancer patients who were immobilized with thermoplastic mask were selected in this study. Stored data from July 2021 to July 2022 were obtained from ARIA patient management system in the Varian 2300CD Unit at Apeksha Hospital. In order to calculate systematic and random errors, translational errors in all directions were collected utilizing 303 pairs of orthogonal portal images. Moreover, three different algorithms were used to obtain the margin of clinical target volume (CTV) to planning target volume (PTV). The estimated systematic and random errors in the directions of antero-posterior, superior-inferior and medio-lateral are 0.13 cm, 0.10 cm and 0.08 cm, and 0.22 cm, 0.21 cm, and 0.19 cm respectively. Less than 0.5 cm margin were obtained by applying three different algorithms. This study indicates that using a 0.5 cm margin for head and neck cancer patients treating in 2300CD Varian Unit at Apeksha Hospital is safe. Further, this study recommends to developing institutional CTV to PTV margin for all sites of cancer to reduce unnecessary radiation to the surrounding normal healthy tissues.

Determining optimal clinical target volume margins based on microscopic extracapsular extension of metastatic nodes in patients with non-small-cell lung cancer after chemotherapy or chemotherapy combined with immunotherapy

BackgroundNo standard has been established for the clinical target volume (CTV) margins of lymph nodes (LNs) in patients with non-small-cell lung cancer (NSCLC) receiving chemotherapy or chemotherapy combined with immunotherapy followed by radiotherapy. This study aimed to discuss the CTV range of NSCLC after chemotherapy or chemotherapy combined with immunotherapy by observing the microscopic extent of tumor spread beyond the LN capsule.MethodsWe retrospectively analyzed the data of 240 patients with stage II and III NSCLC who underwent surgery without neoadjuvant therapy, with neoadjuvant chemotherapy (NAC), or with NAC combined with immunotherapy (NACI). We measured the maximal distance of extracapsular extension (ECE) using a digital microscope, analyzed the correlation between clinicopathological features and ECE distance, and determined the CTV margins of metastatic LN under different treatment methods.ResultsThe ECE distance differed significantly among the three groups (p < 0.001). We determined appropriate margin widths based on a 5% error risk as 3.00, 2.30, and 1.40 mm for direct surgery, NAC, and NACI, respectively. Multivariate analysis revealed that the ECE of metastatic LN correlated with the treatment methods and LN size.ConclusionThe existing CTV delineation standards may increase the radiation toxicity of patients. We believe that different CTV margins should be used for LN in patients with NSCLC receiving different treatments. To ensure 95% coverage of ECE, the gross tumor volume of untreated, chemotherapy-treated, and chemotherapy combined with immunotherapy-treated patients should be expanded by 3.00, 2.30, and 1.40 mm, respectively, to obtain the CTV.

Initial Experience of Implementing a Pre-treatment Dry Run for HyperArc Stereotactic Radiosurgery Treatments With Optical Surface Imaging for Intra-fraction Motion Monitoring.

Linac-based stereotactic radiosurgery (SRS) with planning target volume (PTV) margins <1 mm has become increasingly common in recent years. Optical surface imaging for surface-guided radiation therapy (SGRT) is often used for intra-fraction motion monitoring during these treatments to facilitate the use of a smaller PTV margin by providing real-time quantitative patient positioning information. However, rotating the couch introduces errors to SGRT-reported translations and rotations that can be problematic for SRS treatments with non-coplanar arcs and very small PTV margins. This work presents a novel approach for decreasing the magnitude of these errors by performing a pre-treatment dry run and capturing reference surfaces with the SGRT system at each couch angle included in the treatment plan. Time from cone beam computed tomography (CBCT) to treatment initiation and total treatment session time were reviewed for 30 single-fraction brain SRS cases treated using this technique to determine the effect of including the dry run on treatment session times. Out of the 30 cases treated between April 2023 and January 2024, 23 treatments required only a single CBCT prior to treatment, with no additional mid-treatment imaging required to verify patient positioning after motion. The median time between CBCT and treatment initiation was 7.98 minutes (interquartile range (IQR) = 7.28 to 8.93 minutes). The median time from CBCT to treatment completion was 15.43 minutes (IQR = 13.67 to 21.97 minutes). In the six patients that required one additional CBCT, the treatment session times ranged from 24.32 to 32.83 minutes. There was one patient who required three mid-treatment CBCTs, and the treatment session time was 67.87 minutes. Incorporating the pre-treatment dry run with the acquisition of reference surfaces at each treatment angle decreased errors in SGRT-reported translations and rotations associated with couch rotation without significantly increasing treatment session times.

Cone Beam Computed Tomography-Based Online Adaptive Radiation Therapy of Esophageal Cancer: First Clinical Experience and Dosimetric Benefits

Radiation therapy (RT) plays a key role in the management of esophageal cancer (EC). However, toxicities caused by proximity of organs at risk (OAR) and daily target coverage caused by interfractional anatomic changes are of concern. Daily online adaptive RT (oART) addresses these concerns and has the potential to increase OAR sparing and improve target coverage. We present the first clinical experience and dosimetric investigations of cone beam CT-based oART in EC using the ETHOS platform. Treatment fractions of the first 10 EC patients undergoing cone beam CT-based oART at our institution were retrospectively analyzed. The prescription dose was 50.4 Gy in 28 fractions. The same clinical target volume (CTV) and planning target volume (PTV) margins as for nonadaptive treatments were used. For all sessions, the timestamp of each oART workflow step, PTV size, target volume doses, mean heart dose, and lung V20Gy of both the scheduled and the adapted treatment plan were analyzed. Following automatic propagation, the CTV was adapted by the physician in 164 (59%) fractions. The adapted treatment plan was selected in 276 (99%) sessions. The median time needed for an oART session was 28 minutes (range, 14.8-43.3). Compared to the scheduled plans, a significant relative reduction of 9.5% in mean heart dose (absolute, 1.6 Gy; P = .006) and 16.9% reduction in mean lung V20Gy (absolute, 2.3%; P < .001) was achieved with the adapted treatment plans. Simultaneously, we observed a significant relative improvement in D99%PTV and D99%CTV by 15.3% (P < .001) and 5.0% (P = .008), respectively, along with a significant increase in D95%PTV by 5.1% (P = .003). Although being resource-intensive, oART for EC is feasible in a reasonable timeframe and results in increased OAR sparing and improved target coverage, even without a reduction of margins. Further studies are planned to evaluate the potential clinical benefits.

Intrafraction motion and impact of margin reduction for MR-Linac online adaptive radiotherapy for pancreatic cancer treatments.

Online adaptive radiotherapy is well suited for stereotactic ablative radiotherapy (SABR) in pancreatic cancer due to considerable intrafractional tumour motion. This study aimed to assess intrafraction motion and generate adjusted planning target volume (PTV) margins required for online adaptive radiotherapy in pancreatic cancer treatment using abdominal compression on the magnetic resonance linear accelerator (MR-Linac). Motion monitoring images obtained from 67 fractions for 15 previously treated pancreatic cancer patients were analysed. All patients received SABR (50 Gy in five fractions) on the MR-Linac using abdominal compression. The analysis included quantification of intrafraction motion, leading to the development of adjusted PTV margins. The dosimetric impact of implementing the adjusted PTV was then evaluated in a cohort of 20 patients. Intrafraction motion indicated an average target displacement of 1-3 mm, resulting in an adjusted PTV margin of 2 mm in the right-left and superior-inferior directions, and 3 mm in the anterior-posterior direction. Plans incorporating these adjusted margins consistently demonstrated improved dose to target volumes, with improvements averaging 1.5 Gy in CTV D99%, 4.9 Gy in PTV D99% and 1.2 Gy in PTV-high D90%, and better sparing of the organs at risk (OAR). The improved target volume coverage and reduced OAR dose suggest potential for reducing current clinical margins for MR-Linac treatment. However, it is important to note that decreasing margins may reduce safeguards against geographical misses. Nonetheless, the continued integration of gating systems on MR-Linacs could provide confidence in adopting reduced margins.

Modeling and prediction of set‑up errors in breast cancer image‑guided radiotherapy using the Gaussian mixture model.

The aim of the present study was to develop a prediction model for set-up error distribution in breast cancer image-guided radiotherapy (IGRT) using a Gaussian mixture model (GMM). To achieve this, the image-guided set-up errors data of 80 patients with breast cancer were selected, and the GMM was used to develop the set-up errors distribution prediction model. The predicted error center points, covariance and probability were calculated and compared with the planning target volume (PTV) margin formula. A total of 1,200 sets of set-up errors in IGRT for breast cancer were collected. The results of the Gaussian model parameters showed that the set-up errors were mainly in the direction of µ1-µ4 center points. All the raw errors in the lateral, longitudinal and vertical directions were -6.30-4.60, -5.40-1.47 and -2.70-1.70 mm, respectively. According to the probability of each center, the set-up error was most likely to shift in the µ1 direction, reaching 0.53. The set-up errors of the other three centers, µ2, µ3 and µ4, were 0.11, 0.34 and 0.12, respectively. According to the covariance parameters of the GMM, the maximum statistical standard deviation of the set-up errors reached 29.06. In conclusion, the results of the present study demonstrated that the GMM can be used to quantitatively describe and predict the distribution of set-up errors in IGRT for breast cancer, and these findings could be useful as a reference for set-up error control and tumor PTV expansion in breast cancer radiotherapy without routine, daily IGRT.

Fractionated Stereotactic Intensity-Modulated Radiotherapy for Large Brain Metastases: Comprehensive Analyses of Dose–Volume Predictors of Radiation-Induced Brain Necrosis

Background: The objective was to explore dosimetric predictors of brain necrosis (BN) in fractionated stereotactic radiotherapy (SRT). Methods: After excluding collinearities carefully, multivariate logistic models were developed for comprehensive analyses of dosimetric predictors in patients who received first-line fractionated SRT for brain metastases (BMs). The normal brain volume receiving an xx Gy biological dose in 2 Gy fractions (VxxEQD2) was calculated from the retrieved dose–volume parameters. Results: Thirty Gy/3 fractions (fr) SRT was delivered to 34 patients with 75 BMs (median target volume, 3.2 cc), 35 Gy/5 fr to 30 patients with 57 BMs (6.4 cc), 37.5 Gy/5 fr to 28 patients with 47 BMs (20.2 cc), and 40 Gy/10 fr to 20 patients with 37 BMs (24.3 cc), according to protocols, depending on the total target volume (p &lt; 0.001). After excluding the three-fraction groups, the incidence of symptomatic BN was significantly higher in patients with a larger V50EQD2 (adjusted odds ratio: 1.07, p &lt; 0.02), V55EQD2 (1.08, p &lt; 0.01), or V60EQD2 (1.09, p &lt; 0.01) in the remaining five- and ten-fraction groups. The incidence of BN was also significantly higher in cases with V55EQD2 &gt; 30 cc or V60EQD2 &gt; 20 cc (p &lt; 0.05). These doses correspond to 28 or 30 Gy/5 fr and 37 or 40 Gy/10 fr, respectively. Conclusions: In five- or ten-fraction SRT, larger V55EQD2 or V60EQD2 are BN risk predictors. These biologically high doses may affect BN incidence. Thus, the planning target volume margin should be minimized as much as possible.

The accuracy of cone beam computed tomography (CBCT) technique as an image-guided radiation therapy (IGRT) technique.

To measure the setup error of the patient's positioning using cone-beam computed tomography during radiation therapy treatment fractions by finding systematic, random errors and the planned target volume errors. The observational, longitudinal cohort study was conducted at the Al-Warith International Cancer Institute, Karbala, Iraq, from January to May 2022, and comprised patients with head and neck cancer who underwent radiation therapy. The oncologist delineated and the medical physicist planned. Then the medical physicist modified the positioning system using the cone beam computed tomography option workstation. The vertical value was taken in anteroposterior site, longitudinal in superoinferior, and lateral in e mediolateral. The SPSS 25 were used to analyse data. Of the 31 patients, 17(54.8%) were females and 14(45.2%) were males. The overall mean age was 48.3 ± 10.22 (range: 4-77 years), and 22(70.96%) patients had been treated previously with chemotherapy. The lateral shifting inaccuracy 2.501mm was above the limit, whereas the vertical shifting 1.164mm was within acceptable limits (±2mm). The longitudinal shifting had the smallest displacement 0.436mm. Random error displayed longitudinal moving 1.965mm, lateral shifting 0.623mm and vertical shifting 0.276mm. The planned target volume margins were too wide in longitudinal shifting 3.333mm. Vertical shifting 0.481mm was greater than lateral 1.092mm, but both were within limits (±2mm). Radiation-induced errors in normal tissues must be reduced by reducing planned target volume margins, especially for longitudinal and lateral directions.