Image-guided Adaptive Radiotherapy Research Articles

O.32 - A dosimetric evaluation of IGART strategies for cervix cancer treatment

Introduction: Substantial organ motion and tumor shrinkage may occur during external beam radiotherapy treatment of cervix cancer. The quality of radiation delivery is influenced by these anatomical changes which warrant treatment adaptation. Imaging information obtained prior to the actual treatment can be used in off-line or on-line adaptive strategies to include the temporal aspects of radiotherapy treatment. This will ensure sufficient coverage of the target without compromising organ at risk (OAR) sparing. The aim of this study is to dosimetrically evaluate these two image guided adaptive radiotherapy (IGART) strategies.

Clinical outcome of image-guided adaptive radiotherapy in the treatment of lung cancer patients

ABSTRACTBackground. Adaptive strategy with daily online tumour match is a treatment option when treating locally advanced lung cancer patients with curative intended radiotherapy (RT).Material and methods. Fifty-two consecutive lung cancer patients treated with soft tissue match, adaptive RT and small planning target volumes (PTV) margins were analysed. A control group of 52 consecutive patients treated with bone match, no adaptive strategy and larger margins was included. Patients were followed with computed tomography (CT) scans every third month. CT-images showing loco-regional recurrences were identified. The recurrence gross tumour volume was delineated and registered with the original radiation treatment plan to identify site of failure. All patients were toxicity-scored using CTCAE 4.03 grading scale. Data were analysed using the Kaplan-Meier analysis.Results. The median follow-up time was 16 months (3–35). Within a year, 35% of the patients in the adaptive group (ART-group) and 53% in the control group (No-ART-group) experienced loco-regional failure, showing improved loco-regional control in the ART group (p = 0.05). One patient in the ART-group and four patients in the No-ART-group showed marginal failure. Median overall progression-free survival time for the ART-group was 10 months (95% CI 8–12), and 8 months (95% CI 6–9) for the No-ART-group. Severe pneumonitis (grade 3–5) decreased from 22% in the No-ART-group to 18% in the ART-group (non-significant, p = 0.6). No significant difference in severe dysphagia was found between the two groups.Conclusion. In the first small cohort of patients investigated, implementation of soft-tissue tumour match and adaptive strategies for locally advanced lung cancer patients increased the loco-regional control rate without increasing treatment-related toxicity.

WE-AB-BRA-09: Sensitivity of Plan Re-Optimization to Errors in Deformable Image Registration in Online Adaptive Image-Guided Radiation Therapy

Purpose: Online adaptive therapy (ART) relies on auto-contouring using deformable image registration (DIR). DIR’s inherent uncertainties require user intervention and manual edits while the patient is on the table. We investigated the dosimetric impact of DIR errors on the quality of re-optimized plans, and used the findings to establish regions for focusing manual edits to where DIR errors can Result in clinically relevant dose differences. Methods: Our clinical implementation of online adaptive MR-IGRT involves using DIR to transfer contours from CT to daily MR, followed by a physicians’ edits. The plan is then re-optimized to meet the organs at risk (OARs) constraints. Re-optimized abdomen and pelvis plans generated based on physician edited OARs were selected as the baseline for evaluation. Plans were then re-optimized on auto-deformed contours with manual edits limited to pre-defined uniform rings (0 to 5cm) around the PTV. A 0cm ring indicates that the auto-deformed OARs were used without editing. The magnitude of the variations caused by the non-deterministic optimizer was quantified by repeat re-optimizations on the same geometry to determine the mean and standard deviation (STD). For each re-optimized plan, various volumetric parameters for the PTV, the OARs were extracted along with DVH and isodose evaluation. A plan was deemed acceptable if the variation from the baseline plan was within one STD. Results: Initial results show that for abdomen and pancreas cases, a minimum of 5cm margin around the PTV is required for contour corrections, while for pelvic and liver cases a 2–3 cm margin is sufficient. Conclusion: Focusing manual contour edits to regions of dosimetric relevance can reduce contouring time in the online ART process while maintaining a clinically comparable plan. Future work will further refine the contouring region by evaluating the path along the beams, dose gradients near the target and OAR dose metrics.

EP-1240: Clinical outcomes of image guided adaptive radiotherapy (IGART) for hypofractionated treatment of bladder cancer

EP-1240: Clinical outcomes of image guided adaptive radiotherapy (IGART) for hypofractionated treatment of bladder cancer

Accelerated gradient-based free form deformable registration for online adaptive radiotherapy

The registration of planning fan-beam computed tomography (FBCT) and daily cone-beam CT (CBCT) is a crucial step in adaptive radiation therapy. The current intensity-based registration algorithms, such as Demons, may fail when they are used to register FBCT and CBCT, because the CT numbers in CBCT cannot exactly correspond to the electron densities. In this paper, we investigated the effects of CBCT intensity inaccuracy on the registration accuracy and developed an accurate gradient-based free form deformation algorithm (GFFD). GFFD distinguishes itself from other free form deformable registration algorithms by (a) measuring the similarity using the 3D gradient vector fields to avoid the effect of inconsistent intensities between the two modalities; (b) accommodating image sampling anisotropy using the local polynomial approximation-intersection of confidence intervals (LPA-ICI) algorithm to ensure a smooth and continuous displacement field; and (c) introducing a ‘bi-directional’ force along with an adaptive force strength adjustment to accelerate the convergence process. It is expected that such a strategy can decrease the effect of the inconsistent intensities between the two modalities, thus improving the registration accuracy and robustness. Moreover, for clinical application, the algorithm was implemented by graphics processing units (GPU) through OpenCL framework. The registration time of the GFFD algorithm for each set of CT data ranges from 8 to 13 s. The applications of on-line adaptive image-guided radiation therapy, including auto-propagation of contours, aperture-optimization and dose volume histogram (DVH) in the course of radiation therapy were also studied by in-house-developed software.

Application of image-guided adaptive radiation therapy in lung cancer

Adaptive radiotherapy （ART） is the product of excellence technology.It reschedules,registers according to tumor＇s size,shape and alteration of relative position with the surrounding tissue during the treatment.Thereby it raises the accuracy of the conformal radiotherapy.The technology is gradually being chosen by more and more patients.The technology can increase the rate of local control significantly,while reducing the occurrence of radiation-related adverse reactions. Key words: Lung neoplasms; Therapeutic uses; Adaptive radiotherapy

Validation of a deformable image registration technique for cone beam CT-based dose verification.

As radiation therapy evolves toward more adaptive techniques, image guidance plays an increasingly important role, not only in patient setup but also in monitoring the delivered dose and adapting the treatment to patient changes. This study aimed to validate a method for evaluation of delivered intensity modulated radiotherapy (IMRT) dose based on multimodal deformable image registration (dir) for prostate treatments. A pelvic phantom was scanned with CT and cone-beam computed tomography (CBCT). Both images were digitally deformed using two realistic patient-based deformation fields. The original CT was then registered to the deformed CBCT resulting in a secondary deformed CT. The registration quality was assessed as the ability of the dir method to recover the artificially induced deformations. The primary and secondary deformed CT images as well as vector fields were compared to evaluate the efficacy of the registration method and it's suitability to be used for dose calculation. plastimatch, a free and open source software was used for deformable image registration. A B-spline algorithm with optimized parameters was used to achieve the best registration quality. Geometric image evaluation was performed through voxel-based Hounsfield unit (HU) and vector field comparison. For dosimetric evaluation, IMRT treatment plans were created and optimized on the original CT image and recomputed on the two warped images to be compared. The dose volume histograms were compared for the warped structures that were identical in both warped images. This procedure was repeated for the phantom with full, half full, and empty bladder. The results indicated mean HU differences of up to 120 between registered and ground-truth deformed CT images. However, when the CBCT intensities were calibrated using a region of interest (ROI)-based calibration curve, these differences were reduced by up to 60%. Similarly, the mean differences in average vector field lengths decreased from 10.1 to 2.5 mm when CBCT was calibrated prior to registration. The results showed no dependence on the level of bladder filling. In comparison with the dose calculated on the primary deformed CT, differences in mean dose averaged over all organs were 0.2% and 3.9% for dose calculated on the secondary deformed CT with and without CBCT calibration, respectively, and 0.5% for dose calculated directly on the calibrated CBCT, for the full-bladder scenario. Gamma analysis for the distance to agreement of 2 mm and 2% of prescribed dose indicated a pass rate of 100% for both cases involving calibrated CBCT and on average 86% without CBCT calibration. Using deformable registration on the planning CT images to evaluate the IMRT dose based on daily CBCTs was found feasible. The proposed method will provide an accurate dose distribution using planning CT and pretreatment CBCT data, avoiding the additional uncertainties introduced by CBCT inhomogeneity and artifacts. This is a necessary initial step toward future image-guided adaptive radiotherapy of the prostate.

Tissue Density Mapping of Cone Beam CT Images for Accurate Dose Calculations

Purpose: To improve the accuracy in megavoltage photon beam dose calculation in CBCT-based radiation treatment (RT) plans, using a kilovoltage cone-beam computed tomography (CBCT)-to-density-step (CBCT-SF) function. Materials and Methods: The CBCT-SF table is constructed from differential histograms of the voxel values of CBCT and Fan-beam CT (FBCT). From the CBCT histograms, frequency peaks representing air, lung, soft tissue and bone are observed and their widths in CT numbers are assigned to the lower and higher bounds of the steps in the CBCT-SF. The CBCT-SF is entered into a planning system as an alternative to the clinical CT-to-density table. The CT image sets studied in this work consist of FBCT and CBCT scans of three patients: a prostate cancer patient, a lung cancer patient and a head and neck patient; and of a humanoid phantom at sections of the pelvis, the thorax and the head. Deformable image registration is used to map the patient FBCT scans to the corresponding CBCT images to minimize anatomical variations. Three-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiotherapy (IMRT) plans are made on the FBCT image sets of the patients and the phantom. The plans are recalculated on the CBCT scans using both the conventional CT-to-density table and the CBCT-SF. Dose calculations on the CBCT images and FBCT images are compared using dose differences, distance to agreement (DTA), Gamma analyses and dose volume histogram (DVH) analyses. Results: The results show that IMRT plans optimized using CBCT scans and FBCT scans agree dosimetrically within 1% when the CBCT-SF is used for the CBCT-based plans, including thoracic IMRT plan. In contrast, up to 5% dose difference is observed between IMRT plans optimized on FBCT scans and CBCT scans for thoracic cases if conventional CT-to-density table is used on CBCT images. Conclusions: The simple stepwise mapping of the CBCT numbers to density using the CBCT-SF resolves the inaccuracies in dose calculations previously reported in CBCT-based RT plans. CBCT-SF can be used in Image-Guided adaptive radiotherapy planning.

Individualized altered fractionation as a more effective radiotherapy for non-small cell lung cancer.

Individualized altered fractionation as a more effective radiotherapy for non-small cell lung cancer.

Improving radiotherapy planning, delivery accuracy, and normal tissue sparing using cutting edge technologies.

In the United States, more than half of all new invasive cancers diagnosed are non-small cell lung cancer, with a significant number of these cases presenting at locally advanced stages, resulting in about one-third of all cancer deaths. While the advent of stereotactic ablative radiation therapy (SABR, also known as stereotactic body radiotherapy, or SBRT) for early-staged patients has improved local tumor control to >90%, survival results for locally advanced stage lung cancer remain grim. Significant challenges exist in lung cancer radiation therapy including tumor motion, accurate dose calculation in low density media, limiting dose to nearby organs at risk, and changing anatomy over the treatment course. However, many recent technological advancements have been introduced that can meet these challenges, including four-dimensional computed tomography (4DCT) and volumetric cone-beam computed tomography (CBCT) to enable more accurate target definition and precise tumor localization during radiation, respectively. In addition, advances in dose calculation algorithms have allowed for more accurate dosimetry in heterogeneous media, and intensity modulated and arc delivery techniques can help spare organs at risk. New delivery approaches, such as tumor tracking and gating, offer additional potential for further reducing target margins. Image-guided adaptive radiation therapy (IGART) introduces the potential for individualized plan adaptation based on imaging feedback, including bulky residual disease, tumor progression, and physiological changes that occur during the treatment course. This review provides an overview of the current state of the art technology for lung cancer volume definition, treatment planning, localization, and treatment plan adaptation.

A Systematic Review of Organ Motion and Image-guided Strategies in External Beam Radiotherapy for Cervical Cancer

Advanced radiotherapy techniques, such as intensity-modulated radiotherapy (IMRT), may significantly benefit cervical cancer patients, in terms of reducing late toxicity and potentiating dose escalation. Given the steep dose gradients around the planning target volume (PTV) with IMRT planning, internal movement of organs during treatment may cause geographical miss of the target and unnecessary organs at risk (OAR) inclusion into high dose regions. It is therefore important to consider the extent and patterns of organ motion and to investigate potential image-guided radiotherapy (IGRT) solutions before implementing IMRT for cervical cancer. A systematic literature search was carried out using Medline, Embase, Cochrane Library, Web of Science, Cinahl and Pubmed. Database-appropriate search strategies were developed based upon terms for uterine neoplasms, IGRT, organ motion and target volume. In total, 448 studies were identified and screened to find 39 relevant studies, 12 of which were abstracts. These studies show that within the target volume for cervical cancer radiotherapy, uterine motion is greater than cervical. Uterine motion is predominantly influenced by bladder filling, cervical motion by rectal filling. Organ motion patterns are patient specific, with some having very little (5 mm) and others having much larger shifts (40 mm) of the target volume. Population-based clinical target volume (CTV)–PTV margins would be large (up to 4 cm around the uterus), resulting in unnecessary OAR inclusion within the PTV, reducing the benefits of IMRT. Potential solutions include anisotropic margins with increased margins in the anteroposterior and superoinferior directions, or greater PTV margins around the uterine fundus than the cervix. As pelvic organ motion seems to be patient specific, individualised PTV margins and adaptive IGRT strategies have also been recommended to ensure target volume coverage while increasing OAR sparing. Although these strategies are promising, they need significant validation before they can be adopted into clinical practice.

Phase I dose-escalated image-guided adaptive bladder radiotherapy study: Results of first dose cohort (68Gy).

291 Background: Prospective study assessing the safety of dose escalation in the treatment of localised muscle-invasive bladder cancer using image-guided adaptive radiotherapy (RT) with a reduced high-dose tumour volume. Methods: Radical RT was planned and delivered in 3 phases. Phase I was delivered to the empty whole bladder (10Gy 5#). Bladder variation assessed on phase I daily imaging was used to determine the adaptive approach for phase III (composite volume or plan of the day). Phase II was delivered to a tumour boost volume on a partially filled bladder (18Gy 9# if normal tissue dose-constraints were met, or to 14Gy 7# if they were not). The adaptive phase III (40Gy 20#) was delivered to the empty whole bladder. The primary endpoint was late RTOG toxicity. Results: Of the twenty-six patients recruited to 68Gy, 20 patients met the normal tissue constraints for dose escalation. For the phase III adaptive technique, 14 patients were treated with a composite volume approach and 12 with a plan of the day. At median follow-up of 24 months, 13 (65%) dose escalated patients remain alive and disease free. Within this dose cohort there has been 1 muscle invasive local recurrence, 1 superficial recurrence and 1 patient has developed metastases. 5 patients in the dose-escalated group have died; 3 from cardiac events, 1 from hospital acquired pneumonia and 1 from metastatic bladder cancer. 1 patient in the dose-escalated group has experienced grade 3 late toxicity (cystitis) occurring 24.6 months after the end of radiotherapy. Conclusions: Image-guided adaptive RT techniques with reduced high-dose volume allow tumour dose-escalation. Toxicity data to date suggest tolerability of 68Gy. Local disease control rates are promising. Trial recruitment continues at 70Gy using intensity modulated radiotherapy technique to deliver a simultaneous integrated boost. Clinical trial information: 7653.

Progress in the management of limited‐stage small cell lung cancer

Approximately 15% of lung cancer cases are of the small cell subtype, but this variant is highly aggressive and is often diagnosed at advanced stages. Outcomes after current treatment regimens have been poor, with 5-year survival rates as low as 25% for patients with limited-stage disease. Advances in therapy for small cell lung cancer have included the development of more effective chemotherapeutic agents and radiation techniques. For example, hyperfractionated radiotherapy given early in the course of the disease can reduce local recurrence and extend survival. Other technologic advances in radiation planning and delivery such as intensity-modulated radiotherapy, image-guided adaptive radiotherapy, and 4-dimensional computed tomography/positron emission tomography have facilitated the design of treatment volumes that closely conform to the shape of the tumor, which allows higher radiation doses to be given while minimizing radiation-induced toxicity to adjacent structures. Future improvements in outcomes will require clarifying the molecular basis for this disease.

Results of Image-Guided, Dose-Escalated Radiation Therapy for Localized Prostate Cancer: 10-Year Results Using an Off-Line, Adaptive Technique

Image guided radiation therapy (IGRT) using real-time imaging with on-line correction has become standard practice and is the driving force in refinement of external beam prostate radiation therapy. Technologies that facilitate this practice are new, and little long-term data is available documenting their outcomes. Since 1997, a successful form of off-line, image-guided, adaptive radiation therapy (IGART) has been used at our institution, and we report the largest series of prostate cancer patients treated with dose-escalated IGRT available to date.

Intrafraction Variability and Deformation Quantification for Accelerated Partial-Breast Irradiation (APBI)

While IGRT reduces setup error for linac-based APBI, IGRT cannot fully account for deformations arising from intrafraction motion. This study sought to evaluate intra-fraction variability and local deformations of the lumpectomy cavity (LC), breast, and nearby organs at risk. Sixteen early-stage (T0-T2N0M0) left-sided post-lumpectomy breast cancer cases and 1 bilateral case with discernible LCs were retrospectively evaluated on an IRB-approved study. During simulation, each patient underwent a free-breathing CT and 10-phase 4DCT. For each patient, treated breast, heart, lungs, and LC were delineated on the end-inhale (EI) 4DCT phase. B-spline multi-resolution deformable image registration was employed to propagate contours to all phases. For the entire population, respiration-induced motion was calculated via centroid analysis. To quantify deformation, displacement vector fields (DVFs) were analyzed between EI and end-exhale (EE) datasets for the breast and LC volumes by classifying the vector magnitudes as < 2 mm, 2-4 mm, and > 4 mm. For the patient population, LC intra-fraction motion was 0.7 ± 0.7 mm, 1.1 ± 0.7 mm, and 1.8 ± 1.2 mm in lateral (LAT), anterior-posterior (A-P), and superior-inferior (S-I) directions, respectively. Similar intra-fraction motion results were obtained for the treated breast: 0.4 ± 0.3 mm, 0.9 ± 0.5 mm, and 1.6 ± 0.9 mm LAT, A-P, and S-I directions, respectively. In the most extreme case, LC and breast excursions of ∼2 mm and ∼3 mm were observed in the A-P and S-I directions, respectively. Respiration-induced motion was dominant S-I for the heart (3.8 ± 1.7 mm) and lungs (3.6 ± 2.0 mm). Over all patients, DVF analysis revealed that 51.2 ± 20.3% (range, 9.6 - 82.6%) of the treated breast volume deformed between 2-4 mm, whereas 11.3 ± 12.7% deformed > 4 mm (range, 0 to 36.7%). The majority of LC volumes (60.7 ± 37.6%) deformed 2-4 mm. However, a small subset demonstrated > 4 mm deformation (6.8 ± 14.6%). The patient with the largest deformation (∼45% of the LC volume deformed > 4 mm) also demonstrated the largest intra-fraction cavity and breast excursions. Overall, LC and breast demonstrated < 3 mm intra-fraction centroid motion, whereas heart and lung were slightly more mobile. Deformation characterization of the LC and breast revealed large variability among patients, suggesting value in patient-specific margins for APBI. A subset of volumes showed substantial deformation (> 4 mm), which, if unaccounted for, could potentially impact dose distribution. Further work is needed to resolve dominant regions of deformation. Coupling these results with inter-fraction setup analysis will likely yield better margin design, and potentially support the need for image-guided adaptive radiation therapy for APBI.

Translationale Uroradioonkologie

Recent developments and innovations in modern radiation oncology enable radiation oncologists to deliver a biologically, physically and technically stratified individualized treatment. In urological oncology, radiotherapy can be offered as a valid treatment alternative for a number of tumors, such as prostate cancer or urothelial cancer/bladder cancer. By improving the precision of radiotherapy, significant improvement in tumor control can be achieved, coupled with a reduction of the risk of treatment-related side effects. Techniques such as high precision radiotherapy and combinations of radiation and modern imaging, such as image-guided radiotherapy (IGRT) and adaptive radiotherapy (ART), make daily individually tailored treatment possible. In the future the biological characteristics of tumors and of normal tissue as well as molecular markers can be used for treatment stratification.

Dosimetric and Radiobiological Consequences of Computed Tomography–Guided Adaptive Strategies for Intensity Modulated Radiation Therapy of the Prostate

To examine a range of scenarios for image-guided adaptive radiation therapy of prostate cancer, including different schedules for megavoltage CT imaging, patient repositioning, and dose replanning. We simulated multifraction dose distributions with deformable registration using 35 sets of megavoltage CT scans of 13 patients. We computed cumulative dose-volume histograms, from which tumor control probabilities and normal tissue complication probabilities (NTCPs) for rectum were calculated. Five-field intensity modulated radiation therapy (IMRT) with 18-MV x-rays was planned to achieve an isocentric dose of 76 Gy to the clinical target volume (CTV). The differences between D95, tumor control probability, V70Gy, and NTCP for rectum, for accumulated versus planned dose distributions, were compared for different target volume sizes, margins, and adaptive strategies. The CTV D95 for IMRT treatment plans, averaged over 13 patients, was 75.2 Gy. Using the largest CTV margins (10/7 mm), the D95 values accumulated over 35 fractions were within 2% of the planned value, regardless of the adaptive strategy used. For tighter margins (5 mm), the average D95 values dropped to approximately 73.0 Gy even with frequent repositioning, and daily replanning was necessary to correct this deficit. When personalized margins were applied to an adaptive CTV derived from the first 6 treatment fractions using the STAPLE (Simultaneous Truth and Performance Level Estimation) algorithm, target coverage could be maintained using a single replan 1 week into therapy. For all approaches, normal tissue parameters (rectum V(70Gy) and NTCP) remained within acceptable limits. The frequency of adaptive interventions depends on the size of the CTV combined with target margins used during IMRT optimization. The application of adaptive target margins (<5 mm) to an adaptive CTV determined 1 week into therapy minimizes the need for subsequent dose replanning.

Comparison of dose-escalated, image-guided radiotherapy vs. dose-escalated, high-dose-rate brachytherapy boost in a modern cohort of intermediate-risk prostate cancer patients

We compared outcomes in intermediate-risk prostate cancer patients treated with dose-escalated adaptive image-guided radiation therapy (IGRT) or dose-escalated high-dose-rate brachytherapy boost (HDR-B). Patients with intermediate-risk prostate cancer by National Comprehensive Cancer Network criteria were treated with either CT-based off-line adaptive IGRT (n = 734) or HDR-B (n = 282). IGRT was delivered with 3D-conformal or intensity-modulated radiation therapy with a median dose of 77.4 Gy. For HDR-B, the whole pelvis received a median 46 Gy, and the prostate 2 implants of 9.5 Gy (n = 71), 10.5 Gy (n = 155), or 11.5 Gy (n = 56). Median followup was 3.7 years for IGRT and 8.0 years for HDR-B (p < 0.001). Eight-year biochemical control was 86% for IGRT and 91% for HDR-B (p = 0.22), disease-free survival 67% for IGRT and 79% for HDR-B (p = 0.006), and overall survival 75% for IGRT and 86% for HDR-B (p = 0.009). Cause-specific survival (8-year, 100% vs. 99%), freedom from distant metastases (98% vs. 97%), and freedom from local recurrence (98% vs. 98%) did not differ (p > 0.50 each). A worse prognosis group was defined by percent positive prostate biopsy cores >50%, perineural invasion, or stage T2b-c, encompassing 260 (35%) IGRT and 171 (61%) HDR-B patients. These patients evidenced a 5-year biochemical control of 96% for HDR-B and 87% for IGRT (p = 0.002). Dose-escalated IGRT and HDR-B both yield excellent clinical outcomes for patients with intermediate-risk prostate cancer. Improved biochemical control with HDR-B for patients with worse pretreatment characteristics suggests that a subgroup of intermediate-risk prostate cancer patients may benefit from dual-modality treatment.

Experience with adaptive image-guided radiotherapy (IGRT) at RGCI and RC, India

Experience with adaptive image-guided radiotherapy (IGRT) at RGCI and RC, India

SU-E-J-102: Evaluation of a Free-Form Intensity Based Deformable Contouring Method for Prostate Image-Guided Adaptive Radiation Therapy (IGART): Multiple Observer Comparison

Purpose: To evaluate a deformable contouring method for prostate CBCT as part of an automated image‐guided adaptive radiation therapy (IGART) workflow using manual contours from multiple observers for comparison. Methods: Three CBCTs were randomly selected from 6 patients with prostate cancer. Prostate, bladder, and rectum contours were manually defined on the first CBCT by a single observer and on the second and third CBCTs by 3 observers. A commercial free‐form intensity based deformable contouring method was used to deform contours from the first CBCT to the second and third CBCTs. The Dice coefficients and the absolute differences in measured daily delivered dose values were calculated among the deformed and manual contours. Two‐sided t‐tests were performed to determine whether the automated deformable method has similar variability to manual contours from multiple observers. Results: The mean(SD) Dice coefficients calculated between the deformed contours and each of the manual contours were 0.72(0.10), 0.75(0.13), 0.64(0.10) for the prostate, bladder, and rectum, respectively, and were 0.76(0.08), 0.87(0.05), 0.68(0.10), respectively, averaged between multiple observer comparisons. These differences were not significant for the rectum (p=0.095) but were significant for the prostate (p=0.025) and bladder (p 0.05). The difference was significant for the bladder (p <0.05). Conclusion: A free‐form intensity based deformable contouring method for IGART was shown to have similar variability to that of multiple observers both for delivered dose evaluation of the prostate and rectum as well as contour similarity of the rectum while a significant difference existed for bladder dose evaluation. This method has the potential to show deviations from planned treatment as part of an automated prostate IGART workflow. J Piper, D Nelson, and A Nelson are owners of MIM Software Inc. Z Shen and S Pirozzi are employees of MIM Software Inc.