Daily Cone-beam CT Images Research Articles

Organ sample generator for expected treatment dose construction and adaptive inverse planning optimization

To create an organ sample generator (OSG) for expected treatment dose construction and adaptive inverse planning optimization. The OSG generates random samples of organs of interest from a distribution obeying the patient specific organ variation probability density function (PDF) during the course of adaptive radiotherapy. Principle component analysis (PCA) and a time-varying least-squares regression (LSR) method were used on patient specific geometric variations of organs of interest manifested on multiple daily volumetric images obtained during the treatment course. The construction of the OSG includes the determination of eigenvectors of the organ variation using PCA, and the determination of the corresponding coefficients using time-varying LSR. The coefficients can be either random variables or random functions of the elapsed treatment days depending on the characteristics of organ variation as a stationary or a nonstationary random process. The LSR method with time-varying weighting parameters was applied to the precollected daily volumetric images to determine the function form of the coefficients. Eleven h&n cancer patients with 30 daily cone beam CT images each were included in the evaluation of the OSG. The evaluation was performed using a total of 18 organs of interest, including 15 organs at risk and 3 targets. Geometric variations of organs of interest during h&n cancer radiotherapy can be represented using the first 3 ∼ 4 eigenvectors. These eigenvectors were variable during treatment, and need to be updated using new daily images obtained during the treatment course. The OSG generates random samples of organs of interest from the estimated organ variation PDF of the individual. The accuracy of the estimated PDF can be improved recursively using extra daily image feedback during the treatment course. The average deviations in the estimation of the mean and standard deviation of the organ variation PDF for h&n cancer radiotherapy were less than 2 and 1 mm, respectively, for most organs after the second week of treatment. After the first three weeks of treatment, the mean discrepancy of the dose estimation accuracy was within 1% for most of organs, the corresponding standard deviation was within 2.5% for parotids, the brain stem and the cochleae, and within 1% for other organs. A patient specific OSG is feasible and can be used to generate random samples of organs of interest for the expected treatment dose construction and adaptive inverse planning. The accuracy of the OSG can be improved continuously and recursively during the adaptive treatment course using daily volumetric image feedback.

Daily Cone Beam CT Image Guidance for Oncoplastic BCT IMRT Treatments — Should It Be a Routine?

Daily Cone Beam CT Image Guidance for Oncoplastic BCT IMRT Treatments — Should It Be a Routine?

Residual rotational set-up errors after daily cone-beam CT image guided radiotherapy of locally advanced cervical cancer

PurposeDue to the often quite extended treatment fields in cervical cancer radiotherapy, uncorrected rotational set-up errors result in a potential risk of target miss. This study reports on the residual rotational set-up error after using daily cone beam computed tomography (CBCT) to position cervical cancer patients for radiotherapy treatment. Methods and materialsTwenty-five patients with locally advanced cervical cancer had daily CBCT scans (650 CBCTs in total) prior to treatment delivery. We retrospectively analyzed the translational shifts made in the clinic prior to each treatment fraction as well as the residual rotational errors remaining after translational correction. ResultsThe CBCT-guided couch movement resulted in a mean translational 3D vector correction of 7.4mm. Residual rotational error resulted in a target shift exceeding 5mm in 57 of the 650 treatment fractions. Three patients alone accounted for 30 of these fractions. Nine patients had no shifts exceeding 5mm and 13 patients had 5 or less treatment fractions with such shifts. ConclusionTwenty-two of the 25 patients have none or few treatment fractions with target shifts larger than 5mm due to residual rotational error. However, three patients display a significant number of shifts suggesting a more systematic set-up error.

WE-E-213CD-05: A Non-Rigid Image Registration Algorithm That Accommodates Organ Segmentation Error.

To introduce a new deformable image registration algorithm based on surface matching that accommodates organ delineation error in daily Cone-beam CT images based on a priori knowledge of inter-observer segmentation uncertainty. The dataset includes four prostate cancer patients who underwent primary external beam radiotherapy and had tumors that were confined to the prostate. All imaging was performed without intravenous contrast. Organ surface segmentation errors in a multiple observer-contouring study on the pelvic organs in Fan-beam CT (FBCT) and Cone-beam CT (CBCT) were estimated from the training dataset. A novel deformable image registration algorithm is presented where the organ surface matching is penalized by this error. Portions of the organ surface that are delineated reliably are used to guide the registration whereas the portions that are highly uncertain are ignored. This approach reduces the impact of delineation errors in CBCT. An evaluation experiment compares three algorithms, namely intensity-only registration (INT), equally-weighted surface and image registration (EWSIR) and the proposed uncertainty- weighted surface and image registration. The surface dissimilarity was reduced from 0.172 to 0.134, 0.043 and 0.044 respectively after registration. The Jacobian of the transformation found by the proposed method was closer to one than that of EWSIR in the prostate. In prostate external-beam radiotherapy, slice-by-slice 2D manual contouring has variable spatial accuracy. For deformable image registration methods that match segmented surfaces, regions of high inaccuracy can misguide the registration. In contrast to the image registration methods where the FBCT and CBCT surfaces (or other features) are assumed to be exact, our method takes this uncertainty into account. Preliminary results show an improved registration performance suggesting a potential use in IGRT. This work was supported by National Cancer Institute Grant No. P01 CA 116602.

Evaluation of adaptive radiotherapy of bladder cancer by image-based tumour control probability modelling

Clinical implementation of adaptive radiotherapy strategies could benefit from extended tools for plan evaluation and selection. For this purpose we investigated the feasibility of image-based tumour control probability (TCP) modelling using the bladder as example of a tumour site with potential benefit from adaptive strategies. Material and methods. Two bladder cancer patients that underwent planning CT and daily cone beam CT (CBCT) imaging during the treatment course were included. The bladder was outlined in every image series. Following a previously published procedure, various adaptive planning target volumes (PTVs) were generated from the inter-fractional bladder variation observed during the first four CBCT sessions. Intensity modulated treatment plans delivering 60 Gy to a given PTV were generated. In addition, simultaneous integrated boost (SIB) plans giving a 10 Gy boost to the tumour were created. Using the daily CBCT images and polynomial warping, the dose in each bladder volume element was tracked fraction by fraction. TCP calculations employing the tracked accumulated dose distributions, together with radiosensitivity parameters estimated from published data on local control of bladder cancer were performed. The dependence of TCP on the simulated clonogenic cell distribution was also explored. Results. For a uniform clonogenic cell density in the whole bladder, TCP varied between 53% and 58% for the 60 Gy plans, while it was between 51% and 64% for the SIB plans. The lowest values were found when using the smallest PTVs, as they did not geometrically enclose the clinical target volume in all fractions. When increasing the clonogenic cell density in the tumour relative to that in the remaining bladder, the TCP saturated at approximately 75% for the SIB plans. Conclusion. Dose tracking and TCP calculation provided additional information to standard criteria such as geometrical coverage for the selected cases. TCP modelling may be a useful tool in plan evaluation and for selection between multiple plans.

WE-C-BRA-01: Evaluation of Inter-Fraction Setup Uncertainty of the Cervical Spine Using a Novel Relocatable Head Frame Intended for Intra-Cranial Stereotactic Radiotherapy

Purpose: To quantify inter-fraction variation in upper-spine (C1–C3) positioning when using a novel vacuum bite-block relocatable head-frame (RHF), that was originally designed for use in intra-cranial stereotactic radiotherapy (SRT). Methods and Materials: The RHF (Extend™) was designed for multi-session treatments on Perfexion™. As part of an ethics-approved study, the RHF was used to set up and immobilize patients undergoing linac-based intra-cranial SRT. For each fraction, cone-beam CT (CBCT) was used to guide the skull to the treatment room isocentre based on anatomical 3D image-matching. In this study, the same CBCT data were analyzed to quantify the relative displacement of the upper-spine to an ideally well-aligned skull as follows. For each patient, each daily CBCT image set was first co-registered to the planning CT images based on bony anatomy in the skull. From this reference position, the mean difference and standard deviation between planning and daily treatment was determined for each of the geometrical centroid of the C2 process, as well as the spinal canal at level of C3. Results: Data from three patients were analyzed (total of 80 fractions). The mean relative displacement of the C2 process after skull alignment was 0.9±0.78 (range: −1.84–2.7) and 1.2±0.57 (range: −1.15–3.2) mm in the lateral and anterior-posterior directions, respectively. The mean displacement at the level of C3 was 0.35±2.04 (range: −4.1–6.5) and 1.59±1.2 (range: −4.2–4.6) mm in the lateral and anterior-posterior directions, respectively. Conclusion:_Relative to an ideal skull-based alignment as determined with 3D-CBCT, systematic differences in upper-spine position were observed, as indicated by the non-zero mean deviations. Furthermore, a larger variability in daily positioning of C3 compared to C1/C2 was observed, as indicated by the larger standard deviations. These results indicate that applying translational corrections intended for skull localization may be insufficient for localization of the upper-spine when using the RHF.

On-Line Adaptive Radiation Therapy: Feasibility and Clinical Study

The purpose of this paper is to evaluate the feasibility and clinical dosimetric benefit of an on-line, that is, with the patient in the treatment position, Adaptive Radiation Therapy (ART) system for prostate cancer treatment based on daily cone-beam CT imaging and fast volumetric reoptimization of treatment plans. A fast intensity-modulated radiotherapy (IMRT) plan reoptimization algorithm is implemented and evaluated with clinical cases. The quality of these adapted plans is compared to the corresponding new plans generated by an experienced planner using a commercial treatment planning system and also evaluated by an in-house developed tool estimating achievable dose-volume histograms (DVHs) based on a database of existing treatment plans. In addition, a clinical implementation scheme for ART is designed and evaluated using clinical cases for its dosimetric qualities and efficiency.

SU‐FF‐T‐490: The Effect of Daily Cone Beam CT Imaging Dose On the Secondary Cancer Risk for Patients Receiving Prostate IMRT Treatments

Purpose: The impact of daily kilovoltage cone beam CT (CBCT) imaging dose on the radiation induced cancer risk (RICR) was evaluated for patients receiving prostate IMRT treatments. Materials and Methods: Increased secondary cancer risk was modeled by employing effective dose concept described in ICRP report 103 as basis. A prostate IMRT plan was generated in the treatment planning software to calculate dose to organs within the treatment volume. Scattered and leakage dose to organs outside the treatment volume, and CBCT imaging dose were obtained from data in published literature. Both linear and nonlinear risk‐dose response models were considered to establish a relationship between RICR and absorbed dose in the therapeutic dose range. ICRP 103 tissue weighting factors were assumed to stay the constant as a function of absorbed dose. Results: We calculated the relative increase in RICR for various RICR‐dose response models. If the secondary cancer risk was assumed to be proportional to absorbed dose, the increase in RICR due to daily CBCT imaging was in the order of 5%. If the secondary cancer risk was assumed to fall off exponentially at therapeutic doses (beyond 4 Gy), RICR increased by 21%. If RICR was assumed to be constant at therapeutic doses, the increase in RICR was 13%. Finally, if only out‐of‐treatment field organs are considered for risk estimations, RICR increased only 2%. Conclusion: Based on the risk‐dose response models that we evaluated, increased secondary cancer risk due to daily CBCT imaging during prostate IMRT treatments was in the 2% to 21% range. Although such models were known to contain inherently large uncertainties, our study might be a plausible starting point to evaluate the effect of CBCT imaging dose on the secondary cancer risk for radiation therapy patients.

SU‐FF‐J‐23: Treatment Margins for Pancreatic Cancer Based On Daily IGRT with Cone Beam CT (CBCT)

PURPOSE: To evaluate interfraction pancreatic motion as evidence by daily image guided radiotherapy (IGRT) using daily CBCT. MATERIALS AND METHODS: Daily kilovoltage CBCT was used during the definitive treatment of seven patients. Daily CBCT images were registered to planning CT with automatic software tools that put higher importance on boney anatomy as proxy for pancreatic motion. Validation scans were employed when preset constraints were not met. A single treating physician reviewed and approved each fusion. Data regarding interfraction deviation from these daily shifts are analyzed for translation and rotation in the anterior‐posterior (AP), right‐left (RL), and superior‐inferior (SI) directions. RESULTS: One hundred and eighty registration scans were utilized and revealed average shifts of 0.24, 0.15, and .01 cm in the SI, AP, and RL directions, respectively with standard deviations of 0.42, 0.27, and 0.45 cm. Using this data in the Van‐Herk model suggests treatment (PTV) margins of 0.39 cm in RL, 0.91 cm in SI, and 0.58 cm in AP direction to ensure that 90% of all of the clinical target volumes (CTV) receive 95% of the prescribed dose. The average vector displacement was 0.61 cm with a standard deviation of 0.41 cm. The maximum observed vector shift was 1.95 cm in the SI direction. The registration yielded an average rotation of 0.97°, −.06°, and −.20° in the coronal, sagittal, and axial planes respectively with standard deviations of 0.8°, 1.25°, and 1.1°. The maximum rotation noted in any direction was 4.6°. CONCLUSIONS: CBCT is an important tool to monitor interfraction pancreatic motion. Bony fusion as proxy provides some confidence and may translate into smaller margins. However, further assessment with soft tissue fusion and the use of internal fiducials are needed to help provide further confidence.

A cone beam CT-guided online plan modification technique to correct interfractional anatomic changes for prostate cancer IMRT treatment

The purpose of this work is to develop an online plan modification technique to compensate for the interfractional anatomic changes for prostate cancer intensity-modulated radiation therapy (IMRT) treatment based on daily cone beam CT (CBCT) images. In this proposed technique, pre-treatment CBCT images are acquired after the patient is set up on the treatment couch using an in-room laser with the guidance of the setup skin marks. Instead of moving the couch to rigidly align the target or re-planning using the CBCT images, we modify the original IMRT plan to account for the interfractional target motion and deformation based on the daily CBCT image feedback. The multileaf collimator (MLC) leaf positions for each subfield are automatically adjusted in the proposed algorithm based on the position and shape changes of target projection in the beam's eye view (BEV). Three typical prostate cases were adopted to evaluate the proposed technique, and the results were compared with those obtained with bony-structure-based rigid translation correction, prostate-based correction and CBCT-based re-planning strategies. The study revealed that the proposed modification technique is superior to the bony-structure-based and prostate-based correction techniques, especially when interfractional target deformation exists. Its dosimetric performance is closer to that of the re-planned strategy, but with much higher efficiency, indicating that the introduced online CBCT-guided plan modification technique may be an efficient and practical method to compensate for the interfractional target position and shape changes for prostate IMRT.

Is Daily Cone-beam CT Image Guidance Required to Correct Setup Error in Conventionally Fractionated Lung Radiotherapy?

Is Daily Cone-beam CT Image Guidance Required to Correct Setup Error in Conventionally Fractionated Lung Radiotherapy?

SU‐GG‐J‐146: Piecewise Rigid‐Body Image Registration for Adaptive Radiation Therapy

Purpose: To accelerate image deformation and registration for Adaptive Radiation Therapy (ART), we propose a piecewise rigid‐body image registration (PRIR). This algorithm is faster than full deformable registration by up to a factor of ten without sacrificing accuracy. Method and Materials: Deformable Image Registration (DIR) may not reflect the biomechanical characteristics of the anatomy. In short, DIR may try to deform the body in ways that are not physically possible. Fluid dynamic models were adopted to address this problem. However, DIR consumes too much time to be utilized for routine clinical ART. PRIR considers the biomechanical characteristics of the anatomy while retaining the speed of rigid‐body transformations. First, we import contours from a Radiation Therapy Plan (RTP) through DICOM‐RT. We assume that the contour delineates an organ (or tumor), which is modeled as an independent rigid‐body. Two image sets (planning CT and Conebeam CT) were aligned with our rigid‐body registration using Spatial Weighted Mutual Information (SWMI). Then, rigid‐body registration was performed with each organ independently from the rest of image using Mutual Information or SWMI. Users can select a specific type of registration: full translation and rotation, 3‐D scaling, or 2‐D scaling by considering biomechanical characteristics. One prostate case (9 image sets) and one head‐and‐neck case (7 image sets) were tested. Results and Discussion: Our image registration for 6 contoured organs took 95seconds on average. We succeeded in transforming deformable contours from RTP images to daily cone beam CT images. Our method can be utilized to re‐optimize plans for Adaptive Radiation Therapy. The speed can be further improved with parallel processing. Accelerating deformable registrations is critical for implementing real‐time ART. (This work is partly supported by Susan G. Komen Breast Foundation Grant: BTCR126506)

SU‐GG‐I‐102: Evaluation of Nine Similarity Metrics for Intensity‐Based CT‐To‐CBCT Prostate Registrations

Purpose: The prostate can be localized automatically on daily cone‐beam CT (CBCT) images by means of intensity‐based 3D/3D image registrations. The goal of this study is to evaluate nine different similarity metrics for the best localization accuracy. Method and Materials: Seven prostate patients were retrospectively selected with natural prostate calcifications, which resulted in a dataset of 220 CBCT and 7 simulation CT (SimCT) images. On each SimCT image, a region of interest (ROI) was defined by expanding the treatment planning prostate structure by 6 mm and subtracting the rectum with 10 mm expansion. The expansions were chosen empirically. Then, all CBCT images were registered to the corresponding SimCT images by maximizing the similarity within the defined ROI. Downhill simplex method was employed for optimization with full 6D rigid‐body transformation. The pixel values corresponding to bone and air were also excluded from similarity calculations. After each registration, the calcification mismatch was recorded as a measure of prostate localization error. The registrations were repeated for each similarity metric. The nine similarity metrics investigated were: normalized cross‐correlation (NCC), entropy of difference (EOD), mutual information (MI), correlation ratio (CR), sum of absolute difference (SAD), sum of squared difference (SSD), gradient correlation (GC), gradient difference (GD), and pattern intensity (PI). Results: The absolute error means (±standard deviations) were 2.1(±1.6) mm for GD and GC, 2.6(±2.8) mm for PI, 3.9(±4.3) mm for NCC, and 6.4 (±8.0) mm for MI. The others showed over 10 mm mean errors. The directional errors of GD were −0.1(±0.5) mm, 0.1(±1.4) mm, and −0.2(±2.1) mm in the left‐right, anterior‐posterior, and superior/inferior directions respectively. Conclusion: Among the investigated similarity metrics, registrations using GD and GC showed the most accurate prostate localizations on daily CBCT images.

Improving image-guided target localization through deformable registration

Purpose. To quantify the improvements in online target localization using kV cone beam CT (CBCT) with deformable registration. Methods and material. Twelve patients treated under a 6 fraction liver cancer radiation therapy protocol were imaged in breath hold using kV CBCT at each treatment fraction. The images were imported into the treatment planning software and rigidly registered by fitting the liver, identified on the daily kV CBCT image, into the liver contours, previously drawn on the planning CT. The liver was then manually contoured on each CBCT image. Deformable registration was automatically performed, aligning the CT liver to the liver on each CBCT image using MORFEUS, a biomechanical model based deformable registration algorithm. The tumor, defined on planning CT, was mapped onto the CBCT, through MORFEUS. The center of mass (COM) displacement of the tumor was computed. Results. The mean (SD) displacement magnitude (absolute value) of the COM following deformable registration was 0.08 (0.07), 0.10 (0.11), and 0.10 (0.17) cm in the left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions, respectively. The maximum displacement of the COM was 0.34, 0.65, and 0.97 cm in the LR, AP, and SI directions, respectively. Fifteen percent of the treatment fractions had a COM displacement of greater than 0.3 cm and 33% of patients had at least 1 fraction with a displacement of greater than 0.3 cm. The deformable registration, excluding the manual contouring of the liver, was performed in less than 1 minute, on average. Discussion. Rigid registration of the liver volume between planning CT and verification kV CBCT localizes the tumor to within 0.3 cm for the majority (66%) of patients; however, larger offsets in tumor position can be observed due to liver deformation.

Evaluation of a Motion-Encompassing Treatment Strategy for Treating Non-Small Cell Lung Cancer

This study analyzes the setup error of a motion-encompassing treatment strategy for treating non-small cell lung cancer. Using data from patients who were treated with daily KV cone-beam CT (CBCT) image-guidance (IG) for setup correction, the residual setup errors for a hybrid off-line IG protocol were assessed. 22 patients with NSCLC were treated with a motion-encompassing daily CBCT IG strategy during 4/2006 to 3/2007. 4D-CT imaging was used for treatment planning. Patients were scanned in the supine position during normal relaxed free-breathing and were immobilized with a wing-board. The internal target volume (ITV) was determined using the maximum-intensity-projection (MiP) CT data to account for organ motion. A 5-mm PTV margin was used to account for setup error. On-line setup correction was based on the daily CBCT imaging prior to treatment, with an action level of 3 mm. The systematic and random errors were analyzed retrospectively. The results were then fit to a hybrid off-line IG strategy where the patient underwent CBCT-IG for the first 4 fractions. The treatment setup was corrected on-line if the setup error exceeded the action level of 3 mm. A mean shift was then applied on the 5th and the subsequent fractions to correct for the systematic error. The residual errors of the hybrid off-line protocols were assessed. The systematic and random errors were found to be 5.2 ± 3.2 mm, 4.4 ± 2.7 mm and 4.4 ± 3.4 mm for anterior-posterior (AP), medial-lateral (ML) and superior-inferior (SI) directions respectively. The 3D displacement vectors (DV) were 9.9 mm ± 4.7 mm (systematic ± random). The residual error was within 2 mm in all directions for the daily IG strategy. For the hybrid off-line IG strategy, the frequency of the residual errors is shown in the figure. The occurrence frequency was 61% for the 5-mm DV, indicating that the hybrid off-line IG strategy was inadequate. The use of a 5 mm PTV margin on the ITV determined by MiP dataset combined with daily CBCT on-line correction is adequate to avoid geometric miss of the tumor for NSCLC patients. However, the hybrid off-line IG strategy is inadequate unless a larger PTV margin is used.

SU‐FF‐J‐06: Use of Daily KV Cone‐Beam CT Data to Analyze the Effectiveness of a Weekly Correction Strategy for Lung Cancer Radiotherapy

Purpose: A common practice in lung radiotherapy is the correction of setup errors based on the initial and weekly 2D portal images. We have evaluated the effectiveness of this correction strategy using data collected from daily kV cone‐beam CT (CBCT) imaging. Material and Method: Eighteen lung cancer patients immobilized with a wing board were treated on an Elekta Synergy LINAC using our daily on‐line IGRT protocol. Daily CBCT imaging was performed and setup variations were corrected before treatment. A total of 282 CBCT images were collected and the daily setup errors along the anterior‐posterior, superior‐inferior and right‐left axes were calculated using volumetric image registration. With this data we simulated the weekly correction strategy where patient setup error is adjusted once every five fractions based on weekly 2D portal imaging after the initial setup. 3D shift vectors for the whole treatment course were calculated. The frequencies with which the 3D shifts exceeded 5 mm, 10 mm, 15 mm and 20 mm were used to evaluate the effectiveness of this correction strategy. Results: For these eighteen patients, the setup errors along right‐left, superior‐inferior and anterior‐posterior axes range from −1.17 to 1.34 cm, −1.29 to 1.30 cm and −1.71 to 1.21 cm respectively. The maximum value of the 3D shift vectors was 1.78 cm. Out of a total of 282 treatments, the 3D shift vectors were within 5 mm for 144 fractions (51.1%), within 10 mm for 250 fractions (88.7%), and within 15 mm for 278 fractions (98.6%). Four fractions had errors greater than 15 mm. Conclusion: Daily CBCT data can be used to evaluate different correction strategies. For the lung patients, the setup errors can be large along R‐L, S‐I and A‐P directions. A large margin is warranted to ensure adequate tumor coverage if weekly correction scheme is used.

1023: Automatic and Accurate Prostate Localization on Daily CBCT Images Using Intensity-Based 3D/3D Image Registration

To develop a fully automatic and accurate prostate localization method on daily cone-beam CT (CBCT) images. The localization is achieved by an intensity-based 3D/3D image registration method, which maximizes the similarity between CBCT and simulation CT images (SimCT).

SU‐FF‐I‐93: Comparison of Seven Similarity Measures for Intensity‐Based 3D/3D Image Registrations On CBCT

Purpose: The goal of this study is to evaluate seven similarity measure functions for an intensity‐based rigid‐body 3D/3D image registration. The registration method was specifically designed to localize prostates on cone‐beam CT (CBCT) images, acquired for the use of daily patient setup, accomplished by maximizing the similarity from the corresponding simulation CT in an iterative fashion. Methods and Materials: Registrations were conducted with seven different similarity measure functions (NCC‐normalized cross‐correlation, EOD‐entropy of difference, MI‐mutual information, CR‐correlation ratio, GC‐gradient correlation, GD‐gradient difference, and PI‐pattern intensity), three different transformations (translation only, translation plus couch rotation, and translation plus three rotations), and two preprocessing methods (window‐leveling and histogram equalization). For accuracy and robustness estimation, prostate calcifications were identified on the images from four prostate patients (4 simulation CTs + 27 CBCTs), and the average calcification distances (errors) were measured after registrations. Each registration was marked as “converged” if it had less than 5 mm error, and the average distance was used as an accuracy index. Results: In general the histogram equalization and higher degree of transformation freedom improved the accuracy. The only exception was NCC, which performed better without histogram equalization. In the order of better performance, we found the convergence rate and accuracy were (96%, 1.7±1.0mm) for GC, (96%, 1.8±1.0mm) for GD, (96%, 1.8±1.0mm) for PI, (93%, 2.0±1.0mm) for NCC, (81%, 2.9mm) for CR, (78%,2.5mm) for MI, (44%, 2.1mm) for EOD. Conclusions: Among the seven similarity measure functions, registrations utilizing GC, GD, and PI showed the least prostate localization error on daily CBCT images. Such 3D‐3D image registration will be critical in order to make the future use of CBCT more objective, efficient, and accurate.