Bony Registration Research Articles

A comparison of rigid registration methods for prostate localization on CBCT and the dependence on rectum distension

We evaluated automatic three-dimensional intensity-based rigid registration (RR) methods for prostate localization on CBCT scans and studied the impact of rectum distension on registration quality. 106 CBCT scans of 9 prostate patients were used. Each one was registered to the planning computed tomography (CT) scan using different methods: (a) global registration, (b) pelvis bony structure registration, (c) bony registration refined by a local prostate registration using the CT clinical target volume (CTV) expanded with 1, 3, 5, 8, 10, 12, 15 or 20-mm margin. Automatic CBCT contours were generated after propagation of the manual CT contours. To evaluate results, a radiation oncologist was asked to manually delineate the CTV on the CBCT scans (gold standard). The Dice similarity coefficients between propagated and manual CBCT contours were calculated.

Image guided radiation therapy for bladder cancer: Assessment of bladder motion using implanted fiducial markers

PurposeTo determine bladder wall position variability during external beam radiation therapy (EBRT) for bladder cancer with intravesical fiducial markers using 2-dimensional (2D) and volumetric (3D) imaging registration. Methods and MaterialsTwenty T2-4aN0-1 bladder cancer patients underwent definitive EBRT with concurrent chemotherapy between May 2001 and January 2012, and had intravesical fiducial marker placement before simulation. Computed tomographic (CT)-based treatment planning was used for an initial phase to deliver 45 Gy (1.8 Gy/fraction) to the bladder and pelvic lymphatics followed by a boost to the involved bladder wall for an additional 21.6 Gy (1.8 Gy/fraction). Orthogonal kilovoltage radiograph images (34-37 images/patient) were obtained daily, registered with digitally reconstructed radiographs from the planning scan. Translational corrections were made daily. A kilovoltage cone-beam CT (kVCBCT) was acquired weekly and its registration with the planning scan was compared with that day's 2D registration results. ResultsOf 739 treatments, 6% resulted in ≥15 mm displacement in 1 or more directions and 26% resulted in ≥10 mm displacement in 1 or more directions. Based on 2D registrations, the average millimeter difference between bony registration and fiducial marker registration (BR-FMR) in the right-left (RL) (R+), anterior-posterior (AP) (A+), and superior-inferior (SI) (I+) directions were: 0.5 ± 1.0 (range, −2.0 to +3.8), 1.7 ± 4.4 (range, −8.1 to +13.5) and −3.7 ± 5.8 (range, −16.8 to +8.3), respectively. For kVCBCT registrations, the average mm difference in the RL, AP, and SI directions were 0.3 ± 2.1 (range, −2.4 to +5.1), 3.1 ± 5.9 (range, −2.9 to +13.3) and −4.8 ± 8.0 (range, −16.4 to +9.5), respectively. ConclusionsUsing intravesical fiducial markers, the largest difference in bladder motion based BR-FMR differences was in the superior-inferior direction. Because fiducial markers are target surrogates, setup using bony anatomy alone can lead to target displacements up to 13.5 mm anteriorly and 16.8 mm superiorly. This confirms a 1.5-1.7 cm minimum in planning target margins. These findings suggest a significant advantage in using intravesical fiducial markers to determine daily translational corrections.

Inter-tester reproducibility of tumour change in small cell lung cancer patients undergoing chemoradiotherapy

Background. Tumour volume change during delivery of chemoradiotherapy is observed in small cell lung cancer (SCLC) patients. In this study, we have compared tumour volume and anatomical changes, e.g. atelectasis or pleural effusions determined by three different methods. Method. A total of 37 SCLC patients undergoing thoracic radiotherapy during 2010–2011 were included. The patients were treated based on a daily three-dimensional (3D) cone beam computed tomography (CBCT) bony anatomy registration. The CBCT scans were retrospectively reviewed visually by a radiation therapist (Visual-RTT) in order to register tumour volume changes. Furthermore, the tumour volume changes were obtained by either deformable image registration (DIR) or delineation by a radiation oncologist (RO). Kappa (κ) statistics and paired t-tests were used for evaluation of the inter-tester agreement. Results. The tumour volume change between the Visual-RTT, the DIR and the RO assessments obtained 84–97% agreement (κ = 0.68–0.95). Furthermore, there was no statistically significant difference between the tumour change assessment of the RO (mean 13.6 ml) and the DIR (mean 14.5 ml), p = 0.59. Tumour shrinkage was observed in 15 (41%) patients and anatomical changes in seven (19%) patients. Conclusion. The inter-tester reproducibility of tumour volume change between the three methods is excellent. Visual-RTT on-line inspection may be used to determine tumour shrinkage and anatomical changes as atelectasis or pleural effusions during the radiotherapy course by use of daily CBCT scans.

SU‐E‐T‐593: Comparison of Three IMRT Adaptive Methods: The Case of Prostate Cancer

Purpose: Independant movements of prostate and pelvic lymph nodes during IMRT can limit margin reduction and affect the protection of organs‐at‐risk (OAR). In this study we perform an analysis of three adaptive treatments that combine information from both bony and gold marker registrations. The efficiency of those treatments against interfraction prostate movements was evaluated. Methods: A retrospective study was conducted on four prostate cancer patients with 5 to 10 daily CBCTs. Clinical target volumes (CTVs) consisting of pelvic lymph nodes, prostate and seminal vesicles (SV) and OAR were delineated on each CBCT and on the initial CT. Three adaptive methods were analyzed. Two methods relied on a double patient positioning at each fraction. For these, nodal CTVs prescription was delivered on bony registration. Gold markers match was then used either to: (1) complete the dose delivered to prostate and SV (Complement); (2) give almost the entire prescription to prostate and SV with slow gradient between targets to compensate for motions (Controlled Gradient). The third method (COR) used a pool of pre‐calculated plans from anterior fractions where the plan with the prostate center of mass closest to the daily anatomy was chosen. Adaptive techniques were compared to the standard non‐corrected CT plan. Results: With adaptive techniques, prostate and SV V(100%) is improved by 4% compared to non‐corrective method. This improvement does not Result in dose increase to OAR. EUD to bladder is reduced with an average reduction of 15% for COR. Conclusion: Because of their increase in CTV coverage and OAR sparing, COR and Complement methods may be good candidates for simple and effective adaptive treatment strategies for prostate cancer. Further improvements may be obtained by tuning CTV margins. A cumulative dose to simulate a complete treatment will show real effects and allow better comparison between each method. Ministere de la Sante et des Services Sociaux

SU‐E‐J‐27: Residual Interfraction Errors for Prostate SBRT with 6 Dimensional Couch Shifts Using Stereoscopic X‐Ray and Cone Beam CT

Purpose: To evaluate residual interfraction alignment errors following prostate SBRT patient setup with stereoscopic kV‐X‐ray (Exactrac), kV‐CBCT, and a 6‐D robotic treatment couch (RTC). Methods: Data was reviewed from seven prostate cancer patients undergoing five‐fraction SBRT with pre‐and daily‐treatment endorectal balloon placement. Setup protocol, using stereoscopic X‐ray images, consisted of an initial RTC displacement based on bony registration (3 shifts, 3 rotations) followed by a RTC displacement based on implanted gold seeds (3 shifts only). Final image verification of the prostate position was performed with kV‐CBCT, upon which additional shifts prior to treatment delivery were permitted. The prostate (CTV), rectum wall and bladder were delineated on each daily CBCT and planning CT by the same physician. Shifts recorded at the time of treatment (X‐ray and X‐ray plus CBCT) were applied. Planning CT and CBCT contours were then superimposed and evaluated with dice similarity coefficient (DSC). Daily CTVs were also compared with the PTV (simulation CTV + 3mm). Paired student t test was used for statistical comparison. ResultsThe average of 96 stereoscopic X‐ray shifts were 0.96±4.08cm, 0.72±3.42cm and 0.58±3.64cm in the lateral, vertical and longitudinal directions, and 0.78±2.42°, 0.55±1.60° and 0.21±1.09° for the pitch, roll and yaw angles, respectively. CBCT‐guided shifts were performed in 25.7% of the fractions, with an average of 0.01±0.03cm, 0.23±0.21cm, and 0.14±0.26cm in the lateral, vertical, and longitudinal directions, respectively. With X‐ray only and X‐ray plus CBCT shifts, 95.1±3.7% and 96.2±3.2% (p=0.01) of daily CTVs were covered by PTV, respectively. The DSC were 77.1±5.8% and 79.2±4.6% for CTV (p<0.01), 40.1±12.8% and 42.1±12.7% for rectal wall (p=0.03), and 65.2±17.5% and 64.5±18.0% for bladder (p=0.34), respectively. Conclusion Interfraction error following Exactrac‐guided RTC shifts was <3mm, resulting in protocol‐appropriate CTV coverage at treatment based on a 3mm PTV margin. Further CBCT‐based shifts improve prostate alignment, but not significantly.

Interfractional Rectal Motion During External Beam Radiation Therapy for Patients With Rectal Cancer

PurposeDuring external beam radiation therapy (EBRT), rectum position and shape undergoes daily change due to patient bladder and rectal filling. Accommodating these changes is a contributing factor for the planning margins applied for rectal cancer EBRT treatment. We aim to find the magnitude of these changes and design treatment strategies to accommodate these changes.Methods and MaterialsNine patients who underwent treatment with IMRT were selected for this retrospective study. KV CBCT was acquired on every fraction of the treatment. The rectum, bladder, and anus were contoured on one fraction of each week of treatment. The CBCTs were rigidly registered through bony registration and the contours were transferred to the planning CT. The rectum contours on the planning CT were expanded uniformly and the expansions needed to cover all the transferred rectum contours were obtained.ResultsForty-five CBCTs were used for this study. For 5 patients, an expansion of 1 cm of the simulation day rectum completely covers the treatment day rectum for all 5 fractions. The maximum expansion needed for all the patients is 3.25 cm. An expansion of 1 cm, 1.25 cm and 1.5 cm covers 71% (32 of 45), 89% (40 of 45) and 91% (41 of 45) of the treatment fractions respectively.ConclusionFor majority of fractions, an expansion of 1.25 cm of planning day rectum is sufficient to accommodate the interfractional rectal motion and deformation. However, large interfractional rectal movement exists and may need separate strategy, such as adaptive therapy, to accommodate. PurposeDuring external beam radiation therapy (EBRT), rectum position and shape undergoes daily change due to patient bladder and rectal filling. Accommodating these changes is a contributing factor for the planning margins applied for rectal cancer EBRT treatment. We aim to find the magnitude of these changes and design treatment strategies to accommodate these changes. During external beam radiation therapy (EBRT), rectum position and shape undergoes daily change due to patient bladder and rectal filling. Accommodating these changes is a contributing factor for the planning margins applied for rectal cancer EBRT treatment. We aim to find the magnitude of these changes and design treatment strategies to accommodate these changes. Methods and MaterialsNine patients who underwent treatment with IMRT were selected for this retrospective study. KV CBCT was acquired on every fraction of the treatment. The rectum, bladder, and anus were contoured on one fraction of each week of treatment. The CBCTs were rigidly registered through bony registration and the contours were transferred to the planning CT. The rectum contours on the planning CT were expanded uniformly and the expansions needed to cover all the transferred rectum contours were obtained. Nine patients who underwent treatment with IMRT were selected for this retrospective study. KV CBCT was acquired on every fraction of the treatment. The rectum, bladder, and anus were contoured on one fraction of each week of treatment. The CBCTs were rigidly registered through bony registration and the contours were transferred to the planning CT. The rectum contours on the planning CT were expanded uniformly and the expansions needed to cover all the transferred rectum contours were obtained. ResultsForty-five CBCTs were used for this study. For 5 patients, an expansion of 1 cm of the simulation day rectum completely covers the treatment day rectum for all 5 fractions. The maximum expansion needed for all the patients is 3.25 cm. An expansion of 1 cm, 1.25 cm and 1.5 cm covers 71% (32 of 45), 89% (40 of 45) and 91% (41 of 45) of the treatment fractions respectively. Forty-five CBCTs were used for this study. For 5 patients, an expansion of 1 cm of the simulation day rectum completely covers the treatment day rectum for all 5 fractions. The maximum expansion needed for all the patients is 3.25 cm. An expansion of 1 cm, 1.25 cm and 1.5 cm covers 71% (32 of 45), 89% (40 of 45) and 91% (41 of 45) of the treatment fractions respectively. ConclusionFor majority of fractions, an expansion of 1.25 cm of planning day rectum is sufficient to accommodate the interfractional rectal motion and deformation. However, large interfractional rectal movement exists and may need separate strategy, such as adaptive therapy, to accommodate. For majority of fractions, an expansion of 1.25 cm of planning day rectum is sufficient to accommodate the interfractional rectal motion and deformation. However, large interfractional rectal movement exists and may need separate strategy, such as adaptive therapy, to accommodate.

An Audit to Assess Electronic Portal Imaging (EPI) and Cone Beam CT (CBCT) Image Verification Methods in 16 Breast Patients

16 breast patients receiving whole breast radiotherapy (tangential beam arrangement with matched supra clavicular fossa SCF if required) were imaged on treatment fractions 0, 1 and 8 and 12 (fraction 0 being a pre treatment verification appointment). Patients were supine and immobilised on a Sinmed Posiboard with ipsilateral arm raised only. CBCT was acquired and assessed online, consisting of an automatic bone match in comparison to the planning CT. EPIs were taken of both tangential fields. One patient had SCF irradiation and EPIs were acquired of this region. Offline CBCT assessment consisted a manual tumour bed match compared to the planning CT. The EPI image registration method consisted of a bony match compared with digitally reconstructed radiographs (DRR). The bony registrations of the CBCT and EPI methods were evaluated. All data was transformed into the U/V co-ordinate system as carried out by Topolnjak et al(1). U refers to the lateral field placement error and V refers to the longitudinal position.

SU-E-J-30: Using Shifting Planned Dose Matrix to Evaluate Daily Dose Changes for IMRT Prostate Treatment.

Summation of daily DVH from KV-cone beam CT (KV-CBCT) to obtain a composite dose volume histogram (DVH) is challenging. Directly translating the planned dose matrix according to measured daily prostate displacements provided a common reference frame for a composite DVH from daily DVHs. The purpose of this study is to evaluate the shifting planned dose matrix method compared to the dose recalculation method using daily KV-CBCT. Six patients, who received concurrent IMRT treatment for prostate and pelvic lymph nodes with 124 daily CBCTs, were selected for this study. Contours for CBCT's were transferred from the planning CT after soft tissue registration for prostate and bony registration for pelvic lymph nodes. Using the same planning beam configurations, we re-calculated doses for these CBCTs after shifting to corrected treatment isocenters. The planned dose matrix translation was performed by an in-house program written in MATLAB and incorporated with Computational Environment for Radiotherapy Research (CERR) software. The corresponding daily DVH was obtained by shifting the planned dose matrix according to shifts of treatment iso-centers. To compare these two methods, selected endpoint doses for tumor targets and sensitive structures were extracted from DVHs. For prostate displacement less then 1.5 cm, the dose matrix shifting method resulted in 93% and 98% fractions within 5% differences from the recalculation method for D95 of prostate and pelvic lymph nodes, respectively. These numbers decreased to 58% and 71% when 2% dose difference criterion was used. Allowing 5% daily dose difference, shifting planned dose matrix provides effective means to evaluating daily dose changes for concurrent IMRT treatment for prostate and pelvic lymph nodes. The utility of this tool is to provide a common coordinate frame to obtain composite dose distributions.

SU-E-J-22: Effect of MLC Leaf Width on MLC Leaf Shifting Algorithm for Concurrent Treatment of Prostate and Pelvic Lymph Nodes.

Our previous study showed that adjusting selected MLC leaf pairsto follow prostate movement is an effective strategy to account for daily prostate displacement during concurrent treatment with pelvic lymph nodes. MLC leaf width affects the quality of MLC shifting plans for longitudinal prostate motion compensation. This study is to investigate the effect of the MLC leaf width in compensation of the prostate movement. Fifty-one daily CT on-rail scans from three patients were available for this study. On these CTs, the prostate, bladder and rectum were manually contoured, and the lymph nodes contours were transferred from the planning CT after rigid bony registration. For each patient, three different IMRT plans were created based on a planning CT using leaf width of 2.5, 5, and 10 mm, respectively. For each CT, the prostate displacement was determined by dual imaging registration and compensated by shifting MLC resulting in a total of 153 MLC shifted plans. Among 51 daily CTs, the average prostate movement along the superior/inferior direction was 1.1±3.7 mm (range: -6 to 6.5 mm). The differences in D99 of the prostate between the dose of the day and dose of the plan were 2.3±3.3%, 1.3±2.0%, and 4.4±5.1% for 2.5, 5, and 10 mm leaf width plans, respectively (p<<0.05). The corresponding differences in D99 of the lymph nodes were 0.7±0.9%, 0.6±0.9%, and 1.4±0.8%. The mean differences in D50 were 0.8%, 1.6%, and 2.7% for the bladder, and 10.0%, 3.9%, and 5.7% for the rectum, respectively. Using the MLC Shifting method to compensate for prostate movement in the longitudinal direction depends on the MLC leaf width and the magnitude of the prostate motion. The use of leaf width of 5 mm can provide sufficient tumor coverage without significantly affecting doses to the critical structures.

Lung tumor reproducibility with active breath control (ABC) in image-guided radiotherapy based on cone-beam computed tomography with two registration methods

Purpose To study the inter- and intrafraction tumor reproducibility with active breath control (ABC) utilizing cone-beam computed tomography (CBCT), and compare validity of registration with two different regions of interest (ROI). Methods and materials Thirty-one lung tumors in 19 patients received conventional or stereotactic body radiotherapy with ABC. During each treatment, patients had three CBCT scanned before and after online position correction and after treatment. These CBCT images were aligned to the planning CT using the gray scale registration of tumor and bony registration of the thorax, and tumor position uncertainties were then determined. Results The interfraction systematic and random translation errors in the left–right (LR), superior–inferior (SI) and anterior–posterior (AP) directions were 3.6, 4.8, and 2.9 mm; 2.5, 4.5, and 3.5 mm, respectively, with gray scale alignment; 1.9, 4.3, 2.0 mm and 2.5, 4.4, 2.9 mm, respectively, with bony alignment. The interfraction systematic and random rotation errors with gray scale and bony alignment groups ranged from 1.4° to 3.0° and 0.8° to 2.3°, respectively. The intrafraction systematic and random errors with gray scale registration in LR, SI, AP directions were 0.9, 2.0, 1.8 mm and 1.5, 1.7, 2.9 mm, respectively, for translation; 1.5°, 0.9°, 1.0° and 1.2°, 2.2°, 1.8°, respectively, for rotation. The translational errors in SI direction with bony alignment were significantly larger than that of gray scale ( p < 0.05). Conclusions With CBCT guided online correction the interfraction positioning errors can be markedly reduced. The intrafraction errors were not diminished by the use of ABC. Rotation errors were not very remarkable both inter- and intrafraction. Gray scale alignment of tumor may provide a better registration in SI direction.

Quantifying the Reproducibility of Heart Position During Treatment and Corresponding Delivered Heart Dose in Voluntary Deep Inhalation Breath Hold for Left Breast Cancer Patients Treated With External Beam Radiotherapy

Voluntary deep inhalation breath hold (VDIBH) reduces heart dose during left breast irradiation. We present results of the first study performed to quantify reproducibility of breath hold using bony anatomy, heart position, and heart dose for VDIBH patients at treatment table. Data from 10 left breast cancer patients undergoing VDIBH whole-breast irradiation were analyzed. Two computed tomography (CT) scans, free breathing (FB) and VDIBH, were acquired to compare dose to critical structures. Pretreatment weekly kV orthogonal images and tangential ports were acquired. The displacement difference from spinal cord to sternum across the isocenter between coregistered planning Digitally Reconstructed Radiographs (DRRs) and kV imaging of bony thorax is a measure of breath hold reproducibility. The difference between bony coregistration and heart coregistration was the measured heart shift if the patient is aligned to bony anatomy. Percentage of dose reductions from FB to VDIBH: mean heart dose (48%, SD 19%, p = 0.002), mean LAD dose (43%, SD 19%, p = 0.008), and maximum left anterior descending (LAD) dose (60%, SD 22%, p = 0.008). Average breath hold reproducibility using bony anatomy across the isocenter along the anteroposterior (AP) plane from planning to treatment is 1 (range, 0-3; SD, 1) mm. Average heart shifts with respect to bony anatomy between different breath holds are 2 ± 3 mm inferior, 1 ± 2 mm right, and 1 ± 3 mm posterior. Percentage dose changes from planning to delivery: mean heart dose (7%, SD 6%); mean LAD dose, ((9%, SD 7%)S, and maximum LAD dose, (11%, SD 11%) SD 11%, p = 0.008). We observed excellent three-dimensional bony registration between planning and pretreatment imaging. Reduced delivered dose to heart and LAD is maintained throughout VDIBH treatment.

A comparison of three different adaptive strategies in image-guided radiotherapy of bladder cancer

The urinary bladder shows considerable individual variation in shape and position during a course of radiotherapy (RT). In this study we have developed and compared three different adaptive RT (ART) strategies for bladder cancer involving daily cone beam CT (CBCT) imaging and plan selection. Material and methods. Ten patients treated for bladder cancer had daily CBCTs acquired that were registered online using bony anatomy registration. Seven patients received intensity modulated RT (IMRT) with a simultaneous integrated boost (SIB) technique to the bladder and pelvic lymph nodes. Three patients received treatment to the bladder only. Retrospectively, we compared three ART strategies that were all based on daily selection of the most suitable plan from a library consisting of three IMRT-plans corresponding to a small, medium and large target volume. ART method A utilised population-based margins while methods B and C used the bladder as seen on CBCT-scans from the first week of treatment; method B without delineation of the bladder on CBCT and method C with delineation of the bladder. Total dose distributions were calculated using the planning CT. For each patient, we calculated ratios of the dose volume histograms (DVHs) for the three ART strategies relative to non-adaptive therapy. Results. The inter-patient variation was large for all three ART strategies. The mean ratios of the volumes receiving 57 Gy or more (corresponding to 95% of prescribed dose) for methods A, B and C were 0.66 (SD: 0.11), 0.67 (SD: 0.13) and 0.67 (SD: 0.16) respectively when compared to the non-adaptive plan. Conclusion. When using any of the ART strategies, it is possible to reduce significantly the volumes receiving high doses compared to the use of a standard non-adaptive plan. The differences in dose volume parameters between the three methods were small compared with the differences from the standard plan.

Characterizing Interfraction Variations and Their Dosimetric Effects in Prostate Cancer Radiotherapy

To quantitatively characterize the interfraction variations and their dosimetric effects in radiotherapy for prostate cancer. A total of 486 daily computed tomography (CT) sets acquired for 20 prostate cancer patients treated with daily CT-guided repositioning using a linear accelerator and CT-on-rail combination were analyzed. The prostate, rectum, and bladder, delineated on each daily CT data set, were compared with those from the planning CT scan. Several quantities, including Dice's coefficient and the maximal overlapping rate, were used to characterize the interfraction variations. The delivered dose was reconstructed by applying the original plan to the daily CT scan with consideration of proper repositioning. The mean prostate Dice's coefficient and maximal overlapping rate after bony registration was 69.7%±13.8% (standard deviation) and 85.6%±7.8% (standard deviation), respectively. The daily delivered dose distributions were generally inferior to the planned dose distribution for target coverage and/or normal structure sparing. For example, for approximately 5% of the treatment fractions, the prostate volume receiving 100% of the prescription dose decreased dramatically (15-20%) compared with its planned value. The magnitudes of the interfraction variations and their dosimetric effects indicated that, statistically, current standard repositioning using prostate alignment might be adequate for two-thirds of the fractions, but for the rest of the fractions, better on-line correction strategies, such as on-line replanning, are needed. Different adaptive correction schemes for prostatic interfraction changes can be used according to the anatomic changes, as quantified by the organ displacement and deformation parameters. On-line replanning is needed for approximately one-third of the treatment fractions.

Tumor motion prediction with the diaphragm as a surrogate: a feasibility study

We have previously assessed the use of the diaphragm as a surrogate for predicting real-time tumor position with linear models built with training data extracted from the same treatment fraction (Cerviño et al 2009 Phys. Med. Biol. 54 3529–41). However, practical use in the clinical setting requires the capability of predicting tumor position throughout the treatment course using a model built at the beginning of the course. We evaluate the inter-fraction applicability of linear models to predict superior–inferior tumor position based on diaphragm position using 21 fluoroscopic sequences from five lung cancer patients. Tumor position is predicted with models built during the first fluoroscopic sequence of each patient. Other fluoroscopic sets are registered to the first set with five different methods. The mean localization prediction error and maximum error at a 95% confidence level averaged over all patients are found to be 1.2 mm and 2.9 mm, respectively, for bony registration and 1.2 mm and 2.8 mm, respectively, for registration based on the mean position of the tumor in the first two breathing cycles. Other registration methods produce larger prediction errors. In the clinical setting, this prediction error could be added as a margin to the target volume. We therefore conclude that it is feasible to predict lung tumor motion with diaphragm with sufficient accuracy in the clinical setting.

WE-C-303A-04: Quantifying the Reproducibility of Heart Position with Respect to Bony Anatomy in Daily Set Up and the Corresponding Delivered Heart Dose in Voluntary Deep Inhalation Breath Hold for Left Breast Cancer Patients Via External Beam Radiotherapy

Purpose: Voluntary deep inhalation breath hold (DIBH) technique help reduce the heart toxicity during radiation treatment of the left breast. We present the results of the first study performed to quantify the reproducibility of heart position and the heartdose in daily setup for voluntary DIBH patients. Method and Materials: Ten left breast patients undergoing treatment with voluntary DIBH were studied. Each patient had two CT scans, one with free breathing and one with voluntary DIBH, to evaluate the dose under these two conditions to the Heart, anterior most point of left anterior descending (LAD) coronary artery, and left Lung. At the treatment machine, daily and weekly KV orthogonal images were acquired using onboard imaging for each patient. Mosaiq™ software was used to register the full bony anatomy of thorax (including the spine, ribs, anterior chest wall and heart) with KV images to the planning DRRs. Once the patient is aligned on the treatment machine using bony anatomy registration, the difference between the bony anatomy registration and the heart registration is the daily heart position. These differences were transferred into treatment planning system to obtain the delivered dose.Results: The dosimetric evaluation shows clinically significant reduced dose to heart,lung, and LAD in the DIBH case compared to the free breathing case, as previous studies have shown. The weekly heart position shifts are in the order of few mm. Dosimetric evaluations for these shifts indicate that dose to the heart in the daily treatment with DIBH remains low. Conclusion: The setup and breath hold accuracy and the daily delivered dose to the heart was evaluated for voluntary DIBH breast patients. Results indicate a clear reduction of dose delivered to the heart throughout the treatment when voluntary DIBH technique is employed.

SU‐FF‐J‐47: Assessment of Diaphragm as An Internal Surrogate for Lung Tumor Motion Prediction

Purpose: We have previously assessed the use of diaphragm as a surrogate for predicting real‐time tumor position with linear models built with training data extracted from the same treatment fraction. In this study, we evaluate inter‐fraction variability of model parameters and the prediction error for different fractions from the model built on the first fraction. Method and Materials: 21 fluoroscopic sequences from 5 lung cancer patients were used for this study. Each patient has from 2 to 6 fluoroscopic sequences from at least 2 different treatment fractions. Each sequence consists of 352 to 690 frames at 15 frames per second. Superior‐inferior motions of the diaphragm and the tumor were identified in every frame. A model to predict tumor position was built for each sequence, and the parameters from different sequences were compared. Then, the model built from the first fluoroscopic sequence was used to predict tumor position in the subsequent fluoroscopic sets of each patient. Prediction error was calculated for three different ways of registering fluoroscopic sets: manual bony anatomy registration, mean tumor position registration, and mean diaphragm position registration. Results: Average variability of the correlation and slope of linear models built from different fluoroscopic sequences are 1.3% and 8.6% respectively. Prediction based on the model built with data from the first sequence leads to an average mean localization error and average error at a 95% confidence level of 1.2mm and 2.9mm respectively for bony registration, 1.5mm and 3.3mm for mean tumor position registration, and 2.5mm and 4.7mm for mean diaphragm position registration. Conclusion: For the patients studied, diaphragm has shown to be a good surrogate for tumor motion. Variability of model parameters and prediction error from one sequence to another are small. Bony registration seems the most accurate method for patient setup when using diaphragm as a surrogate.

SU-FF-J-94: Clinical Application of Image Guidance to Radiotherapy of Pre-Operative Rectal Cancer : From Day-To-Day Positioning to Biological Modelling

Introduction and purpose Pre‐operative (chemo)radiotherapy is standard in stage II/III rectal cancer, but associated with significant small bowel toxicity. This study focuses on the ability of helical tomotherapy to spare the small bowel compared to the conventional approach, and to explore the positioning and radiobiological benefit of daily MVCT scanning and positioning for these patients. Material and methods A planning protocol was developed allowing minimization of the volume of small bowel irradiated with a dose >15Gy. To define the image‐guidance strategy for the treatment of rectal cancer on tomotherapy MVCT‐scans of 10 patients were acquired before and after treatment. Using automatic bony registration the setup‐error was determined and by delineating the mesorectal area on both MV and kV(planning)‐images an estimation was made of the internal margin by assessing the movement of the internal organs. Suitable margins were extracted and applied for future patients. To assess the biological relevance or benefit of the application of daily scans a NTCP model was applied to the 10 patients, comparing NTCP linked to small bowel toxicity for conventional treatments, and tomotherapy treatments with and without image guidance. Results Analysis of dose volume histograms show the ability to reduce the 15Gy‐volume of the small bowel with helical tomotherapy. Assessment of setup errors and internal movement show a possible margin reduction from 15mm to 8mm (lateral), 16mm to 11mm (anterior) and 14mm to 7mm(posterior) when using daily MVCT positioning. TCP and NTCP calculations show a statistically significant benefit going from classical (39.5%) to rotational IMRT(26.5%), and also between IMRT and IGRT with daily MVCT‐positioning(18%). Conclusion This study shows that treatment of pre‐operative rectal cancer on tomotherapy is feasible and that daily MVCT‐positioning can reduce margins and has a significant biological effect on small bowel toxicity.

Tumor Localization Using Cone-Beam CT Reduces Setup Margins in Conventionally Fractionated Radiotherapy for Lung Tumors

To determine whether setup margins can be reduced using cone-beam computed tomography (CBCT) to localize tumor in conventionally fractionated radiotherapy for lung tumors. A total of 22 lung cancer patients were treated with curative intent with conventionally fractionated radiotherapy using daily image guidance with CBCT. Of these, 13 lung cancer patients had sufficient CBCT scans for analysis (389 CBCT scans). The patients underwent treatment simulation in the BodyFix immobilization system using four-dimensional CT to account for respiratory motion. Daily alignment was first done according to skin tattoos, followed by CBCT. All 389 CBCT scans were retrospectively registered to the planning CT scans using automated soft-tissue and bony registration; the resulting couch shifts in three dimensions were recorded. The daily alignment to skin tattoos with no image guidance resulted in systematic (Sigma) and random (sigma) errors of 3.2-5.6 mm and 2.0-3.5 mm, respectively. The margin required to account for the setup error introduced by aligning to skin tattoos with no image guidance was approximately 1-1.6 cm. The difference in the couch shifts obtained from the bone and soft-tissue registration resulted in systematic (Sigma) and random (sigma) errors of 1.5-4.1 mm and 1.8-5.3 mm, respectively. The margin required to account for the setup error introduced using bony anatomy as a surrogate for the target, instead of localizing the target itself, was 0.5-1.4 cm. Using daily CBCT soft-tissue registration to localize the tumor in conventionally fractionated radiotherapy reduced the required setup margin by up to approximately 1.5 cm compared with both no image guidance and image guidance using bony anatomy as a surrogate for the target.

Analysis of Interfraction Prostate Motion Using Megavoltage Cone Beam Computed Tomography

Determine the degree of interfraction prostate motion and its components measured by using daily megavoltage (MV) cone beam computed tomography (CBCT) imaging. A total of 984 daily MV CBCT images from 24 patients undergoing definitive intensity-modulated radiotherapy for localized prostate cancer were analyzed retrospectively. Pretreatment couch shifts, based on physician registration of MV CBCT to planning CT data sets, were used as a measure of daily interfraction motion. Off-line bony registration was performed to separate bony misalignment from internal organ motion. Interobserver and intraobserver variation studies were performed on 20 MV CBCT images. Mean interfraction prostate motion was 6.7 mm, with the greatest single-axis deviation in the anterior-posterior (AP) direction. The largest positional inaccuracy was accounted for by systematic deviations in bony misalignment, whereas random deviations occurred from bony misalignment and internal prostate motion. In the aggregate, AP motion did not correlate with days elapsed since beginning therapy or on average with rectal size at treatment planning. Interobserver variation was greatest in the AP direction, decreased in experienced observers, and further decreased in intraobserver studies. Mean interfraction motion during the first 6 days of therapy, when used as a subsequent offset, reduced acceptable AP planning target volume margins by 50%. The MV CBCT is a practical direct method of daily localization that shows significant interfraction motion with respect to conventional three-dimensional conformal and intensity-modulated radiotherapy margins, similar to that measured in other modalities.

An Analysis of Inter-Operator Registration Variability in Helical Tomotherapy

This study aims to determine the image registration interoperator variability using the image fusion interface on the TomoTherapy system. The study focused on the registration results among five Ottawa Hospital Cancer Centre personnel comparing 15 retrospective prostate, bladder, and head and neck patients from day 2 megavoltage computed tomography images with planning computed tomography images. Personnel were instructed to manually adjust the fused images after automatic bony registration matching disease-specific criteria. The Pearson correlation matrix was applied to the summed vector lengths of the personnel's overall translations in all three disease sites. The results were compared with a chosen benchmark personnel and were analyzed for significance in correlation. Finally, the overall translations compared with the automatic bony registration were analyzed for any trends in the data. In each disease site, the average correlation from all personnel compared to a benchmark registering prostate, bladder, and head and neck patients were 0.94, 0.94, and 0.64, respectively. Then, analysis of the interuser overall translations illustrated that each disease site had its own trends with regard to interfraction image fusion corrections. Finally, the mean translational variations were assessed over all personnel for each disease site in X (lateral), Y (longitudinal), and Z (vertical) directional planes. The results demonstrate that consistent, accurate image registration is dependent on factors involving overall user experience with TomoTherapy software and user knowledge of human sectional anatomy, among others.