Daily CT Images Research Articles

Feasibility of online IMPT adaptation using fast, automatic and robust dose restoration

Intensity-modulated proton therapy (IMPT) offers excellent dose conformity and healthy tissue sparing, but it can be substantially compromised in the presence of anatomical changes. A major dosimetric effect is caused by density changes, which alter the planned proton range in the patient. Three different methods, which automatically restore an IMPT plan dose on a daily CT image were implemented and compared: (1) simple dose restoration (DR) using optimization objectives of the initial plan, (2) voxel-wise dose restoration (vDR), and (3) isodose volume dose restoration (iDR). Dose restorations were calculated for three different clinical cases, selected to test different capabilities of the restoration methods: large range adaptation, complex dose distributions and robust re-optimization. All dose restorations were obtained in less than 5 min, without manual adjustments of the optimization settings. The evaluation of initial plans on repeated CTs showed large dose distortions, which were substantially reduced after restoration. In general, all dose restoration methods improved DVH-based scores in propagated target volumes and OARs. Analysis of local dose differences showed that, although all dose restorations performed similarly in high dose regions, iDR restored the initial dose with higher precision and accuracy in the whole patient anatomy. Median dose errors decreased from 13.55 Gy in distorted plan to 9.75 Gy (vDR), 6.2 Gy (DR) and 4.3 Gy (iDR). High quality dose restoration is essential to minimize or eventually by-pass the physician approval of the restored plan, as long as dose stability can be assumed. Motion (as well as setup and range uncertainties) can be taken into account by including robust optimization in the dose restoration. Restoring clinically-approved dose distribution on repeated CTs does not require new ROI segmentation and is compatible with an online adaptive workflow.

Positioning accuracy and daily dose assessment for prostate cancer treatment using in-room CT image guidance at a proton therapy facility.

To evaluate the effectiveness of CT image-guided proton radiotherapy for prostate cancer by analyzing the positioning uncertainty and assessing daily dose change due to anatomical variations. Patients with prostate cancer were treated by opposed lateral proton beams based on a passive scattering method using an in-room CT image-guided system. The system employs a single couch for both CT scanning and beam delivery. The patient was positioned by matching the boundary between the prostate and the rectum's anterior region identified in the CT images to the corresponding boundary in the simulator images after bone matching. We acquired orthogonal kV x-ray images after couch movement and confirmed the body position by referring to the bony structure prior to treatment. In offline analyses, we contoured the targeted anatomical structures on 375 sets of daily in-room CT images for 10 patients. The uncertainty of the image-matching procedure was evaluated using the prostate contours and actual couch corrections. We also performed dose calculations using the same set of CT images, and evaluated daily change of dose-volume histograms (DVHs) to compare the effectiveness of the treatment using prostate matching to the bone-matching procedure. The isocenter shifts by prostate matching after bone matching were 0.5±1.8 and -0.8±2.6mm along the superior-inferior (SI) and anterior-posterior (AP) directions, respectively. The body movement errors (σ) after couch movement were 0.7, 0.5, and 0.3mm along the lateral, SI and AP direction, respectively, for 30 patients. The estimated errors (σ) in the prostate matching were 1.0 and 1.3mm, and, in conjunction with the movement errors, the total positioning uncertainty was estimated to be 1.0 and 1.4mm along the SI and AP directions, respectively. Daily DVH analyses showed that in the prostate matching, 98.7% and 86.1% of the total 375 irradiations maintained a dose condition of V95% >95% for the prostate and a dose constraint of V77% <18% for the rectum, whereas 90.4% and 66.1% of the total irradiations did so when bone matching was used. The dose constraint of the rectum and dose coverage of the prostate were better maintained by prostate matching than bone matching (P<0.001). The daily variation in the dose to the seminal vesicles (SVs) was large, and only 40% of the total irradiations maintained the initial planned values of V95% for high-risk treatment. Nevertheless, the deviations from the original value were -4±7% and -5±11% in the prostate and bone matching, respectively, and a better dose coverage of the SV was achieved by the prostate matching. The correction of repositioning along the AP and SI direction from conventional bone matching in CT image-guided proton therapy was found to be effective to maintain the dose constraint of the rectum and the dose coverage of the prostate. This work indicated that prostate cancer treatment by prostate matching using CT image guidance may be effective to reduce the rectal complications and achieve better tumor control of the prostate. However, an adaptive approach is desirable to maintain better dose coverage of the SVs.

Deformable Image Registration and Dose Accumulation for Locally Advanced Non-Small Cell Lung Cancer, Comparing Delivered and Planned Radiotherapy Doses

Uncertainty in the radiotherapy doses actually received over the course of treatment for locally-advanced non-small cell lung cancer (LA-NSCLC) may explain, in part, low long term survival rates for these patients. This is because target and nodal dose coverage, up to this point, have been assumed constant; however disease volume and location are known to change during the course of radiation treatment. In this study, deformable registration (DIR) and dose accumulation (DA) were used to calculate the accumulated dose received over the course of treatment. These accumulated doses are then compared with the clinical treatment plan against acceptable tolerances. This single institution retrospective study randomly selected 26 patients diagnosed with LA-NSCLS receiving radical chemo-radiotherapy. 9 Patients received a dose of 60Gy/30fx and the other 17 patients received a dose of 66/33fx. The clinical delivered plan was exported into RayStation 4.5.2. Using this system's deformable image registration (DIR) tools, doses calculated on daily cone-beam CT images acquired for daily verification are deformed and mapped onto the planning CT images, thus accounting for daily treatment incorporating set-up shifts and changes in the patients anatomy. The accumulated dose delivered was then compared to the clinical treatment plan in terms of target coverage and avoidance of organs at risk. This study demonstrates successfully that, on average, the accumulated doses calculated were not statistically different from the clinical plan. Furthermore, the accumulated doses to the clinical target volumes (CTV's) for both targets and nodes are within acceptable range, illustrating that our current PTV margins are robust against anatomical changes observed during the course of radiation therapy treatment. Differences between CTV doses calculated for the clinical treatment plan compared to the accumulated plan was within 2%. The accumulated OAR doses are also maintained as compared to the clinical plan. We successfully demonstrated that DIR and DA is a useful and valued tool confirming doses received for patients receiving radiation for LA-NSCLC. We have confirmed that our PTV margins are adequate and robust from a dosimetric perspective. Given our relatively small sample size, our results may not be representative of the entire population. Further accrual would confirm our initial findings.

Quantification of Volumetric Changes Occurring During Radiation Therapy for Esophageal Cancer

Many patients undergoing radiation therapy for esophageal cancer have marked anatomic changes during their course of treatment including shrinking of the primary tumor, edematous changes of the treated esophagus, and changes in body habitus. Changes in contour, volume, and location of the esophageal tumor over the total course of radiation therapy could have a potential dosimetric impact if highly conformal treatment techniques are used. We conducted a retrospective repeated measures analysis on daily cone-beam CT imaging to quantify the magnitude of these volumetric changes. Utilizing an institutional retrospective database, fourteen patients were identified for this study. All patients were treated using a linear accelerator with integrated on-board cone beam CT imaging that allows CT imaging during the daily treatment sessions while the patient is immobilized in the treatment position. Gross tumor volumes of the primary tumor were contoured on every axial slice on the primary planning CT and the CT acquired on treatment days 1, 6,11,16,21, and 25. The changes in primary tumor volume over time were analyzed with a repeated measures ANOVA analysis. Gross tumor volumes generally decreased at an average rate of roughly 6% volume loss per week which ultimately resulted in a 23% total average reduction in volume over the total course of therapy. Analysis of variance revealed a statistically significant reduction in tumor volume occurring by cone beam CT 11 with a mean volume of 91% of original (p=0.0011 95% CI 0.89-0.98).The volumes measured on cone beam CTs taken on treatment days 16, 21, and 25 were also statistically significant (p<0.0001) with mean residual volumes of 89% (95% CI = 0.83-0.96), 83% (95% CI=0.79-0.89), and 77% of original volume (95% CI= 0.72-0.83) respectively. Statistically significant volume reductions were noted to begin by radiation treatment day 11 with meaningful dosimetric implications. These data may, therefore, be useful in the implementation of an adaptive radiation therapy planning scheme that periodically adjusts the conformal treatment plan to match these anatomic

Comparing Delivered and Planned Radiation Therapy Doses Using Deformable Image Registration and Dose Accumulation for Locally Advanced Non–small Cell Lung Cancer

There are uncertainties in radiotherapy doses received over the course of treatment for locally-advanced non-small cell lung cancer (LA-NSCLC). This may explain poor long term survival rates for these patients as target and nodal dose coverage, have been assumed constant throughout treatment. However, disease volume and location are known to change. In this study, deformable registration (DIR) and dose accumulation (DA) were used to calculate the dose delivered over the course of treatment. These delivered doses were then compared with the clinical treatment plan. This single institution, retrospective study selected 26 sequential patients diagnosed with LA-NSCLC receiving radical chemo-radiotherapy. Using RayStation 4.5.2, doses were calculated on daily cone-beam CT images acquired for daily verification were then deformed and mapped back onto the planning CT images. DA results were compared to the clinical treatment plan for both target coverage and organs at risk. Findings are summarized in Table 1. On average, DA calculations were not statistically different from the clinical plan (p<0.05). The DA to the clinical target volumes for both targets (CTV-T) and nodes (CTV-N) demonstrated that coverage remained close to that planned. The DA for OARs was also maintained as compared to the clinical plan. While, on average, delivered doses remained within our routine clinical tolerances, outliers were observed. DA for each patient indicated a small decrease of the delivered dose with respect to their clinical plan, although large variations were observed for organs at risk. We successfully demonstrated that DIR and DA can assess delivered doses to patients receiving radiotherapy for LA-NSCLC. We have confirmed that, on average, our PTV margins are adequate and robust from a dosimetric perspective. Given our LA-NSCLC sample, our results may not be representative Non-Small Cell (NSCLC) population. Although no average differences seen, outliers persist and will be the focus of future research.Abstract 3659; Table 1Planned vs Accumulated DosesROIDelivered Dose(±SD)[Min – Max]Planned Dose(±SD)[Min – Max]% change from plan/patient (±SD)[Min – Max]ProtocolD99% CTV-T95.5 ± 3.9%[88 – 102%]97 ± 3.1%[91 – 102%]-1.3 ± 1.9[-6 – 2 %]92%D99% CTV-N94.7 ± 4.8%[79 – 100%]95.8 ± 3.2%[87 – 100%]-1.3 ± 2.2[-9 – 2 %]92%Spinal canal4450 ± 691 cGy[1317 – 5378 cGy]4438 ± 276 cGy[4008 – 5110 cGy]-2.0 ± 2.1[-5 – 5 %]4500 cGyMean lung dose1465 ± 412 cGy[657 – 1911 cGy]1401 ± 390 cGy[611 – 1876 cGy]-1.4 ± 3.5[-11 – 6 %]1800 cGyV20 lung25.9 ± 7.6%[9 – 34%]26.3 ± 8.9%[9 – 45%]-0.3 ± 4.4[-6 – 12%]30%V40 heart13.4 ±13.2%[0 – 50%]14.7 ± 13.1%[0 – 49%]-10.6 ± 24[-89 – 53%]60%V40 esophagus37 ± 17%[6 – 73%]36 ± 19%[0 – 74%]-0.8 ± 8%[-10 – 28%]60% Open table in a new tab

Daily CT guidance improves target coverage during definitive radiation therapy for gastric MALT lymphoma

PurposeRadiation therapy (RT) for gastric mucosa–associated lymphoid tissue (MALT) lymphoma is challenging because of variation in the stomach’s position, size, and shape. We investigated the interfractional changes in stomach location, consequent dosimetric effects, and impact of daily computed tomography image guidance RT (CT-IGRT). Methods and materialsTwelve patients treated for gastric MALT lymphoma with intensity modulated radiation therapy, using a breath-hold technique and restriction of oral intake, were studied retrospectively. The planning target volume (PTV) comprised a 0.5 to 1.0 cm expansion of the stomach. The prescription dose was 30 Gy in 15 to 20 fractions. CT-IGRT was performed daily using CT-on-Rails. Dosimetry was calculated on 229 daily CT images after bony versus CT-based soft tissue alignment, and doses delivered to the target and adjacent structures were compared with the treatment plan. Target coverage was expressed as the percent of the clinical target volume (CTV) and PTV receiving ≥95% of the prescribed dose (V95%). ResultsThe average change in stomach volume was –12.4% (range, –47.6% to 38.6%). The average shift required for target coverage was 1.0 cm (maximum, 2.2 cm). With CT-based alignment to the stomach, the average V95% was 98.5% for CTV and 94.9% for PTV; with bony alignment, these values were 94.5% and 90.4%, respectively (P < .01 for CTV and PTV). With bony alignment, the PTV V95% was ≤90% in 4 patients (33%) over the course of treatment and was as low as 72.5% for 1 fraction. The kidney position varied with respect to the stomach and bony anatomy. Consequently, the dose to the left kidney was higher based on daily CT scans than on planning scans. Dose to other organs at risk did not vary significantly. ConclusionsSubstantial interfractional variation in stomach volume was observed, despite treatment with breath-hold and restriction of oral intake. Daily CT-IGRT improved target coverage, enabling excellent coverage despite the use of small PTV margins.

SU‐C‐202‐02: A Comprehensive Evaluation of Adaptive Daily Planning for Cervical Cancer HDR Brachytherapy

Purpose:To evaluate adaptive daily planning for cervical cancer patients who underwent high‐dose‐rate intra‐cavitary brachytherapy (HDR‐ICBT).Methods:This study included 22 cervical cancer patients who underwent 5 fractions of HDR ICBT. Regions of interest (ROIs) including high‐risk clinical tumor volume (HR‐CTV) and organs‐at‐risk (OARs) were manually contoured on daily CT images. All patients were treated with adaptive daily plans, which involved ROI delineation and dose optimization at each treatment fraction. Single treatment plans were retrospectively generated by applying the first treatment fraction's dwell times adjusted for decay and dwell positions of the applicator to subsequent treatment fractions. Various existing similarity metrics were calculated for the ROIs to quantify interfractional organ variations. A novel similarity score (JRARM) was established, which combined both volumetric overlap metrics (DSC, JSC, and RVD) and distance metrics (ASD, MSD, and RMSD). Linear regression was performed to determine a relationship between inter‐fractional organ variations of various similarity metrics and D2cc variations from both plans. Wilcoxon Signed Rank Tests were used to assess adaptive daily plans and single plans by comparing EQD2 D2cc (α/β=3) for OARs.Results:For inter‐fractional organ variations, the sigmoid demonstrated the greatest variations based on the JRARM and DSC similarity metrics. Comparisons between paired ROIs showed differences in JRARM scores and DSCs at each treatment fraction. RVD, MSD, and RMSD were found to be significantly correlated to D2cc variations for bladder and sigmoid. The comparison between plans found that adaptive daily planning provided lower EQD2 D2cc of OARs than single planning, specifically for the sigmoid (p=0.015).Conclusion:Substantial inter‐fractional organ motion can occur during HDR‐BT, which may significantly affect D2cc of OARs. Adaptive daily planning provides improved dose sparing for OARs compared to single planning.

SU‐F‐J‐57: Effectiveness of Daily CT‐Based Three‐Dimensional Image Guided and Adaptive Proton Therapy

Purpose:Daily CT‐based three‐dimensional image‐guided and adaptive (CTIGRT‐ART) proton therapy system was designed and developed. We also evaluated the effectiveness of the CTIGRT‐ART.Methods:Retrospective analysis was performed in three lung cancer patients: Proton treatment planning was performed using CT image datasets acquired by Toshiba Aquilion ONE. Planning target volume and surrounding organs were contoured by a well‐trained radiation oncologist. Dose distribution was optimized using 180‐deg. and 270‐deg. two fields in passive scattering proton therapy. Well commissioned Simplified Monte Carlo algorithm was used as dose calculation engine. Daily consecutive CT image datasets was acquired by an in‐room CT (Toshiba Aquilion LB). In our in‐house program, two image registrations for bone and tumor were performed to shift the isocenter using treatment CT image dataset. Subsequently, dose recalculation was performed after the shift of the isocenter. When the dose distribution after the tumor registration exhibits change of dosimetric parameter of CTV D90% compared to the initial plan, an additional process of was performed that the range shifter thickness was optimized. Dose distribution with CTV D90% for the bone registration, the tumor registration only and adaptive plan with the tumor registration was compared to the initial plan.Results:In the bone registration, tumor dose coverage was decreased by 16% on average (Maximum: 56%). The tumor registration shows better coverage than the bone registration, however the coverage was also decreased by 9% (Maximum: 22%) The adaptive plan shows similar dose coverage of the tumor (Average: 2%, Maximum: 7%).Conclusion:There is a high possibility that only image registration for bone and tumor may reduce tumor coverage. Thus, our proposed methodology of image guidance and adaptive planning using the range adaptation after tumor registration would be effective for proton therapy.This research is partially supported by Japan Agency for Medical Research and Development (AMED).

A comparison between two clinically applied plan library strategies in adaptive radiotherapy of bladder cancer

Background and purposeThe predominant approach to clinically applied adaptive radiotherapy (ART) for bladder cancer is daily selection of treatment plans from a plan library. In this study we have compared two clinical strategies for creating multiple planning target volumes (PTV) for ART of bladder cancer. Material and methodsOnline ART delivering 60Gy in 30 fractions to the whole bladder was simulated for ten patients using two methods of creating plan libraries. In the RepeatCT method four planning CT scans were acquired at 15-min intervals, generating four CTVs with different bladder volumes. In the RepeatCBCT method one planning CT and four daily cone-beam CT images were combined using Boolean operators to form three composite CTVs. Plan selection rates and PTV volumes were evaluated, with the selected volumes averaged across 30 treatment fractions (PTVmean). ResultsThe PTVmean volume was on average 80cm3 smaller (p<0.001) in the RepeatCT method than in the RepeatCBCT method. Compared to the non-adaptive treatment, the PTVmean was reduced by 46% (range 33–53%, RepeatCT) and 36% (range 27–44%, RepeatCBCT). ConclusionsBoth methods reduced the PTVmean volume compared to the non-adaptive approach, but the reduction was larger using the strategy with repeat planning CT imaging. However, the strategy with combined CT and repeat CBCT imaging produced a more adequate range of PTV volumes.

Ultratight Margin (2-3 mm) Intensity Modulated Radiation Therapy With Daily Cone Beam CT Image Guidance Yields Excellent Locoregional Control in Head and Neck Cancer

Ultratight Margin (2-3 mm) Intensity Modulated Radiation Therapy With Daily Cone Beam CT Image Guidance Yields Excellent Locoregional Control in Head and Neck Cancer

SU‐E‐J‐33: Comparison Between Soft Tissue Alignment and Bony Alignment for Pancreatic Cancer Radiotherapy

PurposeAn IGRT modality for pancreatic cancer treatment with dose escalation at our institution is in‐room daily CT imaging. The purpose of this study is to assess the difference between soft tissue alignment and bony alignment for pancreatic tumor localization.MethodsEighteen patients with pancreatic tumors who underwent IMRT treatment with an inspiration breath‐hold technique between July 2012 and February 2015 are included in this study. Prior to each treatment, a CT scan was acquired. The CT image guidance started with auto‐alignment to either the bony anatomy (vertebral bodies) or fiducials (for the six patients with the stent in/near the tumor) and then, when necessary, manual adjustments were made based on soft tissue alignment using clinical software (CT‐Assisted Targeting system). The difference between soft tissue alignment and bony/fiducial alignment was evaluated.ResultsOf all 380 treatments, manual adjustment was made in 225 treatments, ranging from 11% (3 treatments out of 28) to 96% (27 treatments out of 28) per patient. The mean of the difference between soft tissue alignment and bony/fiducial alignment per patient ranged from −3.6 to 0.3 mm, −1.5 to 2.8 mm, and −3.3 to 3.4 mm in the AP, SI, and RL directions, respectively. The maximum difference over all treatments was −9.5, −14.6, and −14.6 mm in the AP, SI, and RL directions, respectively.ConclusionAbout 60% of the time, manual adjustment based on soft tissue alignment was required. The extent of manual adjustment was usually small but varied significantly from patient to patient. The ultimate goal of the IGRT modality using daily CT imaging is not to fully cover the target but to spare organs‐at‐risk as much as possible to avoid them moving into higher dose gradients than accepted in the treatment plan. To this end, manual adjustment based on soft tissue alignment is critically important.

SU-C-210-06: Quantitative Evaluation of Dosimetric Effects Resulting From Positional Variations of Pancreatic Tumor Volumes

Purpose: The aim of this study is to quantify dosimetric effects resulting from variation in pancreatic tumor position assessed by bony anatomy and implanted fiducial markers Methods: Twelve pancreatic cancer patients were retrospectively analyzed for this study. All patients received modulated arc therapy (VMAT) treatment using fiducial-based Image Guided Radiation Therapy (IGRT) to the intact pancreas. Using daily orthogonal kV and/or Cone beam CT images, the shift needed to co-register the daily pre-treatment images to reference CT from fiducial to bone (Fid-Bone) were recorded as Left-Right (LR), Anterior-Posterior (AP) and Superior-Inferior (SI). The original VMAT plan iso-center was shifted based on KV bone matching positions at 5 evenly spaced fractions. Dose coverage of the planning target volumes (PTVs) (V100%), mean dose to liver, kidney and stomach/duodenum were assessed in the modified plans. Results: A total of 306 fractions were analyzed. The absolute fiducial-bone positional shifts were greatest in the SI direction, (AP = 2.7 ± 3.0, LR = 2.8 ± 2.8, and SI 6.3 ± 7.9 mm, mean ± SD). The V100% was significantly reduced by 13.5%, (Fid-Bone = 95.3 ± 2.0 vs. 82.3 ± 11.8%, p=0.02). This varied widely among patients (Fid-Bone V100% Range = 2–60%), where 33% ofmore » patients had a reduction in V100% of more than 10%. The impact on OARs was greatest to the liver (Fid-Bone= 14.6 vs. 16.1 Gy, 10%), and stomach, (Fid-Bone = 23.9 vx. 25.5 Gy, 7%), however was not statistically significant (p=0.10 both). Conclusion: Compared to matching by fiducial markers, matching by bony anatomy would have substantially reduced the PTV coverage by 13.5%. This reinforces the importance of online position verification based on fiducial markers. Hence, implantation of fiducial markers is strongly recommended for pancreatic cancer patients undergoing intensity modulated radiation therapy treatments.« less

TU‐F‐CAMPUS‐J‐04: Setup Uncertainties in the Mediastinum Area for IMRT Treatment of Lymphoma Patients

Purpose:Despite daily image guidance setup uncertainties can be high when treating large areas of the body. The aim of this study was to measure local uncertainties inside the PTV for patients receiving IMRT to the mediastinum region.Methods:Eleven lymphoma patients that received radiotherapy (breath‐hold) to the mediastinum were included in this study. The treated region could range all the way from the neck to the diaphragm. Each patient had a CT scan with a CT‐on‐rails system prior to every treatment. The entire PTV region was matched to the planning CT using automatic rigid registration. The PTV was then split into 5 regions: neck, supraclavicular, superior mediastinum, upper heart, lower heart. Additional auto‐registrations for each of the 5 local PTV regions were performed. The residual local setup errors were calculated as the difference between the final global PTV position and the individual final local PTV positions for the AP, SI and RL directions. For each patient 4 CT scans were analyzed (1 per week of treatment).Results:The residual mean group error (M) and standard deviation of the inter‐patient (or systematic) error (Σ) were lowest in the RL direction of the superior mediastinum (0.0mm and 0.5mm) and highest in the RL direction of the lower heart (3.5mm and 2.9mm). The standard deviation of the inter‐fraction (or random) error (σ) was lowest in the RL direction of the superior mediastinum (0.5mm) and highest in the SI direction of the lower heart (3.9mm) The directionality of local uncertainties is important; a superior residual error in the lower heart for example keeps it in the global PTV.Conclusion:There is a complex relationship between breath‐holding and positioning uncertainties that needs further investigation. Residual setup uncertainties can be significant even under daily CT image guidance when treating large regions of the body.

Online updating of context-aware landmark detectors for prostate localization in daily treatment CT images.

In image guided radiation therapy, it is crucial to fast and accurately localize the prostate in the daily treatment images. To this end, the authors propose an online update scheme for landmark-guided prostate segmentation, which can fully exploit valuable patient-specific information contained in the previous treatment images and can achieve improved performance in landmark detection and prostate segmentation. To localize the prostate in the daily treatment images, the authors first automatically detect six anatomical landmarks on the prostate boundary by adopting a context-aware landmark detection method. Specifically, in this method, a two-layer regression forest is trained as a detector for each target landmark. Once all the newly detected landmarks from new treatment images are reviewed or adjusted (if necessary) by clinicians, they are further included into the training pool as new patient-specific information to update all the two-layer regression forests for the next treatment day. As more and more treatment images of the current patient are acquired, the two-layer regression forests can be continually updated by incorporating the patient-specific information into the training procedure. After all target landmarks are detected, a multiatlas random sample consensus (multiatlas RANSAC) method is used to segment the entire prostate by fusing multiple previously segmented prostates of the current patient after they are aligned to the current treatment image. Subsequently, the segmented prostate of the current treatment image is again reviewed (or even adjusted if needed) by clinicians before including it as a new shape example into the prostate shape dataset for helping localize the entire prostate in the next treatment image. The experimental results on 330 images of 24 patients show the effectiveness of the authors' proposed online update scheme in improving the accuracies of both landmark detection and prostate segmentation. Besides, compared to the other state-of-the-art prostate segmentation methods, the authors' method achieves the best performance. By appropriate use of valuable patient-specific information contained in the previous treatment images, the authors' proposed online update scheme can obtain satisfactory results for both landmark detection and prostate segmentation.

Evaluation of Online/Offline Image Guidance/Adaptation Approaches for Prostate Cancer Radiation Therapy

To evaluate online/offline image-guided/adaptive treatment techniques for prostate cancer radiation therapy with daily cone-beam CT (CBCT) imaging. Three treatment techniques were evaluated retrospectively using daily pre- and posttreatment CBCT images on 22 prostate cancer patients. Prostate, seminal vesicles (SV), rectal wall, and bladder were delineated on all CBCT images. For each patient, a pretreatment intensity modulated radiation therapy plan with clinical target volume (CTV)=prostate+SV and planning target volume (PTV)=CTV+3mm was created. The 3 treatment techniques were as follows: (1) Daily Correction: The pretreatment intensity modulated radiation therapy plan was delivered after online CBCT imaging, and position correction; (2) Online Planning: Daily online inverse plans with 3-mm CTV-to-PTV margin were created using online CBCT images, and delivered; and (3) Hybrid Adaption: Daily Correction plus an offline adaptive inverse planning performed after the first week of treatment. The adaptive plan was delivered for all remaining 15 fractions. Treatment dose for each technique was constructed using the daily posttreatment CBCT images via deformable image registration. Evaluation was performed using treatment dose distribution in target and critical organs. Treatment equivalent uniform dose (EUD) for the CTV was within [85.6%, 100.8%] of the pretreatment planned target EUD for Daily Correction; [98.7%, 103.0%] for OnlinePlanning; and [99.2%, 103.4%] for Hybrid Adaptation. Eighteen percent of the 22 patients in Daily Correction had a target dose deficiency >5%. For rectal wall, the mean±SD of the normalized EUD was 102.6%±2.7% for Daily Correction, 99.9%±2.5% for Online Planning, and 100.6%±2.1% for Hybrid Adaptation. The mean±SD of the normalized bladder EUD was 108.7%±8.2% for Daily Correction, 92.7%±8.6% for Online Planning, and 89.4%±10.8% for Hybrid Adaptation. Both Online Planning and Hybrid Adaptation can achieve comparable target coverage and normal tissue sparing and are superior to the Daily Correction technique. The Daily Correction technique using a 3-mm target margin in the pretreatment plan is not appropriate to compensate for residual variations in CBCT image-guided prostate cancer radiation therapy.

Comparing Preliminary Acute Toxicity between Two Recent Prospective Radiotherapy Treatments for Patients with Prostate Cancer: Reduced PTV Margins vs GTV Boost

To compare the acute toxicity of the two clinical trials of prospective radiotherapy treatment of prostate cancer: Integrated Advanced Imaging for Treatment Planning and Guidance (Margin) study, and the Tumour-Targeted Radiotherapy (Target) study. 23 patients enrolled in the Margin study were treated using an IMRT plan with prescription dose of 78 Gy with reduced PTV margins of 3 mm to left/right and 5 mm to anterior-posterior and crania-caudal direction guided by means of daily cone beam CT (CBCT) imaging prior to treatment. A subset of patients in the Target study received only external beam radiation therapy. These 10 patients were treated using volumetric modulated arc therapy (VMAT) with a prescription dose of 76 Gy to the CTV and 95 Gy to the GTV (integrated VMAT boost). The acute toxicity for GI and GU was measured using CTCAE V4.0 toxicity assessments. Margin study showed GI toxicity of 30.43%, 60.87%, and 8.7% for grade 0, 1 and 2, respectively. GU toxicity was 4.35%, 78.26%, and 17.39%. The Target study showed a more promising preliminary result. GI toxicity in the Target study was 60.5.43%, and 37.5% for grade 0 and 1. The GU distribution showed 12.5% and 87.5% for grade 0 and 1. No grade 2 or higher toxicity was reported in Target patients. Target study showed a lower acute toxicity in both GI and GU compared to Margin study. Due to the limited sample size in both studies, however, further investigation is warranted for both acute toxicity and late toxicity with a larger sample size.

Online updating of context-aware landmark detectors for prostate localization in daily treatment CT images: Online updating of context-aware landmark detectors in daily treatment CT images

Online updating of context-aware landmark detectors for prostate localization in daily treatment CT images: Online updating of context-aware landmark detectors in daily treatment CT images

Digital reconstruction of high-quality daily 4D cone-beam CT images using prior knowledge of anatomy and respiratory motion

Conventional in-room cone-beam computed tomography (CBCT) lacks explicit representation of patient respiratory motion and usually has poor image quality and inaccurate CT numbers for target delineation and/or adaptive treatment planning. In-room four-dimensional (4D) CBCT image acquisition is still time consuming and suffers the same issue of poor image quality. To overcome this limitation, we developed a computational framework to digitally synthesize high-quality daily 4D CBCT images using the prior knowledge of motion and appearance learned from the planning 4D CT dataset. A patient-specific respiratory motion model was first constructed from the planning 4D CT images using principal component analysis of displacement vector fields across different respiratory phases. Subsequently, the respiratory motion model as well as the image content of the planning CT was spatially mapped onto the daily CBCT using deformable image registration. The synthesized 4D images possess explicit patient motion while maintaining the geometric accuracy of patient's anatomy at the time of treatment. We validated our model by quantitatively comparing the synthesized 4D CBCT against the 4D CT dataset acquired in the same day from protocol patients undergoing daily in-room CBCT setup and weekly 4D CT for treatment evaluation. Our preliminary results have demonstrated good agreement of contours in different motion phases between the synthesized and acquired scans. Various imaging artifacts were also suppressed and soft-tissue visibility was enhanced.

SU-E-J-263: Dosimetric Analysis On Breast Brachytherapy Based On Deformable Image Registration

Purpose: To quantitatively compare and evaluate the dosimetry difference between breast brachytherapy protocols with different fractionation using deformable image registration. Methods: The accumulative dose distribution for multiple breast brachytherapy patients using four different applicators: Contura, Mammosite, Savi, and interstitial catheters, under two treatment protocols: 340cGy by 10 fractions in 5 days and 825cGy by 3 fractions in 2days has been reconstructed using a two stage deformable image registration approach. For all patients, daily CT was acquired with the same slice thickness (2.5mm). In the first stage, the daily CT images were rigidly registered to the initial planning CT using the registration module in Eclipse (Varian) to align the applicators. In the second stage, the tissues surrounding the applicator in the rigidly registered daily CT image were non-rigidly registered to the initial CT using a combination of image force and the local constraint that enforce zero normal motion on the surface of the applicator, using a software developed in house. We calculated the dose distribution in the daily CTs and deformed them using the final registration to convert into the image domain of the initial planning CT. The accumulative dose distributions were evaluated by dosimetry parameters including D90, V150 and V200, as well as DVH. Results: Dose reconstruction results showed that the two day treatment has a significant dosimetry improvement over the five day protocols. An average daily drop of D90 at 1.3% of the prescription dose has been observed on multiple brachytherapy patients. There is no significant difference on V150 and V200 between those two protocols. Conclusion: Brachytherapy with higher fractional dose and less fractions has an improved performance on being conformal to the dose distribution in the initial plan. Elongated brachytherapy treatments need to consider the dose uncertainty caused by the temporal changes of the soft tissue.

SU‐E‐I‐87: Automated Liver Segmentation Method for CBCT Dataset by Combining Sparse Shape Composition and Probabilistic Atlas Construction

Purpose:The aiming of this study was to extract liver structures for daily Cone beam CT (CBCT) images automatically.Methods:Datasets were collected from 50 intravenous contrast planning CT images, which were regarded as training dataset for probabilistic atlas and shape prior model construction. Firstly, probabilistic atlas and shape prior model based on sparse shape composition (SSC) were constructed by iterative deformable registration. Secondly, the artifacts and noise were removed from the daily CBCT image by an edge‐preserving filtering using total variation with L1 norm (TV‐L1). Furthermore, the initial liver region was obtained by registering the incoming CBCT image with the atlas utilizing edge‐preserving deformable registration with multi‐scale strategy, and then the initial liver region was converted to surface meshing which was registered with the shape model where the major variation of specific patient was modeled by sparse vectors. At the last stage, the shape and intensity information were incorporated into joint probabilistic model, and finally the liver structure was extracted by maximum a posteriori segmentation.Regarding the construction process, firstly the manually segmented contours were converted into meshes, and then arbitrary patient data was chosen as reference image to register with the rest of training datasets by deformable registration algorithm for constructing probabilistic atlas and prior shape model. To improve the efficiency of proposed method, the initial probabilistic atlas was used as reference image to register with other patient data for iterative construction for removing bias caused by arbitrary selection.Results:The experiment validated the accuracy of the segmentation results quantitatively by comparing with the manually ones. The volumetric overlap percentage between the automatically generated liver contours and the ground truth were on an average 88%–95% for CBCT images.Conclusion:The experiment demonstrated that liver structures of CBCT with artifacts can be extracted accurately for following adaptive radiation therapy.This work is supported by National Natural Science Foundation of China (No. 61201441), Research Fund for Excellent Young and Middle‐aged Scientists of Shandong Province (No. BS2012DX038), Project of Shandong Province Higher Educational Science and Technology Program (No. J12LN23), Jinan youth science and technology star (No.20120109)