Daily CT Images Research Articles

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?

Does Changing Image Guidance Processes Impact Patient Setup Practice?

Does Changing Image Guidance Processes Impact Patient Setup Practice?

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)

WE-B-350-01: Adaptive Management of Patient Motion in Radiotherapy

Patient‐specific target margin and 4D inverse planning are two typical methods to include treatment relevant patient motion in the treatment planning. Internal target volume (ITV) is probably the earliest technique in constructing patient‐specific target volume to compensate for target motion. However, this method only considers motion geometric information in the construction without utilizing inter‐patient heterogeneity of dose distribution, therefore the target margin has not been minimized. In contrast, a recent development in patient‐specific target margin construction includes motion probability density function (pdf) directly in the 4D dose evaluation; therefore individual dose distribution is included in the target margin design. Utilizing 4D dose calculation, treatment plan can also be optimized by modulating beam intensity with respect to a pre‐measured patient motion pdf. Concept of utilizing non‐uniform beam intensity to compensate for temporal displacements of target position has been pointed out long time ago, however it has not been implemented until recent years. Process of designing beam intensity by including patient geometric variation or motion pdf in the dose calculation and inverse planning has been called 4D inverse planning. Since this planning process is based on organ displacement information in a frequency domain, instead of the spatial domain used in the gating and tracking techniques, it is insensitive to the uncertainties in the motion phase. However, uncertainties in pdf characteristics could significantly detriment the treatment qualities. The most significant variation in motion pattern is the target position baseline variation. Study has shown that inter‐treatment target baseline variation can be minimized efficiently using daily free breathing cone beam CTimaging location and patient repositioning. However, the relative position variation between target and normal organs, the dose response related organ volume variation, as well as intra‐treatment motion pattern variation cannot be easily corrected by repositioning the patient. Therefore, treatment feedback and adaptive planning modification become necessary. In principle, 4D inverse planning is a central part of adaptive planning modification, however to fully accomplish an adaptive planning, the dose distribution in organs of interest which has been delivered previously and may be delivered in future is also important factor and needs to be included in the objective evaluation for 4D planning optimization. Educational Objectives: 1. Understand the options of 4D planning. 2. Understand the sensitivity of 4D planning to the motion uncertainties. 3. Understand the key components of adaptive treatment process and their functions. Research on adaptive management of patient motion is partially supported by Elekta and Philips Research Funds.

SU‐GG‐J‐50: Comparing IGRT Shifts with MVision and CT On Rails Systems Using Anthropomorphic Phantom

Purpose: To evaluate the IGRT repositioning shifts determined by MV Cone Beam CT and by CT on rails using an anthropomorphic phantom. Method and Materials: Two IGRT systems installed in the same Linac room were studied. The CT based IGRT has been used in our Siemens Primus since 2004. Recently, a MV conebeam CT (Siemens MVision) was implemented with the objective to reduce the therapist time to obtain the IGRT shifts. The shifts and the corresponding total time were evaluated using an anthropomorphic PIXY® phantom. CTs of the phantom were obtained for head‐and‐neck, pelvis and lung and treatment planning was generated in XIO‐CMS for each site. The set of images, structures and plans was sent to both IGRT stations. Bbs were placed on the phantom surface at the isocenter axis, as a reference for the daily CT images. For each plan the MVCBCT was obtained three times to determine its reproducibility for 5 and 8 MU protocols. The registration for MVCBCT was done automatically and verified with manual registration. The table was then flipped to the CT position and the images acquired. In this case the registration is done manually and the isocenter shift is obtained through the bbs position. The procedures ware repeated for all sites. The total time to obtain the shifts and the image quality were also registered. Results and Discussion: The reproducibility of the MVCBCT for different protocols was within 2 mm. The maximum difference for MVCBCT and CT was 4 mm. The MVCBCT has poor image for soft tissue, and the fusion registration is based on mutual information. The total time to obtain the shifts is about 3 min for the new system and 7 to 8 min for the CT based system. This study excludes any deviation due to patient rotation and internal changes.

WE-E-AUD A-06: Image and Dose Processing for Image Guided Adaptive Radiation Therapy and Outcome Research

Purpose: To develop a general image processing and dose computation procedure that allows for the accumulation of accurate dose distributions for purposes of both outcome research and IG‐ART (image guided adaptive radiation therapy). We applied the procedure to cone‐beam CT head‐neck cancertreatment cases, as well as Tomotherapy MVCT (mega‐voltage CT) GYN and prostate cancertreatment cases. Method and Materials: The image processing procedures include preprocessing, rigid and deformable registration, mesh‐ based structure contour propagation, and image volume composition for dose re‐computation. The dose processing includes dose recomputation on the daily CT or the composed CT volume, deforming and register the daily dose to the planning dose space, dose accumulation, evaluating of the accumulated dose against the planned dose. Accuracy of image registration is very important for the entire procedure because it determines the accuracy of all later subsequent steps. In order to improve the accuracy, we preprocessed the CTimages before the registration step by using various methods, including edge‐preserving smoothing, Gaussian lowpass smoothing, contrast enhancement, window‐level intensity transformation, and bowel gas pocket painting for abdominal regions. We used MI (mutual information) based algorithm for rigid image registration, and applied the Horn‐Schunck optical flow algorithm for deformable image registration. We used a mesh‐based algorithm for structure contour deformation, interpolation and smoothing. Results: By applying these preprocessing procedures we were able to achieve improved image registration results. The computed image deformation fields were then used to propagate the structure contours from the kVCT to the current daily CTimage, which could be used for dose deformation or re‐computation. Conclusion: We have developed a procedure of image processing and dose computation to support the accumulation of ‘true’ dose distributions. These methods could be used in both radiotherapy outcome research and adaptive radiotherapy applications. Partially supported by NIH R01 grant CA85181.

Do Anatomic Changes During Radiation Impact Normal Tissue and Target Dosimetry? An Analysis of Calculated Cumulative Dose in Head and Neck Patients

To analyze the magnitude of the dosimetric impact of anatomic changes occuring in H&N cancer patients throughout a course of radiotherapy. Eight H&N cancer patients treated definitively on a helical tomotherapy unit constituted the study population. Intensity modulated radiation therapy was planned to be delivered. Planning was performed using the following parameters: 1) Prescription gross target volume (GTV) dose: 64.0 to 74.4 Gy delivered at 2.12–2.00 Gy per fraction; 2) 54–57 Gy to cover ≥95% of low risk PTV; 3) 60 Gy to cover ≥95% of high risk PTV; and 4) <30% of parotid glands receiving 26 Gy. In cases with ipsilateral disease, planning was prioritized to spare the contralateral parotid. Daily MVCT imaging was performed on all cases as a part of routine image guided radiation therapy. At total of 331 daily megavoltage CT scans were evaluated. Daily MVCT scans were processed through a novel deformable image registration (DIR) algorithm to register daily megavoltage CT images to the original planning simulation kilovotage CT image. The deformable registration process was verified by reviewing the adequacy of automatically generated contours on the daily megavoltage CT images. Cumulative dose volume histograms for target volumes and organ at risk volumes were generated and the relative deviation of these DVHs from dosimetry conceived during initial treatment planning was carefully analyzed. The DIR algorithm adequately predicted the location of target and organs at risk volumes in >98% of daily MVCTs reviewed. The average D95 of the high dose target volumes was 68.8 Gy at initial planning versus 67.5 Gy after DIR was applied. In all cases, the difference between planned and actual delivered dose for the target areas was less than 3%. The average parotid dose was higher than initially planned (Fig 1); i.e.113% of planned dose. During the initial planning process in all patients, the dose to at least 1 parotid was kept <25–30 Gy. However, after cumulative DVH evaluation, in 50% of patients, average parotid dose changed from 25–30 Gy (at initial planning) to exceed 30 Gy. Authors have described anatomic changes during a course of radiation therapy1. Using a novel deformable registration algorithm, the impact of these changes on actual delivered doses was evaluated. Anatomic changes throughout the course of treatment had little effect on delivered target doses either to the primary or nodal areas. However, the actual delivered parotid doses were substantially higher than what was planned, exceeding the significant threshold of 25–30 Gy in spared parotids in 50% of cases.1.Barker IJROBP 2004

Probabilistic Analysis of Cord Position in Daily On-Line Cone Beam CT Imaging for Head and Neck Radiotherapy

Probabilistic Analysis of Cord Position in Daily On-Line Cone Beam CT Imaging for Head and Neck Radiotherapy

SU-FF-T-202: EUD-Based Indication for Re-Planning During Image-Guided Adaptive Radiotherapy

Purpose: A quantitative real-time indicator for re-planning due to the inter-fractional anatomic changes (e.g., volume and shape changes) during the course of radiation treatment is not available for image-guided adaptive radiotherapy (IG-ART). In this work, we propose a EUD-based indicator to address this issue. Methods and Materials: A EUD-based quantity is introduced to assess the complicated EUD changes due to the inter-fractional changes collected with daily CT images. Both target(s) and normal structures are considered. IEUD is calculated using the DVHs based on the original planning CT and the daily CT images. To demonstrate feasibility, we have calculated IEUD for selected representative cases in three tumor types: lung, head-and-neck and sarcoma, based on daily CT images acquired with a tomotherapy (HiArt, Tomotherapy) and a CT-on-Rail (Primatom, Siemens). The clinical target volumes (CTVs) and various normal structures were contoured on a series of daily CTs. The verification DVHs were generated by applying the original (or previous re-planning) beam setups to the CT set of the day using either the XiO (CMS) planning system (for the patients treated with the Primatom) or the tomotherapy Planned Adaptive software (for those treated with Tomotherapy). Results: Significant inter-fractional anatomic changes were observed on all selected cases, and these changes can be quantified by the values of IEUD. For a soft-tissue sarcoma at chest, for example, the CTV in the middle of the treatment was found approximately doubled from that on the planning day. Such dramatic volume changes resulted in significant decreases in the CTV coverage (95 reduced to 30%), the IEUD value dropped by 15.6% (compared to that of the original plan), indicating the necessity for re-planning. Conclusions: Dramatic anatomic changes may be observed during the treatment. The presently proposed EUD-based indicator, capable of quantifying such changes, can be used to trigger re-planning for IG-ART.

TH‐D‐M100F‐07: Topology Preserving Diffusion Registration Incorporating with Gradient Orientation Information

Purpose: The goal of this study was to develop an image intensity‐based diffusion registration algorithm that can be used for reliable automatic delineation of anatomical structures on daily CT images. The constraint of the topology preservation method used in this algorithm ensures that the transformations are reliable and accurate. Method and Materials: To achieve accurate deformable image registration using a diffusion‐based method, we proposed to incorporate the gradient orientation information into the driving force in diffusion registration. This is similar to the symmetric force in the demons algorithm. We also introduced to use the positive Jacobian constraint, which is the fundamental requirement for topology preservation, as a guideline to determine the smoothing parameter in the diffusion algorithm to ensure that a realistic registration can be achieved. The planning contours were mapped onto the daily CT image using the displacement field after the deformable image registration. The contour mapping serves as a validation of the algorithm proposed in this study. The performance of the proposed algorithm for register 3D CT images of prostate and head and neck cancer patients were evaluated by visually assessing the agreement of the anatomical structures with deformed contours in target image. Results: The contours deformed with the topology preservation method are more accurate segmentation of the anatomical structures, compared to a similar method without this constraint. The positivity of Jacobian provided a way to evaluate the performance of the registration and to serve as guidance for selecting smoothing parameters. Conclusion: We proposed a diffusion registration algorithm incorporated with the intensity gradient information and the positive Jacobian constraint that ensures the transformations to topologically preservation of anatomical structures. Compared to demons algorithm, the proposed algorithm is more computational efficient and accurate, especially for prostate CT images registration.The research is in part sponsored by Varian.

SU-FF-J-45: A Comparison of Daily Megavoltage CT and Ultrasound Imaging Guided Radiation Therapy for Prostate Cancer

Purpose: To compare the prostate localization capabilities of ultrasound imaging and megavoltage CT (MVCT) for on-line daily image-guided radiotherapy (IGRT) of the prostate cancer. Method and Materials: Daily shifts were analyzed to account for inter-fractional variations collected from a total of 129 prostate cancer patients, 106 with ultrasound-based imaging (BAT, B-mode Acquisition and Targeting) and 23 with MVCT of a TomoTherapy (HiArt) unit. The shifts indicated by the two systems were compared statistically along the right/left (R/L), superior/inferior (S/I), and anterior/posterior (A/P) directions. The systematic and random variations among the daily alignments were calculated. Margins to account for these shifts were estimated. The daily shifts from the first 25 treatment fractions and from the remaining fractions were compared for both systems to examine whether a difference in the prostate motion pattern exists between the first 25 fractions and the rest. Results: The mean shifts and standard deviations along the R/L, S/I, and A/P directions were −0.11±3.80mm, 0.67±4.67mm, and 2.71±6.31mm for BAT localizations and −0.88±5.36mm, 0.41±3.38mm, and 0.91±3.33mm for MVCT localizations, respectively. It is clear that the systematic and random variations in daily shifts based on MVCT were generally smaller than those based on BAT, especially along the A/P direction. A t-test showed this difference to be statistically significant. The PTV margins estimated to account for daily variations were 6.69 mm and 14.66 mm based on MVCT and BAT data, respectively. There was no statistically significant difference in the daily prostate movement pattern between the first 25 fractions and the remaining. Conclusion: The MVCT technique results in smaller variations in daily shifts than ultrasound imaging, indicating that MVCT is more reliable and precise for prostate localization. Ultrasound-based localization may overestimate the daily prostate motion, particularly in the A/P direction. This work is supported partially by MCW Cancer Center Fotsch Foundation.

2747

Offline image guided adaptive treatment technique with single treatment plan modification has been designed to manage patient specific anatomical motion that has a stationary feature during the process of external beam radiation delivery. However, this feature cannot be always guaranteed, and therefore it is important to evaluate the potential site-specific treatment quality prior to clinical implementation. Offline adaptive radiation treatment of prostate cancer with single inverse planning modification was mimicked using multiple daily CT images (15 ∼ 18 per patient) from 21 prostate cancer patients. A pretreatment planning CT image and four extra CTs obtained during the first week treatment were used for adaptive inverse planning. The remaining CT images, acquired twice per week during the entire treatment course, were used to evaluate this adaptive treatment plan. Daily subvolume displacements of prostate (target), rectal wall and bladder wall were calculated by model based deformable organ registration. The subvolume displacements of the first five days were used to approximate a probability density function and included in the objectives of the adaptive inverse planning. Optimal intensity of five co-planar beams was then iteratively searched maximizing the target EUD while minimizing the EUD of rectal wall and bladder wall. The differences between the estimated (planned) EUD and the ‘treatment EUD’ for each organ of interest were used to evaluate this image guided treatment technique. In addition, the evaluation was also performed as a time-sequence (weekly) of the treatment delivery to determine the potential time trend effect of the anatomical variation, as well as the confidence of image guided treatment evaluation. The EUD differences between the treatment and estimation are -0.07±0.45 Gy, -0.76±2.47 Gy and -0.31±3.65 Gy for prostate, bladder wall and rectal wall respectively. The adaptive plan demonstrates a high confidence on target dose estimation, meanwhile the bladder and rectal wall EUD calculated from the adaptive inverse planning have relatively weaker representation. However, their estimation power can be improved using weekly image-guided treatment evaluation (Figure). Target dose in offline image guided radiotherapy with single adaptive inverse planning modification can be guaranteed with very high confidence. Comparably, the bladder and rectal wall doses estimated from the adaptive planning could have relatively large biases. However, these biases can be minimized during the treatment using weekly image feedback.

2237: Daily Variations in Delivered Doses In Patients Treated With Radiotherapy for Localized Prostate Cancer

To study the variations in delivered doses to the prostate gland, rectum and bladder during a full course of image-guided external beam radiotherapy. Ten localized prostate cancer patients were treated with helical tomotherapy to 78 Gy at 2 Gy per fraction, in 39 fractions. Prior to each fraction, a megavoltage pelvic CT study was obtained, resulting in a total of 390 CT studies. The prostate, rectum and bladder were manually contoured on each CT study by a single physician. Daily dosimetric analysis was performed with dose recalculation. The specific study endpoints were D95 (dose to 95% of the prostate), rV2 (absolute rectal volume receiving 2 Gy), and bV2 (absolute bladder volume receiving 2 Gy). The average + S.D. and range of each parameter were analyzed either per patient or in the entire study cohort. For the entire cohort and all 390 fractions, the average daily D95 (+S.D.) was 2.02 + 0.04 Gy (range: 1.79 - 2.20 Gy). This shows remarkable constancy of delivered prostate doses throughout the course of treatment in this group of patients with daily alignment performed on intraprostatic fiducials. For the entire cohort, the average rV2 (+S.D.) was 7.0 + 8.1 cc (range: 0.1 - 67.3 cc). For the entire cohort, the average bV2 (+S.D.) was 8.7 + 6.8 cc (range: 0.3 - 36.8 cc). Fig. 1 shows the variation in rectal doses by displaying the rectal volume receiving 2 Gy in the individual 10 patients; the rV2 value as per the initial plan, the mean rV2 for all 39 fractions per patient, the S.D. of the rV2 for all 39 fractions per patients, the minimum rV2 for all 39 fractions per patient, and the maximum rV2 for all 39 fractions per patient. This shows stiriking variation in rectal doses from patient to patient, and large deviations from day to day in individual patients, e.g. patient #2. In this cohort of patients, rectal doses were, on average, higher than expected in the large majority of patients. Daily CT images can be used to validate doses delivered duing a course of external beam radiotherapy for localized prsotate cancer. Unlike for the prostate gland, there is significant day to day variation in rectal and bladder doses, mostly due to daily variations in volume and shape of these organs. Image guidance for the targeting of the prostate, even with intraprostatic fiducials, does not take into account the variation in actual rectal and bladder doses. Techniques that take such dosimetric parameters into account in daily patient set-ups should be investigated.

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).

2424: Assessment of Setup Uncertainties for Head-and-Neck Cancer Patients Using Daily Megavoltage CT Imaging

2424: Assessment of Setup Uncertainties for Head-and-Neck Cancer Patients Using Daily Megavoltage CT Imaging

SU‐FF‐J‐23: An Image‐Intensity Modification Method to Deal with Non‐Correspondence Problem in Deformable Image Registration of the Gaseous Rectum

Purpose: The goal of this study is to develop an image intensity modification method together with the intensity based deformable registration algorithm for auto‐segmentation of the rectum on daily CT images. In CT‐guided prostate cancer radiotherapy, the planning CT image may contain an empty rectum while there may exist bowel gases in the rectum on any given treatment day (daily CT). The intensity based image registration algorithms alone were insufficient to produce the correct spatial transformations for all objects, especially for the rectum since there was no one‐to‐one correspondence in the gaseous region. Material and Methods: In this study, we proposed a diffusion‐based deformable image registration algorithm combined with an automatic intensity modification method that introduces artificial gas pockets in the planning CT based on prior knowledge of the previously contoured rectum. This will allow for the establishment of correspondence between the two CT images. A multi‐grid method was used for solving the nonlinear partial differential equation for the displacement field, and this displacement field was used to map the manual rectum contours from the planning CT to the daily CT. 30 CT images with the largest bowel gas fillings from 15 prostate cancer patients were chosen to test the algorithm. Results: The intensity modification technique is demonstrated to be effective in correctly delineating the daily rectum. Compared with the deformable image registration method without rectum image modification, the average volume overlap index was improved from 50.6% to 71.2%. This was visually verified by overlaying the segmented contours onto the daily CT images. Conclusion: We developed an effective auto‐segmentation technique for rectum using a deformable image registration algorithm combined with an image intensity modification method. The approach is fully automatic and capable of handling the special non‐correspondence problem in CT images of prostate cancer patients.

SU‐DD‐A3‐03: A Dose‐Guided Adaptive Therapy Process for Treatment Evaluation and Correction

Purpose: To develop an adaptive therapy process for off‐line radiotherapy evaluation and modification. Method and Materials: An adaptive therapy process was developed to analyze and adjust a patient treatment. This included:Daily Online Processes• Daily CT imaging• Patient repositioningWeekly Offline Processes• Automatic dose recalculation on each daily image• Automatic deformable registration of each image with the planning image• Automatic deformation‐based recontouring of each image• Automatic deformation‐based dose accumulation• Cumulative plan evaluation• Replanning, as neededThe on‐line imaging and repositioning was performed with the TomoTherapy MVCT system and integrated registration software. The off‐line processes were performed on a standalone workstation.Head and neck cases were studied with this process. Cumulated doses were typically analyzed at the end of each week, and modifications were performed mid‐course. Remaining treatments were then performed with the adapted plan. Results: It was found that CT‐guided soft‐tissue positioning alone did not protect against dosimetric changes due to patient weight loss. Without plan adaptation, the right parotid gland would have received a dose of 10 Gy above the plan, due to its medial motion towards the target region. However, since mid‐course adaptive replanning was used, the dose was only 2.5 Gy above the original plan. The use of an additional plan adaptation could have further reduced this discrepancy. Conclusion: An adaptive therapy process was developed for off‐line contouring, dose recalculation, dose accumulation, and replanning. This process was applied to clinical head and neck patients to evaluate on‐going treatments, adjust plans, and retrospectively assess the results. This process reduced unexpected parotid dose for these patients. This process also indicated that ability to recalculate to accumulate doses on daily CT images is important for addressing systematic errors, such as anatomical changes, that may arise during a treatment.Supported by TomoTherapy, Inc.

TU‐C‐ValB‐08: Modeling Dose Delivery Accuracy of IMRT Head‐And‐Neck Cancer Treatment

Purpose: To evaluate the impact of internal organ variations on IMRT treatments of head‐and‐neck cancers using different daily alignment techniques. Method and Materials: Eleven head‐and‐neck cancer patients were imaged twice weekly during their course of treatment (141 CT scans total) using an integrated CT‐linear accelerator (EXaCT, Varian Oncology Systems). The clinical IMRT plans were copied onto the daily CT images. The plans were aligned with (1) the daily marked isocenter using three radio‐opaque markers (BBs) and (2) bone alignment, using in‐house software to align the cervical vertebrae. Daily dose distributions were mapped from the daily CT images onto the planning CT image with an in‐house deformable image registration algorithm. Cumulative dose‐volume histograms from the planning CT image were analyzed. Results: The differences in the clinical target volumes (CTV) gEUD between the planned and delivered doses (with BB or bone alignment) were typically ⩽1Gy; therefore the differences in target coverage were most likely clinically insignificant. However, the alignment method did have a statistically significant impact on the percentage‐volume of the CTV at the prescription dose. Neither BB alignment nor bone alignment maintained the planned coverage (average=98.2%), which was reduced to 95.6% with bone alignment (p=0.000) and 94.3% with BB alignment (p=0.000).BB alignment significantly increased the average percentage‐volume receiving ⩾25Gy above the original plan for the ipsilateral (59.6% vs. 51.4%, p=0.003) and contralateral (42.0% vs. 36.4%, p=0.016) parotid glands. The parotid gland gEUD increased by more than 5Gy in 35% of BB alignments and 15% of bone alignments. However, there was no statistically significant difference between BB and bone alignments in parotid dose received. Conclusions: The differences in CTV coverage between bone and BB alignment were statistically significant but small. Bone alignment more closely reproduced the planned parotid dose than BB alignment, although both gave higher dose than the original plan.

SU‐FF‐T‐262: Image‐Guided Non‐Invasive Stereotactic Radiosurgery

Purpose: The objective of this study is to demonstrate that the use of image registration and a non‐invasive head frame for SRS can achieve the same accuracy as the current invasive head‐ring SRS technique. Method and Materials: First, a head phantom was used to evaluate the accuracy of this procedure for aiming the target. Then, two SRT patients fixed with the GTC frame were treated with the proposed technique for ten treatments. The five‐point GTC alignment device was used to aid the repositioning of the GTC frame with respect of the original setup. This alignment device minimized the misalignment of the GTC frame in AP, lateral, and axial translations and the angular deviations about each of these axes on roll, tilt, and spin. Prior to each treatment, a daily CT scan with a localization device was acquired. Daily CT images were registered with planning CT and MRI images. The 9 rods on the daily CT images were then identified, such that all the target, critical structures, and external contours from the planning images were transferred to the daily stereotactic coordinates. A new plan was generated. Results: In the head phantom study, the daily isocenter setup was verified to be within 0.2 mm accuracy based upon the portal images verse the planned DRRs. Optimized the dose distributions for each daily CT images of ten treatments, the mean deviations of the daily isocenter from the planned isocenter are 0.2±0.12, −0.4±0.24, and −0.1±0.07 mm in AP‐, LAT‐, and VERT‐direction, respectively. The comparison of dose‐volume histograms is clearly revealed the prescribed dose enclosed the PTV for every treatment. Conclusions: The image‐guided noninvasive frame SRS has the capability of delivering high level accuracy of dose to the lesion without the discomfort from the pins fixed to the patient's skull.

TH‐C‐ValB‐06: Thoracic Proton Treatment Planning Strategies Based On the 4D CT Information

Purpose: Particularly in the case of thoracic radiation therapy, there are substantial inter‐ and intra‐fractional variations in shape, volume and position of treatment targets and the intervening and surrounding normal tissues. The purpose of this work is to develop proton treatment planning strategies for mobile tumors with and without mobile intervening structures based on 4D CT, and to assess the planning strategies using 4D CT data and daily in‐room CT information. Method and Materials: Five treatment planning strategies were evaluated based on (1) free breathing CT with small smearing, (2) free breathing CT with large smearing, (3) average CT with small smearing, (4) average CT with CT numbers inside the tumor volume replaced by higher CT numbers, and (5) maximum intensity projection CT. For a lung patient with large, 1.6 cm, tumor motion and immobile surrounding tissues, treatment plans were designed using strategies 1, 2 and 4. Each treatment plan was recalculated on five daily CT images using bony structure alignment. For an esophagus patient with 3.5 cm tumor motion and large cardiac, liver and spleen motion, treatment plans were designed using strategies 1, 2, 3 and 5. Each treatment plan was recalculated in all 10 phases of the 4D CT. Results: For the lung patient, the tumor coverage evaluated using the five daily CT's is superior when using strategies 2 and 4 as compared to strategy 1. However, the lung sparing is superior using strategy 4 compared to strategy 2. For the esophagus patient, the treatment plans using strategy 2 and 5 covered the targets for each 4D CT phase while the plan using strategy 1 and 3 caused significant under‐dosing. Conclusion: With the strategies developed using 4D CT data, good tumor coverage was achieved for the thoracic patients with large tumor and surrounding tissue motion using proton therapy.