Intrafractional Variations Research Articles

TU-AB-BRB-02: Stochastic Programming Methods for Handling Uncertainty and Motion in IMRT Planning

The accepted clinical method to accommodate targeting uncertainties inherent in fractionated external beam radiation therapy is to utilize GTV-to-CTV and CTV-to-PTV margins during the planning process to design a PTV-conformal static dose distribution on the planning image set. Ideally, margins are selected to ensure a high (e.g. >95%) target coverage probability (CP) in spite of inherent inter- and intra-fractional positional variations, tissue motions, and initial contouring uncertainties. Robust optimization techniques, also known as probabilistic treatment planning techniques, explicitly incorporate the dosimetric consequences of targeting uncertainties by including CP evaluation into the planning optimization process along with coverage-based planning objectives. The treatment planner no longer needs to use PTV and/or PRV margins; instead robust optimization utilizes probability distributions of the underlying uncertainties in conjunction with CP-evaluation for the underlying CTVs and OARs to design an optimal treated volume. This symposium will describe CP-evaluation methods as well as various robust planning techniques including use of probability-weighted dose distributions, probability-weighted objective functions, and coverage optimized planning. Methods to compute and display the effect of uncertainties on dose distributions will be presented. The use of robust planning to accommodate inter-fractional setup uncertainties, organ deformation, and contouring uncertainties will be examined as will its use to accommodate intra-fractional organ motion. Clinical examples will be used to inter-compare robust and margin-based planning, highlighting advantages of robust-plans in terms of target and normal tissue coverage. Robust-planning limitations as uncertainties approach zero and as the number of treatment fractions becomes small will be presented, as well as the factors limiting clinical implementation of robust planning. Learning Objectives: 1. To understand robust-planning as a clinical alternative to using margin-based planning. 2. To understand conceptual differences between uncertainty and predictable motion. 3. To understand fundamental limitations of the PTV concept that probabilistic planning can overcome. 4. To understand the major contributing factors to target and normal tissue coverage probability. 5. To understand the similarities and differences of various robust planning techniques 6. To understand the benefits and limitations of robust planning techniques

SU-C-210-03: Impact of Breathing Irregularities On Gated Treatments

Purpose: To evaluate the effect of breathing irregularities on target location in gated treatments using amplitude and phase gating. Methods: 111 breathing patterns acquired using RPM system were categorized based on period and amplitude STD as regular (STD period ≤ 0.5 s, STD amplitude ≤ 1.5 mm), medium (0.5 s 1 s, STD amplitude > 3 mm). One pattern representative of the average defined population was selected per category and corresponding target motion reproduced using Quasar Respiratory Motion Phantom. Phantom in motion underwent 4D-CT scan with phase reconstruction. Gated window was defined at end of exhale and DRRs reconstructed in treatment planning at 40% (beam on) and 60% phase (beam off). Target location uncertainty was assessed by comparing gated kV triggered images continuously acquired at beam on/off on a True Beam 2.0 with corresponding DRRs. Results: Average target uncertainty with amplitude gating was in [0.4 – 1.9] mm range for the different scenarios with maximum STD of 1.2 mm for the irregular pattern. Average target uncertainty with phase gating was [1.1 – 2.2] mm for regular and medium patterns, while itmore » increased to [3.6 – 9.6] mm for the irregular pattern. Live gated motion was stable with amplitude gating, while increasing with phase gating for the irregular pattern. Treatment duration range was [68 – 160] s with amplitude and [70 – 74] s with phase gating. Conclusion: Breathing irregularities were found to affect gated treatments only when using phase gating. For regular and medium patterns no significant difference was found between the two gating strategies. Amplitude gating ensured stable gated motion within the different patterns, thus reducing intra-fraction target location variability for the irregular pattern and resulting in longer treatment duration.« less

SU‐E‐T‐237: Deformable Image Registration and Deformed Dose Composite for Volumetric Evaluation of Multimodal Gynecological Cancer Treatments

Purpose:Radiotherapy plans for patients with cervical cancer treated with EBRT followed by HDR brachytherapy are optimized by constraining dose to organs at risk (OARs). Risk of treatment related toxicities is estimated based on the dose received to the hottest 2cc (D2cc) of the bladder, bowel, rectum, and sigmoid. To account for intrafractional variation in OAR volume and positioning, a dose deformation method is proposed for more accurate evaluation of dose distribution for these patients.Methods:Radiotherapy plans from five patients who received 50.4Gy pelvic EBRT followed by 30Gy in five fractions of HDR brachytherapy, using split‐ring and tandem applicators, were retrospectively evaluated using MIM Software version 6.0. Dose accumulation workflows were used for initial deformation of EBRT and HDR planning CTs onto a common HDR planning CT. The Reg Refine tool was applied with user‐specified local alignments to refine the deformation. Doses from the deformed images were transferred to the common planning CT. Deformed doses were scaled to the EQD2, following the linear‐quadratic BED model (considered α/β ratio for tumor as 10, and 3 for rest of the tissues), and then combined to create the dose composite. MIM composite doses were compared to the clinically‐reported plan assessments based upon the American Brachytherapy Society (ABS) guidelines for cervical HDR brachytherapy treatment.Results:Bladder D2cc exhibited significant reduction (−11.4%±3.85%, p< 0.02) when evaluated using MIM deformable dose composition. Differences observed for bowel, rectum, and sigmoid D2cc were not significant (−0.58±7.37%, −4.13%±13.7%, and 8.58%±4.71%, respectively and p>0.05 for all) relative to the calculated values used clinically.Conclusion:Application of deformable dose composite techniques may lead to more accurate total dose reporting and can allow for elevated dose to target structures with the assurance of not exceeding dose to OARs. Further study into deformable dose composition and correlation with clinical outcomes is warranted.

Four-dimensional layer-stacking carbon-ion beam dose distribution by use of a lung numeric phantom.

To extend layer-stacking irradiation to accommodate intrafractional organ motion, we evaluated the carbon-ion layer-stacking dose distribution using a numeric lung phantom. We designed several types of range compensators. The planning target volume was calculated from the respective respiratory phases for consideration of intrafractional beam range variation. The accumulated dose distribution was calculated by registering of the dose distributions at respective phases to that at the reference phase. We evaluated the dose distribution based on the following six parameters: motion displacement, direction, gating window, respiratory cycle, range-shifter change time, and prescribed dose. All parameters affected the dose conformation to the moving target. By shortening of the gating window, dose metrics for superior-inferior (SI) and anterior-posterior (AP) motions were decreased from a D95 of 94%, Dmax of 108%, and homogeneity index (HI) of 23% at T00-T90, to a D95 of 93%, Dmax of 102%, and HI of 20% at T40-T60. In contrast, all dose metrics except the HI were independent of respiratory cycle. All dose metrics in SI motion were almost the same in respective motion displacement, with a D95 of 94%, Dmax of 108%, Dmin of 89%, and HI of 23% for the ungated phase, and D95 of 93%, Dmax of 102%, Dmin of 85%, and HI of 20% for the gated phase. The dose conformation to a moving target was improved by the gating strategy and by an increase in the prescribed dose. A combination of these approaches is a practical means of adding them to existing treatment protocols without modifications.

PO-1029: Intra-fractional organ position variation evaluation during HDR intracavitary brachytherapy of cervical cancer

PO-1029: Intra-fractional organ position variation evaluation during HDR intracavitary brachytherapy of cervical cancer

EP-1494: Evaluation of intra-fraction breathing pattern variability in Helical Tomotherapy by means of optical tracking

EP-1494: Evaluation of intra-fraction breathing pattern variability in Helical Tomotherapy by means of optical tracking

Markerless tracking of small lung tumors for stereotactic radiotherapy.

(1) To validate retrospective markerless tracking software for small lung tumors by comparing tracked motion in 4-dimensional planning computed tomography (4DCT) derived kV projection images and known tumor motion in the same 4DCT. (2) To evaluate variability of tumor motion using kV projection images from cone-beam computed tomography (CBCT) scans acquired on different days. Nonclinical tumor tracking software (TTS) used a normalized cross correlation algorithm to track the tumor on enhanced kV projection images (e.g., from a CBCT scan). The reference dataset consisted of digitally reconstructed radiographs (DRRs) from one phase of a planning 4DCT. TTS matches two in-plane coordinates and obtains the out-of-plane coordinate by triangulating with match results from other projections. (1) To validate TTS, tracking results were compared with known 4DCT tumor motion for two patients (A and B). Projection images (1 image/1°) were digitally reconstructed for each 4DCT phase. From these, kV projection series were composed simulating full breathing cycles every 20° of gantry rotation [breathing period = 20°/(6°/s) = 3.33 s]. Reference templates were 360 "tumor enhanced" DRRs from the 4DCT expiration phase. TTS-derived tumor motion was compared to known tumor motion on 4DCT. (2) For five patients, TTS-assessed motion during clinical CBCT acquisition was compared with motion on the planning 4DCT, and the motion component in the Y (cranio-caudal)-direction was compared with the motion of an external marker box (RPM, real-time position management). (1) Validation results: TTS for case A (tumor 6.2 cm(3), 32 mm axial diameter) over 360° showed mean motion X (medial-lateral) = 3.4, Y = 11.5, and Z (ventral-dorsal) = 4.9 mm (1 SD < 1.0 mm). Corresponding 4DCT motion was X = 3.1, Y = 11.3, and Z = 5.1 mm. Correlation coefficients between TTS tumor motion and displacement of the tumor's center of mass (CoM) on 4DCT were 0.64, 0.96, and 0.82 (X, Y, and Z, respectively). For case B (4.1 cm(3), 20 mm diameter), due to temporarily decreased tumor visibility preventing TTS from resolving the tumor, robust tracking data were only available between angles 300°-40° and 120°-220°. Mean motion according to TTS was X = 2.0, Y = 7.7, and Z = 8.2 mm (1 SD < 0.9 mm). Tumor motion on 4DCT was X = 1.8, Y = 7.6, and Z = 9.5 mm and correlation coefficients between TTS motion and CoM displacement were 0.59, 0.95, and 0.93 (X, Y, and Z, respectively). (2) CLINICAL RESULTS: TTS revealed a mean intrafraction variation in tumor motion in Y-direction of >2.0 mm (1 SD) in four of five patients. In addition, clinical tumor motion amplitude differed from that seen on planning 4DCT. Internal and external structures that create abrupt density change (e.g., table-top edge, interface between lung/mediastinum and lung/heart) were observed to prevent 360° tracking of the tumor. Correlation coefficients between TTS motion in the Y-direction and the RPM signal (22 observations) ranged from 0.78 to 0.96. In 2D, 241 TTS matches at end-inspiration and end-expiration were visually validated: mean difference was 0.8 mm (SD = 0.7) for both. TTS can track small lung tumors if these are visible in kV projections. A 4DCT dataset can be used to validate kV tracking of moving targets. TTS and 4DCT displacement agreed to within 2 mm. TTS and RPM motion were closely associated but tumor motion during CBCT can vary from the planning 4DCT.

Intrafraction Variability and Deformation Quantification in the Breast

To evaluate intrafraction variability and deformation of the lumpectomy cavity (LC), breast, and nearby organs. Sixteen left-sided postlumpectomy and 1 bilateral breast cancer cases underwent free-breathing CT (FBCT) and 10-phase 4-dimensional CT (4DCT). Deformable image registration was used for deformation analysis and contour propagation of breast, heart, lungs, and LC between end-exhale and end-inhale 4DCT phases. Respiration-induced motion was calculated via centroid analysis. Two planning target volumes (PTVs) were compared: PTV(FBCT) from the FBCT volume with an isotropic 10 mm expansion (5 mm excursion and 5 mm setup error) and PTV(4DCT) generated from the union of 4DCT contours with isotropic 5 mm margin for setup error. Volume and geometry were evaluated via percent difference and bounding box analysis, respectively. Deformation correlations between breast/cavity, breast/lung, and breast/heart were evaluated. Associations were tested between cavity deformation and proximity to chest wall and breast surface. Population-based 3-dimensional vector excursions were 2.5 ± 1.0 mm (range, 0.8-3.8 mm) for the cavity and 2.0 ± 0.8 mm (range, 0.7-3.0 mm) for the ipsilateral breast. Cavity excursion was predominantly in the anterior and superior directions (1.0 ± 0.8 mm and -1.8 ± 1.2 mm, respectively). Similarly, for all cases, LCs and ipsilateral breasts yielded median deformation values in the superior direction. For 14 of 17 patients, the LCs and breast interquartile ranges tended toward the anterior direction. The PTV(FBCT) was 51.5% ± 10.8% larger (P<.01) than PTV(4DCT). Bounding box analysis revealed that PTV(FBCT) was 9.8 ± 1.2 (lateral), 9.0 ± 2.2 (anterior-posterior), and 3.9 ± 1.8 (superior-inferior) mm larger than PTV(4DCT). Significant associations between breast and cavity deformation were found for 6 of 9 axes. No dependency was found between cavity deformation and proximity to chest wall or breast surface. Lumpectomy cavity and breast deformation and motion demonstrated large variability. A PTV(4DCT) approach showed value in patient-specific margins, particularly if robust interfraction setup analysis can be performed.

Modeling Pancreatic Tumor Motion Using 4-Dimensional Computed Tomography and Surrogate Markers

To assess intrafractional positional variations of pancreatic tumors using 4-dimensional computed tomography (4D-CT), their impact on gross tumor volume (GTV) coverage, the reliability of biliary stent, fiducial seeds, and the real-time position management (RPM) external marker as tumor surrogates for setup of respiratory gated treatment, and to build a correlative model of tumor motion. We analyzed the respiration-correlated 4D-CT images acquired during simulation of 36 patients with either a biliary stent (n=16) or implanted fiducials (n=20) who were treated with RPM respiratory gated intensity modulated radiation therapy for locally advanced pancreatic cancer. Respiratory displacement relative to end-exhalation was measured for the GTV, the biliary stent, or fiducial seeds, and the RPM marker. The results were compared between the full respiratory cycle and the gating interval. Linear mixed model was used to assess the correlation of GTV motion with the potential surrogate markers. The average ± SD GTV excursions were 0.3 ± 0.2 cm in the left-right direction, 0.6 ± 0.3 cm in the anterior-posterior direction, and 1.3 ± 0.7 cm in the superior-inferior direction. Gating around end-exhalation reduced GTV motion by 46% to 60%. D95% was at least the prescribed 56 Gy in 76% of patients. GTV displacement was associated with the RPM marker, the biliary stent, and the fiducial seeds. The correlation was better with fiducial seeds and with biliary stent. Respiratory gating reduced the margin necessary for radiation therapy for pancreatic tumors. GTV motion was well correlated with biliary stent or fiducial seed displacements, validating their use as surrogates for daily assessment of GTV position during treatment. A patient-specific internal target volume based on 4D-CT is recommended both for gated and not-gated treatment; otherwise, our model can be used to predict the degree of GTV motion.

Monitoring ABC-assisted deep inspiration breath hold for left-sided breast radiotherapy with an optical tracking system.

Recent knowledge on the effects of cardiac toxicity warrants greater precision for left-sided breast radiotherapy. Different breath-hold (BH) maneuvers (abdominal vs thoracic breathing) can lead to chest wall positional variations, even though the patient's tidal volume remains consistent. This study aims to investigate the feasibility of using optical tracking for real-time quality control of active breathing coordinator (ABC)-assisted deep inspiration BH (DIBH). An in-house optical tracking system (OTS) was used to monitor ABC-assisted DIBH. The stability and localization accuracy of the OTS were assessed with a ball-bearing phantom. Seven patients with left-sided breast cancer were included. A free-breathing (FB) computed tomography (CT) scan and an ABC-assisted BH CT scan were acquired for each patient. The OTS tracked an infrared (IR) marker affixed over the patient's xiphoid process to measure the positional variation of each individual BH. Using the BH within which the CT scan was performed as the reference, the authors quantified intra- and interfraction BH variations for each patient. To estimate the dosimetric impact of BH variations, the authors studied the positional correlation between the marker and the left breast using the FB CT and BH CT scans. The positional variations of 860 BHs as measured by the OTS were retrospectively incorporated into the original treatment plans to evaluate their dosimetric impact on breast and cardiac organs [heart and left anterior descending (LAD) artery]. The stability and localization accuracy of the OTS was within 0.2 mm along each direction. The mean intrafraction variation among treatment BHs was less than 2.8 mm in all directions. Up to 12.6 mm anteroposterior undershoot, where the patient's chest wall displacement of a BH is less than that of a reference BH, was observed with averages of 4.4, 3.6, and 0.1 mm in the anteroposterior, craniocaudal, and mediolateral directions, respectively. A high positional correlation between the marker and the breast was found in the anteroposterior and craniocaudal directions with respective Pearson correlation values of 0.95 and 0.93, but no mediolateral correlation was found. Dosimetric impact of BH variations on breast coverage was negligible. However, the mean heart dose, mean LAD dose, and max LAD dose were estimated to increase from 1.4/7.4/18.6 Gy (planned) to 2.1/15.7/31.0 Gy (delivered), respectively. In ABC-assisted DIBH, large positional variation can occur in some patients, due to their different BH maneuvers. The authors' study has shown that OTS can be a valuable tool for real-time quality control of ABC-assisted DIBH.

Evaluation of respiratory pattern during respiratory-gated radiotherapy.

The respiratory cycle is not strictly regular, and generally varies in amplitude and period from one cycle to the next. We evaluated the characteristics of respiratory patterns acquired during respiratory gating treatment in more than 300 patients. A total 331 patients treated with respiratory-gated carbon-ion beam therapy were selected from a group of patients with thoracic and abdominal conditions. Respiratory data were acquired for a total of 3,171 fractions using an external respiratory sensing monitor and evaluated for respiratory cycle, duty cycle, magnitude of baseline drift, and intrafractional/interfractional peak inhalation/exhalation positional variation. Results for the treated anatomical sites and patient positioning were compared. Mean±SD respiratory cycle averaged over all patients was 4.1±1.3s. Mean±SD duty cycle averaged over all patients was 36.5±7.3%. Two types of baseline drift were seen, the first decremental and the second incremental. For respiratory peak variation, the mean intrafractional variation in peak-inhalation position relative to the amplitude in the first respiratory cycle (15.5±9.3%) was significantly larger than that in exhalation (7.5±4.6%). Interfractional variations in inhalation (17.2±18.5%) were also significantly greater than those in exhalation (9.4±10.0%). Statistically significant differences were observed between patients in the supine position and those in the prone position in mean respiratory cycle, duty cycle, and intra-/interfractional variations. We quantified the characteristics of the respiratory curve based on a large number of respiratory data obtained during treatment. These results might be useful in improving the accuracy of respiratory-gated treatment.

Inter- and intra-fractional bladder motion during radiotherapy for bladder cancer: A comparison of full and empty bladders

PurposeTo compare inter- and intra-fraction bladder volume variations and bladder wall motion during radiotherapy (RT) for bladder cancer with full and empty bladder protocols. Materials and methodsBladder volumes, filling rates and bladder wall movement were retrospectively analyzed for 24 patients with at least 4 sets of delineable pre and post treatment cone beam CT (CBCT)-scans. Eight patients were treated with an ‘empty bladder’ (EB) protocol and sixteen patients with a ‘full bladder’ (FB) protocol. Results24 planning CT-scans and 356 CBCT-scans (178 sets) were analyzed. The average time between pre and post irradiation CBCT was 8min (range 6–18min). Median filling rate was 1.94ml/min and did not differ between EB and FB. Random variation in bladder volume and inter-fraction wall movement was slightly but not significantly larger for FB, whereas intra-fraction bladder wall movement was slightly but not significantly smaller for FB. The largest inter- and intra-fraction bladder wall movement was found in the cranial anterior direction. ConclusionEmpty and full bladder protocols show similar inter- and intra-fraction wall motion, and therefore treatment choices could be purely based on organ at risk criteria.

Detection of Tumor Volume Regression and Motion Changes During Radiation Therapy for Primary Thoracic Esophageal Cancer Based on 4DCT Scans: Mesothelioma, Thymic Malignancies, and Other Thoracic Malignancies

To investigate the magnitudes of the changes in volume and the GTV intrafractional displacement of the primary thoracic esophageal cancer in response to conventional fractioned radiation therapy with repeated 4-dimensional computed tomography (4D-CT) during the course of treatment. The 4D-CT data sets were acquired at the time of treatment simulation and after every ten fractions for each of the 38 patients throughout treatment. Scans were registered to the baseline (simulation) 4D-CT scans by using bony landmarks. The gross tumor volumes (GTVs) were delineated on each data set, and the GTVmean was the average of all 10 GTVs of each 4D-CT scan. All 10 GTVs were combined to form the internal gross tumor volume (IGTV). Coordinates and volumes of the GTV and IGTV were acquired from the treatment planning system (TPS). The intrafraction variations in the displacement of the GTV, the volumes of the GTVmean and the IGTV were computed during radiation therapy. The tumor volumes decreased significantly at the twentieth fraction, which was up to 26.09% (P<.001) in GTVmean and 27.78% (P<.001) in IGTV, respectively. The 95th percentile values from the cumulative distributions were used to define minimum margins to account for gross tumor volume motion during treatment planning. According to this study, 2.5 mm RL, 2.3 mm AP, 5.0 mm SI, 5.5 mm 3D vector in the total, 2.3 mm RL, 2.1 mm AP, 4.9 mm SI, 5.4 mm 3D vector in the upper, 2.4 mm RL, 2.3 mm AP, 4.1 mm SI, 4.7 mm 3D vector in the middle, 2.5 mm RL, 2.5 mm AP, 5.4 mm SI, 6.3 mm 3D vector in the distal thoracic esophagus provided coverage of tumor motion in 95% of the cases in our study population. The intrafraction GTV centroid displacements in the SI direction were greater than in the LR and AP direction during treatment. However, no significant displacement was observed during radiation therapy in any direction (P=.616, RL; P=.471, AP; and P=.134, SI). The intrafractional variations of GTV centroid displacement were not significant throughout treatment, but the tumor volume decreased significantly at the 20th fraction. Repeated 4D-CT scans might be beneficial for target correction and planning modification.

Implementation of a target volume design function for intrafractional range variation in a particle beam treatment planning system.

Treatment planning for charged particle therapy in the thoracic and abdominal regions should take account of range uncertainty due to intrafractional motion. Here, we developed a design tool (4Dtool) for the target volume [field-specific target volume (FTV)], which accounts for this uncertainty using four-dimensional CT (4DCT). Target and normal tissue contours were input manually into a treatment planning system (TPS). These data were transferred to the 4Dtool via the picture archiving and communication system (PACS). Contours at the reference phase were propagated to other phases by deformable image registration. FTV was calculated using 4DCT on the 4Dtool. The TPS displays FTV contours using digital imaging and communications in medicine files imported from the PACS. These treatment parameters on the CT image at the reference phase were then used for dose calculation on the TPS. The tool was tested in single clinical case randomly selected from patients treated at our centre for lung cancer. In this clinical case, calculation of dose distribution with the 4Dtool resulted in the successful delivery of carbon-ion beam at the reference phase of 95% of the prescribed dose to the clinical target volume (CTV). Application to the other phases also provided sufficient dose to the CTV. The 4Dtool software allows the design of the target volume with consideration to intrafractional range variation and is now in routine clinical use at our institution. Our alternative technique represents a practical approach to four-dimensional treatment planning within the current state of charged particle therapy.

Sci-Fri AM: Mountain - 06: Optimizing planning target volume in lung radiotherapy using deformable registration

A four dimensional computed tomography (4DCT) image is acquired for all radically treated, lung cancer patients to define the internal target volume (ITV), which encompasses tumour motion due to breathing and subclinical disease. Patient set-up error and anatomical motion that is not due to breathing is addressed through an additional 1 cm margin around the ITV to obtain the planning target volume (PTV). The objective of this retrospective study is to find the minimum PTV margin that provides an acceptable probability of delivering the prescribed dose to the ITV. Acquisition of a kV cone beam computed tomography (CBCT) image at each fraction was used to shift the treatment couch to accurately align the spinal cord and carina. Our method utilized deformable image registration to automatically position the planning ITV on each CBCT. We evaluated the percentage of the ITV surface that fell within various PTVs for 79 fractions across 18 patients. Treatment success was defined as a situation where at least 99% of the ITV is covered by the PTV. Overall, this is to be achieved in at least 90% of the treatment fractions. The current approach with a 1cm PTV margin was successful ∼96% of the time. This analysis revealed that the current margin can be reduced to 0.8cm isotropic or 0.6×0.6×1 cm3 non-isotropic, which were successful 92 and 91 percent of the time respectively. Moreover, we have shown that these margins maintain accuracy, despite intrafractional variation, and maximize CBCT image guidance capabilities.

Evaluation of intrafraction motion of the organs at risk in image-based brachytherapy of cervical cancer.

To assess the variation in the doses received by the organs at risk (OARs) that can occur during treatment planning of cervical cancer by image-based brachytherapy. After intracavitary application, two sets of images-CT and MRI-were obtained. The two sets of images were fused together with respect to the applicator. Contouring was done separately on CT and MR images. Dose received by the OARs on CT images with respect to the plans made on the MR images was estimated and compared with those on the MR images. Although there was always a difference between the dose received by the OARs based on the CT and MRI contours, it was not significant for the bladder and rectum; 2cc doses differed by 0.49Gy (±0.44) p= 0.28 for the bladder and 0.30Gy (±0.29) p= 0.16 for the rectum. The 1cc and 0.1cc differences were also not significant. However for the sigmoid colon, there was significant intrafraction variation in the 2cc doses 0.61 (±0.6) p= 0.001, 1cc doses 0.73 (±0.67) Gy p= 0.00, and 0.1cc dose 0.97 (±0.93) Gy p= 0.009. The variation in the doses to the OARs must be considered while weighing target coverage against overdose to the OARs. Although not significant for the bladder and rectum, it was significant for the sigmoid colon. Estimated doses to OARs on the planning system may not be the same dose delivered at the time of treatment.

TU‐F‐17A‐09: Four‐Dimensional Cone Beam CT Ventilation Imaging Can Detect Interfraction Lung Function Variations for Locally Advanced Lung Cancer Patients

Purpose:Four‐dimensional cone beam CT ventilation imaging (4D‐CBCT VI) is a novel functional lung imaging modality requiring validation. We hypothesize that 4D‐CBCT VI satisfies a necessary condition for validity: that intrafraction variations (e.g. due to poor 4D‐CBCT image quality) are substantially different to interfraction variations (e.g. due to changes in underlying function). We perform the first comparison of intrafraction (pre/post fraction) and interfraction (week‐to‐week) 4D‐CBCT VIs for locally advanced non small cell lung cancer (LA NSCLC) patients undergoing radiation therapy.Methods:A total of 215 4D‐CBCT scans were acquired for 19 LA NSCLC patients over 4‐6 weeks of radiation therapy, including 75 pairs of pre‐/post‐fraction scans on the same day. 4D‐CBCT VIs were obtained by applying state‐of‐the‐art, B‐spline deformable image registration to obtain the Jacobian determinant of deformation between the end‐exhale and end‐inhale phases. All VIs were deformably registered to the corresponding first day scan, normalized between the 10th and 90th percentile values and cropped to the ipsilateral lung only. Intrafraction variations were assessed by computing the mean and standard deviation of voxel‐wise differences between all same‐day pairs of pre‐/post‐fraction VIs. Interfraction differences were computed between first‐day VIs and treatment weeks 2, 4 and 6 for all 19 patients. We tested the hypothesis by comparing cumulative distribution functions (CDFs) of intrafraction and interfraction ventilation differences using two‐sided Kolmogorov‐Smirnov goodness‐of‐fit tests.Results:The (mean ± std. dev.) of intrafraction differences was (−0.007 ± 0.079). Interfraction differences for weeks 2, 4 and 6 were (−0.035 ± 0.103), (−0.006 ± 0.094) and (−0.019 ± 0.127) respectively. For week 2, the changes in CDFs for intrafraction and interfraction differences approached statistical significance (p=0.099).Conclusion:We have shown that 4D‐CBCT VI can satisfy a necessary condition for validity; intrafraction variations do not limit the ability to measure interfraction variations in LA NSCLC patients.This work is supported by a Cancer Institute NSW Early Career Fellowship, an NHMRC Australia Fellowship and NIH P01CA116602.

TH-C-BRD-12: Robust Intensity Modulated Proton Therapy Plan Can Eliminate Junction Shifts for Craniospinal Irradiation

Purpose: The passive scattering proton therapy (PSPT) technique is the commonly used radiotherapy technique for craniospinal irradiation (CSI). However, PSPT involves many numbers of junction shifts applied over the course of treatment to reduce the cold and hot regions caused by field mismatching. In this work, we introduced a robust planning approach to develop an optimal and clinical efficient techniques for CSI using intensity modulated proton therapy (IMPT) so that junction shifts can essentially be eliminated. Methods: The intra-fractional uncertainty, in which two overlapping fields shift in the opposite directions along the craniospinal axis, are incorporated into the robust optimization algorithm. Treatment plans with junction sizes 3,5,10,15,20,25 cm were designed and compared with the plan designed using the non-robust optimization. Robustness of the plans were evaluated based on dose profiles along the craniospinal axis for the plans applying 3 mm intra-fractional shift. The dose intra-fraction variations (DIV) at the junction are used to evaluate the robustness of the plans. Results: The DIVs are 7.9%, 6.3%, 5.0%, 3.8%, 2.8% and 2.2%, for the robustly optimized plans with junction sizes 3,5,10,15,20,25 cm. The DIV are 10% for the non-robustly optimized plans with junction size 25 cm. The dose profiles along the craniospinalmore » axis exhibit gradual and tapered dose distribution. Using DIVs less than 5% as maximum acceptable intrafractional variation, the overlapping region can be reduced to 10 cm, leading to potential reduced number of the fields. The DIVs are less than 5% for 5 mm intra-fractional shifts with junction size 25 cm, leading to potential no-junction-shift for CSI using IMPT. Conclusion: This work is the first report of the robust optimization on CSI based on IMPT. We demonstrate that robust optimization can lead to much efficient carniospinal irradiation by eliminating the junction shifts.« less

SU-E-T-564: Statistical Variation and Acceptance of Non Invasive Immobilization Systems for Multi-Fraction Gamma Knife EXtend Stereotactic Radiosurgery

Purpose: Gamma Knife perfexion (GKPFX) with extend system is being used for multi-fraction or fractionated stereotactic radiosurgery (fGKSRS). Patient surveillance Unit (PSU) is used to attain a non invasive immobilization for fractionation. Calibrated digital probe is used to measure encoded mechanical positions on repositioning check tool (RCT). The objective of this study is to check consistency and statistical position variation during multi-fraction stereotactic radiosurgery using GKPFX extend system. Methods: 31 fractions through 7 patients being delivered to check the positional variation during patient setup. Variation is calculated after sampling the collected data at same point measurement for all the patients. Average value of three stage measurements or the reference values are compiled with the sample data using SPSS (statistical software). Measurement of encoded points has been taken and various statistical parameters are observed. Reproducibility is checked, comparing the reference to measured values through encoded points before treatment. Intra-fraction variation is measured on compiling pre and post treatment RCT measurements while inter-fractionation variation compiled along with fractions before treatment. Results: Pre-treatment radial setup difference auto calculated by the system varies from 0.5mm to 0.8 mm with mean variation of 0.7 mm. Positional variation was observed for post-treatment and was found that it varies from 0.9 mm - 1.0 mm. Paired sample test results the paired difference mean from the reference (for pre and post measurements) was 0.26 mm (0.1775 mm- 0.3475 mm) with only 25% of error significance. The distribution of the random component of observer uncertainty was 0.06mm. Conclusion: Post treatment positional variation is 20%-30% of pre treatment measurements, i.e. about treatment measurements, i.e. about 0.2 mm. Explaining complete procedure to patient, limiting head rotation/lift and preventing other movements, may maximize positional accuracy during fractionations. Contending positional accuracy, supports the extend system for fractionated stereotactic radiosurgery among other multisession radiosurgery techniques.

SU-E-J-215: Onboard Split PET Including the Effects of Attenuation, Scatter, and Random Events

Purpose: Positron emission tomography (PET) using split PET geometry was investigated as an on-board system for functional imaging and PET marker tracking, specifically with tomotherapy. The open dual ring PET would allow measurement of both inter and intra-fractional variation, improving the delineation of tumor volume at any stage in the radiation treatment delivery process. We present results from data obtained using Monte Carlo simulations including the effects of attenuation, random events, and scatter. Methods: PET design was accomplished via Monte Carlo simulations with GATE, the Geant4 Application for Emission Tomography. Images were reconstructed using Software for Image Reconstruction (STIR) with a fully 3D ordered subset expectation maximization (OSEM) image reconstruction technique. Results: Monte Carlo simulations of the split PET geometry indicate that including physical factors that degrade image quality such as attenuation, random events, and scatter still prove the feasibility of near real-time PET for both inter and intra-fractional radiation delivery. The image quality of scan times under 1 minute reveals that it is possible to utilize PET scanning and reconstruction during the treatment session intrafractionally. GATE also simulates the depth information but does not correct obliqueness of the path of line of response so the data is much moremore » realistic than the data obtained with ray tracing. Conclusion: Onboard split PET with TomoTherapy can generate quality images under 1 minute scan times without the need to correct for attenuation, scatter, random events, or depth information of the interaction.« less