Lung SBRT Patients Research Articles

SU‐E‐T‐486: Effect of the Normalized Prescription Isodose Line On Target Dose Deficiency in Lung SBRT Based On Monte Carlo Calculation

Purpose:To investigate the impact of normalized prescription isodose line on target dose deficiency calculated with Monte Carlo (MC) vs. pencil Beam (PB) in lung SBRT. RTOG guidelines recommend prescription lines between 60% and 90% for lung SBRT. How this affects the magnitude of MC‐calculated target dose deficiency has never been studied.Methods:Under an IRB‐approved protocol, four lung SBRT patients were replanned following RTOG0813 by a single physicist. For each patient, four alternative plans were generated based on PB calculation prescribing to 60–90% isodose lines, respectively. Each plan consisted of 360o coplanar dynamic conformal arcs with beam apertures manually optimized to achieve similar dose coverage and conformity for all plans of the same patient. Dose distribution was calculated with MC and compared to that with PB. PTV dose‐volume endpoints were compared, including Dmin, D5, Dmean, D95, and Dmax. PTV V100 coverage, conformity index (CI), and heterogeneity index (HI) were also evaluated.Results:For all 16 plans, median (range) PTV V100 and CI were 99.7% (97.5–100%) and 1.27 (1.20–1.41), respectively. As expected, lower prescription line resulted in higher target dose heterogeneity, yielding median (range) HI of 1.26 (1.05–1.51) for all plans. Comparing MC to PB, median (range) D95, Dmean, D5 PTV dose deficiency were 18.9% (11.2–23.2%), 15.6% (10.0–22.7%), and 9.4%(5.5–13.6%) of the prescription dose, respectively. The Dmean, D5, and Dmax deficiency was found to monotonically increase with decreasing prescription line from 90% to 60%, while the Dmin deficiency monotonically decreased. D95 deficiency exhibited more complex trend, reaching the largest deficiency at 80% for all patients.Conclusion:Dependence on prescription isodose line was found for MC‐calculated PTV dose deficiency of lung SBRT. When comparing reported MC dose deficiency values from different institutions, their individual selections of prescription line should be considered in addition to other factors affecting the deficiency magnitude.

SU-E-T-86: Comparison of Two Commercially Available Programs for the Evaluation of Delivered Daily Dose Using Cone Beam CT (CBCT)

Purpose: In this study, two commercially available programs were compared for the evaluation of delivered daily dose using cone beam CT (CBCT). Methods: Thirty (n=30) patients previously treated in our clinic (10 prostate, 10 SBRT lung and 10 abdomen) were used in this study. The patients' plans were optimized and calculated using the Pinnacle treatment planning system. The daily CBCT scans were imported into Velocity and RayStation along with the corresponding planning CTs, structure sets and 3D dose distributions for each patient. The organs at risk (OAR) were contoured on each CBCT by the prescribing physician and were included in the evaluation of the daily delivered dose. Each CBCT was registered to the planning CT, once with rigid registration and then again, separately, with deformable registration. After registering each CBCT, the dose distribution from the planning CT was overlaid and the dose volume histograms (DVH) for the OAR and the planning target volumes (PTV) were calculated. Results: For prostate patients, we observed daily volume changes for the OARs. The DVH analysis for those patients showed variation in the sparing of the OARs while PTV coverage remained virtually unchanged using both Velocity and RayStation systems. Similar results were observed for abdominalmore » patients. In contrast, for SBRT lung patients, the DVH for the OARs and target were comparable to those from the initial treatment plan. Differences in organ volume and organ doses were also observed when comparing the daily fractions using deformable and rigid registrations. Conclusion: By using daily CBCT dose reconstruction, we proved PTV coverage for prostate and abdominal targets is adequate. However, there is significant dosimetric change for the OARs. For lung SBRT patients, the delivered daily dose for both PTV and OAR is comparable to the planned dose with no significant differences.« less

SU‐F‐BRD‐05: Dosimetric Comparison of Protocol‐Based SBRT Lung Treatment Modalities: Statistically Significant VMAT Advantages Over Fixed‐ Beam IMRT

Purpose:The purpose of this study is to inter‐compare and find statistically significant differences between flattened field fixed‐beam (FB) IMRT with flattening‐filter free (FFF) volumetric modulated arc therapy (VMAT) for stereotactic body radiation therapy SBRT.Methods:SBRT plans using FB IMRT and FFF VMAT were generated for fifteen SBRT lung patients using 6 MV beams. For each patient, both IMRT and VMAT plans were created for comparison. Plans were generated utilizing RTOG 0915 (peripheral, 10 patients) and RTOG 0813 (medial, 5 patients) lung protocols. Target dose, critical structure dose, and treatment time were compared and tested for statistical significance. Parameters of interest included prescription isodose surface coverage, target dose heterogeneity, high dose spillage (location and volume), low dose spillage (location and volume), lung dose spillage, and critical structure maximum‐ and volumetric‐dose limits.Results:For all criteria, we found equivalent or higher conformality with VMAT plans as well as reduced critical structure doses. Several differences passed a Student's t‐test of significance: VMAT reduced the high dose spillage, evaluated with conformality index (CI), by an average of 9.4%±15.1% (p=0.030) compared to IMRT. VMAT plans reduced the lung volume receiving 20 Gy by 16.2%±15.0% (p=0.016) compared with IMRT. For the RTOG 0915 peripheral lesions, the volumes of lung receiving 12.4 Gy and 11.6 Gy were reduced by 27.0%±13.8% and 27.5%±12.6% (for both, p<0.001) in VMAT plans. Of the 26 protocol pass/fail criteria, VMAT plans were able to achieve an average of 0.2±0.7 (p=0.026) more constraints than the IMRT plans.Conclusions:FFF VMAT has dosimetric advantages over fixed beam IMRT for lung SBRT. Significant advantages included increased dose conformity, and reduced organs‐at‐risk doses. The overall improvements in terms of protocol pass/fail criteria were more modest and will require more patient data to establish difference trends of more statistical significance.

SU‐E‐T‐286: Robustness of One‐ Versus Two‐Isocenter SBRT for Two Lung Lesions with Rotational Setup Errors

Purpose:Evaluate the robustness of two different planning techniques for SBRT patients with two lung lesions: with both PTVs treated simultaneously with one isocenter, or separately with an isocenter for each PTV.Methods:One‐ and two‐isocenter IMRT and VMAT SBRT plans were created in Varian Eclipse for five patients who each had two lung lesions. PTV coverage for the one‐ and two‐isocenter plans was similar when possible. Rotational setup errors were simulated using couch rotations of various angles, and the resulting dose distributions were calculated.Results:With a 2° setup error, the average decrease in PTV volume receiving at least the prescribed dose (V100%) was 1.66% for the oneisocenter plans, and 0.05% for the two‐isocenter plans. The changes in V100% ranged from +0.33% to 4.06% for one‐isocenter plans, and from +0.23% to −0.30% for two‐isocenter plans. With a 5° setup error, the average decrease in V100% was 6.46% for a one‐isocenter plan (range: −0.19% to −13.18%), and 0.42% for a two‐isocenter plan (range: +0.21% to −1.33%). For one‐isocenter plans, the decrease in V100% tended to increase as PTV separation increased. The three patients with PTV separation less than 10 cm each showed a decrease in V100% of less than 1% for a 2° error, while the two patients with larger PTV separations showed decreases of more than 3%.Conclusion:As could be predicted by geometric calculations, when a rotational setup error occurs, the PTV coverage decreased more for a one‐isocenter plan than for a two‐isocenter plan. These findings suggest that, in general, for lung SBRT patients with two lesions, the two‐isocenter technique may be preferable. However, they may also suggest that if the PTVs are separated by less than 10 cm, and there is high confidence that setup errors are small (≤ 2°), a one‐isocenter plan may deliver similar coverage.

SU-E-J-266: A Pitfall of a Deformable Image Registration in Lung Cancer

Purpose: For four-dimensional (4D) planning and adaptive radiotherapy, deformable image registration (DIR) is needed and the accuracy is essential. We evaluated the accuracy of one free-downloadable DIR software library package (NiftyReg) and one commercial DIR software (MIM) in lung SBRT cancer patients. Methods: A rigid and non-rigid registrations were implemented to our in-house software. The non-rigid registration algorithm of the NiftyReg and MIM was based on the free-form deformation. The accuracy of the two software was evaluated when contoured structures to peak-inhale and peak-exhale 4DCT image data sets were measured using the dice similarity coefficient (DSC). The evaluation was performed in 20 lung SBRT patients. Results: In our visual evaluation, the eighteen cases show good agreement between the deformed structures for the peak-inhale phase and the peak-exhale phase structures (more than 0.8 DSC value). In the evaluation of the DSC in-house software, averaged DSC values of GTV and lung, heart, spinal cord, stomach and body were 0.862 and 0.979, 0.932, 0.974, 0.860, 0.998, respectively. As the Resultof the registration using the MIM program in the two cases which had less than 0.7 DSC value when analyzed using the in-house software, the DSC value were improved to 0.8. The CT images in a case with low DSC value shows the tumor was surrounded by the structure with the similar CT values, which were the chest wall or the diaphragm. Conclusion: Not only a free-downloadable DIR software but also a commercial software may provide unexpected results and there is a possibility that the results would make us misjudge the treatment planning. Therefore, we recommend that a commissioning test of any DIR software should be performed before clinical use and we should understand the characteristics of the software.

SU‐C‐BRE‐02: BED Vs. Local Control: Radiobiological Effect of Tumor Volume in Monte Carlo (MC) Lung SBRT Planning

Purpose:SBRT with hypofractionated dose schemata has emerged a compelling treatment modality for medically inoperable early stage lung cancer patients. It requires more accurate dose calculation and treatment delivery technique. This report presents the relationship between tumor control probability(TCP) and size‐adjusted biological effective dose(sBED) of tumor volume for MC lung SBRT patients.Methods:Fifteen patients who were treated with MC‐based lung SBRT to 50Gy in 5 fractions to PTVV100%=95% were studied. ITVs were delineated on MIP images of 4DCT‐scans. PTVs diameter(ITV+5mm margins) ranged from 2.7–4.9cm (mean 3.7cm). Plans were generated using non‐coplanar conformal arcs/beams using iPlan XVMC algorithm (BrainLABiPlan ver.4.1.2) for Novalis‐TX with HD‐MLCs and 6MVSRS(1000MU/min) mode, following RTOG‐0813 dosimetric guidelines. To understand the known uncertainties of conventional heterogeneities‐corrected/uncorrected pencil beam (PBhete/ PB‐homo) algorithms, dose distributions were re‐calculated with PBhete/ PB‐homo using same beam configurations, MLCs and monitor units. Biologically effective dose(BED10) was computed using LQ‐model with α/β=10Gy for meanPTV and meanITV. BED10‐c*L, gave size‐adjusted BED(sBED), where c=10Gy/cm and L=PTV diameter in centimeter. The TCP model was adopted from Ohri et al.(IJROBP, 2012): TCP = exp[sBEDTCD50]/ k /(1.0 + exp[sBED‐TCD50]/k), where k=31Gy corresponding to TCD50=0Gy; and more realistic MC‐based TCP was computed for PTV(V99%).Results:Mean PTV PB‐hete TCP value was 6% higher, but, mean PTV PB‐homo TCP value was 4% lower compared to mean PTV MC TCP. Mean ITV PB‐hete/PB‐homo TCP values were comparable (within ±3.0%) to mean ITV MC TCP. The mean PTV(V99%)had BED10=90.9±3.7%(median=92.2%),sBED=54.1±8.2%(median=53.5%) corresponding to mean MC TCP value of 84.8±3.3%(median=84.9%) at 2‐ year local control.Conclusion:The TCP model which incorporates BED10 and tumor diameter indicates that radiobiological effect of target volume and dose calculation algorithm significantly affects TCP for lung SBRT patients. Dose calculation using MC‐based algorithm is more realistic with tissue heterogeneities and is routinely performed in our clinic. Patients will be followed up to determine whether TCP prediction correlate clinical outcomes.

SU‐E‐J‐30: Benchmark Image‐Based TCP Calculation for Evaluation of PTV Margins for Lung SBRT Patients

Purpose:Tumor control probability (TCP) calculated with accumulated radiation doses may help design appropriate treatment margins. Image registration errors, however, may compromise the calculated TCP. The purpose of this study is to develop benchmark CT images to quantify registration‐induced errors in the accumulated doses and their corresponding TCP.Methods:4DCT images were registered from end‐inhale (EI) to end‐exhale (EE) using a “demons” algorithm. The demons DVFs were corrected by an FEM model to get realistic deformation fields. The FEM DVFs were used to warp the EI images to create the FEM‐simulated images. The two images combined with the FEM DVF formed a benchmark model. Maximum intensity projection (MIP) images, created from the EI and simulated images, were used to develop IMRT plans. Two plans with 3 and 5 mm margins were developed for each patient. With these plans, radiation doses were recalculated on the simulated images and warped back to the EI images using the FEM DVFs to get the accumulated doses. The Elastix software was used to register the FEM‐simulated images to the EI images. TCPs calculated with the Elastix‐accumulated doses were compared with those generated by the FEM to get the TCP error of the Elastix registrations.Results:For six lung patients, the mean Elastix registration error ranged from 0.93 to 1.98 mm. Their relative dose errors in PTV were between 0.28% and 6.8% for 3mm margin plans, and between 0.29% and 6.3% for 5mm‐margin plans. As the PTV margin reduced from 5 to 3 mm, the mean TCP error of the Elastix‐reconstructed doses increased from 2.0% to 2.9%, and the mean NTCP errors decreased from 1.2% to 1.1%.Conclusion:Patient‐specific benchmark images can be used to evaluate the impact of registration errors on the computed TCPs, and may help select appropriate PTV margins for lung SBRT patients.

SU‐E‐J‐188: Impact of Unexpected Dose Warping with Deformable Image Registration in Lung Cancer

Purpose:In our previous study of dose warping validation, four cases in twenty lung SBRT patients shows unexpected result. We clarify the impact of dose warping with a free‐downloadable deformable image registration (DIR) program and a commercially‐available DIR software program.Methods:Our in‐house software was implemented with the blockmatching algorithm for rigid registration and the free‐form deformation algorithm for non‐rigid registration from the NiftyReg. The deformation algorithm implemented in the MIM software was also based on the freeform deformation algorithm. In order to assess the accuracy of dose warping from the two software programs, generalized equivalent uniformed dose (gEUD) of the deformed dose distribution for peak inhale phase was compared with the gEUD of the dose distribution for peak inhale phase base on a assumption that a radiobiological equivalent dose would be stable before and after the deformation. The peak‐inhale deformed dose distribution was superimposed on peak‐exhale phase CT and structure datasets.Results:The in‐house software shows that the difference of gEUD for the tumor was less than 1% in the previous study. The difference was 21.4% on average in the four cases (Max: 51.2%). In the four cases in the MIM, the result was improved, however was still more than 1% (Average: 7.3%, Max: 24.3%). In the visual evaluation of the case with large gEUD difference, the shape of dose distribution from dose warping was not in nature from the motion of the tumor. The CT images of a case, which have large difference of gEUD, shows that the tumor was surrounded by the structure with the similar CT values, which were the chest wall or the diaphragm.Conclusion:A commissioning test of not only a freedownloadable DIR program but also a commercial software is essential and we should understand pitfalls of dose warping from the DIR.

Deformable image and dose registration evaluation using two commercial programs

Purpose: To evaluate the daily dose delivered to the patients using daily imaging. Methods : Thirty (n = 30) patients that were previously treated in our clinic (10 prostate, 10 SBRT lung and 10 abdomen) were used in this study. The patients’ plans were optimized and calculated using the Pinnacle treatment planning system. The daily CBCT scans were retrieved and imported into the Velocity and RayStation software along with the corresponding planning CTs, structure sets and 3D dose distributions. In addition, the critical structures were contoured on each CBCT by the prescribing physician and were included in the evaluation of the daily delivered dose. After registering each CBCT scan to the planning CT using deformable registration, the dose volume histograms (DVH) for the organs at risk (OAR) and the respective planning target volumes (PTV) were calculated in Velocity and Raystation. Results: For the prostate patients, we observed daily volume changes for the bladder, rectum and sigmoid. The DVH analysis for those patients showed variation in the sparing of the critical structures while PTV coverage showed no significant changes. Similar results were observed for patients with abdominal targets. In contrast, in SBRT lung patients, the DVH for the critical structures and the PTV were comparable to those from the initial treatment plan. By using daily CBCT dose reconstruction, we proved PTV coverage for prostate and abdominal targets is adequate. However, there is significant dosimetric change for the OAR. These changes were random with no apparent trending. For lung SBRT patients, the delivered daily dose for both PTV and OAR is comparable to the planned dose with no significant differences. Conclusion : Daily tracking of the delivered dose is feasible. The doses can be evaluated only if the OARs have been segmented taken into account any daily anatomical changes and not by deformation of the structures along. ------------------- Cite this article as: Tuohy R, Bosse C, Mavroidis P, Shi Z, Crownover R, Papanikolaou N, Stathakis S. Deformable image and dose registration evaluation using two commercial programs. Int J Cancer Ther Oncol 2014; 2(2):020242. DOI: 10.14319/ijcto.0202.42

Daily fraction dose recalculation based on rigid registration using Cone Beam CT

Purpose: To calculate the daily fraction dose for CBCT recalculations based on rigid registration and compare it to the planned CT doses. Methods : For this study, 30 patients that were previously treated (10 SBRT lung, 10 prostate and 10 abdomen) were considered. The daily CBCT images were imported into the Pinnacle treatment planning system from Mosaic. Pinnacle was used to re-contour the regions of interest (ROI) for the specific CBCT by copying the contours from the original CT plan, planned by the prescribing physician, onto each daily CBCT and then manually reshaping contours to match the ROIs. A new plan is then created with the re-contoured CBCT as primary image in order to calculate the daily dose delivered to each ROI. The DVH values are then exported into Excel and overlaid onto the original CT DVH to produce a graph. Results: For the SBRT lung patients, we found that there were small daily volume changes in the lungs, trachea and esophagus. For almost all regions of interest we found that the dose received each day was less than the predicted dose of the planned CT while the PTV dose was relatively the same each day. The results for the prostate patients were similar, showing slight differences in the DVH values for different days in the rectum and bladder but similar PTV. Conclusion : By comparing daily fraction dose between the re-contoured CBCT images and the original planned CT show that PTV coverage for both prostate and SBRT, it has been shown that for PTV coverage, a planned CT is adequate. However, there are differences between the dose for the organs surrounding the PTV. The dose difference is less than the planned in most instances. ----------------------- Cite this article as: Bosse C, Tuohy R, Mavroidis P, Shi Z, Crownover R, Gutierrez A, Papanikolaou N, Stathakis S. Daily fraction dose recalculation based on rigid registration using Cone Beam CT. Int J Cancer Ther Oncol 2014; 2 (2):020217. DOI : 10.14319/ijcto.0202.17

Real‐time soft tissue motion estimation for lung tumors during radiotherapy delivery

To provide real-time lung tumor motion estimation during radiotherapy treatment delivery without the need for implanted fiducial markers or additional imaging dose to the patient. 2D radiographs from the therapy beam's-eye-view (BEV) perspective are captured at a frame rate of 12.8 Hz with a frame grabber allowing direct RAM access to the image buffer. An in-house developed real-time soft tissue localization algorithm is utilized to calculate soft tissue displacement from these images in real-time. The system is tested with a Varian TX linear accelerator and an AS-1000 amorphous silicon electronic portal imaging device operating at a resolution of 512 × 384 pixels. The accuracy of the motion estimation is verified with a dynamic motion phantom. Clinical accuracy was tested on lung SBRT images acquired at 2 fps. Real-time lung tumor motion estimation from BEV images without fiducial markers is successfully demonstrated. For the phantom study, a mean tracking error <1.0 mm [root mean square (rms) error of 0.3 mm] was observed. The tracking rms accuracy on BEV images from a lung SBRT patient (≈20 mm tumor motion range) is 1.0 mm. The authors demonstrate for the first time real-time markerless lung tumor motion estimation from BEV images alone. The described system can operate at a frame rate of 12.8 Hz and does not require prior knowledge to establish traceable landmarks for tracking on the fly. The authors show that the geometric accuracy is similar to (or better than) previously published markerless algorithms not operating in real-time.

SU‐E‐J‐121: Assess Interfractional Tumor Motion in SBRT Lung Treatment Using 4D Cone‐Beam CT

Purpose: The aim of the study was to assess interfractional tumor motion in SBRT lung treatment using 4D cone‐beam computed tomography (CBCT) to check if tumor motion changes during treatment course. Methods: 4D CBCT scans of eight SBRT lung patients were studied. Each patient had 3‐6 fraction treatments. The patients were scanned on an Elekta XVI cone‐beam imaging system with 4D acquisition module before each fraction treatment. Reconstructed 4D CBCT images were binned into 10 phases (frames). Tumor motion was assessed on the XVI system by measuring the maximum displacements of tumor border positions among 10 frame images, in superior‐inferior (SI), anterior‐posterior (AP), and lateral (LAT) directions. Interfactional tumor motion change was determined by calculating the difference of tumor motion of each fraction and that measured on planning 4DCT. Results: The tumor motion changes were in the ranges of −0.5∼0.4 cm, −0.3∼0.2 cm, and −0.3∼0.2 cm, in the SI, AP, and LAT directions, respectively. Conclusion: 4D CBCT was used to assess interfractional tumor motions. The results showed that the tumor motion changes were clinically insignificant, with respect to the 1‐cm margin used to generate planning target volume from internal target volume.

TU‐C‐141‐11: Dosimetric Consequences of Deformable Image Registration Errors for Lung SBRT Patients

Purpose: To quantify the consequence of deformable image registration (DIR) errors on the total dose calculated for lung SBRT patients. Methods: 4DCT images were acquired from three lung SBRT patients who had lesions in the upper, middle and lower lobes of lungs. From their end‐inhale (EI) CT image, finite element models were developed to generate different sets of displacement vector fields (FEM‐DVFs). With these DVFs, mid‐and end‐exhale (EE) phase images were created from the EI images using a subvoxel‐based, mass mapping method. The simulated images were registered to the EI image using Elastix software, and the resultant DVFs were compared with the FEM‐DVFs to quantify registration errors at each image voxel. IMRT plans were developed with Pinnacle on simulated maximum intensity projection (MIP) images, and recalculated on each simulated phase image. The resultant dose was warped back to the EI image using both FEM and Elastix DVFs. Relative dose error was quantified via the difference between the two warped doses divided by the prescribed target dose. The dosimetric impact index was then calculated as the ratio of the relative dose error to the registration displacement error at each image voxel. Results: The mean registration errors evaluated in PTV for the three patients were 1.6, 1.9 and 1.7 mm, and their corresponding dosimetric impact indices were 4.2, 6.1 and 4.9%, respectively. The dosimetric impact indices averaged in PTV, ITV, lung and patient volume for these patients were 5.1, 4.4, 0.4 and 0.7%, and their corresponding standard deviations were 8.1, 7.0, 3.3 and 4.3%, respectively. Conclusion: The impact of DIR errors on dose accumulation has been quantified with patient‐specific FEM‐based breathing models. The dosimetric impact indices derived could help assess the quality of dose mapping for lung SBRT patients. The research was supported by NIH/NCI R01CA140341.

SU‐E‐T‐612: Integral Dose Study of Non‐Coplanar 4pi Radiotherapy

Purpose: To quantify the integral doses of 4π highly non‐coplanar treatment plans and compare them to traditional coplanar IMRT plans. Methods: A systematic study was conducted using an elliptical cylinder, with a major axis length of 35.4 cm and minor axis length of 23.6cm. A 4.5cm diameter spherical tumor was embedded at 4 equally spaced radial locations on the long axis from the center to the periphery. Twenty 6MV conformal non‐coplanar beams were selected in a fashion so that their trajectories would form a circular cone 24° with respect to the cone axis that converged on the tumor. A coplanar plan using twenty evenly spaced conformal 6MV coplanar beams was conducted for comparison. The dose distribution was calculated using the convolution/superposition algorithm. A patient study was also conducted for 10 liver and 12 lung SBRT cases. A column generation algorithm was used to optimize both beam orientations and the fluence map. The dosimetry was compared to clinical plans using 2‐arc volumetric modulated arc therapy (VMAT). Results: For the centrally‐located tumor, non‐coplanar plans delivered up to 37% greater integral dose but the ratio decreased rapidly with the tumor locating more superficially. Approximately the same integral dose resulted from both planning methods at a tumor depth of 10 cm. For the tumor at 6cm depth, the non‐coplanar plan integral dose was approximately 10% lower. Compared to VMAT, the integral dose to the liver and lung SBRT patients were reduced by 17% and 42% using 4π, respectively. Conclusion: Non‐coplanar plans can reduce integral doses as exemplified by both the phantom and patient studies. The effect is most significant with shallower tumors, e.g. the tumor center within 10 cm of a given surface in the simplified phantom model. Actual patient treatments benefit more due to the optimization of the non‐coplanar beam angles.

SU-E-J-207: Dosimetric Effects of Diaphragm Motion On Spine SBRT Treatments

Purpose: To evaluate the dosimetric effects of diaphragm motion on spine SBRT treatments with IMRT and VMAT deliveries. Methods: Five 4D-CTs were acquired for lung and liver SBRT patients. The vertebral body closest to the diaphragm level was contoured as the CTV. The thecal sac was used as a surrogate for the spinal cord PRV. A 3 mm expansion was used for the PTV with the PTV cropped to the cord PRV. Plans were optimized for PTV maximal coverage at 1800 cGy and cord PRV Dmax was no more than 1200 cGy. Plans were created for 7 and 11 field IMRT and partial arc VMAT modalities, all on the average intensity CT (AvgCT) and recalculated on the 4D CT data set. D95, D1, and Davg were recorded for the different delivery methods and all 11 CT sets (AvgCT+10 phases). Dose differences were calculated for each phase versus the planning AvgCT and for the maximum difference amongst the phases to evaluate worst-case scenarios. Results: Regarding the PTV coverage, the IMRT-7 plans always exhibited the largest dosimetric differences amongst metrics with Davg having the largest range (Davg[max-min]: 63 cGy) versus the IMRT-11 (51 cGy) and VMAT (49 cGy) out of the prescription dose of 1800 cGy. All other dosimetric points had smaller differences for PTV metrics. The maximum difference always occurred between exhale and inhale phases. The cord PRV D1% had smaller differences than PTV metrics with the maximum dose difference of 44 cGy for the IMRT-7 plans, and IMRT-11 had a maximum of 17 cGy and VMAT 23 cGy. Conclusions: Clinically relevant spine SBRT plans display less than 4% maximum difference in PTV dose (Davg) and 3% variability in the cord PRV D1% metric between inhale and exhale phases for IMRT-7 plans in the worst case scenario.

TH-A-137-07: Local Control Differences for SBRT Lung Patients Planned with Pencil Beam Vs. Collapsed Cone Convolution Algorithms

Purpose: There are significant differences between pencil beam and convolution algorithms when used in the thorax. However, these differences have not been previously directly correlated with lung SBRT outcomes. Methods: Data from 237 patients treated with SBRT for NSCLC or metastatic disease from other sites were retrospectively analyzed. Of those, 138 were planned using Brainlab treatment planning software (TPS) with the pencil beam (PB) algorithm and treated on Novalis with fiducial marker-based, dual planar radiograph image guidance. Ninety-nine were planned using Pinnacle TPS with collapsed cone convolution (CCC) algorithm (considered a more accurate model) and treated on Varian linacs with CBCT image guidance. Target delineation/expansion guidelines, nominal prescription values, and dose restrictions were identical between the two groups. Median follow-up was 28 and 17 months for the PB and CCC groups, respectively. Kaplan-Meier survival curves were used to determine the statistical differences in local/marginal control between the two groups. Forty-nine plans done in Brainlab (24 with and 25 without local recurrence) were reproduced in Pinnacle with the same MUs. D99 (PTV, GITV), V90 (PTV, GITV), D95 (PTV) and the isocenter dose (D_ISO) were compared. Results: Twenty-seven patients (19.6%) planned with the PB and 5 patients (5.1%) planned with the CCC algorithms developed local recurrence. Mantel-Cox and Wilcoxon tests show significant differences between the two groups (P< 0.02). Differences between the two algorithms for target coverage were: D99_GITV=13.9%, D99_PTV=20.8%, V90_GITV=13.7%, V90_PTV=37.5%, D95_PTV=18.9% and D_ISO=6%. Conclusion: The outcome differences between the patients planned with the PB vs CCC algorithms were statistically significant. Even after controlling for potential variables between the two groups, there appears to be a loss of local control when the delivered dose is about 20% lower than the prescription. Conversely, potential benefit with dose escalation cannot be excluded.

TU‐E‐141‐01: Using Respiratory Motion as a Degree of Freedom Through Weighted Phase‐Correlated 4D Planning

Purpose: We investigate a novel 4D treatment planning strategy that exploits respiratory motion as an additional degree of freedom. The key idea is to deliver more (less) fluence when the tumor target is out‐of‐line (in‐line) with an organ‐at‐risk (OAR). We have developed our weighted‐phase‐correlated (WPC) 4D treatment planning paradigm on a commercial platform (Eclipse, Varian) and designed it for an “open‐gate” delivery, where beam is on throughout the respiratory cycle. Methods: 4DCT data were collected from twenty lung SBRT patients who exhibited &gt;= 5mm tumor motion despite the use of abdominal compression. For each patient, (i)f × θ 3D‐conformal plans were created and corresponding 3D dose distributions were calculated, where θ = number of respiratory phases (10) and f = number of fields (7 – 12) (ii) For each beam, the dose from nine phases was registered to the reference phase (end‐exhale) using a GPU‐based B‐spline deformable registration, NiftyReg. (iv) using dynamic‐penalized‐likelihood optimization, fluence weights were optimized over all dose distributions. This key step ensured that for each motion‐induced state of the anatomy, an optimal fluence would be delivered. (v) A summed fluence map was imported back into Eclipse and an MLC leaf sequence was generated. In order to ensure deliverability, a maximum leaf velocity constraint (v &lt;= 3.5 cm/s) was imposed. Each WPC‐4D plan was compared with the corresponding, clinically‐delivered, ITV‐based plan. Results: In all 20 patients, WPC‐4D plans gave PTV conformity comparable to ITV‐based plans. The percent dose‐sparing (maximum, average) achieved using WPC‐4D compared to ITV‐based planning: Spinal Cord Dmax=(66,20); Esophagus Dmax=(94,29), Heart Dmax=(84,30); Heart Dmean=(82,35); Lung V20=(27,7); Lung Dmean=(27,13). Conclusion: We have developed a novel 4D‐planning solution that exploits rather than trying to mitigate respiratory motion. Initial results indicate that this approach achieves significant dose‐sparing in OARs compared to conventional motion‐management (abdominal compression) + ITV‐based planning. This research was partially supported by Varian Medical Systems, Palo Alto, CA

SU‐E‐T‐694: Correlation Between Geometric Information and Dose Fractionation in Lung SBRT Treatment Plans

Purpose: Previously, a database‐driven decision support tool to assist clinical staff in assessing and improving lung SBRT treatment plans was developed. Using planning data collected by this software, we determined a simple metric that can be used for early identification of cases for which it will be particularly difficult to achieve desired dosimetric goals. Early detection of these cases will be of clinical value. Methods: This retrospective IRB‐approved study utilizes an existing database of treatment planning data for 121 lung SBRT patients. Patients who had multiple SBRT treatments or more than one lung tumor were excluded. In our department, SBRT patients receive treatment in either 3 fractions of 18Gy or 5 fractions of 10‐12Gy. Five fractions are generally used for cases in which planning goals are difficult to achieve in terms of target coverage vs. dose to OAR. Using the specific fractionation as a surrogate for plan difficulty, we examined possible correlations between fractionation and parameters including target size, total prescribed dose, number of fields and distance to OAR. Results: 84 patients received 3 fractions of treatment and 37 received 5 fractions. Patients with PTVs greater than 70 cm3 in size and ITVs greater than 40 cm3 were always treated with 5 fractions. These patients constituted approximately 20% of the 5 fraction patients. Tumors smaller than this size were sometimes (e.g. when situated close to an OAR) treated with 5 fractions. No correlation was observed between the fractionation and the number of fields. Conclusion: With large volume targets we can predict a treatment of 5 fractions. However, this represents a minority of patients. More powerful predictions could be possible by combining target size information with other metrics such as distance to nearby OARs. The ability to predict fractionation early in planning would allow for more efficient scheduling of LINAC time. Kaye Scholar Grant

SU-E-T-715: Evaluation of Off-Target Isocenter Placement On TArget Coverage and Critical Structure Dose

Purpose: To evaluate the effect of placing the isocenter away from the PTV (so that collision is avoided during VMAT treatment) on tumor coverage or critical structure doses in SBRT. Method: CT images of lung SBRT patients were acquired for pre-treatment alignment using CT-on-rails and imported back in to the planning system. The daily images were registered with the original simulation CT using the GTV as the alignment structure. We re planned the patient (on original CT) using 2 additional scenarios 1) isocenter at the midline of the patient at the AP and Sup-Inf level of the GTV and 2) The isocenters at the center of the vertebral body at the Sup-Inf level of the GTV. In both cases, the plans were normalized so that PTV coverage is similar to the original clinical plan. The treatment plans were recomputed on the daily CTs using original isocenter and the two off target isocenters and doses to target and serial critical structure cord were tabulated. The MU units remained the same between the plans on original CT and daily CT. Result: The target doses were observed to be not affected by the off target placement of isocenter. On average, the dose to 99% GTV and 95% PTV differed from planned value by 0.25 and 0.35 Gy respectively. The 0.1 and 1cc cord doses differed by .23 and .85 Gy respectively. Conclusions: Off center placement of isocenter does not affect target coverage and serial critical structure dose such as cord. However, care should be exercised not to beam penumbra adjacent to critical structure.

SU‐E‐T‐422: Evaluation of PTV Dose Margins for Lung SBRT On Cyberknife Treatment

Purpose: The purpose of this study is to retrospectively evaluate the PTV margins used on CyberKnife for lung SBRT treatment. Methods: The CyberKnife system uses an active tracking system that models the motion of the target based on information from orthogonal images and the skin markers. Deviations between the model predicted position and the real target position show up when patient breaths irregularly. This discrepancy can be up to 1 cm for some situations. Some previous research suggested that 5 mm or 6 mm PTV margins are needed to cover the target 96∼98% of the time. The CK lung SBRT treatment usually prescribed at 55–70% isodose line (Monte Carlo dose calculation), that means the region covered by the full prescription dose in real delivery will be larger than the planned PTV region when the tracking uncertainty is accounted. This indicates that the PTV margin could be less than 5∼6 mm when the real dose is considered. 20 previous treated lung SBRT patients were examined. The real target location relatives to the model position at that time were extracted and the dose was recalculated based on the real target position using an in‐house developed Monte Carlo program. Results: Analysis of all the CyberKnife 20 patients (94 treatments) showed that by using 2 mm physical PTV margin, the region received full prescription dose was 5.4 mm +/− 1.6mm from the GTV. The planning criteria are 100% GTV volume receives at least 100% prescription dose. By consider the tracking uncertainty, for 2 mm physical PTV margin, 99.5 +/− 0.4% GTV volume still receives at least 100% prescription dose. The maximum dose reduced by about 12% on average. Conclusion: Conclusions: Image tracking log from the CyberKnife indicate that a 5mm margin may be overkill when motion tracking is used.