GTV Coverage Research Articles

P1.18-15 Dosimetric and Toxicity Benefits of Adaptive IMRT in Patients with Stage III Non-Small Cell Lung Cancer

Multiple studies observed anatomical changes or tumor shrinkage during concurrent chemoradiotherapy in patients with non-small cell lung cancer (NSCLC). Mid-treatment CT based adaptive radiotherapy targeting to the shrunken tumor can reduce the dose to adjacent normal tissue or potentially deliver a higher dose to the tumor. We aimed to quantitatively analyze the benefit of intensity-modulated radiotherapy (IMRT) adapting to CT changes at the 20th fraction in stage III NSCLC patients. We retrospectively evaluated consecutive patients with unresectable stage III NSCLC treated with adaptive IMRT from November 2017 to August 2018. The eligibility criteria included a mid-treatment CT simulation for replanning at the 20th fraction and a follow-up of at least 6 months. The prescribed dose was 64-66 Gy in 30 fractions unless exceeding the dose limit. Normal tissues were delineated according to RTOG1106 atlas on organs at risk under the supervision of a senior physician. Dose-volume histograms were calculated for the initial plans, composite adaptive plans, and lung isotoxic boost plans. Radiation pneumonitis (RP) and esophagitis (RE) were graded per CTCAE v4.03. Univariate logistic regression was applied to analyze the correlation between dosimetric factors and adverse events. 53 patients were eligible in this study. The average GTV shrinkage was -40.9% at the 20th fraction. Comparing the dosimetric factors of the composite adaptive plans to the initial ones, the GTV coverage was found marginally higher (P=0.002). The doses to normal tissues were significantly lower (all Ps<0.001) in heart mean dose by 109.5 cGy, esophagus V60 by 1.53%, cord maximum dose by -272.7 cGy, lung V20 and mean lung dose (MLD) by 1.11% and 79.2 cGy, respectively. The tumor targets could potentially get an average lung isotoxic boost of 481 cGy. Eight patients (15.1%) had grade 2 RP while no grade 3 or higher RP occurred. Twenty-three patients (43.4%) developed grade ≥ 2 RE. MLD was significantly associated with grade 2 RP with an odds ratio of 1.39 per 100 cGy increase (95% CI, 1.01 to 1.91; P=0.042). Esophagus V60 was significantly associated with grade ≥ 2 RE with an odds ratio of 1.15 per 1% increase (95% CI, 1.04 to 1.28; P=0.009). (Table 1)Tabled 1FactorsInitial PlansAdaptive PlansMean difference95%CIP ValueTargetsPGTV (%)92.9693.810.850.331.370.002PTV(%)94.1394.540.410.350.800.033LungV5(%)46.7745.72-1.05-0.70-1.41<0.001V20(%)25.1524.04-1.11-0.80-1.42<0.001V30(%)18.6217.60-1.02-0.77-1.27<0.001MLD (cGy)1411.41332.2-79.2-60.1-98.4<0.001HeartV30(%)17.4015.11-2.29-0.94-3.620.001V40(%)10.879.06-1.81-0.98-2.64<0.001V55(%)4.062.79-1.27-0.76-1.77<0.001Mean Dose(cGy)1504.51395.0-109.5-67.88-151.22<0.001PericardiumV30(%)32.1730.43-1.74-0.77-2.700.001V40(%)25.7024.00-1.7-0.76-2.640.001V55(%)13.8711.87-2-1.34-2.66<0.001Mean Dose(cGy)2192.92091.2-101.7-60.00-143.3<0.001EsophagusV40(%)39.4336.49-2.94-1.62-4.27<0.001V50(%)27.8924.08-3.81-2.36-5.27<0.001V60(%)7.576.04-1.53-0.96-2.09<0.001Max Dose(cGy)6498.36336.7-161.6-101.99-221.3<0.001CordMax Dose(cGy)4113.03840.3-272.7-209.51-335.93<0.001 Open table in a new tab By adapting to the changes on CT scans at the 20th fraction, the adaptive IMRT approach provides significant dosimetric benefits and has the potential to lower the risk of symptomatic pneumonitis and esophagitis in stage III NSCLC.

Experimental Verification of Dosimetric Uncertainty Related to Rotational Error of Single Isocenter for Multiple Targets Technique

Treatment of multiple brain mets using single isocenter on Linac is a more efficient technique than using multiple isocenters on robotic radiosurgery. This study investigated the uncertainties in target coverage due to the rotational set up errors with film measurements. A commercial radiosurgery head phantom built with a stack of film inserts within the brain was CT scanned at 1mm spacing. Three spherical targets with the diameter of 1cm (2) and 2cm (1) were generated at a distance of 4 ∼ 6cm from the isocenter to simulate multiple mets in the brain. A volumetric modulated arc therapy (VMAT) plan was optimized with a 6X beam to have a minimum 92% coverage for each GTV. The head phantom was aligned on 6 degrees of freedom (6DoF) couch with CBCT. Three pieces of Galchromic films were cut exactly the size of film insert and placed at the locations across each target along the Sup/Inf direction (film plane being close to the CT coronal cut). Once the positioning was confirmed by CBCT, the VMAT plan was delivered first at the planned setup (0° rotational error). A rotational error of 1° and 2° in yaw was then introduced and verified by CBCT. The same VMAT plan was then delivered again to both setups with the 1° and 2° of rotational error in yaw, respectively, with a new set of Galchromic films in place. All films were analyzed to have 1D absolute dose profiles extracted along a line of interest for comparison with the calculation. Comparison of dose profiles between the measurement and the calculation shows a good agreement for the planned delivery, i.e., no rotational errors introduced. With the rotational error of 1° and 2°, the measured dose profiles are seen an obvious shift from the planned. The max shift at the prescription dose was 0.6mm and 1.2mm for the yaw error of 1° and 2°, respectively. The deviation for each target is consistent with the magnitude of rotational errors. Table 1 summarized the max measured deviations of the prescription isodose line in distance from the planned for each GTV at a rotational error of 0°, 1° and 2°, respectively. Experiment for the delivery of a VMAT plan with single isocenter has confirmed the accuracy of the setup using 6DoF and CBCT. Distant targets are susceptible to the deviation of dose coverage due to the rotational errors. A planning margin up to 2mm should be sufficient to ensure the GTV coverage should the rotational tolerance of 6DoF is maintained less than 2°.Abstract 3813; Table 1Rotational Error in YawMax deviations (mm) relative to the planGTV1GTV2GTV30°0.20.30.31°0.60.40.42°0.91.21.0 Open table in a new tab

Individual lymph nodes: “See it and Zap it”

Background and purposeWith magnetic resonance imaging (MRI)-guided radiotherapy systems such as the 1.5T MR-linac the daily anatomy can be visualized before, during and after radiation delivery. With these treatment systems, seeing metastatic nodes with MRI and zapping them with stereotactic body radiotherapy (SBRT) comes into reach. The purpose of this study is to investigate different online treatment planning strategies and to determine the planning target volume (PTV) margin needed for adequate target coverage when treating lymph node oligometastases with SBRT on the 1.5T MR-linac. Materials and methodsTen patients were treated for single pelvic or para-aortic lymph node metastases on the 1.5T MR-linac with a prescribed dose of 5x7Gy with a 3 mm isotropic GTV- PTV margin. Based on the daily MRI and actual contours, a completely new treatment plan was generated for each session (adapt to shape, ATS). These were compared with plans optimized on pre-treatment CT contours after correcting for the online target position (adapt to position, ATP). At the end of each treatment session, a post-radiation delivery MRI was acquired on which the GTV was delineated to evaluate the GTV coverage and PTV margins. ResultsThe median PTV V35Gy was 99.9% [90.7–100%] for the clinically delivered ATS plans compared to 93.6% [76.3–99.7%] when using ATP. The median GTV V35Gy during radiotherapy delivery was 100% [98–100%] on the online planning and post-delivery MRIs for ATS and 100% [93.9–100%] for ATP, respectively. The applied 3 mm isotropic PTV margin is considered adequate. ConclusionFor pelvic and para-aortic metastatic lymph nodes, online MRI-guided adaptive treatment planning results in adequate PTV and GTV coverage when taking the actual patient anatomy into account (ATS). Generally, GTV coverage remained adequate throughout the treatment session for both adaptive planning strategies. “Seeing and zapping” metastatic lymph nodes comes within reach for MRI-guided SBRT.

Identification of patients with locally advanced pancreatic cancer benefitting from plan adaptation in MR-guided radiation therapy

Background and purposeMR-guided radiation therapy (MRgRT) with daily plan adaptation is a novel but time- and resource-intensive treatment for locally advanced pancreatic cancer (LAPC). We analyzed the benefit in target coverage and organ-at-risk (OAR) sparing of daily plan adaptation in 36 consecutive LAPC patients treated with MRgRT to 40 Gy in 5 fractions. Materials and methodsAdaptive planning was assessed for 180 fractions by comparing non-adapted plans with re-optimized plans using (a) GTV coverage and OAR high-doses, and (b) compliance with institutional objectives for GTV coverage and high-dose OAR constraints. Using these criteria, plan adaptation for each fraction was characterized as “not needed”, “beneficial”, or “no benefit”. Decision-tree analysis was performed to identify subgroups most likely or not to benefit from routine plan adaptation. ResultsThe percentage of plans fulfilling institutional constraints increased from 43.9% (non-adapted plans) to 83.3% after online plan adaptation, with significant improvements in GTV coverage and lower V33Gy OAR doses. Adaptive re-optimization was found to be “not needed” in 80 fractions (44.4%), “beneficial” in 95 fractions (52.8%) and of “no benefit” in 5 fractions (2.8%). Decision-tree analysis identified a grouping based on distance from tumor to OAR of ≤3 mm and GTV size, respectively, to be the major determinants for the benefit of daily plan adaptation. ConclusionMRgRT with daily plan adaptation for LAPC was of benefit in approximately half of fractions, improving target coverage and OAR sparing. Plan adaptation appeared to be relevant mainly in cases where the GTV to adjacent OAR distance was ≤3 mm.

217. Differences on GTV and PTV dose coverage for lung tumor stereotactic cyberknife treatments using Raytracing and MC algorithm: A retrospective study

Purpose The aim of this work is to compare the coverage of GTVs and corresponding PTVs lung lesions in 86 patients treated at the National Tumor Institute of Naples “G. PASCALE”, between January 2015 and June 2016, with CyberKnife using two different algorithms (Ray-tracing and MonteCarlo) with the same beams and the same number of monitor units per beam. Methods The patients were treated with CyberKnife Linear accelerator in a ipofractionated radiotherapy regimen. We first divided the patients in two groups depending on tumor localization ( peripheral and central lesions) and then, in each group, we created three volume classes (GTV⩽10 cc; 10 cc Results We noted that the prescription dose always cover more than 95% of both GTV and corresponding PTV in each volume class, not depending on tumor localization, using RayTracing Algorithm. On the contrary, using MonteCarlo algorithm, we noted a leakage in the coverage of GTV, but especially for PTV with undercoverage of until 80%, (probably due to the air included inside the volume) in each volume class for both peripheral and central lesions. Conclusions The first follow up of this patients show, in many cases, reductions or disappearance of the lesions so, probably using MonteCarlo algorithm we should think of reducing the dose prescription. Furthermore, the costraints to organs at risk should be revised.

Dose escalation for locally advanced pancreatic cancer: How high can we go?

PurposeThere are limited treatment options for locally advanced, unresectable pancreatic cancer (LAPC) and no likelihood of cure without surgery. Radiation offers an option for local control, but radiation dose has previously been limited by nearby bowel toxicity. Advances in on-board imaging and treatment planning may allow for dose escalation not previously feasible and improve local control. In preparation for development of clinical trials of dose escalation in LAPC, we undertook a dosimetric study to determine the maximum possible dose escalation while maintaining known normal tissue constraints. Methods and MaterialsTwenty patients treated at our institution with either SBRT or dose-escalated hypofractionated IMRT (DE-IMRT) were re-planned using dose escalated SBRT to 70 Gy in 5 fractions to the GTV and 40 Gy in 5 fractions to the PTV. Standard accepted organ at risk (OAR) constraints were used for planning. Descriptive statistics were generated for homogeneity, conformality, OAR's and GTV/PTV. ResultsMean iGTV coverage by 50 Gy was 91% (±0.07%), by 60 Gy was 61.3% (±0.08%) and by 70 Gy was 24.4% (±0.05%). Maximum PTV coverage by 70 Gy was 33%. Maximum PTV coverage by 60 Gy was 77.5%. The following organ at risk (OAR) constraints were achieved for 90% of generated plans: Duodenum V20 < 30 cc, V30 < 3 cc, V35 < 1 cc; Small Bowel V20 < 15 cc, V30 < 1 cc, V35 < 0.1 cc; Stomach V20 < 20 cc, V30 < 2 cc, V35 < 1 cc. V40 < 0.5 cc was achieved for all OAR. ConclusionsDose escalation to 60 Gy is dosimetrically feasible with adequate GTV coverage. The identified constraints for OAR's will be used in ongoing clinical trials.

Dosimetric Impact of Metal Artifacts From Dental Implants and Clinical Implications for Head And Neck IMRT Treatment Planning

Dental implants are increasingly prevalent in the head and neck cancer patient population and result in the creation of artifacts on CT imaging for treatment planning. In our practice, we have noticed more mucosal toxicity adjacent to dental implants and hypothesized that this observation may be due to errors in dose calculation. The purpose of this study was to retrospectively evaluate dosimetric differences in target coverage and organ at risk (OAR) sparing when adjusting for spurious CT measurements attributed to dental metal artifact. Nine patients with dental implants who were previously treated at our institution with definitive radiation for oropharyngeal cancer were selected for this retrospective study. All patients were treated with 6 MV photons using intensity modulated radiation therapy (IMRT). Dose calculation was performed using an algorithm with heterogeneity corrections. Dental implant and streak artifact volumes were contoured and new IMRT plans were generated with identical beam geometry and monitor units using the uncorrected reference CT data set and a corrected CT data set. Artifacts were assigned a soft tissue electron density value with a Hounsfield unit (HU) of zero. Using descriptive statistics, target volumes and OAR dosimetry were reported and compared between the clinical treatment plans and re-calculated plans. Target volumes analyzed were GTV (D95, D98, D99), intermediate-risk PTV (D95, D98, D99), and high-risk PTV (D95, D98, D99, D2cc). The maximum dose, Dmax, was also collected for each plan. OARs analyzed included the bilateral parotids (mean), oral uninvolved (mean), OAR pharynx (mean), mandible (max), and cord (max). The median artifact volume was 41.6 cc (range, 22.5 to 56.8 cc). When accounting for artifact error, there were small but statistically significant relative increases in Dmax (0.24%, P=.0273), mean oral uninvolved dose (0.24%, P=.0234), and mean left parotid dose (0.20%, P=.0469). On average, the hot spot (Dmax) increased by 181 cGy when accounting for artifact. There were no significant dosimetric differences between plans in terms of GTV, intermediate-risk PTV, and high-risk PTV coverage. There were no significant differences for the mandible max dose, cord max dose, OAR pharynx mean dose, or the right parotid mean dose. In this retrospective study, there was minimal dose variation to target volumes and OAR structures when substituting artifact with soft tissue density HUs. There were significant differences in terms of maximum dose and dose to the uninvolved oral cavity, which suggests that ignoring artifacts in CT images during planning may result in greater hot spots. While small, these differences are worthy of further study as they may at least partially contribute to increased rates of empirically observed mucosal toxicity in the vicinity of dental artifacts.

Treatment planning for MLC based robotic radiosurgery for brain metastases: plan comparison with circular fields and suggestions for planning strategies

Abstract To evaluate the possible range of application of the new InCise2 MLC for the CyberKnife M6 system in brain radiosurgery, a plan comparison was made for 10 brain metastases sized between 1.5 and 9cm3 in 10 patients treated in a single fraction each. The target volumes consist of a PTV derived by expanding the GTV by 1mm and were chosen to have diversity in the cohort regarding regularity of shape, location and the structures needed to be blocked for beam transmission in the vicinity. For each case, two treatment plans were optimized: one using the MLC and one using the IRIS-collimator providing variable circular fields. Plan re-quirements were: dose prescription to the 70% isodose line (18 or 20Gy), 100% GTV coverage, ≥98% PTV coverage, undisturbed central high dose region (95% of maximum dose) and a conformity index as low as possible. Plan com-parison parameters were: conformity index (CI), high-dose gradient index (GIH), low-dose gradient index (GIL), total number of monitor units (MU) and expected treatment time (TT). For all cases, clinically acceptable plans could be gen-erated with the following results (mean±SD) for CI, GIH, GIL, MU and TT, respectively for the MLC plans: 1.09±0.03, 2.77±0.26, 2.61±0.08, 4514±830MU and 27±5min and for the IRIS plans: 1.05±0.01, 3.00±0.35, 2.46±0.08, 8557±1335MU and 42±7min. In summary, the MLC plans were on average less conformal and had a shallower dose gradient in the low dose region, but a steeper dose gradient in the high dose region. This is accompanied by a smaller vol-ume receiving 10Gy. A plan by plan comparison shows that usage of the MLC can spare about one half of the MUs and one third of treatment time. From these experiences and results suggestions for MLC planning strategy can be de-duced.

4D VMAT planning and verification technique for dynamic tracking using a direct aperture deformation (DAD) method.

We developed a four‐dimensional volumetric modulated arc therapy (4D VMAT) planning technique for moving targets using a direct aperture deformation (DAD) method and investigated its feasibility for clinical use. A 3D VMAT plan was generated on a reference phase of a 4D CT dataset. The plan was composed of a set of control points including the beam angle, MLC apertures and weights. To generate the 4D VMAT plan, these control points were assigned to the closest respiratory phases using the temporal information of the gantry angle and respiratory curve. Then, a DAD algorithm was used to deform the beam apertures at each control point to the corresponding phase to compensate for the tumor motion and shape changes. Plans for a phantom and five lung cases were included in this study to evaluate the proposed technique. Dosimetric comparisons were performed between 4D and 3D VMAT plans. Plan verification was implemented by delivering the 4D VMAT plans on a moving QUASAR™ phantom driven with patient‐specific respiratory curves. The phantom study showed that the 4D VMAT plan generated with the DAD method was comparable to the ideal 3D VMAT plan. DVH comparisons indicated that the planning target volume (PTV) coverages and minimum doses were nearly invariant, and no significant difference in lung dosimetry was observed. Patient studies revealed that the GTV coverage was nearly the same; although the PTV coverage dropped from 98.8% to 94.7%, and the mean dose decreased from 64.3 to 63.8 Gy on average. For the verification measurements, the average gamma index pass rate was 98.6% and 96.5% for phantom 3D and 4D VMAT plans with 3%/3 mm criteria. For patient plans, the average gamma pass rate was 96.5% (range 94.5–98.5%) and 95.2% (range 94.1–96.1%) for 3D and 4D VMAT plans. The proposed 4D VMAT planning technique using the DAD method is feasible to incorporate the intra‐fraction organ motion and shape change into a 4D VMAT planning. It has great potential to provide high plan quality and delivery efficiency for moving targets.

Effect of atelectasis changes on tissue mass and dose during lung radiotherapy.

Purpose:To characterize mass and density changes of lung parenchyma in non-small cell lung cancer (NSCLC) patients following midtreatment resolution of atelectasis and to quantify the impact this large geometric change has on normal tissue dose.Methods:Baseline and midtreatment CT images and contours were obtained for 18 NSCLC patients with atelectasis. Patients were classified based on atelectasis volume reduction between the two scans as having either full, partial, or no resolution. Relative mass and density changes from baseline to midtreatment were calculated based on voxel intensity and volume for each lung lobe. Patients also had clinical treatment plans available which were used to assess changes in normal tissue dose constraints from baseline to midtreatment. The midtreatment image was rigidly aligned with the baseline scan in two ways: (1) bony anatomy and (2) carina. Treatment parameters (beam apertures, weights, angles, monitor units, etc.) were transferred to each image. Then, dose was recalculated. Typical IMRT dose constraints were evaluated on all images, and the changes from baseline to each midtreatment image were investigated.Results:Atelectatic lobes experienced mean (stdev) mass changes of −2.8% (36.6%), −24.4% (33.0%), and −9.2% (17.5%) and density changes of −66.0% (6.4%), −25.6% (13.6%), and −17.0% (21.1%) for full, partial, and no resolution, respectively. Means (stdev) of dose changes to spinal cord Dmax, esophagus Dmean, and lungs Dmean were 0.67 (2.99), 0.99 (2.69), and 0.50 Gy (2.05 Gy), respectively, for bone alignment and 0.14 (1.80), 0.77 (2.95), and 0.06 Gy (1.71 Gy) for carina alignment. Dose increases with bone alignment up to 10.93, 7.92, and 5.69 Gy were found for maximum spinal cord, mean esophagus, and mean lung doses, respectively, with carina alignment yielding similar values. 44% and 22% of patients had at least one metric change by at least 5 Gy (dose metrics) or 5% (volume metrics) for bone and carina alignments, respectively. Investigation of GTV coverage showed mean (stdev) changes in VRx, Dmax, and Dmin of −5.5% (13.5%), 2.5% (4.2%), and 0.8% (8.9%), respectively, for bone alignment with similar results for carina alignment.Conclusions:Resolution of atelectasis caused mass and density decreases, on average, and introduced substantial changes in normal tissue dose metrics in a subset of the patient cohort.

SU‐F‐T‐512: A Dosimetric Comparison for Palliative Spine Metastasis Treatments: 3‐D Vs. Conformal Arc

Purpose:Spine metastases have traditionally been treated using a parallel opposed 3‐D technique: an anterior field and a posterior field (AP/PA). This has proven to give the lesions adequate coverage. The downside to this type of treatment is that the full prescription dose covers a large area of the patient and can go through critical organs such as the heart, spine, lungs, bowel, and others depending on the vertebrae being treated. Conformal arc therapy can also be used for this type of treatment. These two techniques will be compared for target coverage and doses to organs at risk (OARs).Methods:20 multi‐vertebra metastases patients were chosen for this study, with a range of vertebrae being treated amongst each case. The original AP/PA treatments were re‐planned with 1–3 conformal arcs placed depending on the location of nearby OARs. The GTV coverage was normalized so that both techniques had equal coverage. The OAR doses and the volume of the 50% prescription dose value were then compared between the two planning techniques.Results:Initial results on average show lower doses for the heart maximum, heart mean, lung V20, spinal cord maximum, and bowel maximum for the conformal arc plans. The AP/PA plans however on average showed lower kidney mean dose, liver mean dose, and parotid mean dose. The 50% prescription dose volume was smaller in all the conformal arc plans except for one.Conclusion:From the preliminary results the conformal arc plans spare critical OARs better than the two field technique, except when treating the vertebrae near the kidneys, liver, and parotids. The conformal arc plans also have a lower volume of the 50% prescription dose value. The data suggests that conformal arcs can be used for certain palliative spine treatments and that this technique should be considered when beginning planning.

SU‐F‐T‐389: Validation in 4D Dosimetry Using Dynamic Phantom

Purpose:Tumor motion due to respiration causes the uncertainties during the radiotherapy. This study aims to find the differences between planning dose by treatment planning and the received dose using dynamic phantom.Methods:Respiratory motion was simulated by the DYNAMIC THORAX PHANTOM (Model 008A). 4D‐CT scans and maximum intensity projection (MIP) images for GTV were acquired for analysis. The amplitude of craniocaudal tumor motion including 2mm, 5mm, 10mm and 20mm with 3cm2 tumor size were performed in this study. The respiratory cycles of 4‐seconds and 6‐seconds were included as the different breathing modes. IMRT, VAMT, and Tomotherapy were utilized for treatment planning. Ion chamber and EBT3 were used to measure the point dose and planar dose. Dose distributions with different amplitudes, respiratory cycles, and planning techniques were all measured and compared to calculations.Results:The variations between the does measurements and calculation dose by treatment planning system were found in both point dose and dose distribution. The 0.83% and 5.46 % differences in dose average were shown on phantom with motions using 2mm amplitude in 4 second respiratory cycle, and 20mm amplitude in 4 second respiratory cycle, respectively. The most point dose overestimation as compared of the calculations was shown the plan generated by Tomotherapy. The underestimations of planar dose as compared of calculations was found in the 100% coverage doses for GTV.Conclusion:The loss of complete (100%) GTV coverage was the predominant effect of respiratory motion observed in this study. Motion amplitude and treatment planning system were the major factors leading the dose measurement variation as compared of planning calculations.

SU‐C‐202‐05: Pilot Study of Online Treatment Evaluation and Adaptive Re‐Planning for Laryngeal SBRT

Purpose:We have instigated a phase I trial of 5‐fraction stereotactic body radiotherapy (SBRT) for advanced‐stage laryngeal cancer. We conducted this pilot dosimetric study to confirm the potential utility of online adaptive re‐planning to preserve treatment quality.Methods:Ten cases of larynx cancer were evaluated. Baseline and daily SBRT treatment plans were generated per trial protocol. Daily volumetric images were acquired prior to every fraction of treatment. Reference simulation CT images were deformably registered to daily volumetric images using Eclipse. Planning contours were then deformably propagated to daily images. Reference SBRT plans were directly copied to calculate delivered dose distributions on deformed reference CT images. In‐house software platform has been developed to calculate cumulative dose over a course of treatment in four steps: 1) deforming delivered dose grid to reference CT images using deformation information exported from Eclipse; 2) generating tetrahedrons using deformed dose grid as vertices; 3) resampling dose to a high resolution within every tetrahedron; 4) calculating dose‐volume histograms. Our inhouse software was benchmarked with a commercial software, Mirada.Results:In all ten cases including 49 fractions of treatments, delivered daily doses were completely evaluated and treatment could be re‐planned within 10 minutes. Prescription dose coverage of PTV was less than intended in 53% of fractions of treatment (mean: 94%, range: 84%–98%) while minimum coverage of CTV and GTV was 94% and 97%, respectively. Maximum bystander point dose limits to arytenoids, parotids, and spinal cord remained respected in all cases, although variances in carotid artery doses were observed in a minority of cases.Conclusion:Although GTV and CTV coverage is preserved by in‐room 3D image guidance of larynx SBRT, PTV coverage can vary significantly from intended plans. Online adaptive treatment evaluation and re‐planning is potentially necessary and our procedure is clinically applicable to fully preserve treatment quality.This project is supported by CPRIT Individual Investigator Research Award RP150386.

SU‐F‐I‐51: CT/MR Image Deformation: The Clinical Assessment QA in Target Delineation

Purpose:To study the deformation effects in CT/MR image registration of head and neck (HN) cancers. We present a clinical indication in guiding and simplifying registration procedures of this process while CT images possessed artifacts.Methods:CT/MR image fusion provides better soft tissue contrast in intracranial GTV definition with artifacts. However, whether the fusion process should include the deformation process is questionable and not recommended. We performed CT/MR image registration of a HN patient with tonsil GTV and nodes delineation on Varian Velocity™ system. Both rigid transformation and deformable registration of the same CT/MR imaging data were processed separately. Physician's selection of target delineation was implemented to identify the variations. Transformation matrix was shown with visual identification, as well as the deformation QA numbers and figures were assessed.Results:The deformable CT/MR images were traced with the calculated matrix, both translation and rotational parameters were summarized. In deformable quality QA, the calculated Jacobian matrix was analyzed, which the min/mean/max of 0.73/0/99/1.37, respectively. Jacobian matrix of right neck node was 0.84/1.13/1.41, which present dis‐similarity of the nodal area. If Jacobian = 1, the deformation is at the optimum situation. In this case, the deformation results have shown better target delineation for CT/MR deformation than rigid transformation. Though the root‐mean‐square vector difference is 1.48 mm, with similar rotational components, the cord and vertebrae position were aligned much better in the deformable MR images than the rigid transformation.Conclusion:CT/MR with/without image deformation presents similar image registration matrix; there were significant differentiate the anatomical structures in the region of interest by deformable process. Though vendor suggested only rigid transformation between CT/MR assuming the geometry remain similar, our findings indicated with patient positional variations, deformation registration is needed to generate proper GTV coverage, which will be irradiated more accurately in the following boost phase.

Feasibility and Initial Dosimetric Findings for a Randomized Trial Using Dose-Painted Multiparametric Magnetic Resonance Imaging–Defined Targets in Prostate Cancer

To compare dosimetric characteristics with multiparametric magnetic resonance imaging-identified imaging tumor volume (gross tumor volume, GTV), prostate clinical target volume and planning target volume, and organs at risk (OARs) for 2 treatment techniques representing 2 arms of an institutional phase 3 randomized trial of hypofractionated external beam image guided highly targeted radiation therapy. Group 1 (n=20) patients were treated before the trial inception with the standard dose prescription. Each patient had an additional treatment plan generated per the experimental arm. A total of 40 treatment plans were compared (20 plans for each technique). Group 2 (n=15) consists of patients currently accrued to the hypofractionated external beam image guided highly targeted radiation therapy trial. Plans were created as per the treatment arm, with additional plans for 5 of the group 2 experimental arm with a 3-mm expansion in the imaging GTV. For all plans in both patient groups, planning target volume coverage ranged from 95% to 100%; GTV coverage of 89.3Gy for the experimental treatment plans ranged from 95.2% to 99.8%. For both groups 1 and 2, the percent volumes of rectum/anus and bladder receiving 40Gy, 65Gy, and 80Gy were smaller in the experimental plans than in the standard plans. The percent volume at 1Gy per fraction and 1.625Gy per fraction were compared between the standard and the experimental arms, and these were found to be equivalent. The dose per fraction to the OARs can be made equal even when giving a large simultaneous integrated boost to the GTV. The data suggest that a GTV margin may be added without significant dose effects on the OARs.

Evaluation of Single Field Uniform Dose (SFUD) Proton Pencil Beam Scanning (PBS) Planning Strategy for Lung Mobile Tumor Using a Digital Phantom

Purpose: To quantitatively evaluate four different Proton SFUD PBS initial planning strategies for lung mobile tumor. Methods and Materials: A virtual lung patient’s four-dimensional computed tomography (4DCT) was generated in this study. To avoid the uncertainties from target delineation and imaging artifacts, a sphere with diameter of 3 cm representing a rigid mobile target (GTV) was inserted into the right side of the lung. The target motion is set in superior-inferior (SI) direction from ?5 mm to 5 mm. Four SFUD planning strategies were used based on: 1) Maximum-In-tensity-Projection Image (MIP-CT); 2) CT_average with ITV overridden to muscle density (CTavg_muscle); 3) CT_average with ITV overridden to tumor density (CTavg_tumor); 4) CT_average without any override density (CTavg_only). Dose distributions were recalculated on each individual phase and accumulated together to assess the “actual” treatment. To estimate the impact of proton range uncertainties, +/?3.5% CT calibration curve was applied to the 4DCT phase images. Results: Comparing initial plan to the dose accumulation: MIP-CT based GTV D98 degraded 2.42 Gy (60.10 Gy vs 57.68 Gy). Heart D1 increased 6.19 Gy (1.88 Gy vs 8.07 Gy); CTavg_tumor based GTV D98 degraded 0.34 Gy (60.07 Gy vs 59.73 Gy). Heart D1 increased 2.24 Gy (3.74 Gy vs 5.98 Gy); CTavg_muscle based initial GTV D98 degraded 0.31 Gy (60.4 Gy vs 60.19 Gy). Heart D1 increased 3.44 Gy (4.38 Gy vs 7.82 Gy); CTavg_only based Initial GTV D98 degraded 6.63 Gy (60.11 Gy vs 53.48 Gy). Heart D1 increased 0.30 Gy (2.69 Gy vs 2.96 Gy); in the presence of ±3.5% range uncertainties, CTavg_tumor based plan’s accumulated GTV D98 degraded to 57.99 Gy (+3.5%) 59.38 Gy (?3.5%), and CTavg_muscle based plan’s accumulated GTV D98 degraded to 59.37 Gy (+3.5%) 59.37 Gy (?3.5%). Conclusion: This study shows that CTavg_Tumor and CTavg_Muscle based planning strategies provide the most robust GTV coverage. However, clinicians need to be aware that the actual dose to OARs at distal end of target may increase. The study also indicates that the current SFUD PBS planning strategy might not be sufficient to compensate the CT calibration uncertainty.

Does Reducing Cochlear Dose Through Beam Sector Blocking Improve Hearing Preservation Following Vestibular Schwannoma Radiosurgery?

Results: For the entire cohort, the 5-year local control (LC) was 69.6% (95% CI 50e89.2%), and overall survival (OS) was 81.4% (95% CI 65.3e97.5%). All tumor failures were in-field and no distant metastases have yet been observed. In multivariate analysis, LC was significantly associated with prescribed RT dose (HR 0.78 per Gy, pZ0.022) and GTV at RT (HR 1.03 per cm, pZ0.018), while OS was significantly associated with D1cm (HR 0.89 per Gy, pZ0.038). Comparing radiation plans by cohort, the median D1cm in patients experiencing failure was 66.75 Gy (RBE) compared to a median of 76.25 Gy (RBE) in those remaining controlled (pZ0.017). Among the 14 patients with a D1cm 67 Gy (RBE) had a median follow-up of 43 months. The 5-year LC in this subset was 81.1% (95% CI 61.7e100%) (pZ0.004 overall comparison by GTV 67 Gy (RBE)). To improve outcomes, multidisciplinary efforts should focus on optimizing tumor resection to facilitate adequate GTV coverage. Author Disclosure: M.W. McDonald: None. O.R. Linton: Research Grant; Jesse N. Jones, III Memorial Fund.

Pulsed Reduced Dose Rate Reirradiation for Recurrent Primary Central Nervous System Tumors

Results: For the entire cohort, the 5-year local control (LC) was 69.6% (95% CI 50e89.2%), and overall survival (OS) was 81.4% (95% CI 65.3e97.5%). All tumor failures were in-field and no distant metastases have yet been observed. In multivariate analysis, LC was significantly associated with prescribed RT dose (HR 0.78 per Gy, pZ0.022) and GTV at RT (HR 1.03 per cm, pZ0.018), while OS was significantly associated with D1cm (HR 0.89 per Gy, pZ0.038). Comparing radiation plans by cohort, the median D1cm in patients experiencing failure was 66.75 Gy (RBE) compared to a median of 76.25 Gy (RBE) in those remaining controlled (pZ0.017). Among the 14 patients with a D1cm 67 Gy (RBE) had a median follow-up of 43 months. The 5-year LC in this subset was 81.1% (95% CI 61.7e100%) (pZ0.004 overall comparison by GTV 67 Gy (RBE)). To improve outcomes, multidisciplinary efforts should focus on optimizing tumor resection to facilitate adequate GTV coverage. Author Disclosure: M.W. McDonald: None. O.R. Linton: Research Grant; Jesse N. Jones, III Memorial Fund.

Effect of dose-escalation of IMRT for unresectable pancreatic cancer 1 cm away from the luminal mucosa on long-term survival.

354 Background: The use of chemoradiation (CXRT) for locally advanced pancreatic cancer (LAPC) is controversial. Delivery of high doses of RT capable of leading to local tumor control is challenging. We reviewed outcomes of treatement with dose-escalated IMRT with curative intent. Methods: Of 211 patients treated from 5/2006 to 8/2014 with CXRT for LAPC, 49(23%) had tumors &gt; 1 cm from the luminal organs ere selected for dose-escalated IMRT using integrated boost (SIB) technique, inspiration breath hold, and computed tomographic (CT) image guidance. Fractionation was optimized for coverage of gross tumor (GTV,Table 1). A 2-5mm margin on the GTV, was treated as an SIB within a microscopic dose. Forty-seven (96%) patients received a median of 4.0 months of induction chemotherapy and 45 (92%) received concurrent capecitabine or gemcitabine. Results: Mean GTV coverage was 86% (95% CI 78% to 94%). Median FU was 32 mo. Median OS and 1, 2, 3 and 5 year OS rates were 22.6mo (95% CI 16.4 to 43.9mo), 83%, 49%,38%, and 18% from the date of diagnosis and 17.8mo, 63%, 38%, 33%, and 18% from the start of RT. Degree of GTV coverage and Biological Equivalent Dose (BED) did not appear to affect outcome. Freedom from progression at 3 y were 40.6% (local) and 37.1% (distant). Acute toxicity was uncommon: grade 2 pain, diarrhea, anorexia, nausea or fatigue in 18 (37%), and grade 3 diarrhea in one patient (2%). Four patients (13%) required transfusion for anemia. One patient had a GI bleed possibly related to treatment. Conclusions: Dose-escalated IMRT across a BED range of 70-100Gy for inoperable patients selected with induction chemotherapy appears well tolerated and may improve the likelihood of long term survival. These results are similar to the best outcomes reported for patients after surgical resection. Incomplete high-dose GTV coverage does not appear to be detrimental. A randomized phase II trial is testing IMRT (RTOG 1201, 63Gy/28fx, with stratification by SMAD4 expression). [Table: see text]

SU-E-T-85: Comparison of Treatment Plans Calculated Using Ray Tracing and Monte Carlo Algorithms for Lung Cancer Patients Having Undergone Radiotherapy with Cyberknife

Purpose: The latest publications indicate that the Ray Tracing algorithm significantly overestimates the dose delivered as compared to the Monte Carlo (MC) algorithm. The purpose of this study is to quantify this overestimation and to identify significant correlations between the RT and MC calculated dose distributions. Methods: Preliminary results are based on 50 preexisting RT algorithm dose optimization and calculation treatment plans prepared on the Multiplan treatment planning system (Accuray Inc., Sunnyvale, CA). The analysis will be expanded to include 100 plans. These plans are recalculated using the MC algorithm, with high resolution and 1% uncertainty. The geometry and number of beams for a given plan, as well as the number of monitor units, is constant for the calculations for both algorithms and normalized differences are compared. Results: MC calculated doses were significantly smaller than RT doses. The D95 of the PTV was 27% lower for the MC calculation. The GTV and PTV mean coverage were 13 and 39% less for MC calculation. The first parameter of conformality, as defined as the ratio of the Prescription Isodose Volume to the PTV Volume was on average 1.18 for RT and 0.62 for MC. Maximum doses delivered to OARs was reduced in the MC plans. The doses for 1000 and 1500 cc of total lung minus PTV, respectively were reduced by 39% and 53% for the MC plans. The correlation of the ratio of air in PTV to the PTV with the difference in PTV coverage had a coefficient of −0.54. Conclusion: The preliminary results confirm that the RT algorithm significantly overestimates the dosages delivered confirming previous analyses. Finally, subdividing the data into different size regimes increased the correlation for the smaller size PTVs indicating the MC algorithm improvement verses the RT algorithm is dependent upon the size of the PTV.