3D Treatment Research Articles

Radiation oncology: a snapshot in time, 2014.

This special series issue of Journal of Clinical Oncology is devoted to the field of radiation oncology. The intention is to apprise our multidisciplinary readership of the latest developments and potential future directions in this field. The contents of this issue represent a snapshot in time, highlighting carefully selected topics in radiation oncology circa 2014. The editors enjoyed the process of considering what to record, given that we had such a broad and exciting assortment of choices available. We regret that we could not cover every aspect of the field. For example, we did not include a piece on lymphomas, although there have been significant changes in recent years with respect to the role of radiotherapy in managing that group of malignancies. Likewise, we did not specifically spotlight brachytherapy, or the contributions of the engineers and medical physicists whose work is the technologic basis of modern radiation oncology. The latter point is worth extra emphasis: the marriage of computed tomography, magnetic resonance, and positron emission tomography imaging to treatment planning, the consequential advent of three-dimensional (3D) conformal radiation therapy, the development of inverse planning software and intensity-modulated radiation therapy delivery hardware, the invention of near real-time image guidance of treatment delivery using cone-beam computed tomography and other techniques, and the adaptation of motion management strategies and 4D treatment planning (3D with a breathing motion component) are just some of the marvelous inventions that are the sine qua non on which the field is now based. Radiation oncology is not alone as a medical specialty that has benefited greatly from computerization and other applied modern technology, but we radiation oncologists owe a particularly large debt to our medical physics colleagues, who work alongside us in the clinic every day. Some of the articles that are included in this special series issue do, in fact, address clinical gains that are enabled by technologic progress (articles on stereotactic radiosurgery, stereotactic body radiation therapy, particle therapy, and novel paradigms for early-stage breast cancer, for example). These high-tech treatments are sometimes highly resource intensive, and we must be mindful of whether we are providing good value to patients and society with these sophisticated innovations—hence the thoughtful contribution by Teckie et al1 concerning how we begin to frame that question. The laws of physics are not changing, nor is human anatomy likely to evolve noticeably in this millennium. Consequently, as pointed out by Giaccia,2 it could be that we are reaching the limits of how far we can go on the basis of advances in treatment delivery technology alone. For further progress we will likely need to emphasize tumor biology from perspectives that vary from the genome to growth factor signaling to the tumor-immune system interface. Morris and Harari3 discuss pioneering efforts to translate such knowledge to the clinic. The ultimate measure of any success in oncology is how patients regard their own outcomes, and a fine discussion of the methodology of analyzing patient-reported outcomes is provided by Siddiqui et al,4 who also discuss important issues to consider in the long-term post-treatment survivorship phase of life—a state that is gratifyingly becoming more and more common. Whereas Salama and Milano5 describe the virtue of maintaining an aggressive approach for many patients with limited metastatic disease, Lutz et al6 offer insights into how radiotherapy is best integrated into the end-of-life phase of care for patients with cancer. In a few contributions in this issue, there are passages that provide background information of a historical nature on the development of treatment strategies and methods that are now in routine clinical use. The value of these sections is twofold: first, they recognize important individuals and schools of thought from the past that have helped to shape our modern approaches; and second, they remind us that future readers of material published today might look back on treatments given in 2014 and consider many to be outdated. Indeed, we hope that this will be the case. Although we are proud of how the field of radiation oncology has progressed, there remain many challenges to overcome before the problem of cancer is a thing of the past.

SU‐E‐J‐54: Dosimetric Evaluation of Amplitude‐Based Gating in Volumetric Modulated Arc Therapy (VMAT)

Purpose:The purpose of this work is to evaluate the dosimetric influence in the gated volumetric modulated arc therapy.Methods:This study is performed in three liver cancer patients. Execution of 4D CT simulation and plan deliveries on one‐dimensional respiratory motion phantom were based on patient's respiratory trace data sets. We chose the regular and irregular respiratory amplitude patterns among the patient respiratory‐gated treatment log file. Dose distributions were evaluated using EBT3 film by comparing the gated delivery with static delivery.Results:The presented results of static measured and calculated dose distributions showed good agreement with gamma‐index passing rates above 97% for 3%/3 m dose accuracy/distance‐to‐agreement criteria. In regular respiratory amplitude pattern, the results of two in three patients evaluated gamma‐index passing rates above 92% except for another patient who was evaluated of 87.16%. Patients whose plans had the largest percentage failing the gamma statistics exhibited irregular respiratory amplitude pattern that evaluated less than 70%. Minimum passing rate was 37.40%. This results showed that the correlation of between sorted 4D CT phase and irregular patient respiratory amplitude pattern may results in considerable dosimetric difference.Conclusion:The effort to maintain regular patient respiratory amplitude pattern between 4D CT simulation and treatment is the most important in gated volumetric modulated arc therapy.

SU-D-18A-01: Tumor Motion Tracking with a Regional Deformable Registration Model for Four Dimensional Radiation Treatment of Lung Cancer

Purpose: To develop a tumor motion model from four-dimensional computed tomography (4DCT) of thoracic patients and demonstrate its impact on 4D radiation therapy simulation. Methods: A regional deformable image registration algorithm was introduced to extract tumor motion out of patient's breathing cycle. The gross target volume (GTV) was manually delineated on a selected phase of 4DCT and a subregion with 10mm margin supplemented to the GTV was created on the Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA). Together with 4DCT the structures were exported into an inhouse research platform. A free form B-Spline deformable registration was carried out to map the subregion to other respiratory phases. The displacement vector fields were employed to propagate GTV contours with which the center of mass (CoM) of the GTV was computed for each breathing phase of 4DCT. The resultant GTV motion and its volumetric shape are utilized to facilitate 4D treatment planning. Five lung cancer patients undergoing stereotactic body radiation therapy were enrolled and their 4DCT sets were included in the study. Results: Application of the algorithm to five thoracic patients indicates that clinically satisfactory outcomes were achievable with a spatial accuracy better than 2mm for GTV contour propagation betweenmore » adjacent phases, and 3mm between opposite phases. The GTV CoM was found to be in the range of 2.0mm through 2.5cm, depending upon the tumor location. Compared to the traditional whole image based registration, the computation of the regional model was found to be an order of magnitude more efficient. Conclusion: A regional deformable registration model was implemented to extract tumor motion. It will have widespread application in 4D radiation treatment planning in the future to maximally utilize the available spatial-tempo information.« less

SU‐E‐T‐502: Biometrically Accepted Patient Records

Purpose:To implement a clinically useful palm vein pattern recognition biometric system to treat the correct treatment plan to the correct patient each and every time and to check‐in the patient into the department to access the correct medical record.Methods:A commercially available hand vein scanning system was paired to Aria and utilized an ADT interface from the hospital electronic health system. Integration at two points in Aria, version 11 MR2, first at the appointment tracker screen for the front desk medical record access and second at the queue screen on the 4D treatment console took place for patient daily time‐out. A test patient was utilized to check accuracy of identification as well as to check that no unintended interactions take place between the 4D treatment console and the hand vein scanning system. This system has been in clinical use since December 2013.Results:Since implementation, 445 patients have been enrolled into our biometric system. 95% of patients learn the correct methodology of hand placement on the scanner in the first try. We have had two instances of patient not found because of a bad initial scan. We simply erased the scanned metric and the patient enrolled again in those cases. The accuracy of the match is 100% for each patient, we have not had one patient misidentified. We can state this because we still use patient photo and date of birth as identifiers. A QA test patient is run monthly to check the integrity of the system.Conclusion:By utilizing palm vein scans along with the date of birth and patient photo, another means of patient identification now exits. This work indicates the successful implementation of technology in the area of patient safety by closing the gap of treating the wrong plan to a patient in radiation oncology.FOJP Service Corporation covered some of the costs of the hardware and software of the palm vein pattern recognition biometric system.

Differences in respiratory-induced pancreatic tumor motion between 4D treatment planning CT and daily cone beam CT, measured using intratumoral fiducials

Background. In radiotherapy, the magnitude of respiratory-induced tumor motion is often measured using a single four-dimensional computed tomography (4DCT). This magnitude is required to determine the internal target volume. The aim of this study was to compare the magnitude of respiratory-induced motion of pancreatic tumors on a single 4DCT with the motion on daily cone beam CT (CBCT) scans during a 3–5-week fractionated radiotherapy scheme. In addition, we investigated changes in the respiratory motion during the treatment course.Material and methods. The mean peak-to-peak motion (i.e. magnitude of motion) of pancreatic tumors was measured for 18 patients using intratumoral gold fiducials visible on CBCT scans made prior to each treatment fraction (10–27 CBCTs per patient; 401 CBCTs in total). For each patient, these magnitudes were compared to the magnitude measured on 4DCT. Possible time trends were investigated by applying linear fits to the tumor motion determined from daily CBCTs as a function of treatment day.Results. We found a significant (p ≤ 0.01) difference between motion magnitude on 4DCT and on CBCT in superior-inferior, anterior-posterior and left-right direction, in 13, 9 and 12 of 18 patients, respectively. In the anterior- posterior and left-right direction no fractions had a difference ≥ 5 mm. In the superior-inferior direction the difference was ≥ 5 mm for 17% of the 401 fractions. In this direction, a significant (p ≤ 0.05) time trend in tumor motion was observed in 4 of 18 patients, but all trends were small (− 0.17–0.10 mm/day) and did not explain the large differences in motion magnitude between 4DCT and CBCT.Conclusion. A single measurement of the respiratory-induced motion magnitude of pancreatic tumors using 4DCT is often not representative for the magnitude during daily treatment over a 3–5-week radiotherapy scheme. For this patient group it may be beneficial to introduce breath-hold to eliminate respiratory-induced tumor motion.

Four-Dimensional Patient Dose Reconstruction for Scanned Ion Beam Therapy of Moving Liver Tumors

Estimation of the actual delivered 4-dimensional (4D) dose in treatments of patients with mobile hepatocellular cancer with scanned carbon ion beam therapy. Six patients were treated with 4 fractions to a total relative biological effectiveness (RBE)-weighted dose of 40 Gy (RBE) using a single field. Respiratory motion was addressed by dedicated margins and abdominal compression (5 patients) or gating (1 patient). 4D treatment dose reconstructions based on the treatment records and the measured motion monitoring data were performed for the single-fraction dose and a total of 17 fractions. To assess the impact of uncertainties in the temporal correlation between motion trajectory and beam delivery sequence, 3 dose distributions for varying temporal correlation were calculated per fraction. For 3 patients, the total treatment dose was formed from the fractional distributions using all possible combinations. Clinical target volume (CTV) coverage was analyzed using the volumes receiving at least 95% (V95) and 107% (V107) of the planned doses. 4D dose reconstruction based on daily measured data is possible in a clinical setting. V95 and V107 values for the single fractions ranged between 72% and 100%, and 0% and 32%, respectively. The estimated total treatment dose to the CTV exhibited improved and more robust dose coverage (mean V95 > 87%, SD < 3%) and overdose (mean V107 < 4%, SD < 3%) with respect to the single-fraction dose for all analyzed patients. A considerable impact of interplay effects on the single-fraction CTV dose was found for most of the analyzed patients. However, due to the fractionated treatment, dose heterogeneities were substantially reduced for the total treatment dose. 4D treatment dose reconstruction for scanned ion beam therapy is technically feasible and may evolve into a valuable tool for dose assessment.

Validation of Automatic Contour Propagation for 4D Treatment Planning Using Multiple Metrics

The aim of this work is to provide insights into multiple metrics clinical validation of deformable image registration and contour propagation methods in 4D lung radiotherapy planning. The following indices were analyzed and compared: Volume Difference (VD), Dice Similarity Coefficient (DSC), Positive Predictive Value (PPV) and Surface Distances (SD). The analysis was performed on three patient datasets, using as reference a ground-truth volume generated by means of Simultaneous Truth And Performance Level Estimation (STAPLE) algorithm from the outlines of five experts. Significant discrepancies in the quality assessment provided by the different metrics in all the examined cases were found. Metrics sensitivity was more evident in presence of image artifacts and particularly for tubular anatomical structures, such as esophagus or spinal cord. Volume Differences did not account for position and DSC exhibited criticalities due to its intrinsic symmetry (i.e. over- and under-estimation of the reference contours cannot be discriminated) and dependency on the total volume of the structure. PPV analysis showed more robust performance, as each voxel concurs to the classification of the propagation, but was not able to detect inclusion of propagated and ground-truth volumes. Mesh distances could interpret the actual shape of the structures, but might report higher mismatches in case of large local differences in the contour surfaces. According to our study, the combination of VD and SD for the validation of contour propagation algorithms in 4D could provide the necessary failure detection accuracy.

A 4D-optimization concept for scanned ion beam therapy

Background and purposeScanned carbon beam therapy offers advantageous dose distributions and an increased biological effect. Treating moving targets is complex due to sensitivity to range changes and interplay. We propose a 4D treatment planning concept that considers motion during particle number optimization. Material and methodsThe target was subdivided into sectors, one for each motion phase of a 4D-CT. Each sector was non-rigidly transformed to its motion phase and there targeted by a dedicated raster field (RST). Therefore, the resulting 4D-RST compensated target motion and range changes. A 4D treatment control system (TCS) was needed for synchronized delivery to the measured patient motion. 4D-optimized plans were simulated for 9 NSCLC lung cancer patients and compared to static irradiation at end-exhale. A prototype TCS was implemented and successfully tested in a film experiment. ResultsThe 4D-optimized treatment plan resulted in only slightly lower dose coverage of the target compared to static optimization, with V95% of 97.9% (median, range 96.5–99.4%) vs. 99.3% (98.5–99.8%), with negligible overdose. The conformity number was comparable at 88.2% (85.1–92.5%) vs. 85.2% (79.9–91.2%) for 4D and static, respectively. ConclusionWe implemented and tested a 4D treatment plan optimization method resulting in highly conformal dose delivery.

Carbon Ion Scanning Lung Therapy (With Respiratory-Gated Phase Controlled Rescanning): Simulation Study for 4D Treatment Planning

To provide mostly satisfactory dose conformity to the moving target, we applied a respiratory-gated strategy to the carbon-ion active scanning lung treatment with multiple phase controlled rescannings (PCR). Here, we quantified the dose assessment using 4DCT data set. Total 14 patients with lung tumor (stage I non-small-cell lung cancer or oligometastatic lung tumor) were randomly selected. Volumetric cine CT images were acquired in 4D mode under free breathing conditions. Pencil-beam scanning carbon-ion radiation therapy (PBS-CIRT) plans were created for each patient. The gross tumor volume, the clinical target volume (CTV) and organ at risks (OARs) were delineated. Field-specific target volumes (FTVs) were calculated for respective beam fields; two types of FTVs were designed for ungated (full respiratory cycle) and gated (T40-T60) strategies. The prescribed dose of 48 Gy (RBE) in single fraction was given to the FTVs with different coplanar four angles. As technique of irradiation for moving target, two methods were used in current study: (1) respiratory gating for motion mitigation, (2) PCR to obtain a conformal dose distribution. We evaluated the dose metrics by changing the number of PCRs and compared them in between ungated and gated strategies. To calculate the accumulated dose distribution, the deformable image registration was applied to transfer dose distributions at respective phases to the reference phase. By defining of FTV, undershooting to the target could be generally avoided in each respiratory phase. As shown in the Table, D95 of the treatment dose was 94.4 ± 1.9% even in 1x PCR ungated plan. In gating plan, significant improvements in all the dose volume indices of CTV and OARs were found compared with ungated plan. Moreover, observe that the CTV coverage improved gradually as the number of PCR increased. Four times or more of PCR resulted in adequate target coverage. As for OARs, the dose indices of lung V20, Dmax of heart, and spinal cord did not change significantly according to increasing PCR. These metrics were improved only with respiratory gating. For instance, dose to lung V20 was decreased 12% (4.3 Gy [RBE] in ungated plan to 3.8 Gy [RBE] in gated). For a moving target, PBS-CIRT system with gating and PCR accurately achieve homogenous dose distribution even in actual patients.Digital Poster Abstract 1093; TableComparison of dose-volume parameters averaged over 14 patients1x PCR4x PCR8x PCRMeanSDMeanSDMeanSDCTVD95 (%)Ungated94.4±1.997.1±0.797.2±0.7Gated96.0±0.896.3±0.996.5±1.2Dmax (%)Ungated109.5±4.5101.8±0.6101.2±0.4Gated103.7±1.6102.2±1.1101.6±0.6Dmin (%)Ungated89.0±3.792.0±3.692.1±3.4Gated91.7±1.592.11.692.4±2.2 Open table in a new tab

4D Treatment Dose Reconstruction for Scanned Ion Beam Therapy

Purpose/Objective(s): To report the initial outcomes and toxicity of SABR for oligometastases. Materials/Methods: Fifty-three patients with a minimum potential followup of 1 year were treated with SABR directed to a total of 99 metastatic sites (lung n Z 65, liver n Z 14, bone n Z 10, lymph node n Z 9, soft tissue n Z 1). The most commonly used SABR regimen was 50 Gy in 5 consecutive daily fractions using conformal arc technique with daily image guidance using CBCT. The most common primary sites were colorectal cancer (CRC; 28%), non-small cell lung carcinoma (NSCLC; 19%), soft tissue sarcoma (STS; 15%), and renal cell carcinoma (RCC; 8%). Median time to metastatic progression from initial diagnosis was 1.2 years (range, 0-13.1 years). The majority of patients were treated for solitary metastases (59%). Prior to SABR, 45% were treated for metastatic disease with surgery, chemotherapy or both. Fourteen patients received additional SABR for limited progression. Results: Median follow-up was 1.3 years (1.4 years for living patients). Local control (LC), distant-progression free survival (DPFS), causespecific survival (CSS), and overall survival (OS) at 1 and 2 years were 92%, 56%, 87%, 85%, and 74%, 40%, 70%, 69%, respectively, after the initial course of SABR. After a second course of SABR for limited progression the 1-year DPFS was only 22%. On univariate analysis there was no significant predictor for LC, although liver metastases and CRC primaries showed a trend toward decreased LC. Two-year DFPS, CSS, and OS were worse for patients with multiple metastatic sites, multiple organ involvement, increased cumulative GTV at initial SABR course, and when all measurable disease was not treated with SABR. Grade 3+ toxicity was observed in only 3 patients. Conclusions: In selected patients with limited metastatic disease, SABR resulted in durable disease control in a significant percentage of patients with minimal toxicity. Author Disclosure: R. Dagan: None. A.R. Yeung: None. R.A. Zlotecki: None. C.G. Morris: None. P. Okunieff: None.

Quantifying variability of intrafractional target motion in stereotactic body radiotherapy for lung cancers

In lung stereotactic body radiotherapy (SBRT), variability of intrafractional target motion can negate the potential benefits of four‐dimensional (4D) treatment planning that aims to account for the dosimetric impacts of organ motion. This study used tumor motion data obtained from CyberKnife SBRT treatments to quantify the reproducibility of probability motion function (pmf) of 37 lung tumors. The reproducibility of pmf was analyzed with and without subtracting the intrafractional baseline drift from the original motion data. Statistics of intrafractional tumor motion including baseline drift, target motion amplitude and period, were also calculated. The target motion amplitude significantly correlates with variations (1 SD) of motion amplitude and baseline drift. Significant correlation between treatment time and variations (1 SD) of motion amplitude was observed in anterior‐posterior (AP) motion, but not in craniocaudal (CC) and left‐right (LR) motion. The magnitude of AP and LR baseline drifts significantly depend on the treatment time, while the CC baseline drift does not. The reproducibility of pmf as a function of time can be well described by a two‐exponential function with a fast and slow component. The reproducibility of pmf is over 60% for the CC motion and over 50% for the AP and LR motions when baseline variations were subtracted from the original motion data. It decreases to just over 30% for the CC motion and about 20% for the AP and LR motion, otherwise. 4D planning has obvious limitations due to variability of intrafractional target motion. To account for potential risks of overdosing critical organs, it is important to simulate the dosimetric impacts of intra‐ and interfractional baseline drift using population statistics obtained from SBRT treatments.PACS number: 87.55.‐x

Simulations using patient data to evaluate systematic errors that may occur in 4D treatment planning: A proof of concept study

The purpose of this work is to present a framework to evaluate the accuracy of four-dimensional treatment planning in external beam radiation therapy using measured patient data and digital phantoms. To accomplish this, 4D digital phantoms of two model patients were created using measured patient lung tumor positions. These phantoms were used to simulate a four-dimensional computed tomography image set, which in turn was used to create a 4D Monte Carlo (4DMC) treatment plan. The 4DMC plan was evaluated by simulating the delivery of the treatment plan over approximately 5 min of tumor motion measured from the same patient on a different day. Unique phantoms accounting for the patient position (tumor position and thorax position) at 2 s intervals were used to represent the model patients on the day of treatment delivery and the delivered dose to the tumor was determined using Monte Carlo simulations. For Patient 1, the tumor was adequately covered with 95.2% of the tumor receiving the prescribed dose. For Patient 2, the tumor was not adequately covered and only 74.3% of the tumor received the prescribed dose. This study presents a framework to evaluate 4D treatment planning methods and demonstrates a potential limitation of 4D treatment planning methods. When systematic errors are present, including when the imaging study used for treatment planning does not represent all potential tumor locations during therapy, the treatment planning methods may not adequately predict the dose to the tumor. This is the first example of a simulation study based on patient tumor trajectories where systematic errors that occur due to an inaccurate estimate of tumor motion are evaluated.

Potential dosimetric benefit of dose-warping based 4D planning compared to conventional 3D planning in liver stereotactic body radiotherapy (SBRT)

Respiratory motion induces dosimetric uncertainties for thoracic and abdominal cancer radiotherapy (RT) due to deforming and moving anatomy. This study investigates the extent of dosimetric differences between conventional 3D treatment planning and path-integrated 4D treatment planning in liver stereotactic body radiotherapy (SBRT). Respiratory-correlated 4DCT image sets with 10 phases were acquired for patients with liver tumours. Path-integrated 4D dose accumulation was performed using dose-warping techniques based on deformable image registration. Dose-volume histogram analysis demonstrated that the 3D planning approach overestimated doses to targets by up to 24% and underestimated dose to normal liver by ~4.5%, compared to the 4D planning methodology. Therefore, 4D planning has the potential to quantify such issues of under- and/or over-dosage and improve treatment accuracy.

SU‐E‐T‐589: A Robust 4D Treatment Planning Approach for Lung Radiotherapy

Purpose: 4D treatment planning optimizes the cumulative dose delivered over the whole respiratory cycle to generate a plan that compensates for a patients individual respiratory motion. However, changes in the time spent in the each respiratory state may render a 4D plan invalid. We introduced and evaluate two robust treatment planning approaches to compensate for respiratory motion. Methods: A 4D optimization method was developed which optimizes the phase‐weighted dose distribution. Two robust 4D treatment planning approaches were tested: (1) planning on the average motion pdf (AVE_PDF); and (2) combining 4D plans designed on the “worst case” pdfs (WC_PDF). The sensitivity of nominal and robust 4D treatment plans to respiratory motion variations was tested for two scenarios where respiratory phase weights were modified to model changes in amplitude as well as the relative proportion of the respiratory cycle spent inhaling vs. exhaling. Results: The DVHs of robust plans show less sensitivity to variation in breathing pattern. Compared to the nominal 4D plan, robust plans improve the V95 by 2 to 6 Gy and CTV min dose by 1 to 5 Gy. Conclusion: Robust 4D plan can be designed either using average pdf approach or worst case pdf approach. We find that nominal 4D plans are very sensitive to the variation in respiration pattern while robust 4D plans are less sensitive under the similar changes. As compared to static 4D plan, healthy tissue sparing is also better in robust plan. This research is supported by the Natural Sciences and Engineering Research Council of Canada (NSERC).

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‐366: Time‐Resolved EPID Dosimetry for Validating ArcIMRT

Purpose: The purpose of this study is to demonstrate time‐resolved dosimetry for verifying arc intensity modulated radiation therapy (arcIMRT) delivery, utilizing the continuous image acquisition of an electronic portal imaging device (EPID). Methods: An arcIMRT field was made of sliding, multi‐leaf collimator (MLC) motion of X1 leaves traveling from −5 cm to +5 cm and back, while Y opening was kept at 10 cm. For this field, this travel was repeated three times during the gantry rotation of 180°. The images were continuously acquired while the EPID was irradiated with 240 MU at a constant dose rate of 300 MU/min. By summing 10 frames, and thus reducing the temporal resolution to 1 second, a longitudinal non‐uniformity in the images was reduced to less than 3%. Dose images in EPID were also calculated by the XVMC code for MLC positions at 1 mm interval using the EPID model developed in our previous study. The calculated images at 1 sec resolution were then correlated to and compared with the images, validating the time‐resolved dosimetry. Results: Over the period of 48 seconds, 481 EPID images were acquired. The MLCs traveled at 1.25 cm/sec. Thus, 10 images were assigned to 1.25 cm of MLC travel for the correlation and for dose evaluation. For all gantry angles, the agreement of the gamma tests was above 90%, given 3 mm distance to agreement and 3% dose difference, when MLCs moved forwardly to the X2 side. However, when they travel back, the pass rates were below 90% due to MLC lagging which was detected in time‐resolved EPID dose imaging. Conclusion: Time‐resolved, four‐dimensional dose validation of arcIMRT was demonstrated, showing the temporal information of dynamic radiation delivery. This can be used for the validation of 4D treatment delivery techniques.

SU‐C‐141‐05: Impact of Respiratory‐Correlated CT Reconstruction Algorithms in the Choice of Margin Definition for Free Breathing Lung Treatment

Purpose: To investigate the impact of phase‐and amplitude‐based reconstruction algorithms in 4D treatment margin strategies in presence of respiratory irregularities. Methods: 4D CT images of Dynamic Thorax Phantom were acquired and the respiratory signal was tracked by the belt. Three irregular breathing patterns were simulated: amplitude, frequency, and combined amplitude and frequency variations. 4D CT image artifacts were analyzed for the phase‐and amplitude based reconstruction algorithms. To assess the impact of the 4D CT image artifacts on the dose distribution, twelve plans (three simulated breathing models for the two reconstruction algorithms and for the two margins strategies) were calculated with a Monte Carlo based TPS. The lung tumor was delineated according to the mid ventilation and the ITV‐based strategy. All the plans were evaluated for Dmin, Dmax, Dmean, homogeneity index and conformity index. Results: The amplitude‐based reconstruction shows a significant reduction of artifacts compared to the phase‐based only for breathing amplitude variations. Most of the observed artifacts are image blurring and incomplete structures. The dosimetric analysis shows that the mid‐ventilation strategy provides better tumor coverage than the ITV‐based strategy. In the mid‐ventilation strategy the selected phase is close to the exhale phase which is less subject to motion artifacts. In the ITV‐based strategy the influence of motion artifacts is more pronounced due to the contribution of different phases and leads to a blurred dose distribution. Conclusion: Amplitude‐based reconstruction algorithm reduces image distortions in the particular case of amplitude variations but not when frequency variations are present. Based on the impact of image artifacts on the definition of margin strategies we showed that the mid‐ventilation strategy is more robust than the ITV‐based strategy. The ITV‐based strategy is more prone to image artifacts which cause significant degradation in the dose distribution compare to the mid ventilation.

Upgrade and benchmarking of a 4D treatment planning system for scanned ion beam therapy

Upgrade and benchmarking of a research 4D treatment planning system (4DTPS) suitable for realistic patient treatment planning and treatment simulations taking into account specific requirements for scanned ion beam therapy, i.e., modeling of dose heterogeneities due to interplay effects and range changes caused by patient motion and dynamic beam delivery. The 4DTPS integrates data interfaces to 4D computed tomography (4DCT), deformable image registration and clinically used motion monitoring devices. The authors implemented a novel data model for 4D image segmentation using Boolean mask volume datasets and developed an algorithm propagating a manually contoured reference contour dataset to all 4DCT phases. They further included detailed treatment simulation and dose reconstruction functionality, based on the irregular patient motion and the temporal structure of the beam delivery. The treatment simulation functionality was validated against experimental data from irradiation of moving radiographic films in air, 3D moving ionization chambers in a water phantom, and moving cells in a biological phantom with a scanned carbon ion beam. The performance of the program was compared to results obtained with predecessor programs. The measured optical density distributions of the radiographic films were reproduced by the simulations to (-2 ± 12)%. Compared to earlier versions of the 4DTPS, the mean agreement improved by 2%, standard deviations were reduced by 7%. The simulated dose to the moving ionization chambers in water showed an agreement with the measured dose of (-1 ± 4)% for the typical beam configuration. The mean deviation of the simulated from the measured biologically effective dose determined via cell survival was (617 ± 538) mGy relative biological effectiveness corresponding to (10 ± 9)%. The authors developed a research 4DTPS suitable for realistic treatment planning on patient data and capable of simulating dose delivery to a moving patient geometry for scanned ion beams. The accuracy and reliability of treatment simulations improved considerably with respect to earlier versions of the 4DTPS.

Experimental evaluations of the accuracy of 3D and 4D planning in robotic tracking stereotactic body radiotherapy for lung cancers

Due to the complexity of 4D target tracking radiotherapy, the accuracy of this treatment strategy should be experimentally validated against established standard 3D technique. This work compared the accuracy of 3D and 4D dose calculations in respiration tracking stereotactic body radiotherapy (SBRT). Using the 4D planning module of the CyberKnife treatment planning system, treatment plans for a moving target and a static off-target cord structure were created on different four-dimensional computed tomography (4D-CT) datasets of a thorax phantom moving in different ranges. The 4D planning system used B-splines deformable image registrations (DIR) to accumulate dose distributions calculated on different breathing geometries, each corresponding to a static 3D-CT image of the 4D-CT dataset, onto a reference image to compose a 4D dose distribution. For each motion, 4D optimization was performed to generate a 4D treatment plan of the moving target. For comparison with standard 3D planning, each 4D plan was copied to the reference end-exhale images and a standard 3D dose calculation was followed. Treatment plans of the off-target structure were first obtained by standard 3D optimization on the end-exhale images. Subsequently, they were applied to recalculate the 4D dose distributions using DIRs. All dose distributions that were initially obtained using the ray-tracing algorithm with equivalent path-length heterogeneity correction (3D EPL and 4D EPL) were recalculated by a Monte Carlo algorithm (3D MC and 4D MC) to further investigate the effects of dose calculation algorithms. The calculated 3D EPL, 3D MC, 4D EPL, and 4D MC dose distributions were compared to measurements by Gafchromic EBT2 films in the axial and coronal planes of the moving target object, and the coronal plane for the static off-target object based on the γ metric at 5%/3mm criteria (γ5%/3mm). Treatment plans were considered acceptable if the percentage of pixels passing γ5%/3mm (Pγ<1) ≥ 90%. The averaged Pγ<1 values of the 3D EPL, 3D MC, 4D EPL, and 4D MC dose calculation methods for the moving target plans are 95%, 95%, 94%, and 95% for reproducible motion, and 95%, 96%, 94%, and 93% for nonreproducible motion during actual treatment delivery. The overall measured target dose distributions are in better agreement with the 3DMC dose distributions than the 4DMC dose distributions. Conversely, measured dose distributions agree much better with the 4D EPL/MC than the 3D EPL/MC dose distributions in the static off-target structure, resulting in higher Pγ<1 values with 4D EPL/MC (91%) vs 3D EPL (24%) and 3D MC (25%). Systematic changes of target motion reduced the averaged Pγ<1 to 47% and 53% for 4D EPL and 4D MC dose calculations, and 22% for 3D EPL/MC dose calculations in the off-target films. In robotic tracking SBRT, 4D treatment planning was found to yield better prediction of the dose distributions in the off-target structure, but not necessarily in the moving target, compared to standard 3D treatment planning, for reproducible and nonreproducible target motion. It is important to ensure on a patient-by-patient basis that the cumulative uncertainty associated with the 4D-CT artifacts, deformable image registration, and motion variability is significantly smaller than the cumulative uncertainty occurred in standard 3D planning in order to make 4D planning a justified option.

Duodenal sparing stereotactic body radiation therapy for the treatment of locally advanced pancreatic cancer.

254 Background: The proximity of the pancreas to bowel presents a unique challenge for pancreatic cancer SBRT. This study explores the safety and effectiveness of a novel approach optimizing pancreatic tumor coverage and duodenal sparing. Methods: Eighteen patients with locally advanced pancreatic cancer (15 head/3 body) were treated with initial chemotherapy (Gemcitabine or 5-FU/Cisplatin) for 2- 6 months. All underwent endoscopic US-guided fiducial placement in and around the tumor and had a 4D treatment planning CT scan with oral contrast. This CT was used in conjunction with EUS findings, PET, and biphasic CT scans to identify the GrossTumorVolume (GTV) on the expiration phases. The planning target volume (PTV) was created by expanding the GTV by 2 mm. Dose-volume histogram (DVH) endpoints were constructed, keeping V7/15/20 (stomach/duodenum volumes that receive 7 Gy/15 Gy/20 Gy) &lt;40%,/&lt;25%/ &lt;15% ;the dose to 1/3 of the duodenal circumference &lt;20Gy and duodenal point max dose &lt;23 Gy. Additional dose constraints included liver D50&lt;5 Gy, ipsilateral kidney D25&lt;5 Gy, cord Dmax&lt;10Gy. Three 10 Gy fractions, normalized to the 85% isodose were delivered to the PTV on consecutive weekdays using fiducial-based respiratory motion tracking on a dedicated 6 MV linac-integrated stereotactic delivery system . The patients were then offered systemic therapy for 6 months or until tolerance or disease progression. Follow-up occurred at 4 weeks, 12 weeks, and every 3 months. Results: All patients completed SBRT and a median of 5 total cycles of pre-and post-SBRT chemotherapy. Gross target volumes ranged from 14.2-205.7cm3 (median 39.5cm3). With a median follow-up of 10 months, 1 patient developed transient gastroparesis, 1 G 2 abdominal pain;1 G 2 hematologic toxicity. No late toxicity was observed in 15 patients with longer follow-up (median 12 month). All patients were free of local progression at the last follow-up visit (range 3-12 months). Conclusions: Linac-delivered organ-sparing SBRT with chemotherapy in locally advanced pancreatic cancer resulted in excellent local control and also was well tolerated acutely and subacutely. Longer follow-up is warranted. A phase I dose-escalation study is underway.