Target Dose-volume Histograms Research Articles

Challenges of Treating Lung Cancer Patients at MR-Linac Using MR-Based Synthetic CT Calculation in the Adaptive Workflow

Magnetic Resonance guided adaptive radiotherapy (MRgART) allows plan adaptation according to the new patient anatomy; the contours of the structures are adjusted based on the patient's daily MRI, and in the adapt to shape (ATS) workflow, the adapted plan is recalculated on the MRI-based synthetic CT (sCT) generated by bulk density assignment. For sites where there is a high electronic density (ED) gradient between the target and surrounding tissues, such as in lung cancer treatments, the assignment of an average ED may not be able to reproduce an accurate dose calculation. This study evaluates the accuracy of the sCT adapted plan calculation for lung cancer patients and assesses whether the assignment of an optimized ED can reduce dosimetric differences should they arise MATERIALS/METHODS: Nine lung cancer patients treated at Unity 1.5 MR-Linac were selected for this retrospective study. The patient's target and organs at risk (OARs) were contoured, and a CT reference plan containing the ED bulk assignment information i.e., the contours to use in the ATS workflow, and their corresponding average ED was generated. To assess the accuracy of the dosimetry of the adapted plan calculated on the sCT, the plan was recalculated on an ideal sCT (sCTref) obtained from the reference CT by forcing the drawn contours to the average ED as defined on the CT reference plan. Targets and OARs dose-volume histogram (DVH) of the CT and sCTref plans and the dose distributions using gamma (γ) analysis with 2%-2mm criteria were compared. In the case of a discrepancy between the DVHs, the average Eds used for the recalculation on the sCTref were adjusted by several attempts to obtain a sCT optimized (sCTopt) for which a superposition of DVHs on CT and sCTopt was achieved. For 7 of the 9 patients CT and sCTref target DVHs were not comparable, with a mean dosimetric difference of 5.55% (range 2.35%-7.46%) in the target volume receiving the prescription dose (VDpre), while OARs DVH dose differences remained below 1% for the nine patients. The adjustment of the ED of the homolateral lung in the sCTopt, reduced the mean target VDpre dosimetric difference between CT and sCTopt to 0.66% (range 0.17%-1.64%). In addition, the results of the gamma analysis increased from values ranging between 39.5%-70.3% to 88.5%-93.2%, as shown in the Table. Dosimetric errors in the use of the sCT calculation for targets in high ED gradient areas may arise; the use of optimized ED for sCT calculation may be a promising strand to investigate in order to proceed with MR-based sCT plan adaptation for lung cancer treatment.

A method for patient-specific DVH verification using a high-sampling-rate log file in an Elekta linac.

We have proposed a method for patient-specific dose-volume histogram (DVH) verification using a 40-ms high-sampling-rate log file (HLF) available in an Elekta linac. Ten prostate volumetric-modulated arc therapy plans were randomly selected, and systematic leaf position errors of ±0.2, ±0.4, or ±0.8mm were added to the 10 plans, thereby producing a total of 70 plans. An RTP file was created by interpolating each leaf position in the HLF to obtain values at each control point, which is subsequently exported to a treatment planning system. The isocenter dose calculated by the HLF-based plan to a phantom (Delta4 Phantom+) was compared to that measured by the diode in the phantom in order to evaluate the accuracy of the HLF-based dose calculation. The D95 of the planning target volume (PTV) was also compared between the HLF-based plans and the original plans with the systematic leaf position errors, the latter being referred to as theory-based plans. Sensitivities of the DVH parameters in the target, the rectum, and the bladder were also calculated with the varied systematic leaf position errors. The relative differences in the isocenter doses between the HLF-based calculations and the measurements among the 70 plans were 0.21%±0.67% (SD). The maximum relative differences in PTV D95 between the HLF-based and the theory-based plans among the 70 cases were 0.11%. The patient-specific DVH verification method detected a change in the target DVH parameters of less than 1% when the systematic leaf position error was ±0.2mm. It is therefore suggested that the proposed DVH verification method may simplify patient-specific dose quality assurance procedures without compromising accuracy and sensitivity.

Learning-Based Prostate Needle Position Prediction for HDR Brachytherapy

During prostate high-dose-rate (HDR) brachytherapy, physicians select needle positions in a peripheral loading technique based on experience but without knowledge of the achievable dose distribution. Sub-optimal needle placement may lead to unfavorable dose distributions even after plan optimization. We propose a new deep learning-based method to predict HDR needle position for prostate HDR brachytherapy.The proposed framework consists of three major steps: (1) deformable registration via registration network (Reg-Net), (2) multi-atlas ranking and (3) needle regression. To model the global spatial relationship among multiple organs, binary masks of the target and organs at risk are transformed into distance maps which describe the distance of each local voxel to the organ surfaces. Reg-Net is utilized to deformably register the distance maps from multi-atlas set to match a patient's distance map and then bring needle maps from multi-atlas to this patient via spatial transformation. Several criteria are used for multi-atlas ranking including prostate volume similarity, multi-organ semantic image similarity and needle position criteria. Finally, needle regression is used to refine the final needle positions. A retrospective study of consecutively treated patients was used to evaluate the feasibility of the proposed method. We performed a five-fold cross validation to evaluate the proposed method. We generated a new dose plan for each patient based on predicted needle location. Target and organ dose volume histogram (DVH) metrics were used to quantify the difference between the clinical and predicted needle plans. All plans were normalized to prostate V100 of 95%.Ninety predicted needle plans were compared to clinical plans. In each case, the needle prediction algorithm completed within 1 minute. See table for comparison of target and organ DVH metrics (mean and standard deviation). Predicted needle plans had slightly greater target dose heterogeneity. The RTOG constraints of bladder V75% < 1 cc, rectum V75% < 1cc, and urethra V125% < 1 cc were met in all predicted plans.It is feasible to use this novel deep-learning-based method to predict needle locations for HDR prostate brachytherapy a priori. This strategy merits further study to improve utilization and quality of prostate brachytherapy.

Impact of Positioning Errors on the Dosimetry of Breath-Hold-Based Volumetric Arc Modulated and Tangential Field-in-Field Left-Sided Breast Treatments.

Heart diseases and cardiovascular events are well-known side effects in left-sided breast irradiation. Deep inspiration breath hold (BH) combined with fast delivery techniques such as volumetric modulated arc therapy (VMAT) or tangential field-in-field (TFiF) can serve as a valuable solution to reduce the dose to the heart. This study aims to compare the impact of positioning errors in VMAT and TFiF plans for BH left-sided breast treatments. Fifteen left-sided breast patients treated in BH with TFiF technique were included in this retrospective study. For each patient, a second plan with VMAT technique was optimized. Eighteen setup variations were introduced in each of these VMAT and TFiF reference plans, shifting the isocenter along six different directions by 3, 5, and 10 mm. A total of 540 perturbed plans, 270 for each technique, were recalculated and analyzed. The dose distributions on the target and organs at risk obtained in the different perturbed scenarios were compared with the reference scenarios, using as dosimetric endpoints the dose-volume histograms (DVH). The results were compared using the Wilcoxon test. Comparable plan quality was obtained for the reference VMAT and TFiF plans, except for low doses to organs at risk for which higher values (p < 0.05) were obtained for VMAT plans. For TFiF plans, perturbations of the isocenter position of 3, 5, or 10 mm produced mean deviations of the target DVH dosimetric parameters up to −0.5, −1.0, and −5.2%, respectively; VMAT plans were more sensitive to positioning errors resulting in mean deviations up to −0.5, −4.9, and −13.9%, respectively, for the same magnitude of the above mentioned perturbations. For organs at risk, only perturbations along the left, posterior, and inferior directions resulted in dose increase with a maximum deviation of +2% in the DVH dosimetric parameters. A notable exception were low doses to the left lung and heart for 10 mm isocenter shifts for which the mean differences ranged between +2.7 and +4.1%. Objective information on how external stresses affect the dosimetry of the treatment is the first step towards personalized radiotherapy.

Investigation of Target Minimum and Maximum Dosimetric Criteria for the Evaluation of Standardized Radiotherapy Plan &amp;lt;br/&amp;gt;—Target Minimum and Maximum Evaluation

Purpose: Standardization of tumor dosimetric coverage is essential for the evaluation of radiotherapy treatment plan quality. National clinical trials network RTOG protocols include tumor target dosimetric criteria that specify the prescription dose and minimum and maximum dose (Dmin and Dmax) coverages. This study investigated the impact of various minimum and maximum dose definitions using tumor control probability (TCP) models. Methods and Materials: Three disease sites (head and neck, lung, and prostate) were studied using target volume dosimetric criteria from the RTOG 0920, 1308, and 0938 protocols. Simulated target dose-volume histograms (DVHs) of Dmin and Dmax were modeled using the protocol specifications. Published TCP models for the three disease sites were applied to the DVH curves. The effects of various dose definitions on TCP were studied. Results: While the prescription dose coverage was maintained, a -3.7% TCP difference was observed for head and neck cancer when the target doses varied by 3.5% of the tumor volume from the point dose. For prostate and lung cancers, -3.3% and -2.2% TCP differences were observed, respectively. The TCPs for head and neck and prostate cancers were more negatively affected by deviations in the Dmin than the TCP for lung cancer. The lung TCP increased to a greater extent with a change in the Dmax compared with the head and neck and prostate TCPs. Conclusions: These results can be used to evaluate plan quality when the target dose only slightly deviates from the dosimetric criteria. When the overall target prescription dose coverage is maintained, the Dmax is recommended to be within 3% of the target volume: 98% (for head and neck and prostate) and 97% (for lung) of the target volume, satisfying the Dmin needed to maintain TCP variations at less than 2.1%. Using 0.03 cc instead of a point dose for Dmin and Dmax criteria minimally impacts TCPs.

Spinal Cord Tolerance Guided by Stereotactic Radiosurgical Treatment of Intramedullary Arteriovenous Malformations

Spinal cord toxicity resulting in neurological deficits remains the most feared complication of spinal region stereotactic radiosurgery (SRS). In treating vertebral body disease, a radiation dose gradient can typically be established between tumor and spinal cord. This is difficult for intramedullary spinal cord arteriovenous malformations (iSCAVMs), which can be integrated throughout the cord. Here we report outcomes of cord tolerance in treatment of integrated iSCAVMs. A retrospective cohort of 17 consecutive patients with integrated iSCAVMs treated once with frameless robotic surgery, SRS between 2007 and 2018 was accrued. Clinical outcomes, dose prescriptions, and target dose-volume histogram (DVH) parameters were acquired. Volumes of interest (VOIs) comprising the proper AVM and cord were registered on treatment planning software. A cord minus AVM (CmA) structure representing independent spinal cord was generated by Boolean subtraction of the respective proper AVM from the cord. Various dose quantifiers were extracted from DVHs, statistical univariate analysis was performed, and Task Group 101 (TG101) constraints were interpolated to two-fraction regimens with α/β of 2-3 Gray (Gy) using Wolfram Mathematica (Wolfram Research, Champaign, IL). Among the 17 patients analyzed (eight male and nine female), all had CmA volumes smaller than cord, confirming the integrated nature of their iSCAVMs. Nine had a nidus in the C-spine; six in the T-spine; and two in the L-spine or lower. Median age at diagnosis was 23, ranging from 9 to 56 years. Nine patients had no prior intervention, while eight had prior embolization(s) and/or microsurgery. Median follow-up time was 40 months. Ten patients (59%) experienced Grade (G) 0 neurological toxicity post-treatment, seven (41%) experienced G1, and zero experienced G2+. Three patients were prescribed three fractions to 21 or 24 Gy. The rest received two fractions (20-22 Gy). Max dose (Dmax) to cord and CmA ranged from 2278-2874 cGy and 2000-2659 cGy, respectively. All patients had Dmax to cord and CmA greater than TG101 constraints. All patients received minimum dose to hottest 0.03 cc volume (D0.03) to cord greater than constraints; 15 patients (88%), to CmA. All patients received minimum dose to hottest 0.35 cc volume (D0.35) to cord greater than threshold; 13 patients (76%), to CmA. At significant level of 0.05, one-tailed binomial test suggests max monthly G2+ toxicity rate of 0.4% in exceeding tolerances. No neurologic toxicity beyond Grade 1 was experienced by any patient, all of whom received Dmax to spinal cord tissue (both independent and integrated with AVM) beyond recommended constraints. Further, all patients exceeded volumetric constraints to cord. Most exceeded volumetric constraints to CmA, as well. Novel tolerance data from rare iSCAVMs may influence current cord constraints to avoid limiting appropriate treatment of more common vertebral metastases.

Derivation of in vivo source tracking error thresholds for TRUS-based HDR prostate brachytherapy through simulation of source positioning errors

PurposeThe purpose of this study was to simulate treatment planning source positioning errors in transrectal ultrasound–based real-time high-dose-rate prostate brachytherapy treatments and determine appropriate in vivo source tracking error thresholds. Methods and MaterialsTreatment planning source positioning errors were simulated for 20 patient plans in the brachytherapy treatment planning system by manually adjusting the dwell position coordinates within selected catheters without plan reoptimization. The change in dose-volume histogram (DVH) indices was calculated as a function of the source positioning error. The magnitude of the change in the DVH indices was then used to derive appropriate in vivo source tracking error thresholds. ResultsSource positioning error thresholds to prevent potentially significant changes in prostate (target) DVH metrics ranged from 2 to 5 mm, dependent on the direction of the source positioning error, as well as the relative weight of the dwell position within the plan, and its position relative to the patient anatomy. Source positioning error thresholds to prevent potentially clinically significant changes in organ at risk DVH metrics were found to be complex and patient-dependent. ConclusionsIn vivo source tracking error thresholds for transrectal ultrasound–based real-time high-dose-rate prostate brachytherapy were investigated via the simulation of treatment planning source positioning errors. These error thresholds were found to be dependent not only on the direction of the error, but also on the endpoint. There is still the potential for larger changes in DVH indices to occur for catheter shifts smaller than the proposed threshold levels in this study.

Impact of TPS calculation algorithms on dose delivered to the patient in proton therapy treatments

To estimate the impact of dose calculation approaches adopted in different treatment planning systems (TPSs) on proton therapy dose delivered with pencil beam scanning (PBS).Treatment plans for six regular volumes in water and 15 clinical cases were optimized with Syngo-VC13 and exported for forward recalculation with Raystation-V7.0 pencil beam (RS-PBA) and Monte Carlo (RS-MC) algorithms and with the independent Fluka-MC engine. To verify clinical consistency between the two TPS dosimetric outcomes, the average percentage variations of clinical target volume (CTV) D98%, D50% and D2%, adopted for plan prescription and evaluation, were considered. Ionization chamber measurements served as a further reference for comparison in homogeneous conditions. CTV dose volume histogram (DVH) analysis and gamma evaluation with 3 mm—3% agreement criteria quantified the dose deviation of TPS calculation algorithms, in heterogeneous conditions, against the Fluka-MC code.CTV D50%, representing the plan dose prescription goal, was higher on average over H&N cases of (3.9 ± 0.9)% and (2.3 ± 0.6)% as calculated with RS-PBA and RS-MC, respectively, compared to Syngo. For tumors located in the pelvis district, average D50% variations of (1.6 ± 0.7)% and (1.2 ± 0.7)% were found. Syngo underestimated target near maximum doses with respect to all computation systems. Calculation accuracy in heterogeneous conditions of RS-PBA H&N plans resulted poor when a range shifter was required. Target DVH and γ-analysis showed excellent agreement between RS-MC and Fluka-MC, with γ-pass rates >98% for all patient groups.Different TPS dose calculation approaches mainly affected dose delivered in H&N proton treatments, while minor deviations were found for pelvic tumors. RS-MC proved to be the most accurate TPS dose calculation algorithm when compared to an independent MC simulation code.

A new inverse planning formalism with explicit DVH constraints and kurtosis-based dosimetric criteria

In inverse planning processes of radiotherapy, dose-volume histograms (DVH) have been commonly used as a measure of plan quality. The exact mathematical formulation of DVHs involves binary variables and is therefore intrinsically nonconvex. In view of the resulting computational intractability, existing treatment planning systems adopt convex surrogate models that involve many parameters. Manual parameter tweaking is then required to determine reasonable values for those parameters in order to generate plans that meet the target DVHs. However, conventional convex formulations for inverse planning entails non-trivial differences from an exact formulation and, therefore, provide only very limited control over the DVHs. In view of this limitation, this paper presents a new formulation that combines a novel folded concave penalty (FCP)-based constraint and a new kurtosis-based optimization criterion. The former presents a much sharper approximation to the exact DVH constraints than the existing counterparts, and the latter penalizes the occurrence of hot and cold spots in dose distributions. We investigated the effectiveness of the proposed planning formulations on three head-and-neck cases in comparison with conventional DVH-based formulations. Significant outperformance in terms of better distributed doses-at-volume was achieved through the proposed scheme.

Knowledge-based automated planning for oropharyngeal cancer.

The purpose of this study was to automatically generate radiation therapy plans for oropharynx patients by combining knowledge-based planning (KBP) predictions with an inverse optimization (IO) pipeline. We developed two KBP approaches, the bagging query (BQ) method and the generalized principal component analysis-based (gPCA) method, to predict achievable dose-volume histograms (DVHs). These approaches generalize existing methods by predicting physically feasible organ-at-risk (OAR) and target DVHs in sites with multiple targets. Using leave-one-out cross validation, we applied both models to a large dataset of 217 oropharynx patients. The predicted DVHs were input into an IO pipeline that generated treatment plans (BQ and gPCA plans) via an intermediate step that estimated objective function weights for an inverse planning model. The KBP predictions were compared to the clinical DVHs for benchmarking. To assess the complete pipeline, we compared the BQ and gPCA plans to both the predictions and clinical plans. To isolate the effect of the KBP predictions, we put clinical DVHs through the IO pipeline to produce clinical inverse optimized (CIO) plans. This approach also allowed us to estimate the complexity of the clinical plans. The BQ and gPCA plans were benchmarked against the CIO plans using DVH differences and clinical planning criteria. Iso-complexity plans (relative to CIO) were also generated and evaluated. The BQ method tended to predict that less dose is delivered than what was observed in the clinical plans while the gPCA predictions were more similar to clinical DVHs. Both populations of KBP predictions were reproduced with inverse plans to within a median DVH difference of 3Gy. Clinical planning criteria for OARs were satisfied most frequently by the BQ plans (74.4%), by 6.3% points more than the clinical plans. Meanwhile, target criteria were satisfied most frequently by the gPCA plans (90.2%), and by 21.2% points more than clinical plans. However, once the complexity of the plans was constrained to that of the CIO plans, the performance of the BQ plans degraded significantly. In contrast, the gPCA plans still satisfied more clinical criteria than both the clinical and CIO plans, with the most notable improvement being in target criteria. Our automated pipeline can successfully use DVH predictions to generate high-quality plans without human intervention. Between the two KBP methods, gPCA plans tend to achieve comparable performance as clinical plans, even when controlling for plan complexity, whereas BQ plans tended to underperform.

Inverse optimization of objective function weights for treatment planning using clinical dose-volume histograms

We developed and evaluated a novel inverse optimization (IO) model to estimate objective function weights from clinical dose-volume histograms (DVHs). These weights were used to solve a treatment planning problem to generate ‘inverse plans’ that had similar DVHs to the original clinical DVHs. Our methodology was applied to 217 clinical head and neck cancer treatment plans that were previously delivered at Princess Margaret Cancer Centre in Canada. Inverse plan DVHs were compared to the clinical DVHs using objective function values, dose-volume differences, and frequency of clinical planning criteria satisfaction. Median differences between the clinical and inverse DVHs were within 1.1 Gy. For most structures, the difference in clinical planning criteria satisfaction between the clinical and inverse plans was at most 1.4%. For structures where the two plans differed by more than 1.4% in planning criteria satisfaction, the difference in average criterion violation was less than 0.5 Gy. Overall, the inverse plans were very similar to the clinical plans. Compared with a previous inverse optimization method from the literature, our new inverse plans typically satisfied the same or more clinical criteria, and had consistently lower fluence heterogeneity. Overall, this paper demonstrates that DVHs, which are essentially summary statistics, provide sufficient information to estimate objective function weights that result in high quality treatment plans. However, as with any summary statistic that compresses three-dimensional dose information, care must be taken to avoid generating plans with undesirable features such as hotspots; our computational results suggest that such undesirable spatial features were uncommon. Our IO-based approach can be integrated into the current clinical planning paradigm to better initialize the planning process and improve planning efficiency. It could also be embedded in a knowledge-based planning or adaptive radiation therapy framework to automatically generate a new plan given a predicted or updated target DVH, respectively.

Investigating the impact of treatment delivery uncertainties on treatment effectiveness for lung SABR.

To quantify the impact of treatment delivery uncertainties on lung stereotactic ablative body radiotherapy (SABR) plans for step-and-shoot intensity-modulated radiotherapy (ssIMRT) and volumetric modulated arc therapy (VMAT). Baseline ssIMRT and VMAT treatment plans were generated for a cohort of 18 lung SABR patients. Modified plans were generated for each baseline plan by systematically varying gantry and collimator angles between -5 and +5 degrees, as well as multi-leaf collimator (MLC) leaf position errors of magnitude between 1 and 5 mm in both directions (i.e. leaf banks shifted either in the same (Type 1) or opposite (Type 2) directions). Planning target volume (PTV), spinal cord and healthy lung dose-volume histogram (DVH) metrics were compared between the modified and baseline plans. Collimator and gantry angle uncertainties did not significantly impact any of the PTV DVH metrics considered. MLC shifts of 5 mm resulted in average V95% changes of [Formula: see text] (Type 1) and [Formula: see text] (Type 2) and average [Formula: see text] changes of [Formula: see text] (Type 1) and [Formula: see text] (Type 2) for ssIMRT and VMAT plans. Comparatively, MLC shifts of - 2 mm resulted in average [Formula: see text] changes of [Formula: see text] (Type 1) and [Formula: see text] (Type 2) and average [Formula: see text] changes of [Formula: see text] (Type 1) and [Formula: see text] (Type 2) for ssIMRT and VMAT plans. ssIMRT gantry angle uncertainties impacted spinal cord DVH metrics the most, with increases in [Formula: see text] of [Formula: see text] occurring for a 1 degree shift. Type 2 MLC modifications impacted all OAR DVH metrics substantially with differences in spinal cord [Formula: see text] (ssIMRT) and healthy lung [Formula: see text] (VMAT) exceeding [Formula: see text] for 5 mm shifts. Uncertainties in MLC leaf positions affected target and OAR DVH metrics more than collimator or gantry angle uncertainties for lung SABR plans. Less patient-to-patient variation occurred from delivery uncertainties in VMAT than ssIMRT.

Dosimetric and radiobiological comparison of TG-43 and Monte Carlo calculations in 192Ir breast brachytherapy applications

PurposeTo investigate the clinical significance of introducing model based dose calculation algorithms (MBDCAs) as an alternative to TG-43 in 192Ir interstitial breast brachytherapy. Materials and methodsA 57 patient cohort was used in a retrospective comparison between TG-43 based dosimetry data exported from a treatment planning system and Monte Carlo (MC) dosimetry performed using MCNP v. 6.1 with plan and anatomy information in DICOM-RT format. Comparison was performed for the target, ipsilateral lung, heart, skin, breast and ribs, using dose distributions, dose-volume histograms (DVH) and plan quality indices clinically used for plan evaluation, as well as radiobiological parameters. ResultsTG-43 overestimation of target DVH parameters is statistically significant but small (less than 2% for the target coverage indices and 4% for homogeneity indices, on average). Significant dose differences (>5%) were observed close to the skin and at relatively large distances from the implant leading to a TG-43 dose overestimation for the organs at risk. These differences correspond to low dose regions (<50% of the prescribed dose), being less than 2% of the prescribed dose. Detected dosimetric differences did not induce clinically significant differences in calculated tumor control probabilities (mean absolute difference <0.2%) and normal tissue complication probabilities. ConclusionWhile TG-43 shows a statistically significant overestimation of most indices used for plan evaluation, differences are small and therefore not clinically significant. Improved MBDCA dosimetry could be important for re-irradiation, technique inter-comparison and/or the assessment of secondary cancer induction risk, where accurate dosimetry in the whole patient anatomy is of the essence.

Spot-Scanning Proton Arc (SPArc) Therapy: The First Robust and Delivery-Efficient Spot-Scanning Proton Arc Therapy

Topresent a novel robust and delivery-efficient spot-scanning proton arc (SPArc) therapy technique. A SPArc optimization algorithm was developed that integrates control point resampling, energy layer redistribution, energy layer filtration, and energy layer resampling. The feasibility of such a technique was evaluated using sample patients: 1 patient with locally advanced head and neck oropharyngeal cancer with bilateral lymph node coverage, and 1 with a nonmobile lung cancer. Plan quality, robustness, and total estimated delivery time were compared with the robust optimized multifield step-and-shoot arc plan without SPArc optimization (Arcmulti-field) and the standard robust optimized intensity modulated proton therapy (IMPT) plan. Dose-volume histograms of target and organs at risk were analyzed, taking into account the setup and range uncertainties. Total delivery time was calculated on the basis of a 360° gantry room with 1 revolutions per minutegantry rotation speed, 2-millisecond spot switching time, 1-nA beam current, 0.01 minimum spot monitor unit, and energy layer switching time of 0.5 to 4seconds. The SPArc plan showed potential dosimetric advantages for both clinical sample cases. Compared with IMPT, SPArc delivered 8% and 14% less integral dose for oropharyngeal and lung cancer cases, respectively. Furthermore, evaluating the lung cancer plan compared with IMPT, it was evident that the maximum skin dose, the mean lung dose, and the maximum dose to ribs were reduced by 60%, 15%, and 35%, respectively, whereas the conformity index was improved from 7.6 (IMPT) to 4.0 (SPArc). The total treatment delivery time for lung and oropharyngeal cancer patients was reduced by 55% to 60% and 56% to 67%, respectively, when compared with Arcmulti-field plans. The SPArc plan is the first robust and delivery-efficient proton spot-scanning arc therapy technique, which could potentially be implemented into routine clinical practice.

Impact of MR geometric distortion on brachytherapy in cervical cancer

Introduction Volumetric imaging and in particular magnetic resonance (MR) has replaced the 2D imaging for brachytherapy treatment planning. However it is recognized that MR images suffer from geometric distortions which can have an impact on target and OARs DVH parameters due to high dose gradient in brachytherapy applications. Objective To assess the potential dosimetric impact of MR geometric distortion on the brachytherapy treatment planning for cervical cancer. Materials and methods MR dataset for 15 cervical cancer patients were acquired based on GEC ESTRO recommendations. Structures were delineated on the paraaxial images and treatment plans were generated using Oncentra™ TPS. In-house software was developed to calculate the deformable registration between CT and MR images of a control point based phantom and to derive a distortion map of the MR system. Geometrically corrected images were then generated by applying the inverse of the distortion map. Structures were copied and adapted to the corrected datasets and the treatment plans recomputed. Dose Volume Histograms (DVH) were evaluated for High Risk CTV (HRCTV), bladder, rectum, and sigmoid. Results Mean geometric distortions were less than 0.5 mm over a Field Of View (FOV) of 350 mm. Differences in D100 and D90 were less than 2% for HRCTV. Differences in D2cc were less than 4% for bladder, rectum, and sigmoid. Preliminary results showed that, for all organs, less than 4% of deviation was observed for D100 and D90. Conclusions For an FOV of 350 mm, MR geometric distortion has no significant dosimetric impact on DVH parameters in brachytherapy for cervical cancer. Disclosure Authors have nothing to disclose.

Robustness quantification methods comparison in volumetric modulated arc therapy to treat head and neck cancer

BackgroundTo compare plan robustness of volumetric modulated arc therapy (VMAT) with intensity modulated radiation therapy (IMRT) and to compare the effectiveness of 3 plan robustness quantification methods. Methods and materialsThe VMAT and IMRT plans were created for 9 head and neck cancer patients. For each plan, 6 new perturbed dose distributions were computed using ±3 mm setup deviations along each of the 3 orientations. Worst-case analysis (WCA), dose-volume histogram (DVH) band (DVHB), and root-mean-square dose-volume histogram (RVH) were used to quantify plan robustness. In WCA, a shaded area in the DVH plot bounded by the DVHs from the lowest and highest dose per voxel was displayed. In DVHB, we displayed the envelope of all DVHs in band graphs of all the 7 dose distributions. The RVH represents the relative volume on the vertical axis and the root-mean-square-dose on the horizontal axis. The width from the first 2 methods at different target DVH indices (such as D95% and D5%) and the area under the RVH curve for the target were used to indicate plan robustness. Results were compared using Wilcoxon signed-rank test. ResultsThe DVHB showed that the width at D95% of IMRT was larger than that of VMAT (unit Gy) (1.59 vs 1.18) and the width at D5% of IMRT was comparable to that of VMAT (0.59 vs 0.54). The WCA showed similar results between IMRT and VMAT plans (D95%: 3.28 vs 3.00; D5%: 1.68 vs 1.95). The RVH showed the area under the RVH curve of IMRT was comparable to that of VMAT (1.13 vs 1.15). No statistical significance was found in plan robustness between IMRT and VMAT. ConclusionsThe VMAT is comparable to IMRT in terms of plan robustness. For the 3 quantification methods, WCA and DVHB are DVH parameter–dependent, whereas RVH captures the overall effect of uncertainties.

Quantifying the performance of in vivo portal dosimetry in detecting four types of treatment parameter variations.

To quantify the ability of electronic portal imaging device (EPID) dosimetry used during treatment (in vivo) in detecting variations that can occur in the course of patient treatment. Images of transmitted radiation from in vivo EPID measurements were converted to a 2D planar dose at isocenter and compared to the treatment planning dose using a prototype software system. Using the treatment planning system (TPS), four different types of variability were modeled: overall dose scaling, shifting the positions of the multileaf collimator (MLC) leaves, shifting of the patient position, and changes in the patient body contour. The gamma pass rate was calculated for the modified and unmodified plans and used to construct a receiver operator characteristic (ROC) curve to assess the detectability of the different parameter variations. The detectability is given by the area under the ROC curve (AUC). The TPS was also used to calculate the impact of the variations on the target dose-volume histogram. Nine intensity modulation radiation therapy plans were measured for four different anatomical sites consisting of 70 separate fields. Results show that in vivo EPID dosimetry was most sensitive to variations in the machine output, AUC = 0.70 - 0.94, changes in patient body habitus, AUC = 0.67 - 0.88, and systematic shifts in the MLC bank positions, AUC = 0.59 - 0.82. These deviations are expected to have a relatively small clinical impact [planning target volume (PTV) D99 change <7%]. Larger variations have even higher detectability. Displacements in the patient's position and random variations in MLC leaf positions were not readily detectable, AUC < 0.64. The D99 of the PTV changed by up to 57% for the patient position shifts considered here. In vivo EPID dosimetry is able to detect relatively small variations in overall dose, systematic shifts of the MLC's, and changes in the patient habitus. Shifts in the patient's position which can introduce large changes in the target dose coverage were not readily detected.

SU‐E‐T‐618: Plan Robustness Study of Volumetric‐Modulated Arc Therapy Vs. Intensity‐Modulated Radiation Therapy for Head and Neck Cancer

Purpose:Lack of plan robustness may contribute to local failure in volumetric‐modulated arc therapy (VMAT) to treat head and neck (H&amp;N) cancer. Thus we compared plan robustness of VMAT with intensity‐modulated radiation therapy (IMRT).Methods:VMAT and IMRT plans were created for 9 H&amp;N cancer patients. For each plan, six new perturbed dose distributions were computed — one each for ± 3mm setup deviations along the S‐I, A‐P and L‐R directions. We used three robustness quantification tools: (1) worst‐case analysis (WCA); (2) dose‐volume histograms (DVHs) band (DVHB); and (3) root‐mean‐square‐dose deviation (RMSD) volume histogram (DDVH). DDVH represents the relative volume (y) on the vertical axis and the RMSD (x) on the horizontal axis. Similar to DVH, this means that y% of the volume of the indicated structure has the RMSD at least × Gy[RBE].The width from the first two methods at different target DVH indices (such as D95 and D5) and the area under the DDVH curves (AUC) for the target were used to indicate plan robustness. In these robustness quantification tools, the smaller the value, the more robust the plan is. Plan robustness evaluation metrics were compared using Wilcoxon test.Results:DVHB showed the width at D95 from IMRT to be larger than from VMAT (unit Gy) [1.59 vs 1.18 (p=0.49)], while the width at D5 from IMRT was found to be slightly larger than from VMAT [0.59 vs 0.54 (p=0.84)]. WCA showed similar results [D95: 3.28 vs 3.00 (p=0.56); D5: 1.68 vs 1.95 (p=0.23)]. DDVH showed the AUC from IMRT to be slightly smaller than from VMAT [1.13 vs 1.15 (p=0.43)].Conclusion:VMAT plan robustness is comparable to IMRT plan robustness. The plan robustness conclusions from WCA and DVHB are DVH parameter dependent. On the other hand DDVH captures the overall effect of uncertainties on the dose to a volume of interest.NIH/NCI K25CA168984; Eagles Cancer Research Career Development; The Lawrence W. and Marilyn W. Matteson Fund for Cancer Research Mayo ASU Seed Grant; The Kemper Marley Foundation

Comparison of target dosimetry and treatment outcome in patients with stage III non-small cell lung cancer treated with three-dimensional conformal radiotherapy and intensity-modulated radiotherapy

Objective To compare the target dosimetric distribution and clinical outcome in patients with stage III non-small cell lung cancer(NSCLC)treated with three-dimensional conformal radiotherapy (3DCRT)and intensity-modulated radiotherapy(IMRT) . Methods The clinical data of 419 patients with stage III NSCLC treated with either 3DCRT or IMRT were retrospectively analyzed. Among them, there were 338 male and 81 female patients, and the median age was 63 years(range: 32-84 years). There were 340patients treated with 3DCRT and 79 with IMRT, and the median prescribed dose was 60 Gy(range: 50-76Gy). One hundred and forty patients were treated with radiotherapy alone and 279 were treated with chemoradiotherapy. The target dosimetric distribution was evaluated with dose-volume histogram(DVH)parameters. The overall survival(OS)rate was calculated using the Kaplan-Meier method and analyzed by the log-rank test. Results When comparing the clinical data, the patients treated with 3DCRT were in older ages, and had advanced N and clinical stages (P=0.01, 0.00, and 0.00, respectively). When comparing the target DVH parameters, the patients treated with IMRT had larger planning target volume(PTV) (P=0.01) , significantly lower clinical target volume(CTV)Dmean, CTV D90, PTV Dmean, and PTV V65-V60 (P=0.05- 0.01) , significantly higher V5-V20 in both lungs, higher esophagus Dmean, longer esophagus in the radiation field, higher linear energy transfer between 45 and 55 keV/µm(LET45-LET55) , and higher spinal cord Dmean (P=0.03 - 0.00). The follow-up rate was 97.4%. After radiotherapy, the 1 -, 3-, and 5-year OS rates were 65.5%, 26.1%, and 18.5%, respectively, and the median survival time was 20 months. There were no significant differences in OS rate and the incidence of acute radiation pneumonitis and radiation esophagitis between patients treated with IMRT and 3 DCRT (P=0.06, 0.73, 0.13). Stratified analysis showed that, when comparing the patients treated with IMRT with those treated with 3DCRT, the survival rate was only lower in male patients, patients in stage T3 -T4 or N0 -N2, and those without chemotherapy (P=0.04, 0.04, 0.02, 0.00) . Conclusions The treatment outcomes of patients with stageⅢ NSCLC undergoing IMRT and 3 DCRT are comparable. IMRT shows a potential dosimetric advantage, but the result needs further investigation. Key words: Lung neoplasms/three-dimensional radiotherapy; Dosimetry; Prognosis

SU‐E‐T‐416: VMAT Dose Calculations Using Cone Beam CT Images: A Preliminary Study

Purpose:Cone beam CT (CBCT) images have been used routinely for patient positioning throughout the treatment course. However, use of CBCT for dose calculation is still investigational. The purpose of this study is to assess the utility of CBCT images for Volumetric Modulated Arc Therapy (VMAT) plan dose calculation.Methods:A CATPHAN 504 phantom (The Phantom Laboratory, Salem, NY) was used to compare the dosimetric and geometric accuracy between conventional CT and CBCT (in both full and half fan modes). Hounsfield units (HU) profiles at different density areas were evaluated. A C shape target that surrounds a central avoidance structure was created and a VMAT plan was generated on the CT images and copied to the CBCT phantom images. Patient studies included three brain patients, and one head and neck (H' N) patient. VMAT plans generated on the patients treatment planning CT was applied to CBCT images obtained during the first treatment. Isodose distributions and dosevolume‐ histograms (DVHs) were compared.Results:For the phantom study, the HU difference between CT and CBCT is within 100 (maximum 96 HU for Teflon CBCT images in full fan mode). The impact of these differences on the calculated dose distributions was clinically insignificant. In both phantom and patient studies, target DVHs based on CBCT images were in excellent agreement with those based on planning CT images. Mean, Median, near minimum (D98%), and near maximum (D2%) doses agreed within 0‐2.5%. A slightly larger discrepancy is observed in the patient studies compared to that seen in the phantom study, (0‐1% vs. 0 – 2.5%).Conclusion:CBCT images can be used to accurately predict dosimetric results, without any HU correction. It is feasible to use CBCT to evaluate the actual dose delivered at each fraction. The dosimetric consequences resulting from tumor response and patient geometry changes could be monitored.