Original Clinical Plans Research Articles

Evaluation of the Geometric and Dosimetric Accuracy of Synthetic Computed Tomography Images for Magnetic Resonance Imaging-only Stereotactic Radiosurgery.

IntroductionStereotactic radiosurgery (SRS) plans created using synthetic computed tomography (CT) images derived from magnetic resonance imaging (MRI) data may offer the advantage of inhomogeneity correction by convolution algorithms, as is done for CT-based plans. We sought to determine and validate the clinical significance and accuracy of synthetic CT images for inhomogeneity correction in MRI-only stereotactic radiosurgery plans for treatment of brain tumors.MethodsIn this retrospective study, data from two patients with brain metastases and one with meningioma who underwent imaging with multiple modalities and received frameless SRS treatment were analyzed. The SRS plans were generated using a convolution algorithm to account for brain inhomogeneity using CT and synthetic CT images and compared with the original clinical TMR10 plans created using MRI images.ResultsSynthetic CT-derived SRS plans are comparable with CT-based plans using convolution algorithm, and for some targets, based on location, they provided better coverage and a lower maximum dose.ConclusionsThe results suggest similar dose delivery results for CT and synthetic CT-based treatment plans. Synthetic CT plans offered a noticeable improvement in target dose coverage and a more gradual dose fall-off relative to TMR10 MRI-based plans. The major disadvantage is a slightly increased dose (by 0.37%) to nearby healthy tissue (brainstem) for synthetic CT-based plans relative to those created using clinical MRI images, which may be a problem for patients undergoing high-dose treatment.

Real-time interactive planning for radiotherapy of head and neck cancer with volumetric modulated arc therapy.

Planning complex radiotherapy treatments can be inefficient, with large variation in plan quality. In this study we evaluated plan quality and planning efficiency using real-time interactive planning (RTIP) for head and neck (HN) volumetric modulated arc therapy (VMAT). RTIP allows manipulation of dose volume histograms (DVHs) in real-time to assess achievable planning target volume (PTV) coverage and organ at risk (OAR) sparing. For 20 HN patients previously treated with VMAT, RTIP was used to minimize OAR dose while maintaining PTV coverage. RTIP DVHs were used to guide VMAT optimization. Dosimetric differences between RTIP-assisted plans and original clinical plans were assessed. Five blinded radiation oncologists indicated their preference for each PTV, OAR and overall plan. To assess efficiency, ten patients were planned de novo by experienced and novice planners and a RTIP user. The average planning time with RTIP was <20 min, and most plans required only one optimization. All 20 RTIP plans were preferred by a majority of oncologists due to improvements in OAR sparing. The average maximum dose to the spinal cord was reduced by 10.5 Gy (from 49.5 to 39.0 Gy), and the average mean doses for the oral cavity, laryngopharynx, contralateral parotid and submandibular glands were reduced by 3.5 Gy (39.1-35.7 Gy), 6.8 Gy (42.5-35.7 Gy), 1.7 Gy (17.0-15.3 Gy) and 3.3 Gy (22.9-19.5 Gy), respectively. Incorporating RTIP into clinical workflows may increase both planning efficiency and OAR sparing.

A Novel Jawless Linac with Fast MLC Collimation Achieved Comparable Plan Quality and Improved Delivery Efficiency Compared to Conventional Linacs for Challenging Head and Neck Cases

We investigated the dosimetric quality and delivery efficiency of head and neck (HN) plans achieved by a novel jawless 6 MV flattening-filter free (6FFF) linac with fast MLC collimation. Five challenging HN cases treated with simultaneous integrated boost (SIB) were retrospectively selected. There were two to three prescription dose levels with the high dose of 60-70 Gy and the low dose of 54-59.4 Gy. The treatment goal was that 395% of both high dose PTV (HD_PTV) and low-dose PTV (LD_PTV) will receive the prescription doses. The OARs included brainstem, spinal cord, parotids, esophagus, larynx, oral cavity, and mandible. The original 9-field IMRT clinical plans were done using a treatment planning system for conventional linacs. Each plan was replanned with a 9-field IMRT technique using an unreleased version of the treatment planning system for the novel jawless linac with 1-cm-wide dual-layer MLCs. The lower layer MLCs were used for beam modulation while the upper layer MLCs were used to block inter-leaf leakage. The original clinical plans and the newly generated replans were compared using dosimetric endpoints (such as PTV coverages and OAR max/mean doses), homogeneity index (HI), conformity index (CI) and total MU. Paired t-tests were performed and the results were considered significant if the p-values were ≤ 0.05. The population averaged dosimetric endpoints, HI, CI, and total MU are summarized in the table below. All newly generated replans for the jawless linac met the clinical dose limit requirements. The PTV coverage, OAR doses, HI, and CI in the replans were all comparable to those in the clinical plans. The total MUs in the replans (2584 ± 407) were significantly greater than those in the clinical plans (1495 ± 207). However, the delivery times for the replans were still shorter than that for the clinical plans because the jawless linac has a much higher dose rate (800 MU/min) compared to the conventional linac (300 MU/min). For challenging HN cases, the novel jawless linac with fast MLC collimation achieved similar plan quality and improved delivery efficiency compared to the conventional linacs.Abstract TU_35_3244: Table 1Plan comparison (n=5)ClinicalReplanClinical vs. Replan (p-value)HD_PTVVHD (%)96.6 ± 1.996.9 ± 1.60.639LD_PTVVLD (%)96.0 ± 1.797.3 ± 1.80.166BrainstemDmax (Gy)31.1 ± 15.629.2 ± 13.70.290Spinal_cordDmax (Gy)39.6 ± 3.938.4 ± 1.10.541Parotid_LDmean (Gy)29.0 ± 6.127.5 ± 6.60.099Parotid_RDmean (Gy)26.4 ± 3.825.5 ± 4.10.484EsophagusDmean (Gy)52.9 ± 18.552.5 ± 19.40.553LarynxDmean (Gy)52.3 ± 22.253.5 ± 22.30.438Oral CavityDmean (Gy)31.2 ± 11.030.4 ± 9.40.497MandibleDmean (Gy)30.5 ± 13.629.4 ± 11.60.530HI1.09 ± 0.011.09 ± 0.020.541CI0.96 ± 0.020.97 ± 0.020.109MU1495 ± 2072584 ± 4070.002 Open table in a new tab

Training and Evaluation of a Knowledge-based Planning System for Treatment Planning of Malignant Pleural Mesothelioma to the Intact Lungs

Radiotherapy treatment planning of malignant pleural mesothelioma (MPM) with intact lungs is challenging not only due to the size and the complexity of the target volume but also because of the need to spare critical organs within and in close proximity. Our purpose was to develop and evaluate a knowledge-based model (KBM) for this purpose. Since June 2010, 62 patients have been treated at our institution with radiation for MPM after pleurectomy/decortication (P/D) and thus having both lungs intact. The treatment planning and delivery technique used was Volumetric Modulated Arc Therapy (VMAT) with 2 coplanar partial arcs and 6 MV photons. Prior clinically accepted treatment plans of these patients served as standard for plan quality. We chose 50 patients to build a dose volume histogram (DVH) estimation model based on the range of prescription doses, PTV volumes, volumes of organs at risk (OARs), essentially the ipsilateral lung, total lung, contralateral lung and the heart, as well as a range of their overlapping volumes with the PTV. Dose constraints for the total lung, contralateral lung and heart were similar to those for planning non-small cell lung cancer (NSCLC) cases while constraints for the liver, kidneys, stomach, esophagus and bowel were similar to those for planning abdominal tumors using conventional fractionation. Coverage constraints were PTV D95 and V95 = 94%. Relative priorities given for OAR sparing and coverage were same across all plans chosen for training. The DVH estimation model was trained and tested using knowledge-based planning software. Treatment plans for 12 patients generated clinically as well as using KBM were compared for dosimetric parameters using the Wilcoxon Rank-sum test. The prescription dose for these plans ranged from 30.6 Gy to 50.4 Gy (median dose 45 Gy). For 9 out of 12 patients (75%), the KBM plans were able to meet acceptable dosimetric criteria at least as well as the original clinical plans. No significant differences (p>0.05) were seen in the mean total lung dose (constraint of ≤ 20 Gy), V20 Gy to the total lung (constraint ≤ 40%), mean heart dose (constraint ≤ 30 Gy) as well as V30 Gy to the heart (constraint ≤ 50%) and the maximum achievable prescription dose (capped at 50.4 Gy). The remaining dosimetric parameters met the clinically acceptable guidelines while using KBM, however the original plans showed a trend towards improved sparing, although not statistically significant. The use of a knowledge-based planning system is capable of achieving clinically acceptable results for treatment planning of MPM with intact lungs having the potential of reducing treatment-planning time. Future work however is needed for testing more new cases and further refinement of the model to improve the current DVH prediction especially with respect to organs other than the lungs and heart.

Clinical Application of a Novel Voxel- and Machine Learning-Based Automated Planning Method for Prostate Volumetric Arc Radiation Therapy

We have developed a novel voxel- and machine learning-based automated planning method that requires the planning image dataset, target contours, relevant organ at risk (OAR) volumes as inputs for dose distribution prediction, and a voxel dose dose-mimicking step that generates a complete treatment plan in 20-25 minutes. As the method has automated determination of appropriate clinical trade-offs, user input during the optimization process is not required. Herein, we validate our method for clinical application in prostate volumetric arc radiotherapy (RT). A dose prediction algorithm was trained and tested using 116 consecutive clinically used plans for prostate cancer radical RT. An independent dataset of 20 patients with localized prostate cancer was randomly selected and the dose prediction algorithm was used to generate RT plans. These were evaluated alongside the original clinical plans by three expert reviewers (2 GU radiation oncologists and 1 medical physicist) blinded to the plan origin. Three plans were excluded prior to any review as they contained hip prosthesis. Evaluation criteria included acceptability and preference with regards to target coverage, dose conformity, dose gradient at rectum/prostate interface, lateral symmetry, OAR sparing, and overall approval. Of the 17 patients reviewed, automated plans were deemed superior in target coverage and OAR sparing, similar in gradient at rectum/prostate interface, and inferior in high-dose conformity and lateral dose symmetry. Both automated and clinical plans were deemed clinically acceptable in 49 of 51 instances across the three reviewers. One automated plan was deemed clinically unacceptable by two reviewers, and two different clinical plans were deemed clinically unacceptable by one reviewer. When compared head-to-head by the three reviewers, automated plans were deemed superior in 38 (74.5%) and equivalent to clinical plans in 3 (5.9%) reviews. Overall, in 15 of 17 (88.2%) patients, automated plans were deemed superior or equivalent to the delivered clinical plans. Our machine learning-based automated planning framework has the potential for clinical application in prostate RT, providing and opportunity to improve efficiency and consistency in the planning process. Furthermore, without the need for user interaction during plan optimization process, can enable increased throughput and liberate human resources for being allocated elsewhere within the RT process. Integration into our clinical practice including prospective performance evaluation of the proposed method is ongoing.

Focal boost to residual gross tumor volume in brachytherapy for cervical cancer—A feasibility study

PurposeImage-guided plan optimization with MRI and CT for interstitial and intracavitary brachytherapy is an established technique in treating cervical cancer. The purpose of this study was to assess the feasibility of boosting the dose to the residual gross tumor volume (GTV-Tres) to 140% of the high-risk clinical target volume (HR-CTV) prescription. Methods and MaterialsBrachytherapy plans from 50 consecutive patients were analyzed in this study. All received external beam radiotherapy followed by brachytherapy (6 Gy × 4 fraction or 7 Gy × 4 fraction to HR-CTV). The original treatment plans were reoptimized escalating the GTV-Tres dose 140% of the original HR-CTV prescription dose to 8.4 Gy and 9.8 Gy/ per fraction, respectively, with the aim of achieving GTV-TresV140 ≥ 90% and D98 ≥ 100 Gy. The HR-CTV coverage and organ at risk (OAR) dose–volume histogram values were kept within the tolerance, which had been accepted for the original clinical plans. ResultsA total of 24 patients (48%) achieved the planning goal after reoptimization. There was no significant difference between the D2cc of the OARs of the clinical plan and the study boost plan. The factors having greatest impact on the delivered dose to the GTV-Tres are proximity of the OAR, intrauterine positioned outside the GTV-Tres, and suboptimal interstitial placement for boosting GTV-Tres. ConclusionsIt is possible to boost the prescription dose to the GTV-Tres achieving 140% increase, which equates to an EQD2α/β=10 > 100 Gy. Plans without both interstitial catheters and/or intrauterine within the GTV-Tres are most likely to be suboptimal. This planning study demonstrates that dose escalation to the GTV-Tres is feasible and further work into clinical application should be considered.

Multicriteria plan optimization in the hands of physicians: a pilot study in prostate cancer and brain tumors

BackgroundThe purpose of this study was to demonstrate the feasibility of physician driven planning in intensity modulated radiotherapy (IMRT) with a multicriteria optimization (MCO) treatment planning system and template based plan optimization. Exploiting the full planning potential of MCO navigation, this alternative planning approach intends to improve planning efficiency and individual plan quality.MethodsPlanning was retrospectively performed on 12 brain tumor and 10 post-prostatectomy prostate patients previously treated with MCO-IMRT. For each patient, physicians were provided with a template-based generated Pareto surface of optimal plans to navigate, using the beam angles from the original clinical plans. We compared physician generated plans to clinically delivered plans (created by dosimetrists) in terms of dosimetric differences, physician preferences and planning times.ResultsPlan qualities were similar, however physician generated and clinical plans differed in the prioritization of clinical goals. Physician derived prostate plans showed significantly better sparing of the high dose rectum and bladder regions (p(D1) < 0.05; D1: dose received by 1% of the corresponding structure). Physicians’ brain tumor plans indicated higher doses for targets and brainstem (p(D1) < 0.05). Within blinded plan comparisons physicians preferred the clinical plans more often (brain: 6:3 out of 12, prostate: 2:6 out of 10) (not statistically significant). While times of physician involvement were comparable for prostate planning, the new workflow reduced the average involved time for brain cases by 30%. Planner times were reduced for all cases. Subjective benefits, such as a better understanding of planning situations, were observed by clinicians through the insight into plan optimization and experiencing dosimetric trade-offs.ConclusionsWe introduce physician driven planning with MCO for brain and prostate tumors as a feasible planning workflow. The proposed approach standardizes the planning process by utilizing site specific templates and integrates physicians more tightly into treatment planning. Physicians’ navigated plan qualities were comparable to the clinical plans. Given the reduction of planning time of the planner and the equal or lower planning time of physicians, this approach has the potential to improve departmental efficiencies.

Experience-Based Quality Control in Radiation Therapy Treatment Planning of High Risk Post-prostatectomy Prostate Cancer with RapidPlan: NRG Oncology RTOG 0621

To develop a knowledge based planning (KBP) model with RapidPlan (Varian Medical Systems, Palo Alto, USA) for the treatment of high risk post-prostatectomy prostate cancer using a knowledge database of high quality treatment plans from the national clinical trial RTOG 0621. To test the model as a quality assurance (QA) tool and use it to establish the feasibility of the compliance criteria for NRG GU003. :An initial dosimetric analysis was carried out to identify high quality treatment plans from RTOG 0621. Treatment plans for patients enrolled in the trial were scored according to the system used by the Imaging and Radiation Oncology Core (IROC) of the National Clinical Trials Network (NCTN) of the NCI to assess adherence to the trial protocol. Of the 80 treatment plans enrolled in the trial a knowledge database of 40 high quality plans was used to train the model. Another subset of plans, orthogonal to the training set, was chosen for the validation set to ensure that the model accurately predicts dose volume histograms (DVHs) for all critical structures. The remaining plans were re-optimized with the model in order to test its effectiveness as a QA tool. The re-optimized plans were analyzed to determine what percentage pass each of the compliance criteria for NRG GU003. The RapidPlan prostate model has been successfully trained and used to re-optimize plans selected from RTOG 0621. Plans re-optimized with the model show increased dose sparing to the bladder and rectum without loss of target coverage. These re-optimized plans show a reduction of V50 of up to 45% for the bladder and 24% for the rectum when compared with the original clinical plans submitted to the trial. The RapidPlan prostate model developed in this study is an effective tool for IMRT treatment planning QA.

Rapid Pre-Clinical Validation of Knowledge-Based Planning Routines across Multiple Disease Sites

Knowledge-based planning (KBP) requires extensive testing and validation prior to clinical implementation. The purpose of this work is demonstrate, across multiple disease sites, a rapid validation method for KBP routines. We retrospectively identified 57 prostate plans, 40 lung stereotactic body radiotherapy (SBRT) plans (equal numbers left and right), and 20 head-and-neck plans, all treated with volumetric modulated arc therapy (VMAT). Using a commercial KBP system and four site-specific routines, we generated deliverable KBP plans for each available case. All field parameters (e.g. arc arrangement, beam energy) and prescription scaling (D95%=100%) were preserved in each case. With site-specific reporting scripts that automatically extract dose-volume histogram (DVH) information from the original clinical plans and the newly created KBP plans, all relevant target and organ-at-risk (OAR) DVH cutpoints can be compared in each disease sample. Aggregate analysis of site-specific DVH metrics illuminate the systematic differences between the proposed KBP routines and the existing clinical planning performance. 57 prostate cases were replanned with the prostate KBP routine in ∼3 hours (∼10 min each case, 4 continuously running in parallel). With similar runtime to prostate, 40 lung SBRT cases were executed in ∼2 hours. The 20 head-and-neck cases required ∼30 minutes per case, completed in <4 hours. The reporting scripts extracted data in <30 seconds/case. A sample of critical metrics for each disease site are summarized in Table 1. In prostate, the rectal DVHs were improved between the KBP and clinical samples with no cost to increased target heterogeneity. In head-and-neck, KBP better spared the parotid glands while delivering slightly higher max doses and equivalent D95 for all targets. Both KBP lung SBRT incarnations created more heterogeneous target doses; this was offset by improved left lung doses at 50%, whereas the right lung KBP plans were the same or worse for V50%. With just one clinician-day of effort 4 KBP routines were tested against existing clinical practice across 117 patients. Future implementations of treatment planning and informatics systems that allow batch replanning of cases in the background could effectively eliminate barriers to safely incorporating new KBP routines (either internally-generated or externally obtained) into clinical practice.Abstract 3556; Table 1ClinicalKBPProstatePTVDmax107.8% ± 1.3%107.3% ± 1.3%RectumV40V65V7526.5% ± 7.0%10.1% ± 3.7%5.9% ± 2.5%23.5% ± 6.0%9.2% ± 3.5%5.2% ± 2.5%Head-and-neckPTV_high (70 Gy)Dmax109.2% ± 2.6%111.0% ± 3.5%PTV_intermediate (59.4 Gy) PTV_low (54.1 Gy)D95101.2% ± 3.2%102.% ± 2.2%101.6% ± 3.1%102.4% ± 2.2%Left Parotid Right ParotidDmean (Gy)25.9 ± 14.425.7 ± 9.823.3 ± 15.021.1 ± 8.9Left Lung SBRTPTVDmax109.6% ± 3.5%122.9% ± 4.5%Left LungV50% (cc)78.5 ± 61.973.4 ± 55.7Right Lung SBRTPTVDmax112.0% ± 3.0%128.0% ± 7.1%Right LungV50% (cc)114.8 ± 77.7117.2 ± 81.9 Open table in a new tab

Clinical implementation of a knowledge based planning tool for prostate VMAT

BackgroundA knowledge based planning tool has been developed and implemented for prostate VMAT radiotherapy plans providing a target average rectum dose value based on previously achievable values for similar rectum/PTV overlap. The purpose of this planning tool is to highlight sub-optimal clinical plans and to improve plan quality and consistency.MethodsA historical cohort of 97 VMAT prostate plans was interrogated using a RayStation script and used to develop a local model for predicting optimum average rectum dose based on individual anatomy. A preliminary validation study was performed whereby historical plans identified as “optimal” and “sub-optimal” by the local model were replanned in a blinded study by four experienced planners and compared to the original clinical plan to assess whether any improvement in rectum dose was observed. The predictive model was then incorporated into a RayStation script and used as part of the clinical planning process. Planners were asked to use the script during planning to provide a patient specific prediction for optimum average rectum dose and to optimise the plan accordingly.ResultsPlans identified as “sub-optimal” in the validation study observed a statistically significant improvement in average rectum dose compared to the clinical plan when replanned whereas plans that were identified as “optimal” observed no improvement when replanned. This provided confidence that the local model can identify plans that were suboptimal in terms of rectal sparing. Clinical implementation of the knowledge based planning tool reduced the population-averaged mean rectum dose by 5.6Gy. There was a small but statistically significant increase in total MU and femoral head dose and a reduction in conformity index. These did not affect the clinical acceptability of the plans and no significant changes to other plan quality metrics were observed.ConclusionsThe knowledge-based planning tool has enabled substantial reductions in population-averaged mean rectum dose for prostate VMAT patients. This suggests plans are improved when planners receive quantitative feedback on plan quality against historical data.

Direction Modulated Brachytherapy for Treatment of Cervical Cancer. II: Comparative Planning Study With Intracavitary and Intracavitary–Interstitial Techniques

To perform a comprehensive comparative planning study evaluating the utility of the proposed direction modulated brachytherapy (DMBT) tandem applicator against standard applicators, in the setting of image guided adaptive brachytherapy of cervical cancer. A detailed conceptual article was published in 2014. The proposed DMBT tandem applicator has 6 peripheral grooves of 1.3-mm width, along a 5.4-mm-thick nonmagnetic tungsten alloy rod of density 18.0g/cm(3), capable of generating directional dose profiles. We performed a comparative planning study with 45 cervical cancer patients enrolled consecutively in the prospective observational EMBRACE study. In all patients, MRI-based planning was performed while utilizing various tandem-ring (27 patients) and tandem-ring-needles (18 patients) applicators, in accordance with the Groupe Européen de Curiethérapie-European Society for Radiotherapy and Oncology recommendations. For unbiased comparisons, all cases were replanned with an in-house-developed inverse optimization code while enforcing a uniform set of constraints that are reflective of the clinical practice. All plans were normalized to the same high-risk clinical target volume D90 values achieved in the original clinical plans. In general, if the standard tandem was replaced with the DMBT tandem while maintaining all other planning conditions the same, there was consistent improvement in the plan quality. For example, among the 18 tandem-ring-needles cases, the average D2cm(3) reductions achieved were -2.48%±11.03%, -4.45%±5.24%, and -5.66%±6.43% for the bladder, rectum, and sigmoid, respectively. An opportunity may also exist in avoiding use of needles altogether for when the total number of needles required is small (approximately 2 to 3 needles or less), if DMBT tandem is used. Integrating the novel DMBT tandem onto both intracavitary and intracavitary-interstitial applicator assembly enabled consistent improvement in the sparing of the OARs, over a standard "single-channel" tandem, though individual variations in benefit were considerable. Although at an early stage of development, the DMBT concept design is demonstrated to be useful and pragmatic for potential clinical translation.

SU‐F‐T‐55: Reproducibility of Interstitial HDR Brachytherapy Plans

Purpose:Treating gynecological cancers with interstitial high‐dose‐rate (HDR) brachytherapy requires precise reconstruction of catheter positions to obtain accurate dosimetric plans. In this study, we investigated the degree of reproducibility of dosimetric plans for Syed HDR brachytherapy.Methods:We randomly selected five patients having cervix‐vaginal cancer who were recently treated in our clinic with interstitial HDR brachytherapy with a prescription dose of 25–30 Gy in five fractions. Interstitial needles/catheters were placed under fluoroscopic guidance and intra‐operative 3T MRI scan was performed to confirm the desired catheter placement for adequate target volume coverage. A CT scan was performed and fused with the MRI for delineating high‐risk CTV (HR‐CTV), intermediate‐risk CTV (IR‐CTV) and OARs. HDR treatment plans were generated using Oncentra planning software. A single plan was used for all five fractions of treatment for each patient. For this study, we took the original clinical plan and removed all the reconstructed catheters from the plan keeping the original contours unchanged. Then, we manually reconstructed all the catheters and entered the same dwell time from the first original clinical plan. The dosimetric parameters studied were: D90 for HR‐CTV and IR‐CV, and D2cc for bladder, rectum, sigmoid and bowel.Results:The mean of absolute differences in dosimetric coverage (D90) were (range): 1.3% (1.0–2.0%) and 2.0% (0.9–3.6%) for HR‐CTV and IR‐CTV, respectively. In case of OARs, the mean of absolute variations in D2cc were (range): 4.7% (0.7–8.9%) for bladder, 1.60% (0.3–3.2%) for rectum, 1.6% (0–3.9%) for sigmoid, and 1.8% (0–5.1%) for bowel.Conclusion:Overall, the reproducibility of interstitial HDR plans was within clinically acceptable limit. Observed maximum variation in D2cc for bladder. If number of catchers and dwell points were relatively low or any one catheter was heavily loaded, then reproducibility of the plan was more sensitive to the accuracy of catheter reconstruction.

SU‐F‐T‐543: The Impact of Density Overrides in IMRT and VMAT Planning for Lung Cancer Patients Treated SBRT

Purpose:To evaluate the dosimetric impact of target volume overrides on the quality of IMRT and VMAT lung, SBRT treatment plans.Methods:Ninety one patients previously treated with lung SBRT at our institution were retrospectively studied. IMRT and VMAT plans were created on 5 image sets for each patient: free breathing scans(FB), average scans(AVE), average scan with internal target volume overridden to tumor density(ITVoverride), average scan with planning target volume overridden(PTVoverride), and a hybrid method where the internal target volume was set to tumor density and the set‐up margin was set to an intermediate density between lung and tumor(HBoverride). All plans were optimized with the same constraints as the original clinical plan AVE. Dose fractionation scheme included 12Gyx4, 10Gyx4, 8Gyx4, 12Gyx3, and 9Gyx5. EQD2 was computed for various target metrics (mean, D1%, D99%) and normal lung metrics (V5Gy, V20Gy, mean lung dose). Plan quality was investigated via conformity index (CI) and gradient index (GI).Results:The ITVoverride plans achieved the highest mean target EQD2(0.38±0.69Gy over AVE) and the highest D1% hotspot(0.73±1.17Gy over AVE) in comparison to all other plans. PTVoverride and HBoverride also demonstrated higher mean target EQD2 than AVE, but lower hotspots. Compared to AVE, PTVoverride and HBoverride resulted in lower coldspots of −9.95±3.78Gy and −9.91±3.68Gy respectively. All override plans achieved lower lung metrics than the AVE plans, with the largest reductions in PTVoverride (−0.31±0.30Gy for V20Gy, −0.44±0.44Gy for V5Gy, −0.23±0.18Gy for mean lung dose). Comparing FB (largest CI,GI) to PTVoverride (smallest CI,GI), CI and GI improved from 1.05 to 1.00, and 4.81 to 4.48 respectively. The differences in these metrics did not correlate with tumor motion or target volume.Conclusion:Despite the improvements in CI and GI, density overrides in target volumes for 91 lung SBRT patients showed limited dosimetric advantages over clinical plans on the average CT.

WE‐DE‐201‐01: BEST IN PHYSICS (THERAPY): A Fast Multi‐Target Inverse Treatment Planning Strategy Optimizing Dosimetric Measures for High‐Dose‐Rate (HDR) Brachytherapy

Purpose:Inverse treatment planning (ITP) for interstitial HDR brachytherapy of gynecologic cancers seeks to maximize coverage of the clinical target volumes (tumor and vagina) while respecting dose‐volume‐histogram related dosimetric measures (DMs) for organs at risk (OARs). Commercially available ITP tools do not support DM‐based planning because it is computationally too expensive to solve. In this study we present a novel approach that allows fast ITP for gynecologic cancers based on DMs for the first time.Methods:This novel strategy is an optimization model based on a smooth DM‐based objective function. The smooth approximation is achieved by utilizing a logistic function for the evaluation of DMs. The resulting nonconvex and constrained optimization problem is then optimized with a BFGS algorithm. The model was evaluated using the implant geometry extracted from 20 patient treatment plans under an IRB‐approved retrospective study. For each plan, the final DMs were evaluated and compared to the original clinical plans. The CTVs were the contoured tumor volume and the contoured surface of the vagina. Statistical significance was evaluated with a one‐sided paired Wilcoxon signed‐rank test.Results:As did the clinical plans, all generated plans fulfilled the defined DMs for OARs. The proposed strategy showed a statistically significant improvement (p&lt;0.001) in coverage of the tumor and vagina, with absolute improvements of related DMs of (6.9 +/− 7.9)% and (28.2 +/− 12.0)%, respectively. This was achieved with a statistically significant (p&lt;0.01) decrease of the high‐dose‐related DM for the tumor. The runtime of the optimization was (2.3 +/− 2.0) seconds.Conclusion:We demonstrated using clinical data that our novel approach allows rapid DM‐based optimization with improved coverage of CTVs with fewer hot spots. Being up to three orders of magnitude faster than the current clinical practice, the method dramatically shortens planning time.

Semiautomated head-and-neck IMRT planning using dose warping and scaling to robustly adapt plans in a knowledge database containing potentially suboptimal plans.

Prior work by the authors and other groups has studied the creation of automated intensity modulated radiotherapy (IMRT) plans of equivalent quality to those in a patient database of manually created clinical plans; those database plans provided guidance on the achievable sparing to organs-at-risk (OARs). However, in certain sites, such as head-and-neck, the clinical plans may not be sufficiently optimized because of anatomical complexity and clinical time constraints. This could lead to automated plans that suboptimally exploit OAR sparing. This work investigates a novel dose warping and scaling scheme that attempts to reduce effects of suboptimal sparing in clinical database plans, thus improving the quality of semiautomated head-and-neck cancer (HNC) plans. Knowledge-based radiotherapy (KBRT) plans for each of ten "query" patients were semiautomatically generated by identifying the most similar "match" patient in a database of 103 clinical manually created patient plans. The match patient's plans were adapted to the query case by: (1) deforming the match beam fluences to suit the query target volume and (2) warping the match primary/boost dose distribution to suit the query geometry and using the warped distribution to generate query primary/boost optimization dose-volume constraints. Item (2) included a distance scaling factor to improve query OAR dose sparing with respect to the possibly suboptimal clinical match plan. To further compensate for a component plan of the match case (primary/boost) not optimally sparing OARs, the query dose volume constraints were reduced using a dose scaling factor to be the minimum from either (a) the warped component plan (primary or boost) dose distribution or (b) the warped total plan dose distribution (primary + boost) scaled in proportion to the ratio of component prescription dose to total prescription dose. The dose-volume constraints were used to plan the query case with no human intervention to adjust constraints during plan optimization. KBRT and original clinical plans were dosimetrically equivalent for parotid glands (mean/median doses), spinal cord, and brainstem (maximum doses). KBRT plans significantly reduced larynx median doses (21.5 ± 6.6 Gy to 17.9 ± 3.9 Gy), and oral cavity mean (32.3 ± 6.2 Gy to 28.9 ± 5.4 Gy) and median (28.7 ± 5.7 Gy to 23.2 ± 5.3 Gy) doses. Doses to ipsilateral parotid gland, larynx, oral cavity, and brainstem were lower or equivalent in the KBRT plans for the majority of cases. By contrast, KBRT plans generated without the dose warping and dose scaling steps were not significantly different from the clinical plans. Fast, semiautomatically generated HNC IMRT plans adapted from existing plans in a clinical database can be of equivalent or better quality than manually created plans. The reductions in OAR doses in the semiautomated plans, compared to the clinical plans, indicate that the proposed dose warping and scaling method shows promise in mitigating the impact of suboptimal clinical plans.

SU‐C‐BRA‐07: Virtual Bronchoscopy‐Guided IMRT Planning for Mapping and Avoiding Radiation Injury to the Airway Tree in Lung SAbR

Purpose:Post‐treatment radiation injury to central and peripheral airways is a potentially important, yet under‐investigated determinant of toxicity in lung stereotactic ablative radiotherapy (SAbR). We integrate virtual bronchoscopy technology into the radiotherapy planning process to spatially map and quantify the radiosensitivity of bronchial segments, and propose novel IMRT planning that limits airway dose through non‐isotropic intermediate‐ and low‐dose spillage.Methods:Pre‐ and ∼8.5 months post‐SAbR diagnostic‐quality CT scans were retrospectively collected from six NSCLC patients (50–60Gy in 3–5 fractions). From each scan, ∼5 branching levels of the bronchial tree were segmented using LungPoint, a virtual bronchoscopic navigation system. The pre‐SAbR CT and the segmented bronchial tree were imported into the Eclipse treatment planning system and deformably registered to the planning CT. The five‐fraction equivalent dose from the clinically‐delivered plan was calculated for each segment using the Universal Survival Curve model. The pre‐ and post‐SAbR CTs were used to evaluate radiation‐induced segmental collapse. Two of six patients exhibited significant segmental collapse with associated atelectasis and fibrosis, and were re‐planned using IMRT.Results:Multivariate stepwise logistic regression over six patients (81 segments) showed that D0.01cc (minimum point dose within the 0.01cc receiving highest dose) was a significant independent factor associated with collapse (odds‐ratio=1.17, p=0.010). The D0.01cc threshold for collapse was 57Gy, above which, collapse rate was 45%. In the two patients exhibiting segmental collapse, 22 out of 32 segments showed D0.01cc &gt;57Gy. IMRT re‐planning reduced D0.01cc below 57Gy in 15 of the 22 segments (68%) while simultaneously achieving the original clinical plan objectives for PTV coverage and OAR‐sparing.Conclusion:Our results indicate that the administration of lung SAbR can Result in significant injury to bronchial segments, potentially impairing post‐SAbR lung function. To our knowledge, this is the first investigation of functional avoidance based on mapping and minimizing dose to individual bronchial segments.The presenting author receives research funding from Varian Medical Systems, Elekta, and VisionRT.

Clinical Realization of Sector Beam Intensity Modulation for Gamma Knife Radiosurgery: A Pilot Treatment Planning Study

To demonstrate the clinical feasibility and potential benefits of sector beam intensity modulation (SBIM) specific to Gamma Knife stereotactic radiosurgery (GKSRS). SBIM is based on modulating the confocal beam intensities from individual sectors surrounding an isocenter in a nearly 2π geometry. This is in contrast to conventional GKSRS delivery, in which the beam intensities from each sector are restricted to be either 0% or 100% and must be identical for any given isocenter. We developed a SBIM solution based on available clinical planning tools, and we tested it on a cohort of 12 clinical cases as a proof of concept study. The SBIM treatment plans were compared with the original clinically delivered treatment plans to determine dosimetric differences. The goal was to investigate whether SBIM would improve the dose conformity for these treatment plans without prohibitively lengthening the treatment time. A SBIM technique was developed. On average, SBIM improved the Paddick conformity index (PCI) versus the clinically delivered plans (clinical plan PCI = 0.68 ± 0.11 vs SBIM plan PCI = 0.74 ± 0.10, P=.002; 2-tailed paired t test). The SBIM plans also resulted in nearly identical target volume coverage (mean, 97 ± 2%), total beam-on times (clinical plan 58.4 ± 38.9 minutes vs SBIM 63.5 ± 44.7 minutes, P=.057), and gradient indices (clinical plan 3.03 ± 0.27 vs SBIM 3.06 ± 0.29, P=.44) versus the original clinical plans. The SBIM method is clinically feasible with potential dosimetric gains when compared with conventional GKSRS.

Extended distance non-isocentric treatment in stereotactic body radiation therapy (SBRT) for lung cancer

Purpose: To obtain the maximum differential non-coplanar beams angle for a faster dose dropping outside Plan Target Volume (PTV) for lung cancer treated by Stereotactic body radiation therapy (SBRT), an extended distance non-isocentric (EDNI) treatment method was explored and developed. Methods: The EDNI requires delivering of the treatment beam at 120 cm or farther for sauce axial distance (SAD) instead of standard 100 cm. This change provides a more compact dose distribution around PTV and the lower toxicity to organs at risk (OAR) due to benefit of 120 cm SAD and more choice of beam and couch angle. A hand calculation formula for the translation between 100 SAD and EDNI was used to verify the treatment plan results. A phantom for end to end study based on this EDNI technique was used to compare with standard 100 SAD deliveries for SBRT. Three patients who underwent SBRT treatment were randomly chosen to demonstrate the benefits of EDNI technique. These treatment re-plans were applied to EDNI and evaluated for conformal index (CI) of PTV, R 50% of PTV , 2 cm distance (D 2cm ) of PTV and Maximum dose (D max )of OARs to compare with original clinical plans. Results: All of the cases delivered by the EDNI technique satisfied dose requirements of RTOG 0263 and showed a faster dose dropping outside of PTV than standard SAD deliveries. The distance from PTV after 1.5 cm for the EDNI technique had a smaller maximum dose and much lower standard deviation for dose distribution. The EDNI applied plans for patients showed less R 50% and D 2cm of PTV (P≤ 0.05), also similar results for D max of esophagus, trachea and spinal cord. Conclusion: The EDNI method enhances the capabilities of linear accelerators as far as the increased gradient of dose drop-off outside of PTV is concerned. More angular separation between beams leads to more compact dose distributions, which allow decreasing volume of high dose exposure in SBRT treatments and better dose distribution on sensitive organs to minimize the treatment toxicity.

Roll and pitch set-up errors during volumetric modulated arc delivery: can adapting gantry and collimator angles compensate?

The purpose of this work was to investigate whether adapting gantry and collimator angles can compensate for roll and pitch setup errors during volumetric modulated arc therapy (VMAT) delivery. Previously delivered clinical plans for locally advanced head-and-neck (H&N) cancer (n = 5), localized prostate cancer (n = 2), and whole brain with simultaneous integrated boost to 5 metastases (WB + 5M, n = 1) were used for this study. Known rigid rotations were introduced in the planning CT scans. To compensate for these, in-house software was used to adapt gantry and collimator angles in the plan. Doses to planning target volumes (PTV) and critical organs at risk (OAR) were calculated with and without compensation and compared with the original clinical plan. Measurements in the sagittal plane in a polystyrene phantom using radiochromic film were compared by gamma (γ) evaluation for 2 H&N cancer patients. For H&N plans, the introduction of 2°-roll and 3°-pitch rotations reduced mean PTV coverage from 98.7 to 96.3%. This improved to 98.1% with gantry and collimator compensation. For prostate plans respective figures were 98.4, 97.5, and 98.4%. For WB + 5M, compensation worked less well, especially for smaller volumes and volumes farther from the isocenter. Mean comparative γ evaluation (3%, 1 mm) between original and pitched plans resulted in 86% γ < 1. The corrected plan restored the mean comparison to 96% γ < 1. Preliminary data suggest that adapting gantry and collimator angles is a promising way to correct roll and pitch set-up errors of < 3° during VMAT for H&N and prostate cancer.

SU-E-T-357: Semi-Automated Knowledge-Based Radiation Therapy (KBRT) Planning for Head-And-Neck Cancer (HNC): Can KBRT Plans Achieve Better Results Than Manual Planning?

Purpose: Knowledge Based Radiation Therapy Treatment (KBRT) planning can be used to semi-automatically generate IMRT plans for new patients using constraints derived from previously manually-planned, geometrically similar patients. We investigate whether KBRT plans can achieve greater dose sparing than manual plans using optimized, organspecific constraint weighting factors. Methods: KBRT planning of HNC radiotherapy cases geometrically matched each new (query) case to one of the 105 clinically approved plans in our database. The dose distribution of the planned match was morphed to fit the querys geometry. Dose-volume constraints extracted from the morphed dose distribution were used to run the IMRT optimization with no user input. In the first version, all constraints were multiplied by a weighting factor of 0.7. The weighting factors were then systematically optimized (in order of OARs with increasing separation from the target) to maximize sparing to each OAR without compromising other OARs. The optimized, second version plans were compared against the first version plans and the clinically approved plans for 45 unilateral/bilateral target cases using the dose metrics: mean, median and maximum (brainstem and cord) doses. Results: Compared to the first version, the second version significantly reduced mean/median contralateral parotid doses (>2Gy) for bilateral cases. Other changes between the two versions were not clinically meaningful. Compared to the original clinical plans, both bilateral and unilateral plans in the second version had lower average dose metrics for 5 of the 6 OARs. Compared to the original plans, the second version achieved dose sparing that was at least as good for all OARs and better for the ipsilateral parotid (bilateral) and oral cavity (bilateral/unilateral). Differences in planning target volume coverage metrics were not clinically significant. Conclusion: HNC-KBRT planning generated IMRT plans with at least equivalent dose sparing to manually generated plans; greater dose sparing was achieved in selected OARs.