Direct Machine Parameter Optimization Research Articles

Analysis of OAR Doses in DMPO-Based Prostate IMRT

Radiotherapy plays an eminent role in the treatment of the prostate cancers with rotational radiotherapy taking the leading edge in the recent years. The study aims to find the relevance of the Direct machine parameter optimization (DMPO) based static gantry IMRT treatment planning in the step and shoot environment for ca prostate case with involved nodes in the era of rotational radiotherapy. Also we aim to determine the relation between the femur dose and the rectum dose, and mean rectum dose and rest rectum volume if any. 32 ca prostate patients were selected retrospectively for this study with prescribed dose of 68Gy (EQD2 72.08Gy α/β 10) and 50Gy in 25 fractions to the prostate and nodes respectively treated with the simultaneous integrated boost technique. The plans were done on a treatment planning system with DMPO algorithm and delivered using step and shoot method in LINAC. For PTV, 95% volume covering 100% prescribed dose was achieved with posterior rectum wall receiving less than 34Gy and organs at risk (OARs) within the tolerance limit. Along with the percentage volume of the rest rectum (rectum minus PTV with 0.5cm margin), the mean doses of rectum, rest rectum, rest bladder (bladder minus PTV with 0.5cm margin) and femur were noted. The relation between rectum mean dose and femur mean dose were studied. Also comparison was done between mean rectum dose and percentage (%)rest rectum volume. The doses received by 5% of the femoral heads were also studied. Plot of mean rectum dose versus mean femur dose received by each patients shows the inverse relation with correlation coefficient of -0.401 (p<0.05). Also the mean rectum dose and %rest rectum volume are inversely related with correlation coefficient of -0.81 (p<0.001). The mean of the dose received by 5% volume of right and left femur is 40.39±4.7Gy and 40.2±3.8Gy respectively with p<0.001. The mean dose of rectum was 36.5±5.5Gy with median dose of 37.6Gy. For rest rectum the mean dose was 28.7±5.6Gy with median dose of 27.8Gy. The mean dose of the left and right femoral heads and rest bladder were 24.2±4.5Gy, 23.7±4.5Gy and 31.6±6.2Gy respectively with p value <0.001. For an optimal plan for ca prostate case achieved on a TPS with DMPO algorithm, we could predict the mean dose of rectum to be 36.5±5.5Gy with 95% Confidence interval for mean femoral head dose of 23.9±4.2Gy. We could also infer that the femur dose and mean rectum dose, and the %rest rectum volume and mean rectum dose were inversely correlated with correlation coefficient of -0.401 and -0.810 respectively. Ca Prostate being the fourth most commonly occurring cancers is widely treated using radiotherapy as an important mode of treatment with different treatment techniques all over the world. Although the rotational radiotherapy delivers the treatment in comparably lower beam on time, the DMPO based static gantry IMRT plans still holds the dosimetric relevance with comparable OAR doses.

187. Evaluation of intrarater and interrater VMAT plans obtained with MCO algorithm and comparison with DMPO plans

Purposeto evaluate Multicriteria Optimization Algorithm (MCO) and its reliability in providing VMAT treatment plans through the technique of Pareto surfaces navigation. Methodsten patients who were previously treated with VMAT for prostate cancer were selected and replanned five times each by three different operators (total 50 plans) in RayStation treatment planning system using MCO [1] that provides a real-time assessment of the tradeoff between different clinical goals. For the optimization of each plan we started from the same template derived according the target coverage and the spare of healthy tissue used in our clinical protocols. Furthermore we calculate also a manual plan for each patient using the algorithm based on Direct Machine Parameter Optimization (DMPO); the quality of the plans was evaluated through D2%, D98%, Conformity Index (CI = [volume that receives 95% of dose prescription/target volume]) and Homogeneity index (HI = [dose that covers 5% of the target/dose that covers 95% of the target]). We evaluated intrarater and interrater reliability of the MCO plans through Intraclass Correlation Coefficient (ICC). Finally we compared the best MCO plan (assigning a score to each of the four indexes) with the one obtained with DMPO optimization estimating Pearson and Linn’s Correlation Coefficients (PCC and LCC). ResultsThe average intrarater ICC over the three operators for CI index was “fair” (0,50). Interrater ICC showed “poor” similarity between operators for CI (0,35). Both PCC and LCC between DMPO plan and MCO best plan were relevant (⩾0,7) for CI for op3. ConclusionsDespite some “poor” statistical similarity found in the analysis, the MCO optimization has the potential to provide high-quality plans comparable to the DMPO algorithm in terms of PTV coverage especially in isodoses conformity achievement.

Physician Driven Multicriteria Optimization of Advanced Lung Cancer Patients VMAT Plans

Multi-Criteria Optimization (MCO), a novel commercially available optimization method for intensity modulated and volumetric arc therapies (IMRT and VMAT) has the potential to improve treatment planning techniques and workflows. By navigating between Pareto solutions, MCO allows planners and physicians to assess in real time the impact and tradeoffs between all clinical goals and organ constraints. The purpose of our work is to investigate a universal set of objectives and constraints for lung cancer MCO VMAT plans and the impact of involving the Physician in the navigation of the generated Pareto plans. The central hypothesis is that physician-driven MCO VMAT plans will allow choice of optimal dosimetric trade-offs for a given individual patient. We randomly selected ten lung cancer patients (Stage II-IV) planned with a two arcs VMAT technique. Prescription doses ranged from 50.4Gy to 66Gy in 1.8 or 2Gy per fraction, with one case of dose painting. Target volumes were generated per clinical standard of care. The ITVs ranged from 18cc-387cc. Using the clinically delivered isocenter and beam set-up, we retrospectively generated MCO plans with a universal set of constraints and objectives that was developed by trying different combinations in a few patients. Two versions of the MCO plan scores (one navigated by the dosimetrist [MCOd] and one by the physician [MCOp]) were compared with clinical plans generated with Direct Machine Parameter Optimization (DMPO). Dosimetric and biological parameters evaluated were PTV V95%, Conformity Index (CI), PTV Dmax and the un-complicated probability (UP) representing the absence of lung and esophagus complications. The cord dose was not evaluated because almost all plans had Dmax doses of less than 45Gy, with negligible probability of complications. MCOd vs clinical comparison: the values for PTV V95 were MCOd: 99%(95-100) vs clinical plans: 98% (94-100),p-value= 0.2, the CI were similar in both cases (p-value=0.5) but the Dmax was higher for the MCO plans (average 109% vs. 107%, p =0.004), the lung-esophagus UP was similar between MCOd (Mean: 74%, range: 55-87%) and the clinical pla(Mean:75%, range: 60-89%), p= 0.1. Corresponding MCOd versus MCOp comparisons were similar for the parameters tested. However, using real time assessment of dosimetric trade-offs, in each individual plan the physician was able to actively choose a preferred combination of dose to target vs organs. When evaluating individual patients, there were five patients for which MCO plans had a clear advantage in reducing dose to lung and/or esophagus while improving/maintaining target coverage, four patients with comparable plans and one patient where MCO was worse. MCO VMAT lung plans obtained with standardized objectives and constraints generated comparable or better plans than the clinical DMPO plans while allowing physicians to make real time comparisons of dosimetric trade-offs and accept a plan optimal for an individual patient.

Dosimetric effect of beam arrangement for intensity-modulated radiation therapy in the treatment of upper thoracic esophageal carcinoma

To evaluate the lung sparing in intensity-modulated radiation therapy (IMRT) for patients with upper thoracic esophageal tumors extending inferiorly to the thorax by different beam arrangement. Overall, 15 patient cases with cancer of upper thoracic esophagus were selected for a retrospective treatment-planning study. Intensity-modulated radiation therapy plans using 4, 5, and 7 beams (4B, 5B, and 7B) were developed for each patient by direct machine parameter optimization (DMPO). All plans were evaluated with respect to dose volumes to irradiated targets and normal structures, with statistical comparisons made between 4B with 5B and 7B intensity-modulated radiation therapy plans. Differences among plans were evaluated using a two-tailed Friedman test at a statistical significance of p < 0.05. The maximum dose, average dose, and the conformity index (CI) of planning target volume 1 (PTV1) were similar for 3 plans for each case. No significant difference of coverage for planning target volume 1 and maximum dose for spinal cords were observed among 3 plans in present study (p > 0.05). The average V5, V13, V20, mean lung dose, and generalized equivalent uniform dose (gEUD) for the total lung were significantly lower in 4B-plans than those data in 5B-plans and 7B-plans (p < 0.01). Although the average V30 for the total lung were significantly higher in 4B-plans than those in 5B-plans and 7B-plans (p < 0.05). In addition, when comparing with the 4B-plans, the conformity/heterogeneity index of the 5B- and 7B-plans were significantly superior (p < 0.05). The 4B-intensity-modulated radiation therapy plan has advantage to address the specialized problem of lung sparing to low- and intermediate-dose exposure in the thorax when dealing with relative long tumors extended inferiorly to the thoracic esophagus for upper esophageal carcinoma with the cost for less conformity. Studies are needed to compare the superiority of volumetric modulated arc therapy with intensity-modulated radiation therapy technique.

An empirical method for automatic determination of maximum number of segments in DMPO-based IMRT for Head and Neck cases

AimAn empirical scheme called “anatomy-guided segment counting (AGSC)” is proposed for automatic selection of maximum number of segments (NOS) for direct machine parameter optimization (DMPO). BackgroundDirect machine parameter optimization (DMPO) requires the user to define the maximum number of segments (NOS) in order to proceed with an optimization process. Till date there is no established approach to arrive at an optimal and case-specific maximum NOS in DMPO, and this step is largely left to the planner's experience. Materials and methodsThe AGSC scheme basically uses the Beam's-eye views (BEVs) and other planning parameters to decide on appropriate number of segments for the beam. The proposed algorithm was tested in eight H&N cases. We used Auto Plan feature available in Pinnacle3 (version 9.10.0) for driving the DMPO optimization. ResultsThere is about 13% reduction in the composite objective value in AGSC plans as compared to the plans employing 6 NOS per beam and 10% increase in the composite objective value in AGSC plans as compared to the plans employing 8 NOS per beam. On the delivery efficiency front, there is about 10% increase in NOS in AGSC plans as compared to the plans employing 6 NOS per beam specification. Similarly, there is about 19% reduction in NOS in AGSC plans as compared to the plans employing 8 NOS per beam specification. ConclusionThe study demonstrates that the AGSC method allows specifying appropriate number of segments into the DMPO module accounting for the complexity of a given case.

Forward planning or inverse planning? In medio stat virtus

Introduction Implementation of inverse planning algorithms into modern treatment planning systems (TPS) has supposed an improvement of the planning process especially in cases where intensity modulated planning solutions are desirable. On the other hand this improvements drastically reduced control that user might have over planning parameters. This situation generated an interesting debate among those who support forward planning and those who support an inverse planning approach. Purpose In this work we explore a third possibility to which we refer as initialized inverse planning and that may be considered a step in the middle between forward and inverse planning strategies. This methods actually combines flexibility of the first and optimization capability of the latter. Materials and methods TPS used in this work is Pinnacle 3 by Philips and its Direct Machine Parameter Optimization (DMPO) inverse planning algorithm. Our initialized inverse planning approach (to which we refer as iDMPO) starts from the solution provided by a forward planning strategy which is then optimized by the DMPO algorithm. In this second step we impose tighter constraints than the one reached with forward planning. Results In this work we compare this three methods (forward, inverse with DMPO and iDMPO) in the specific case of prostate cancer treatments. Nevertheless conclusions are also applicable to other treatments. Conclusion Results show that with iDMPO technique it is possible to improve treatments dosimetric results, compared to the forward planning, imparting lower doses to organs at risk while delivering more homogeneous dose distributions to target volumes and thus reaching the pure inverse planning approach solution. Disclosure Authors disclose any relationship that may bias this work.

Optimization of stereotactic body radiotherapy treatment planning using a multicriteria optimization algorithm

PurposeTo provide high-quality and efficient dosimetric planning for various types of stereotactic body radiotherapy (SBRT) for tumor treatment using a multicriteria optimization (MCO) technique fine-tuned with direct machine parameter optimization (DMPO). Methods and materialsEighteen patients with lung (n=11), liver (n=5) or adrenal cell cancer (n=2) were treated using SBRT in our clinic between December 2014 and June 2015. Plans were generated using the RayStation™ Treatment Planning System (TPS) with the VMAT technique. Optimal deliverable SBRT plans were first generated using an MCO algorithm to find a well-balanced tradeoff between tumor control and normal tissue sparing in an efficient treatment planning time. Then, the deliverable plan was post-processed using the MCO solution as the starting point for the DMPO algorithm to improve the dose gradient around the planning target volume (PTV) while maintaining the clinician's priorities. The dosimetric quality of the plans was evaluated using dose-volume histogram (DVH) parameters, which account for target coverage and the sparing of healthy tissue, as well as the CI100 and CI50 conformity indexes. ResultsUsing a combination of the MCO and DMPO algorithms showed that the treatment plans were clinically optimal and conformed to all organ risk dose volume constraints reported in the literature, with a computation time of approximately one hour. The coverage of the PTV (D99% and D95%) and sparing of organs at risk (OAR) were similar between the MCO and MCO+DMPO plans, with no significant differences (p>0.05) for all the SBRT plans. The average CI100 and CI50 values using MCO+DMPO were significantly better than those with MCO alone (p<0.05). ConclusionsThe MCO technique allows for convergence on an optimal solution for SBRT within an efficient planning time. The combination of the MCO and DMPO techniques yields a better dose gradient, especially for lung tumors.

Determination of optimal number of beams in direct machine parameter optimization-based intensity modulated radiotherapy for head and neck cases.

This paper aims to introduce an algorithm called “sensitivity-based beam number selection (SBBNS)” for fully automated and case-specific determination of an optimal number of equispaced beams in intensity-modulated radiotherapy (IMRT). We tested the algorithm in five head and neck cases of varying complexity. We used direct machine parameter optimization method coupled with Auto Plan feature available in Pinnacle TPS (Version 9.10.0) for optimization. The Pearson correlation test shows a correlation of 0.88 between predicted and actual optimal number of beams, which indicates that SBBNS method is capable of predicting optimal number of beams for head and neck cases with reasonable accuracy. The major advantage of the algorithm is that it intrinsically takes into account various case- and machine-specific factors for the determination of optimal number. The study demonstrates that the algorithm can be effectively applied to IMRT scenarios to determine case specific and optimal number of beams for head and neck cases.

Characteristics of non-coplanar IMRT in the presence of target-embedded organs at risk.

BackgroundThe aim is to analyze characteristics and to study the potentials of non-coplanar intensity modulated radiation therapy (IMRT) techniques. The planning study applies to generalized organ at risk (OAR) – planning target volume (PTV) geometries.MethodsThe authors focus on OARs embedded in the PTV. The OAR shapes are spherically symmetric (A), cylindrical (B), and bended (C). Several IMRT techniques are used for the planning study: a) non-coplanar quasi-isotropic; b) two sets of equidistant coplanar beams, half of beams incident in a plane perpendicular to the principal plane; c) coplanar equidistant (reference); d) coplanar plus one orthogonal beam. The number of beam directions varies from 9 to 16. The orientation of the beam sets is systematically changed; dose distributions resulting from optimal fluence are explored. A selection of plans is optimized with direct machine parameter optimization (DMPO) allowing 120 and 64 segments. The overall plan quality, PTV coverage, and OAR sparing are evaluated.ResultsFor all fluence based techniques in cases A and C, plan quality increased considerably if more irradiation directions were used. For the cylindrically symmetric case B, however, only a weak beam number dependence was observed for the best beam set orientation, for which non-coplanar directions could be found where OAR- and PTV-projections did not overlap. IMRT plans using quasi-isotropical distributed non-coplanar beams showed stable results for all topologies A, B, C, as long as 16 beams were chosen; also the most unfavorable beam arrangement created results of similar quality as the optimally oriented coplanar configuration. For smaller number of beams or application in the trunk, a coplanar technique with additional orthogonal beam could be recommended. Techniques using 120 segments created by DMPO could qualitatively reproduce the fluence based results. However, for a reduced number of segments the beam number dependence declined or even reversed for the used planning system and the plan quality degraded substantially.ConclusionsTopologies with targets encompassing sensitive OAR require sufficient number of beams of 15 or more. For the subgroup of topologies where beam incidences are possible which cover the whole PTV without direct OAR irradiation, the quality dependence on the number of beams is much less pronounced above 9 beams. However, these special non-coplanar beam directions have to be found. On the basis of this work the non-coplanar IMRT techniques can be chosen for further clinical planning studies.Electronic supplementary materialThe online version of this article (doi:10.1186/s13014-015-0494-5) contains supplementary material, which is available to authorized users.

A feedback constraint optimization method for intensity-modulated radiation therapy of nasopharyngeal carcinoma.

Intensity-modulated radiation therapy (IMRT) is able to achieve good target conformance with a limited dose to organs at risk (OARs); however, IMRT increases the irradiation volume and monitor units (MUs) required. The present study aimed to evaluate the use of an IMRT plan with fewer segments and MUs, while maintaining quality in the treatment of nasopharyngeal carcinoma. In the present study, two types of IMRT plan were therefore compared: The direct machine parameter optimization (DMPO)-RT method and the feedback constraint DMPO-RT (fc_DMPO-RT) method, which utilizes compensative feedback constraint in DMPO-RT and maintains optimization. Plans for 23 patients were developed with identical dose prescriptions. Each plan involved synchronous delivery to various targets, with identical OAR constraints, by means of 7 coplanar fields. The average dose, maximum dose, dose-volume histograms of targets and the OAR, MUs of the plan, the number of segments, delivery time and accuracy were subsequently compared. The fc_DMPO-RT exhibited superior dose distribution in terms of the average, maximum and minimum doses to the gross tumor volume compared with that of DMPO-RT (t=62.7, 20.5 and 22.0, respectively; P<0.05). The fc_DMPO-RT also resulted in a smaller maximum dose to the spinal cord (t=7.3; P<0.05), as well as fewer MUs, fewer segments and decreased treatment times than that of the DMPO-RT (t=6.2, 393.4 and 244.3, respectively; P<0.05). The fc_DMPO-RT maintained plan quality with fewer segments and MUs, and the treatment time was significantly reduced, thereby resulting in reduced radiation leakage and an enhanced curative effect. Therefore, introducing feedback constraint into DMPO may result in improved IMRT planning. In nasopharyngeal carcinoma specifically, feedback constraint resulted in the improved protection of OARs in proximity of targets (such as the brainstem and parotid) due to sharp dose distribution and reduced MUs.

SU‐E‐T‐740: The Optimization in Segment Numbers for Nasopharyngeal Carcinoma (NPC) Treated by Intensity Modulated

Purpose:In premise of uninfluenced to dose distribution of tumor target and organ at risk(OAR) in Nasopharyngeal carcinoma (NPC),the number of segment was changed to increase efficacy and optimize treatment method by designing different plan of intensity modulated radiotherapy(IMRT).Methods:10 cases of NPC were confirmed in pathology and treated with step and shoot IMRT. The target volumes were defined: GVT‐T (the primary tumor); GVT‐N (the metastasis lymph nodes in neck); CTV‐T and CTV‐N (the clinical targets of GVT‐T and GTV‐N); and PTV (incidental lymph nodal region). Dose of GTV/CTV/PTV was 70/64/54Gy for 30 fractions.Every patient was designed 7 treatment plans of IMRT by Pinnacle 8.0m planning system,each plan was used with 9 beams of uniform distribution and fixing incidence direction(200°,240°,280°,320°,0°,40°,80°,120°and 160°respectively),and designed for delivery on Elekta Synergy linear accelerator.All plans were optimized with the direct machine parameter optimization(DMPO) algorithm.Minimum MU in each segment was defined at 4MU,and minimal segment area was 4cm2.Number of maximum segment field was defined at 120,110,100,90,80,70,60, respectively.Coverage and conformity of GTV/CTV/PTV,sparing of OARs(spinal cord, brain, parotids, lens and optic nerves) were compared.Results:In this group,mean volume of GTV(GTV‐T+GTV‐N),CTV(CTV‐T+CTV‐N)and PTV were 91.39±71.13cm3,177.27±92.16cm3 and 1028.05±167.03cm3,.respectively. Compared with the number of segment decreased from 120 to 60, there were no difference in MU(p&gt;0.05),and the dose distribution of targets(GTV/CTV/PTV),OARs and CI were not different too(p&gt;0.05).But when the segment number was &lt;60, CI of PTV was changed compared with others(p&lt;0.05).Conclusion:The minimal segment number of IMRT plan designed by Pinnacle 8.0m planning system in NPC should be decreased reasonably and the limit of minimal segment number was recommended by 70.

Volumetric-modulated arc therapy planning using multicriteria optimization for localized prostate cancer.

The purpose of this work is to evaluate the volumetric‐modulated arc therapy (VMAT) multicriteria optimization (MCO) algorithm clinically available in the RayStation treatment planning system (TPS) and its ability to reduce treatment planning time while providing high dosimetric plan quality. Nine patients with localized prostate cancer who were previously treated with 78 Gy in 39 fractions using VMAT plans and rayArc system based on the direct machine parameter optimization (DMPO) algorithm were selected and replanned using the VMAT‐MCO system. First, the dosimetric quality of the plans was evaluated using multiple conformity metrics that account for target coverage and sparing of healthy tissue, used in our departmental clinical protocols. The conformity and homogeneity index, number of monitor units, and treatment planning time for both modalities were assessed. Next, the effects of the technical plan parameters, such as constraint leaf motion CLM (cm/°) and maximum arc delivery time T (s), on the accuracy of delivered dose were evaluated using quality assurance passing rates (QAs) measured using the Delta4 phantom from ScandiDos. For the dosimetric plan's quality analysis, the results show that the VMAT‐MCO system provides plans comparable to the rayArc system with no statistical difference for V95% (p<0.01), D1% (p<0.01), CI (p<0.01), and HI (p<0.01) of the PTV, bladder (p<0.01), and rectum (p<0.01) constraints, except for the femoral heads and healthy tissues, for which a dose reduction was observed using MCO compared with rayArc (p<0.01). The technical parameter study showed that a combination of CLM equal to 0.5 cm/degree and a maximum delivery time of 72 s allowed the accurate delivery of the VMAT‐MCO plan on the Elekta Versa HD linear accelerator. Planning evaluation and dosimetric measurements showed that VMAT‐MCO can be used clinically with the advantage of enhanced planning process efficiency by reducing the treatment planning time without impairing dosimetric quality.PACS numbers: 87.55.D, 87.55.de, 87.55.Qr

Hippocampus Avoidance Whole-brain Radiation Therapy: A Practical Intensity-modulated Radiation Therapy Planning and Delivery Approach to RTOG 0933

PurposeThe goal of this work was to develop a more practical technique to meet the requirements of the Radiation Therapy Oncology Group (RTOG) 0933 protocol for sparing the hippocampus while irradiating the whole brain. Whole-brain radiation therapy (WBRT) has been linked to a decrease in neurocognitive function and increase in memory loss leading to lower quality of life for patients. Hippocampus avoidance (HA) WBRT may help to mitigate these issues by delaying the decline in neurocognitive function. RTOG 0933 is a phase II clinical trial aimed at HA-WBRT for patients with metastatic brain lesions. Recently published results from this study showed significant benefit in preserving short-term memory function for these patients. Although the trial allows several methods of treatment, including tomotherapy, volumetric arc therapy, and linac-based IMRT, treatment at many cancer centers is limited to linear accelerator (linac) based step-and-shoot IMRT. The linac-based method suggested by RTOG involves nine gantry angles on a number of different planes. Our goal was to find a more practical solution for the planning and delivery of HA-WBRT according to the RTOG 0933 protocol. Methods and MaterialsA total of 14 patients were accrued to 0933 at our center. Patients receive magnetic resonance imaging, which is fused with the planning computed tomographic scan to aid in contouring the hippocampus. Immobilization of the patients is achieved using Mayo Mold and Thermoplast shell (CDR Systems, Calgary, AB). Treatment planning is performed using Pinnacle version 9 (Pinnacle3, Fitchburg, WI) and direct machine parameter optimization. Treatment is delivered using a Varian iX (Varian Medical Systems, Palo Alto, CA), 120-leaf multileaf collimator, 6 MV photon beams. Through an iterative process, we developed a technique using seven gantry angles and no couch rotations. The planning process was aided by the use of dose shaping rings and avoidance structures; the delivery time was further optimized by avoiding split fields and keeping the total number of monitor units and beam segments to the minimum required to meet RTOG 0933 dose constraints. The planning and dosimetry were centrally reviewed for the first five patients according to protocol requirement. ResultsWe were able to achieve our goal by developing a technique that requires only seven gantry angles and no couch movements. All of our plans met the protocol requirements and were accepted into the study. We assessed the average delivery time for four patients comparing our technique with the technique proposed by the RTOG. An average time saving of 523 seconds was achieved with the seven gantry angle technique. There was also a modest saving of 102 total monitor units with our technique. ConclusionsThe technique developed in our center is a practical technique for HA-WBRT. Our process gave reproducible results in the patients we accrued to RTOG 0933 and should allow many more centers to be able to deliver HA-WBRT. Should HA become a standard of care this technique is easier to adopt.

Sci-Fri AM: Mountain - 05: Unified Optimization and Delivery of Intensity-modulated Radiation Therapy and Volume-modulated Arc Therapy

Purpose: To study the feasibility of a unified intensity-modulated arc therapy (UIMAT) that combines IMRT and VMAT optimization and delivery in order to produce efficient and superior radiation treatment plans. Methods: Inverse planning for UIMAT was prototyped on the Pinnacle treatment planning system (Philips Medical Systems). UIMAT integrates IMRT and VMAT delivery in the same arc where IMRT was delivered with gantry speed close to zero. Optimal gantry angles for the IMRT phases were selected automatically by the inverse optimization algorithm. Optimization of the VMAT phases and IMRT phases were done simultaneously using Pinnacle's direct machine parameter optimization algorithm. Five treatment plans each for prostate, head and neck, and lung were generated using our unified technique and compared with clinical VMAT or IMRT plans. Delivery verification was performed on an ArcCheck phantom (Sun Nuclear) and delivered in clinical mode on a Varian TrueBeam linear accelerator. Results: In this prototype implementation, compared to the VMAT or IMRT plans, with the plans normalized to the same dose coverage to the planning target volumes, the UIMAT plans produced improved OAR sparing for head and neck cases, while for lung and prostate cases, the dosimetric improvements for OARs were not as significant. In this proof-of-concept work, we demonstrated that a novel radiation therapy delivery technique combining VMAT and IMRT delivery in the same arc is feasible. Initial results showed UIMAT has the potential to be superior to either standard IMRT or VMAT.

SU-E-T-592: Relationship Between Dose of Distribution and Area of Segment Fields Among Different Intensity-Modulated Radiotherapy Planning in Cervix Cancer

Purpose: In premise of uninfluenced to dose distribution of tumor target and organ at risk(OAR) in cervical cancer,area of segment fields was changed to increase efficacy and optimize treatment method by designing different plan of intensity modulated radiotherapy(IMRT). Methods: 12 cases of cervical cancer were confirmed in pathology and treated with step and shoot IMRT. Dose of PTV was 50Gy/25fractions. Every patient was designed 9 treatment plans of IMRT by Pinnacle 8.0m planning system,each plan was used with 9 beams of uniform distribution and fixing incidence direction(200°,240°,280°,320°,0°,40°,80°,120°and 160°respectively),and designed for delivery on Elekta Synergy linear accelerator. All plans were optimized with the direct machine parameter optimization(DMPO) algorithm using the same set of optimization objectives. Number of maximum segment field was defined at 80 and minimum MU in each segment was 5MU,and minimal segment area was 2*1cm2,2*2cm2,3*3cm2,4*4cm2,5*5cm2,6*6cm2,7*7cm2,8*8cm2and 9*9cm2,respectively.Coverage,homogeneity and conformity of PTV,sparing of OAR, MU and number of segment were compared. Results: In this group, mean volume of PTV was 916.8±228.7 cm3. Compared with the area of minimal segment field increased from 2*1cm2 to 9*9 cm2,the number of mean MU was decreased from 1405±170 to 490±47 and the number of segment field was reduced from 76±4 to 39±7 respectively(p 0.05),but when the minimal segment area was 8*8 cm2 and 9*9 cm2,they were changed compared with 7*7 cm2 and below(p<0.05). Conclusion: The minimal segment field of IMRT plan designed by Pinnacle 8.0m planning system in cervical carcinoma should be enlarge reasonably and minimal segment area of 7*7 cm2 was recommend.

Preventive sparing of spinal cord and brain stem in the initial irradiation of locally advanced head and neck cancers

Since reirradiation in recurrent head and neck patients is limited by previous treatment, a marked reduction of maximum doses to spinal cord and brain stem was investigated in the initial irradiation of stage III/IV head and neck cancers. Eighteen patients were planned by simultaneous integrated boost, prescribing 69.3 Gy to PTV1 and 56.1 Gy to PTV2. Nine 6 MV coplanar photon beams at equispaced gantry angles were chosen for each patient. Step‐and‐shoot IMRT was calculated by direct machine parameter optimization, with the maximum number of segments limited to 80. In the standard plan, optimization considered organs at risk (OAR), dose conformity, maximum dose <45 Gy to spinal cord and <50 Gy to brain stem. In the sparing plans, a marked reduction to spinal cord and brain stem were investigated, with/without changes in dose conformity. In the sparing plans, the maximum doses to spinal cord and brain stem were reduced from the initial values (43.5±2.2 Gy and 36.7±14.0 Gy), without significant changes on the other OARs. A marked difference (−15.9±1.9 Gy and −10.1±5.7 Gy) was obtained at the expense of a small difference (−1.3%±0.9%) from initial PTV195% coverage (96.6%±0.9%). Similar difference (−15.7±2.2 Gy and −10.2±6.1 Gy) was obtained compromising dose conformity, but unaffecting PTV195% and with negligible decrease in PTV295% (−0.3%±0.3% from the initial 98.3%±0.8%). A marked spinal cord and brain stem preventive sparing was feasible at the expense of a decrease in dose conformity or slightly compromising target coverage. A sparing should be recommended in highly recurrent tumors, to make potential reirradiation safer.PACS number: 87.55.D

Effects of dose limitation and increased protective weight on dose distribution in auditory organs during intensity-modulated radiotherapy for nasopharyngeal carcinoma

Objective To investigate the protective effects of reducing average radiation dose and increasing protective weight on the auditory system (tympanic cavity,the bony portion of eustachian tube,vestibule,and cochlea) during intensity-modulated radiotherapy (IMRT) for nasopharyngeal carcinoma (NPC).Methods The planning system (ADAC Pinnacle3 8.0m) with direct machine parameter optimization was used to optimize the IMRT planning for 40 patients with NPC (stage Ⅰ + Ⅱ:20 patients ;stage Ⅲ + Ⅳ:20 patients).Without reducing the radiation dose for target volume,the IMRT planning was optimized by limiting the average dose administered to the auditory system or increasing the protective weight for the protected organs in auditory system.The protective effects were assessed by analyzing the average dose received by the auditory system.Results After limiting the average dose administered to the auditory system without reducing the radiation dose for target volume,the average dose received by the auditory system was significantly reduced (3855.5-5391.3 Gy vs 2960.3-4559.6 Gy,P =0.000 for all) ; when the protective weight for the auditory system was increased,the average dose received by the auditory system was even more reduced (3855.5-5391.3 Gy vs 2725.4-4271.4 Gy,P =0.000 for all).For all three regimens,the average dose was significantly higher in stage Ⅲ + Ⅳ patients than in stage Ⅰ + Ⅱ patients (P =0.000 for all).Conclusions For the IMRT planning for NPC,limiting the average dose administered to the auditory system can greatly reduce the average dose received by the auditory system,and increasing the protective weight for the auditory system can further reduce the average dose received by the auditory system.However,the protective effect on the auditory system may be reduced as the stage of NPC increases. Key words: Auditory organ; Dose limitation; Optimized weight; Nasopharyngeal neoplasms/intensity-modulated radiotherapy

Study on multicriteria optimization of intensity-modulated radiotherapy planning comparing with direct machine parameter optimization

Objective The aim of this study was to evaluate the role of multicriteria optimization (MCO) in planning of intensity-modulated radiotherapy (IMRT).Methods Twenty IMRT patients (ten with prostate and ten with lung cancers) were randomly selected.For these patients,the treatment plans were designed with direct machine parameter optimization (DMPO).Based on these plans,new plans were designed with MCO,while keeping the setting conditions unchanged.Comparison was made between the two plans including the dose distribution,the dose volume histogram,the time of optimization and number of monitor unit (MU),but were play by pairing-t test.Results The plan designed in both optimizations satisfied all clinical requirements.For the same or better target coverage,rectum,bladder and small bowel were better with MCO compared with DMPO,MCO reduced 58％ of the time for optimization by average while MU increased 32％ by average for prostate cancer.For lung cancer,the whole lung,heart and spinal cord were better with MCO compared with DMPO,MCO reduced 59％ of the time for optimization by average while MU increased 11％ by average.Conclusions In comparison with DMPO,MCO reduces the dose of organs at risk,shorten the time of optimization. Key words: Multicriteria optimization; Direct machine parameter optimization; Intensity-modulated radiotherapy planning

SU-E-T-719: Dosimetric Differences of Post HDR Prostate IMRT Plans Generated From RayStation Multi-Criteria Optimization (MCO) and Pinnacle Direct Machine Parameter Optimization (DMPO)

Purpose: To evaluate dosimetric differences of post HDR prostate IMRT plans generated from RayStation Multi‐Criteria Optimization (MCO) and Pinnacle3 Direct Machine Parameter Optimization (DMPO) Methods: Ten patients previously treated with DMPO plans on Pinnacle3 v9.0 were selected. CT images with same RT structures from Pinnacle were sent to RayStation v2.4.8.180 for re‐planning using MCO Pareto surface plans with the same prescription and beam angles. The prescription was 50.4 Gy to 95% of PTV for all cases. Planning outcomes such as D95 (95% of volume receiving the prescribed dose), D 5, D2, D1 for PTV, D 53 for bladder, D53 , D30, and D1cc for rectum, D1cc for small and large bowel, conformity index (CI = PIV / PTV), and homogeneity index (HI = D5 / D95) were evaluated according to the clinical protocol. Results: The mean volume of PTV was 222.6 cm3 (range from 178.5 to 276.3 cm3). The D95 of PTV coverage of RayStation vs. Pinnacle was improved and more uniform with 49.8 Gy vs. 49.7 Gy, respectively. D5, D2, and D1 of PTV were lower with RayStation. So were D53 for bladder and rectum, D30 for rectum (p = 0.017), D1cc for rectum and D1cc for small bowel, D1cc for large bowel except one case on RayStation. CI was better with RayStation compared to Pinnacle (1.01 vs. 1.07). HI was better with RayStation compared to Pinnacle (1.06 vs. 1.07). Conclusion: MCO planning automatically generates a set of Pareto optimized solutions for given objectives to allow tradeoffs between targets and critical organs. RayStation can achieve better uniform tumor coverage with fewer hot spots while sparing more critical structures in post HDR prostate plans. MCO Pareto based plan is helpful in determining the best optimized dosimetry solutions.

SU-E-T-720: Maintaining Prostate IMRT Plan Quality Using the Fraction of Rectum Overlapping the PTV

Purpose: It is clinically meaningful to generate the optimal intensity‐modulated radiation therapy (IMRT) treatment plans within the restricted time with equally good quality for different patients, which is planner‐dependent. We developed a planner‐independent method for a clinical decision of prostate IMRT plan using the rectal complication probability with the volume of rectum overlapping the PTV. Methods: Eighteen prostate IMRT plans were retrospectively analyzed. The prescription dose in the all the planning target volumes (PTVs) was normalized with 76 Gy in 2 Gy fractions. We found a correlation between the fraction of rectum overlapping the PTV and the rectal normal tissue complication probability (NTCP), which is the predictable NTCP curve, which means the lower bound of the rectal NTCP. The plans beyond the predictable NTCP criteria were generated again with seven 10 MV beams from the Varian Clinac iX linear accelerator. The beams were arranged coplanar at: 0, 50, 100, 140, 220, 260, and 310. IMRT plans were generated using direct machine parameter optimization in the Pinnacle3. Results: All the regenerated plans came to be included in the predictable NTCP criteria. In our patient data, two plans were re‐planned: the NTCP for patient 7 was changed from 10.9 to 8.8 and the other NTCP for patient 14 was from 15.6 to 12.9. The rectal NTCP is clinically more meaningful than the mean dose of rectum. Conclusion: This model can predict the rectum NTCP before IMRT planning, to control the IMRT plan quality during planning. Hence we can maintain prostate IMRT plan quality using the fraction of rectum overlapping the PTV.