Direct Machine Parameter Optimization Research Articles

SU‐DD‐A1‐02: Parameter Optimization in Head and Neck IMRT for Elekta Linacs

Purpose: Planning and delivery in HN‐IMRT is challenging for Elekta linacs because of numerous constraints on beam delivery systems. The purpose of this study is to find a set of planning parameters that are applicable to most patients and optimal in terms of plan quality, delivery efficiency and dosimetric accuracy. Method and Materials: Four types of plans were created for each of 12 patients: ideal fluence optimization (FO), conventional two‐step optimization (TS) consisting of FO followed by MLC conversion, segment weight optimization (SW) and direct machine parameter optimization (DMPO). Maximum number of segments (NS) and minimum segment area (MSA) were varied in DMPO. Plan quality was evaluated based on score, dose distributions and dosimetric indices. Delivery efficiency was evaluated by irradiation time, and dosimetric accuracy by Mapcheck. Results: Plan quality deviates most from ideal FO for TS, with slight improvement for SW. DMPO is the closest to FO with the least variation among patients. NS of 80–160 in DMPO yield optimal plans. At larger NS (⩾80), plan quality decreases with MSA as expected, except for MSA <8cm2, which suggests presence of local minima in the DMPO algorithm. The irradiation time is strongly dependent on the plan segments (NSactual), weakly dependent on MUs, and independent of MSA. Typical plans with 79 segments and 8cm2 MSA have ∼747 MU and take ∼ 8 minutes to deliver; this increases to ∼13 minutes for 158 segments. Dosimetric accuracy is independent of DMPO parameters. Conclusion: The superior quality of DMPO plans makes it ideal for planning HN‐IMRT on Elekta linacs and its consistency allows development of class solutions. However, the vulnerability of local minima warrants such a study to systematically evaluate the effect of parameters in new planning techniques. The optimal set of parameters should be chosen to balance plan quality and delivery efficiency.

WE-E-AUD A-03: Evaluation of Dosimetric Margins in Prostate IMRT Treatment Plans Generated with Pinnacle DMPO

Purpose: To quantify ‘dosimetric margins’ existing in prostate IMRT plans generated with Pinnacle Direct Machine Parameter Optimization (DMPO), due to imperfect conformance of the planned dose distribution to delineated targets. To demonstrate that when these margins are accounted for, the van Herk margin framework provides accurate target coverage estimates. Method and Materials: DMPO was performed on 27 prostate plans with 5mm PTV margins. Setup errors were simulated via fluence convolution and sampling from a systematic error distribution, and resulting dose volume histograms (DVHs) were used to generate van Herk style Dose Population Histograms (DPHs). The dosimetric margin distribution (DMD) is the 3D margin distribution between the clinical target volume (CTV) and the treated volume (TV). Consistent with ICRU guidelines, the TV was the volume enclosed by the planning target volume (PTV) minimum dose isodose surface. Due to imperfect conformance of the planned dose distribution, the DMD can extend beyond the PTV. This creates an additional ‘buffer zone’ around targets, which can absorb setup and other geometric errors. The DMD was measured by exporting CTV and TVs from Pinnacle as meshes, and measuring collision distances. Results: DPH analysis showed the DMPO plans could tolerate random plus systematic setup errors having standard deviations (SDs) up to 3mm. Naive application of the van Herk margin formula suggests that 5mm PTV margins should absorb errors with SDs up to only 1.6mm. Coverage calculations based on measured DMDs were in agreement with the DPH analysis: the presence of dosimetric margins allows the prostate plans to absorb larger errors than one would naively expect. Conclusion: These DMPO results agree with those previously obtained using an in‐house optimizer. Both optimizers create similarly large dosimetric margins. Accounting for these margins enables accurate coverage estimates, without the need for simulations. (Supported by NIH R01CA98524 and NIH P01CA116602).

TH-D-AUD B-02: A Technique For Creating VMAT Plans Using Direct Machine Parameter Optimization

Purpose: The objective of this study was to develop a technique for single rotation volume modulated arc therapy (VMAT) treatment plans using the current clinical version of the Pinnacle Treatment Planning system. The goal was to create treatment plans using only tools currently available in Pinnacle. Method and Materials:Treatment planning for VMAT was performed using the Direct Machine Parameter Optimization (DMPO) algorithm. With DMPO, inverse planning is performed by optimizing MLC leaf positions and segment weights. The treatment delivery was modeled and optimized as 51 equally spaced fixed beams. This beam geometry was selected to generate comparable plans to helical tomotherapy. Upon completion of optimization, the MLC delivery files were exported and converted into a single arc. Fixed‐gantry IMRT plans were also created for comparison by analyzing dose statistics, volume histograms, and differences. Calculated doses and MLC controller files were extracted from Pinnacle and imported into MATLAB for analysis. Gamma analyses were performed between the fixed‐gantry and VMAT plans. In addition, the modulation factor was calculated for the VMAT plans. Results: A total of 11 treatment plans were created for VMAT and fixed‐gantry IMRT. In all cases, the DMPO algorithm was able to optimize a plan resulting in one beam aperture. For prostate and HN and 2.) The VMAT plans have dose distributions similar to fixed‐gantry IMRT.

SU-GG-T-61: Feasibility Study of Adaptive Intensity-Modulated Radiotherapy Using Field Aperture Re-Optimization

Purpose: To study the feasibility of adaptive intensity‐modulated radiation therapy(IMRT) using field aperture re‐optimization on a new CT set showing target volume change and to evaluate its potential clinical benefit. Method and Materials: A lungcancer patient who had a significant change of target volume during treatment with helical tomotherapy and required a plan adaptation was selected for the study. Two CT scans were performed: one was taken before radiotherapy, and the other was taken after 21 treatment fractions. A six‐field step‐and‐shoot IMRT plan was generated on the initial CT and transferred to the second CT after image registration. The MLC segment field apertures and weights were re‐optimized using direct machine parameter optimization (DMPO) for 10 iterations in Pinnacle inverse planning system on the second CT set. The dose distributions and dose volume histograms were compared between the original and the adaptive plans for both helical tomotherapy and the six‐field IMRT plan to evaluate the potential benefit of adaptation. The method is also applied to a clinical case by propagating contours from planning CT to daily MVCTs and calculating cumulative dose distributions based on a deformable model using both non‐adapted and daily‐adapted IMRT plans. Results: Both the adaptive helical tomotherapy and six‐field IMRT plans give dose distributions that yield better dose coverage of target volumes and spare more healthy lung tissue compared to the original plans for the clinical case studied. Re‐optimization of the six‐field step‐and‐shoot IMRT plan only required 5 minutes using DMPO. The time required for the whole IMRT adaptation process including contour propagation will also be reported. Conclusion: An IMRT plan can be adapted very quickly using field aperture re‐optimization based on the new CT images with contours adjusted to reflect changes in gross tumor volume and sensitive structures.

SU‐GG‐T‐527: Multi‐Objective IMRT Planning Which Produces Deliverable Plans

Purpose: To demonstrate the first instance of the direct production of deliverable plans using multi‐objective IMRT treatment planning. Method and Materials: Three clinical IMRT cases are studied: a pancreas, prostate, and brain. The research software ORBIT Workstation is used to create a database of Pareto optimal treatment plans. For each case, N+1 plans are generated, where N is the number of objective functions. N=6, 8, and 9 for the pancreas, prostate, and brain, respectively. The N+1 plans are the N anchor points solutions (each objective individually minimized) and a single equally weighted solution. For all runs, a minimum dose to the target of approximately 90% of Rx dose is used as a constraint to ensure that each plan has some target coverage. The resulting plan database is navigated with a graphical user interface that allows users to improve any objective by appropriately mixing the plans in the database in real time. The final averaged plan is then used as the input to a dose matching algorithm in ORBIT, which achieves that plan using direct machine parameter optimization (DMPO). Results: We find that the database of size N+1 is adequate to explore the treatment tradeoffs and select a treatment plan. The selected plan can be reproduced with DMPO, and in fact the normal tissue sparing is typically improved during this step. Conclusion: Since smooth navigation of Pareto optimal plans involves averaging plans, navigation can be done only before the plans are sequenced. We demonstrate the success of a two step approach, where first plans are generated and navigated in beamlet space and then the selected mixed plan is made deliverable by a DMPO dose mimicking algorithm. Conflict of Interest: Work done in collaboration with RaySearch Laboratories.

SU‐GG‐T‐524: Initialization Strategy and Concurrent Imaging for Single 360 Degree Arc Intensity Modulated Treatment Delivery

Purpose: Single 360 degree arc treatment with variable MU per degree and changing MLC apertures provides an efficient means for delivering highly conformal radiation therapy. Additionally, the treatment geometry is amenable to CT image acquisition during the delivery enabling verification of patient position. The challenge with the technique is finding the optimal set of apertures over the arc. Aperture based optimization (MLC leaf positions) is the natural approach, but suffers for complex targets if the initial starting condition is simply the fields exposing the target around the arc. The problem occurs when the optimal solution would have the MLC closing off a larger portion of the target requiring large positional changes from the initial condition to the optimal position. In this case gradient based optimizations fall short in that the derivative of the dose distribution with respect to the leaf position is flat for locations further from the leaf tip. In order to remedy this problem, improved initialization of the initial MLC positions around the arc based on the target geometry are proposed. Method and Materials: A research version of the Pinnacle3 RTPS was used to plan the single arc delivery. The approach uses 36 equispaced beams over a 360 degree arc, and optimizes the plan using a single MLC aperture for each beam and the direct machine parameter optimization (DMPO) technique in the system. The Elekta Synergy linear accelerator was used to deliver the treatment and for cone beam CT acquisition. Results: Initial results show dose conformality similar to that of traditional intensity modulated radiotherapy with the 360 degree delivery using geometry based pre‐initialization of the apertures. Additionally, kilovolt image acquisition during delivery was possible. Scattered radiation and the interplay between image acquisition frequency and the pulse rate of the treatment beam influence the image quality but not significantly.

MO-D-351-06: A Generalized Inverse Planning Tool for Arc-Based IMRT Delivery

Purpose: The recent development of new linaccontrol systems that are capable of delivering Volumetric Modulated Arc Therapy (VMAT) has attracted significant attention. There remains, however, a lack of robust inverse planning tools for VMAT. In this study, we will present a generalized inverse planning tool that can provide highly conformal VMAT solutions using either single‐arc or multiple‐arc deliveries for both Varian and Elekta MLCs.Method and Materials: To generate VMAT plans, we first created optimized multi‐field IMRT plans with equal‐spaced beam angles in Pinnacle3 using direct machine parameter optimization (DMPO). A “deliverable” fluence map was reconstructed using the resulting apertures for each beam. Next, we applied our home‐grown arc sequencer to translate these fluence maps into VMAT plans. Based on the user‐defined requirements, the sequencer can provide either single‐arc or multiple‐arc plans that meet the predefined VMAT leaf‐motion constraints. The obtained VMAT plans were then loaded into Pinnacle3 for a final dose calculation. In this study, 10 cases were tested in this study covering a variety of treatment sites including head‐&‐neck(5), prostate(3), lung(1) and brain(1). Results: A total of 24 VMAT plans were created using these 10 cases. Results demonstrated that highly conformal VMAT dose distributions can be achieved with an average sequencing time of under 8 minutes. On average, the VMAT plans required 513 MUs to deliver between 1 to 3 arcs. The average standard deviation in the target dose was 5.79 cGy/fraction; while the average target volume covered by 95% prescribed dose was 98.1%. Our results show that comparable VMAT plans can be achieved using either the Elekta 80‐leaf MLC or the Varian 120‐leaf Millennium MLC.Conclusion: Our generalized arc‐sequencing algorithm serves as a robust inverse planning solution for VMAT. Highly conformal single‐arc or multiple‐arc VMAT plans can be created for Elekta and Varian MLCs.

MO-D-351-02: An Efficient Approach To Volumetric Modulated Arc Therapy Optimization and Sequencing

Purpose: To develop and evaluate an efficient algorithm for inverse treatment planning of volumetric arc therapy (VMAT). Method and Materials: VMAT plans were generated by first applying a direct machine parameter optimization algorithm from a research version of a commercial treatment planning system (Pinnacle 8.1v) to 18 beams spaced equidistantly at 20 degrees. At each beam angle, the number of segments was variable (typically 3 to 8) to match the intensity modulation fluence map. Low weight or small area segments were eliminated during optimization. The resulting segments were redistributed around the arc such that each individual segment was at a unique arc angle. Segments were sorted according to shape similarity in order to reduce leaf travel in between arc angle positions. To ensure accuracy of the final dose calculation, additional interpolated segments were generated, followed by a segment weight optimization to identify beam‐off segments that would otherwise conflict with plan objectives. The approach was evaluated for six cases (three head‐and‐necks, two prostate, and one brain) with respect to treatment delivery time, and overall plan quality in comparison to step‐and‐shoot IMRT using a conventional number of static beams and identical objectives and constraints as in the VMAT case. Results: The dynamic arc plans consistently demonstrated target coverage and critical structure sparing comparable to step‐and‐shoot plans. For the head‐and‐neck cases, sparing of the brainstem and the spinal cord could be improved substantially. Treatment parameter optimization and dose calculation required 15 to 20 minutes on standard hardware. Estimated delivery times were between 1.5 and 4 minutes, which is mainly determined by the leaf trajectories, gantry speed and dose rate. Conclusion: A method for inverse planning for VMAT delivery was developed. The method produces treatment plans with high dosimetric quality and efficient delivery time with clinically feasible user time and effort.

SU‐EE‐A3‐04: On‐Line CT‐Guided Adaptive Re‐Planning Based On Deformed Original Dose Distributions

Purpose: Daily image‐guided setup based on soft tissue target may improve target coverage. However, when significant anatomical changes occur, a simple isocenter shift may not be adequate. In this study, we propose an online IMRT replanning strategy for prostate cancer which uses the deformed dose distributions from the original treatment plan as its objective. Method and Materials: The replanning procedure used was as follows: (1) the dose distribution on the planning CT was deformed to the daily CT and used as the reference objective dose distribution for IMRT replanning; (2) the prescription isodose line on the reference dose distribution was auto‐segmented and used as the fictitious “target volume” to set the initial MLC leaf positions; (3) we developed and implemented a voxel‐by‐voxel dose‐based cost function. The IMRT treatment plan was optimized using the direct machine parameter optimization algorithm to achieve the following goals: (a) inside the region enclosed by the original prescription dose line, the replanned dose distribution was optimized to match with the reference objective; (b) outside this region, the objective function was chosen to lower the dose value and to penalize the dose value exceeding the reference dose for each voxel. The replanning process does not need re‐auto‐segmentation of patient's anatomy, although re‐segmented anatomic contours were used to evaluate the effectiveness of this approach. Results: We compared the dose distributions and the DVHs of the original plan and the daily plans using (1) image‐guided setup based on prostate alignment, (2) deformed dose distribution from the original plan, and (3) our re‐planning method. We found that our re‐planning strategy matched well with the original plan. Conclusion: The replanning strategy using the original dose distribution as the goal for optimization produces dose distributions similar to the original approved plan and is an effective approach for on‐line CT‐guided adaptive radiotherapy.

Clinical Evaluation of Direct Aperture Optimization When Applied to Head-And-Neck IMRT

Direct Machine Parameter Optimization (DMPO) is a leaf segmentation program released as an optional item of the Pinnacle planning system (Philips Radiation Oncology Systems, Milpitas, CA); it is based on the principles of direct aperture optimization where the size, shape, and weight of individual segments are optimized to produce an intensity modulated radiation treatment (IMRT) plan. In this study, we compare DMPO to the traditional method of IMRT planning, in which intensity maps are optimized prior to conversion into deliverable multileaf collimator (MLC) apertures, and we determine if there was any dosimetric improvement, treatment efficiency gain, or planning advantage provided by the use of DMPO. Eleven head-and-neck patients treated with IMRT had treatment plans generated using each optimization method. For each patient, the same planning parameters were used for each optimization method. All calculations were performed using Pinnacle version 7.6c software and treatments were delivered using a step-and-shoot IMRT method on a Varian 2100EX linear accelerator equipped with a 120-leaf Millennium MLC (Varian Medical Systems, Palo Alto, CA). Each plan was assessed based on the calculation time, a conformity index, the composite objective value used in the optimization, the number of segments, monitor units (MUs), and treatment time. The results showed DMPO to be superior to the traditional optimization method in all areas. Considerable advantages were observed in the dosimetric quality of DMPO plans, which also required 32% less time to calculate, 42% fewer MUs, and 35% fewer segments than the conventional optimization method. These reductions translated directly into a 29% decrease in treatment times. While considerable gains were observed in planning and treatment efficiency, they were specific to our institution, and the impact of direct aperture optimization on plan quality and workflow will be dependent on the planning parameters, planning system, and linear accelerators used by a particular institution.

Comparison of direct machine parameter optimization versus fluence optimization with sequential sequencing in IMRT of hypopharyngeal carcinoma

BackgroundTo evaluate the effects of direct machine parameter optimization in the treatment planning of intensity-modulated radiation therapy (IMRT) for hypopharyngeal cancer as compared to subsequent leaf sequencing in Oncentra Masterplan v1.5.MethodsFor 10 hypopharyngeal cancer patients IMRT plans were generated in Oncentra Masterplan v1.5 (Nucletron BV, Veenendal, the Netherlands) for a Siemens Primus linear accelerator.For optimization the dose volume objectives (DVO) for the planning target volume (PTV) were set to 53 Gy minimum dose and 59 Gy maximum dose, in order to reach a dose of 56 Gy to the average of the PTV. For the parotids a median dose of 22 Gy was allowed and for the spinal cord a maximum dose of 35 Gy. The maximum DVO to the external contour of the patient was set to 59 Gy. The treatment plans were optimized with the direct machine parameter optimization ("Direct Step & Shoot", DSS, Raysearch Laboratories, Sweden) newly implemented in Masterplan v1.5 and the fluence modulation technique ("Intensity Modulation", IM) which was available in previous versions of Masterplan already. The two techniques were compared with regard to compliance to the DVO, plan quality, and number of monitor units (MU) required per fraction dose.ResultsThe plans optimized with the DSS technique met the DVO for the PTV significantly better than the plans optimized with IM (p = 0.007 for the min DVO and p < 0.0005 for the max DVO). No significant difference could be observed for compliance to the DVO for the organs at risk (OAR) (p > 0.05). Plan quality, target coverage and dose homogeneity inside the PTV were superior for the plans optimized with DSS for similar dose to the spinal cord and lower dose to the normal tissue. The mean dose to the parotids was lower for the plans optimized with IM. Treatment plan efficiency was higher for the DSS plans with (901 ± 160) MU compared to (1151 ± 157) MU for IM (p-value < 0.05).Renormalization of the IM plans to the mean of the dose to 95% of the PTV (D95) of the DSS plans, resulted in similar target coverage and dose to the parotids for both strategies, at the cost of a significantly higher dose to the normal tissue and maximum dose to the target. The relative volume of the PTV receiving 107% or more of the prescription dose V107 increased to 35.5% ± 20.0% for the IM plan as compared to a mean of 0.9% ± 0.9% for the DSS plan.ConclusionThe direct machine parameter optimization is a major improvement compared to the fluence modulation with subsequent leaf sequencing in Oncentra Masterplan v1.5. The resulting dose distribution complies better with the DVO and better plan quality is achieved for identical specification of DVO. An additional asset is the reduced number of MU as compared to IM.

SU‐FF‐P‐04: Evaluation of Treatment Planning Time Savings Using Direct Machine Parameter Optimization

Purpose: The purpose of this study is to evaluate the capability for increasing treatment‐planning throughput by utilizing Direct Machine Parameter Optimization (DMPO). Traditional inverse treatment planning consists of an optimization phase followed by a separate leaf‐sequencing phase. With DMPO based optimization, there is no leaf‐sequencing step. As such, the treatment planning time for each IMRT case can potentially be decreased and treatment‐planning throughput could be increased. Method and Materials: Ten patients (5 prostate and 5 head &amp; neck) were randomly selected for a comparative treatment planning study. IMRT treatment plans were created using the standard IMRT optimization algorithm with k‐mean clustering based leaf‐sequencing. A second treatment plan was created for each patient using the DMPO optimization algorithm. The only difference between the two plans for each patient was the optimization algorithm. For each case, the time required to optimize the plans, the number of segments required for step‐and‐shoot delivery, and total monitor units were recorded. Results: Both the DMPO and IMRT optimizations yielded similar dose distributions. The mean time to perform inverse planning and leaf sequencing for prostate patients was 8.00 minutes, verses 8.85 minutes for DMPO based optimization. In contrast, the efficiency of DMPO for Head &amp; Neck cases is substantially different. The mean time to perform a DMPO optimization for Head &amp; Neck cases was 12.05 minutes, as compared with 28.63 minutes for IMRT optimization. Conclusion: Over 16 minutes of treatment planning time can be saved per patient by utilizing the DMPO based optimization in Pinnacle for Head &amp; Neck treatment planning. The time advantage can grow to hours saved per patient if multipe treatment plans are created, or if the planners agressively adjust the plan. Depending on the Head &amp; Neck volumes treated, DMPO optimization can increase treatment planning effiency.

SU-FF-T-266: Intensity Modulated Radiotherapy of Breast Cancer Using Direct-Machine-Parameter-Optimization

Purpose: To design an efficient step-and-shot IMRT technique for breast CA and compare the results with other established treatment planning techniques. Methods and Materials: The technique was based on a modified Direct Machine Parameter Optimization (DMPO) method, where some of the inversely optimized segments were adjusted by the user in order to allow for skin “fall-off”. During DMPO, the optimized fluence was converted into segments where the segment shape and weight became part of the optimization; thereby, forcing segments that did not satisfy the requirements to be discarded. A set of 20 breast CA patients was used to develop the technique and compare it with a 3D-conformal with optimized wedges and a forward-optimized technique with manually defined segments. Results: Dose distribution and DVH analysis of all patient plans resulted superior PTV coverage with better dose homogeneity and lower lung and heart doses for our IMRT technique, compared to the two other techniques. The number of segments can be controlled by the user and the resulted MUs for delivery are less that those needed for the 3D-conformal with optimized wedges technique. Conclusions: DMPO can be effectively used on a routine breast IMRT treatment planning since it allows the user to modify the number and shapes of selected segments quickly and efficiently. Thus, this method solves the issue of patient and PTV setup uncertainty and motion by allowing the creation of “fall-off” defined by the user, on patient-to-patient basis. The technique can be easily extended to 4-field plans were the two anterior and posterior oblique fields are added for more complex PTVs. Dose uniformity to the PTV (due to the electronic compensation) and lower doses to the lungs and heart are the major advantages of our method.

SU-EE-A1-01: Dosimetric Comparisons of DMPO and Two-Step Approach Step-And-Shoot IMRT Plans

Purpose: To systematically evaluate step-and-shoot Intensity-Modulated-Radiation-Therapy (IMRT) plans generated by Direct-Machine-Parameter-Optimization (DMPO) and by Two-Step-Approach (TSA) using identical optimization parameters in Pinnacle 3 treatment planning system. Method and Materials: Using Pinnacle 3 version7.6c, TSA plans of total eight patients with Head-and-Neck, Prostate and Lung cancers were generated using identical optimization parameters from clinical plans used DMPO. The dose of planned-target-volume (PTV) in TSA plan was scaled to closely match at prescribed dose volume in the DMPO plan. Three PTV dosimetric indices: dose-coverage, dose-conformity and dose-inhomogeneity, were generated for each plan. Dosimetric comparisons were performed for organ-at-risk (OAR) with both “maximum-dose-objectives” and “dose-volume-based-objectives”. Final dose recalculation using EGS4-based in-house Monte-Carlo program for each plan was performed and corresponding dosimetric data were obtained. Film-based IMRT QA was performed for three patients. Results: On average, total monitor-units (MUs) are about 25% higher of TSA than DMPO. The averaged segment-numbers and PTV dosimetric indices are almost identical between plans from DMPO and TSA. The maximum-dose (defined at 0.1cc) of Head-and-Neck and Lung OARs with “maximum-dose-objectives” of TSA are, on average, ∼2.5Gy and ∼0.9Gy lower than those of DMPO, respectively. The averaged dose difference in prostate OARs with “maximum-dose-objectives” is small. For OARs with “dose-volume-based-objectives”, there is little difference between TSA and DMPO for all sites. The Monte-Carlo dose recalculations showed similar trends. The agreement between Pinnacle3 calculations and film measurements is 99% for all fields using 3%–3mm criteria. Conclusion: Dosimetric comparisons between DMPO and TSA IMRT plans demonstrated that using identical optimization parameters, DMPO plans have less total MUs and similar averaged segment-number as well as almost identical PTV dosimetric index values as TSA plans. For Head-and-Neck and lung plans, TSA has noticeable better sparing of OARs with “maximum-dose-based-objectives”, which is confirmed by Monte-Carlo recalculations. Film QA demonstrated both TSA and DMPO plans are very accurate.

SU-FF-J-102: Evaluation of the Dosimetric Differences Resulting From the Application of Gated IMRT Plans to a Non-Gated Patient Anatomy

Purpose: To evaluate the dosimetric impact of applying a plan optimized for gated delivery to a free‐breathing/different breathing phase CT data set, due to gating failure. Method and Materials: A retrospective investigation was performed on two lungIMRT patients. Each patient had a free‐breathing and a 4D CT scan.Tumors and OARs were contoured on each CT data set. The targets are GTV, CTV, and PTV, while OARs are spinal cord, heart, and each lung.IMRT step‐and‐shoot Direct Machine Parameter Optimization (Pinnacle v.7.4f) method was employed in the treatment planning. Evaluated were DVHs and gEUDs for a single fraction. The point marked by the BBs was used as a reference in the plan transfer from the standard (where the plan was optimized) to the compared CT data set. Thus, beam angles, energies, jaw settings, and MLC files were transferred preserving the relative geometry. Results: The coverage for GTV, CTV, and PTV changes within 1%–2%, 1%–7%, and 3%–10% respectively, as planning and delivery CT data sets are interchanged. The OAR gEUDs in the optimized gating plan are lower by as much as 28% than the corresponding gEUDs in the plan delivered to a free‐breathing CT data set, although it may not be significant if the tolerance doses are not exceeded. The lung sparing deteriorates by up to 14% when a plan optimized for an exhale gating is delivered via an inhale gating. Conclusions: The daily dose variations of the order of 7% to 10% may be non‐negligible and may lead to biological consequences. Therefore, an interlock in the record and verify system, coupled to the gating system, will prevent the occurrences of erroneous deliveries and will maximize the patient benefit form the use of new technologies in radiotherapy.

SU‐EE‐A1‐06: IMRT Dose Gradients For Extracranial Stereotactic Radiosurgery

Purpose: To compare IMRT treatment techniques for a simulated para‐spinal mass located in the thoracic spine of an anthropomorphic phantom and to measure the accuracy of Megavoltage‐CT (MVCT) images for localizing spinal anatomy in the T‐Spine region. Methods and Materials: Treatment planning CT images were acquired on a kilovoltage CT simulator of a whole body anthropomorphic RANDO phantom and used to create a planning target volume (PTV) covering the T7 to T9 vertebral bodies. The fixed gantry IMRT cases were planned using the Pinnacle treatment planning system and delivered on a Varian 21EX with a 120‐leaf multileaf collimator. Inverse treatment plans were created with 9 and 12 equally spaced fixed fields starting at 0‐degrees (IEC Scale). Inverse planning was performed using Direct Machine Parameter Optimization (DMPO), and gradient decent optimization with sliding window leaf sequencing. Helical tomotherapy cases were planned using the TomoTherapy HI‐ART treatment planning system. Relative dose measurements were made using calibrated film placed in the RANDO phantom. MVCT images of the RANDO phantom were acquired with a tomotherapy system and fused with the treatment planning CT images. The phantom was then correctly positioned, and the fusion error was measured by imaging the T7 and T9 phantom vertebrae. A principal component analysis was used to determine the largest factors in image registration. Results: The 9‐Field DMPO and helical tomotherapy cases had PTV uniformities of 10% and maintained a large dose gradient. Conclusions: Helical tomotherapy and 9‐Field DMPO treatments yielded similar dose gradients (10%/mm) and PTV dose uniformity indices (10%). The sliding window treatment deliveries were consistently worse in cord sparing and dose uniformity. Anthropomorphic phantom studies indicated that megavoltage CT images were capable of imaging the spine for placement at isocenter within 1‐mm of the desired position.

WE-D-224C-03: Surface Dose Measurements for Intensity Modulated Radiation Therapy

Purpose: To measure the surface and near-surface doses for a variety of IMRT delivery and planning techniques, and to determine the optimal PTV size and planning scenerio for skin dose maximization or minimization. Methods and Materials: A primary PTV was defined on an anthropomorphic head phantom for a typical parotid-sparing head &neck treatment. Treatment plans were created with 0, 1, 2, 3, 4, and 5-mm separation between the skin surface and PTV boundary. IMRT treatments were planned using the Pinnacle treatment planning system and delivered on a Varian 21EX with a 120-leaf multileaf collimator. Inverse planning was performed using Direct Machine Parameter Optimization (DMPO) for step and shoot IMRT and gradient decent optimization for sliding window leaf sequencing IMRT. Helical tomotherapy cases were planned using the TomoTherapy HI-ART treatment planning system and delivered using a Tomotherpy HI-ART treatment delivery system. Surface doses were measured for each of the treatment deliveries using film placed in an anthropomorphic head phantom and thermoluminescent dosimeters (TLD) chips placed on the phantom's surface. A chip-specific TLD surface calibration factor was determined and applied to the raw TLD readings to account for measurement efficiency at the surface. Relative dose measurements were made using calibrated film placed in the phantom. Surface and near-surface doses were measured from digitized film images for a 1.5-cm range inside the phantom to in-air outside the phantom. Results: Surface dose values measured with film were consistently lower, while TLD measurements were higher than planned for the cases studied. Conclusions: The helical tomotherapy treatment plans were found to have better parotid sparing and PTV dose uniformity than both the step-and-shoot DMPO and sliding window plans. The tomotherapy planning system was observed to overestimate the surface dose from 9 to 18 percent.

SU‐FF‐T‐304: Is 0.5 Cm Leaf Width of MLC Beneficial in IMRT?

Purpose: Evaluate treatment plan quality and quantify dosimetric accuracy of IMRT for different disease sites with MLC of 0.5 and 1.0 cm leaf width. Method and Materials: We tested a hypothesis, quality of IMRT treatment plans with smaller MLC leaf width is better, by developing treatment plans on Pinnacle treatment planning system (TPS) for multiple patients (total 60) with tumor sites located in Head and Neck (H&amp;N), Vertebral Body and Prostate. For each patient, two plans are created using same objective functions and optimization parameters and algorithm (Direct Machine Parameter Optimization): one with 0.5 cm leaf width; the other with 1.0 cm leaf width. The TPS beam modeling of the two virtual machines was exactly the same except for the MLC leaf width. The dose volume histogram (DVH) is used in the evaluations of plan quality. All target coverage is normalized to the criteria of 95% of the target volume receiving prescribed dose. A diode array (Mapcheck) was used to quantify the dosimetric accuracy of all plans created with two MLC leaf width resolutions. Results: DVHs of all the patient plans reveal very minor dosimetric differences between the IMRT plans of the two MLC systems for the same patient. There are almost identical target coverage and similar dose distributions among the critical structures in H&amp;N patients and vertebra body patients. For prostate, plan of 1 cm MLC leaf width has slightly more hot spots in the target with the same dose coverage criteria. Planar doses comparisons with Mapcheck measurements indicate slightly larger uncertainty in 0.5 cm leaf plans. Conclusion: There are negligible dosimetric differences in IMRT plans created with 0.5 cm and 1.0 cm leaf width resolution. However, the dosimetric accuracy of plans generated with 1.0 cm leaf width is better than the plans generated with 0.5 cm leaf width.

Intensity-modulated radiotherapy of breast cancer using direct aperture optimization

Background and purpose To design a clinically reliable and efficient step-and-shoot IMRT delivery technique for the treatment of breast cancer using direct aperture optimization (DAO). Using DAO, segments are created and optimized within the same optimization process. Patients and methods The DAO technique implemented in the Pinnacle treatment planning system, which is called direct machine parameter optimization (DMPO), was used to generate IMRT plans for twelve breast cancer patients. The prescribed dose was 50 Gy. Two DMPO plans were generated. The first approach uses DMPO only; the second technique combines DMPO with two predefined segments (DMPO segm), having shapes identical to the conventional tangential fields. The weight of these predefined segments is optimized simultaneously with DMPO. The DMPO plans were compared with normal two-step (TS) IMRT, creating segments after optimizing the intensity. Results Dose homogeneity within the target volume was 4.8±0.6, 4.3±0.5 and 3.8±0.5 Gy for the TS, DMPO and DMPO segm plans, respectively. Comparing the IMRT plans with an idealized dose distribution obtained using only beamlet optimization, the degradation of the dose distribution was less for the DMPO plans compared with the two-step IMRT approach. Furthermore, this degradation was similar for all patients, while for the two-step IMRT approach it was patient specific. Conclusions An efficient step-and-shoot IMRT solution was developed for the treatment of breast cancer using DMPO combined with two predefined segments.

SU-FF-T-94: Clinical Evaluation of Direct Machine Parameter Optimization Algorithm for Head and Neck IMRT Treatment

Purpose: Because many critical structures are in close proximity to target volumes, cancers of the head and neck (H&N) are often suited for treatment with IMRT. However, the time required to generate and deliver a clinically acceptable IMRT plan can be significantly longer than a conventional plan. This study evaluated a new inverse planning algorithm, DMPO (direct machine parameter optimization), with attention to parameter settings, plan quality and treatment efficiency for H&N cancers. Method and Materials: The Pinnacle treatment planning system version 7.4 was used. The DMPO allows users to limit the number of total MLC segments (N) for treatment. After a user-defined number of iterations (n) for pencil beam optimization, the DMPO generates MLC segments for each field for dose calculations using a convolution algorithm. Both the MLC leaf positions and the weight of each segment are then optimized until cost tolerance or iteration number is reached. Treatment plans generated using DMPO were compared with H&N cases that were previously treated using an older version (6.2). The plan quality was compared using cost functions and DVHs of target volumes and critical structures. The total monitor units and MLC segments for treatment were compared for different combinations of n and N. Results: The DMPO provided plans of DVHs similar to clinical cases with significantly less planning time. More importantly, the total MU and MLC segments for treatment delivery were reduced by 40% to 50%. Cost functions changed only slightly on n and N and total MU increased as n increased, but was independent of N. Our preliminary data indicated a combination of n=10–15 with 10 segments per field appeared to be optimal for most H&N cases. Conclusion: The DMPO algorithm generated more efficient plans while providing equal or better quality than the previous plans for IMRT treatment.