MLC Segments Research Articles

SU-E-T-397: Interplay Effect of Gated Lung Stereotactic Body Radiotherapy with RapidArc Delivery.

To investigate the dosimetric effect of intrafraction tumor motion during gated RapidArc Stereotactic Body Radiotherapy (SBRT) delivery. The realtime tumor motion data were retrieved from 6 lung patients. Each of them received 3 fractions of stereotactic radiotherapy treatments with Cyberknife Synchrony. Phase gating through an external surrogate was simulated with a gating window of 5 mm. The resulting residual tumor motion curves during gating (beam-on) were retrieved. RapidArc SBRT was planned on the platform of Varian Truebeam at 6 MV with 1400 MU/min. Planning target volume (PTV) was defined as physician-contoured clinical target volume (CTV) surrounded by an isotropic 5 mm margin. Each patient was prescribed with 60Gy/3 fractions. The RA plan typically consists of 2 arcs; each contains 90-120 control points. An algorithm was developed to reconstruct the delivered dose with tumor motion. The MLC segment is assumed to move relatively to a static tumor. Each MLC control point, mainly the leaf position were modified according to the probability density function of tumor motion. The newly created MLC control points were written back to the treatment file in the dicom format which was subsequently imported to treatment planning system (Varian Eclipse) for dose recalculation. The magnitude of dose deviation with motion is consistent with the excursion of the residual tumor movement. Overall CTV coverage of the study group is barely affected owing to the 5 mm margin. The fractional PTV dose coverage dropped by 4% at most and that from all fractions by 3%. An examination in the point dose shows an increase of 4% in the maximum dose and decrease of 10% for the minimum dose. With effective gating, interplay effect does not change the target coverage much during gated RapidArc SBRT. However it increases the dose nonuniformity inside target.

SU-E-T-573: Quality and Deliverability of Intensity Modulated Neutron Radiotherapy (IMNRT) Plans

Intensity Modulated Neutron Radiotherapy (IMNRT) has been commissioned for clinical use. The number of allowable segments in IMNRT plans is limited by MLC speed. Quality and deliverability of static IMNRT treatment plans using the TG-119 test suite were evaluated to establish guidelines for the number of segments per plan. Treatment plans were created and optimized to specified constraints for all cases in the TG-119 test suite using the Varian Eclipse TPS. A 4MV photon beam with similar penetration characteristics as the fast neutron beam was used as a surrogate for this optimization. Final dose calculations were performed using an in-house TPS commissioned for neutron dose calculations. Following optimization, MLC segments were created for three ranges of total plan complexity - very limited (15-23 segments), limited (24- 31 segments), and unlimited. Calculated DVHs were then compared for compliance with TG-119 dose constraints. The estimated time of delivery for plans in each range was calculated based on known delivery parameters. The prostate case passes all constraints for each complexity level. All other plans fail to meet at least one constraint for one or more of the complexity levels. For all cases combined, the very limited, limited, and unlimited complexity levels meet 16, 17, and 19 of 23 total dose constraints, respectively. The mean estimated delivery time for the very limited, limited, and unlimited plans is 34 minutes (range: 27-39), 40 minutes (range: 34-45), and 68 minutes (range: 53-81) respectively, neglecting any delay due to therapists entering the treatment room. IMNRT plan quality is limited by current MLC capabilities. IMNRT plans should be limited to 25 segments to ensure a reasonable treatment time of 45 minutes. Even with this small number of segments, we were able to meet most dose constraints set forth in TG-119.

TH‐A‐213AB‐03: Inverse Planning for 3D Intensity‐Modulated Grid‐Therapy

Purpose: To develop an inverse optimization method for 3D intensity‐modulated grid‐therapy to improve dose distribution of grid therapy of advanced stage tumors. Methods: The following process was used to generate a 3D intensity‐modulated grid‐therapy plan. First, based on the geometry of the target volume and organs at risk (OAR), three to six radiation fields were selected to minimize the overlap between the target volume and OARs. Typically, three orthogonal fields were used to minimize the overlap between the fields to maintain grid‐like dose distribution for each field. Second, a step‐and‐shoot IMRT plan was generated with selected fields using Pinnacle treatment planning system (Philips Medical Systems). Third, each MLC segment was converted to a grid field defined by additional MLC segments using an in‐house developed program. Finally, the plan was further optimized using segment weight optimization in Pinnacle treatment planning system, maintaining the grid shape for each IMRT field with optimized intensities for the opening grids. The method was tested for a few clinical cases with bulky tumors. Dose distributions and dose volume histograms were compared between conventional single‐field grid‐therapy plan and 3D intensity‐modulated grid‐therapy plan. Results: Compared to conventional single‐field grid‐therapy plan, 3D intensity‐modulated grid‐therapy plan gives higher minimum dose to the target volume for potential improved tumor control probability and lower space‐fractionated doses to OARs for potential reduction of normal tissue complication probability. The drawback of the method is longer radiation treatment time. Conclusions: A method was developed to generate 3D intensity‐modulated grid‐therapy plan. Comparing to single‐field grid‐therapy plan, it gives higher dose to the target volume and lower space‐fractionated dose to OARs for a potential therapeutic advantage.

Inverse‐optimized 3D conformal planning: Minimizing complexity while achieving equivalence with beamlet IMRT in multiple clinical sites

Inverse planned intensity modulated radiation therapy (IMRT) has helped many centers implement highly conformal treatment planning with beamlet-based techniques. The many comparisons between IMRT and 3D conformal (3DCRT) plans, however, have been limited because most 3DCRT plans are forward-planned while IMRT plans utilize inverse planning, meaning both optimization and delivery techniques are different. This work avoids that problem by comparing 3D plans generated with a unique inverse planning method for 3DCRT called inverse-optimized 3D (IO-3D) conformal planning. Since IO-3D and the beamlet IMRT to which it is compared use the same optimization techniques, cost functions, and plan evaluation tools, direct comparisons between IMRT and simple, optimized IO-3D plans are possible. Though IO-3D has some similarity to direct aperture optimization (DAO), since it directly optimizes the apertures used, IO-3D is specifically designed for 3DCRT fields (i.e., 1-2 apertures per beam) rather than starting with IMRT-like modulation and then optimizing aperture shapes. The two algorithms are very different in design, implementation, and use. The goals of this work include using IO-3D to evaluate how close simple but optimized IO-3D plans come to nonconstrained beamlet IMRT, showing that optimization, rather than modulation, may be the most important aspect of IMRT (for some sites). The IO-3D dose calculation and optimization functionality is integrated in the in-house 3D planning/optimization system. New features include random point dose calculation distributions, costlet and cost function capabilities, fast dose volume histogram (DVH) and plan evaluation tools, optimization search strategies designed for IO-3D, and an improved, reimplemented edge/octree calculation algorithm. The IO-3D optimization, in distinction to DAO, is designed to optimize 3D conformal plans (one to two segments per beam) and optimizes MLC segment shapes and weights with various user-controllable search strategies which optimize plans without beamlet or pencil beam approximations. IO-3D allows comparisons of beamlet, multisegment, and conformal plans optimized using the same cost functions, dose points, and plan evaluation metrics, so quantitative comparisons are straightforward. Here, comparisons of IO-3D and beamlet IMRT techniques are presented for breast, brain, liver, and lung plans. IO-3D achieves high quality results comparable to beamlet IMRT, for many situations. Though the IO-3D plans have many fewer degrees of freedom for the optimization, this work finds that IO-3D plans with only one to two segments per beam are dosimetrically equivalent (or nearly so) to the beamlet IMRT plans, for several sites. IO-3D also reduces plan complexity significantly. Here, monitor units per fraction (MU/Fx) for IO-3D plans were 22%-68% less than that for the 1 cm × 1 cm beamlet IMRT plans and 72%-84% than the 0.5 cm × 0.5 cm beamlet IMRT plans. The unique IO-3D algorithm illustrates that inverse planning can achieve high quality 3D conformal plans equivalent (or nearly so) to unconstrained beamlet IMRT plans, for many sites. IO-3D thus provides the potential to optimize flat or few-segment 3DCRT plans, creating less complex optimized plans which are efficient and simple to deliver. The less complex IO-3D plans have operational advantages for scenarios including adaptive replanning, cases with interfraction and intrafraction motion, and pediatric patients.

Quality assurance analysis of participating centres’ protocol compliance to a UK multicentre hypofractionated breast (FAST) trial

The FAST (FASTer radiotherapy for breast radiotherapy) trial is a UK Phase 2 multicentre randomised clinical trial evaluating a five-fraction schedule of whole-breast radiotherapy following local excision of early breast cancer. The purpose of this quality assurance study was to analyse the radiotherapy planning data in order to confirm compliance with the trial protocol. 915 patients were recruited between 2004 and 2007 from 18 centres. The protocol required that all centres should use three-dimensional dose compensations to optimise radiotherapy plans. Planning techniques, maximum dose (D(max)) and dose-volume histograms from treatment plans were evaluated and compared between centres. The homogeneity of plans was tested by creating a cut-off value of 5% for the percentage of breast volume receiving >105% of the prescribed dose. 672 data sets from 15 centres were available. 93% (624/672) of plans were treated using forward-planned multileaf collimator (MLC) segments, 6% with breast compensators and 1% with inverse-planned MLC segments. 94% (635/672) of patients had a D(max)≤107% of the prescribed dose. 11% (74/672) of plans delivered >105% of the prescribed dose to >5% of the breast volume. Reviewing the data in this study, 95% of plans submitted by centres complied with the protocol. With the improved breast radiotherapy standards shown in FAST centres, the following recommendations were suggested for future UK breast radiotherapy trials: (i) the minimum, mean and maximum dose to the whole-breast planning target volume (PTV) should be recorded and assessed; (ii) apart from having a D(max)≤107% of the prescribed dose, ≤5% of PTV should a receive dose >105% of the prescription dose.

Volumetric Modulated Arc Therapy for Stereotactic Body Radiation Therapy of the Prostate

system. Treatment was delivered using 6/10 MV beam with one single 360 o arc containing 20 sectors. The MLC performed a full dynamic sweep in each sector. All the calculations were performed using a Monte Carlo algorithm, including the QA plans. Plans wereverified using the cylindrical ArcCheck QA device. AQA criterion of 3%-3mm was used to determine the plan passing rate. Results: In all cases at least 95% of the PTV received the prescription dose. The resulting coverage yielded a median prostate mean dose of 53.7 Gy [52.2 - 55.2 Gy] and a median PTV mean dose of 53.5 Gy [51.8 - 54.5 Gy]. Due to the proximity of PTV, the most stringent dose constraints were for the bladder wall and anterior rectal wall. The median bladder wall V25Gy was 9.3cc [8.8 - 13.8] and the median anterior rectum wall V50Gy was 1.9cc [0.7 - 3.5]. The urethra maximum dose was limited to less than 52.5 Gy. The median treatment time was 10:06 min [8:24 - 12:35] and the median number of MUs was 3630 [3137 4620] which indicated that most of the MUs were efficiently delivered at a high dose rate during most of the treatment (max dose rate 480 MU/min). The number of MLC segments decreased progressively as experience built up and the median was 185 [166 1108]. QA passing rates were fully satisfactory and above our threshold of 90% with a median of 95.95% [93.4 - 98.6%]. Conclusions: Highly conformal plans were generated and delivered using Monaco and VMAT. Very short treatment times were achieved, thereby improving patient comfort and reducing the risk of intra-fractional motion associated with long treatments times. QA results show excellent agreement between measurements and plans. Patients tolerated treatments well acutely.

SU‐E‐T‐859: A New Approach to Improve IMRT Delivery Efficiency by Intensity Map Discretization

Purpose: To minimize IMRT treatment monitor units and delivery time without compromising dosimetric quality in an in‐house treatment planning system. Methods: In‐house treatment planning system PLanUNC employs a two‐step process to generate a MLC‐IMRT treatment plan. First, the index‐dose gradient minimization dose optimization process produces a continuous intensity map. Second, MLC segmentation process converts the intensity map into MLC segments. This two‐step process is sensitive to MLC segmentation technique and can lead to excessive number of segments and dosimetric degradation. We developed a new method that discretizes the intensity map during optimization. The new method considered each beam in turn: its intensity map was optimized using the current dose distribution; the optimized intensity map was discretized according to its statistical distribution; the dose distribution was updated after discretization. In this process, the peaks and valleys in the continuous intensity map were converted into discrete plateaus, and the optimization of remaining beams mitigated possible degradation of dose conformity introduced by the discretization of previous beams. MLC segmentation was applied to the discretized fluence maps. This new approach is being validated using prostate and head and neck cases. Results: For the prostate, with no compromise to dose conformity, the number of segments was reduced by 40%, and the MUs reduced by ∼10%. For the head and neck, the number of segments was reduced by 20%, and a small dose escalation was observed in adjacent normal tissues; this may be due to complex constraints used in the optimization process and could be accommodated by re‐optimizing the beam weight using fixed beam apertures in the future. Conclusions: Discretization based on statistical distribution of beamlet intensity could effectively reduce IMRT beam complexity and result in shorter treatment time.

SU-E-T-477: Multileaf Collimator Quality Assurance for Volumetric Modulated Arc Therapy

Purpose: To develop a quality assurance procedure for the multileaf collimator(MLC) in volumetric modulated arc therapy (VMAT) beam delivery. Method and Materials: A general pattern for MLC QA is proposed in this study to test the interaction between gantry speed and the MLC traveling speed on different linacs for VMAT dose delivery. An independently mounted film was used to catch the entire arc movement. To utilize similar concept for picket‐fence test, we divided the gantry 360degrees into two counterclockwise fields, 180 to 0 and 0 to 180degrees, and dynamic abutted segments were delivered on every 15 degree gantry rotation. The MLC traveled 3 cm distance in each segment dynamically, from starting opening at 0 and ending at 3cm. Starting point for each 15 degree segment was from 0th, 3rd, 6th, 9th…to 36th cm with MLC bank B moving. But each segment was delivered twice, first in counterclockwise and second time in clockwise directions with MLC travelling in opposite directions with starting point 3rd, 6th, 9th, 12th …to 39th cm with MLC bank A moving. These two fields had the same MLC segments. The accumulated dose on film was analyzed. Results: With perfect speed combination between gantry and MLC, the total dose profile along the leaf‐ motion direction is uniform. All leaves in each bank move together that will be reflected in the width of the strip and conjunction lines, while one leaf is out of range by 1 mm, the profile will show non‐uniform profile only at that leaf position. The conjunction lines of each segment also represent the reproducibility of the combination of gantry and MLC positions. Conclusion: We have investigated a quality assurance procedure for the MLC used in VMAT treatment. It can detect the mismatch between gantry speed and MLC motion to quantify the accuracy of VMAT dose delivery.

SU-E-T-797: Effects of the Number of Intensity Levels in Step&shoot IMRT for Prostate Treatments

Purpose: Step&shoot IMRTTreatment planning requires the conversion of an optimized fluence into deliverable sequences of MLC segments. The objective of this paper is to study the effects of varying the intensity levels (IL's) number in prostate step&shoot IMRT treatments. Methods: Patient data with prostate cancer and IMRT treatment indication was selected. Treatment planning was done using a 6MV 7‐field arrangement (Primus linac, Optifocus MLC, 10mm leaf‐width) and Konrad v2.2 (Siemens) TPS. The treatment plan includes the simultaneous irradiation of the prostate to 82Gy and the seminal vesicles to 64Gy in 41 fractions. Four plans were generated using 5, 7, 10 and 15 IL's and normalized to deliver the same minimum dose to the prostate. Comparisons between plans were done in terms of total segment number, MU, treatment time and DVH for PTVs and OARs (rectum, bladder and femoral heads). Results: The total segment numbers were 44, 60, 84 and 116 from 5 to 15 IL's. The MU's total numbers decreased with the increase of the IL's (527, 522, 518 and 508). Increasing the number of IL's was accompanied by the treatment times raise (11%, 23% and 37% in relation to 5 IL's, 387seg) and resulted in increases of Prostate PTV dose homogeneity and conformity index (CI). The maximum dose (CI) was 87.7Gy (0.957) and 89.7Gy (0.836) for 15 and 5 IL respectively. No difference in minimum dose and CI was found for the seminal vesicle PTV. All plans showed insignificant OAR DVH differences. Conclusions: The increment in the IL's number resulted in an improvement of the Prostate PTV homogeneity and treatment time without any extra sparing to OAR. The benefit of a better uniform dose in the Prostate PTV should be analyzed for each treatment in order to set planning guidelines.

SU‐E‐T‐843: Direct Segment Optimization of Conformal Plans: Achieving Equivalence with Beamlet IMRT

Purpose: Use of inverse planning to improve conformal treatment plan quality has concentrated on optimization of beamlet‐based IMRT plans. The few direct aperture‐based optimization methods are all based on optimizing a beamlet‐based plan, followed by sequencing which identifies the many apertures to be optimized. In contrast, direct segment optimization (DSO), described here, directly optimizes conformal and few‐segment plans using inverse planning. This work compares DSO‐optimized plans versus beamlet IMRT and forward‐planned 3DCRT plans for a series of clinical sites. Methods: Our in‐house beamlet‐based inverse planning system has been enhanced to support DSO with an improved and reimplemented perturbational version of the Edge/Octree calculation algorithm and a multilayer search strategy. DSO optimizes MLC segment shapes, weights and/or jaw positions for multiple field plans (including VMAT). In our inverse planning infrastructure, beamlet, multi‐segment and conformal plans can all be optimized using the same cost functions, dose points, and plan evaluation metrics, making quantitative comparisons between techniques straightforward. Inverse planned DSO 3DCRT plans are compared to forward planned 3DCRT and beamlet IMRT plans for brain, liver, lung, and other sites. Results: For many tested patient geometries, DSO achieves equivalent plan quality to the beamlet IMRT plan optimized with the same cost function, though there are certain situations where IMRT is expected to (and does) perform better. The DSO plans use significantly fewer MU, require less treatment time, are less sensitive to motion, and do not require pre‐treatment IMRT QA checks, so they may be significantly easier to implement in the clinic. Conclusions: Optimization of 3DCRT and multi‐segment plans using DSO results in plan evaluation metrics virtually equivalent to beamlet IMRT plans optimized with the same cost function and beam directions, for many geometries. DSO may potentially lead to important efficiency savings for many clinical protocols.Supported in part by NIH P01‐CA59827

SU‐E‐T‐777: Clinical Implementation of An Automated Segmented MLC Field Approach to Improve Safety of Radiation Treatment Delivery: Eliminating Wedges, External Shields and Physical Junction Shifts

Purpose: As part of routine quality improvement, departmental radiation therapy techniques were reviewed, and technical and operational modifications identified to enhance radiation delivery safety. Our aim was to standardize planning for conventional techniques that exhibit higher potential for delivery errors via automation without compromising plan quality. This was achieved through the use of segmented intensity modulated radiotherapy (IMRT) fields. Methods: A segmented IMRT solution was developed to simulate common fields requiring: a) wedge‐based modulation (eg breast, rectum), b) shielding of anatomical regions (eg midline blocks, island shields) and c) junction shifts during treatment. For cases requiring more intricate beam modulation than what can be provided by a wedge‐like field, a simple inverse‐planned IMRT optimization was devised. Our approach does not depend on anatomical contours, and uses standard 3D conformal beam geometries for delivery and to define planning volumes. All planning strategies were automated using Pinnacle3 scripting to increase planning efficiency and consistency. Results: Compared to standard techniques using wedges, physical blocks, and junction shifts, the developed IMRT protocols resulted in comparable dose distributions. The implementation of scripts in Pinnacle allowed us to improve quality control, as well as planning and delivery efficiency. Delivery safety improvements were also realized. Physical junction shifts can be replaced by intra‐fraction junction feathering (whereby multiple junction shifts are nested within control points, delivered daily) to reduce the dosimetric impact of field edge misalignments and simplify delivery. Regarding physical beam modifiers, the potential for accidents involving dropped devices or errors due to device omission or misalignment is virtually eliminated. Conclusion: Radiation treatment plans that require wedges, physical blocks and junction shifts can be dosimetrically reproduced using simple segmented IMRT fields. Implementation of an MLC‐based delivery offers important benefits in both workflow efficiency and patient safety.

Comparison of IMRT Techniques in the Radiotherapeutic Management of Head and Neck Cancer: Is Tomotherapy “Better” than Step-and-Shoot IMRT?

Currently, the most common method of delivering intensity-modulated radiotherapy (IMRT) is through step-and-shoot, segmental multi-leaf collimator (SMLC)-based techniques. Although rotational delivery methods such as helical tomotherapy (HT) have been proposed as offering advantages in the treatment of head and neck cancer, a lack of clinical data exists on its potential utility. This study compared dosimetric, clinical, and quality-of-life endpoints among 149 patients treated by HT and SMLC-IMRT for head and neck cancer. Dosimetric analysis revealed that the use of HT resulted in significant improvements with respect to mean dose (23.5 versus 27.9 Gy, p = 0.03) and V30 (30.1 versus 43.9 Gy, p = 0.01) to the contralateral (spared) parotid gland. However, the incidence of grade 3+ xerostomia in the late setting was 10% and 8% among patients treated by HT and SMLC-IMRT, respectively (p = 0.46). There were no significant differences in any of the quality of life endpoints among patients treated by HT and SMLC-IMRT (p > 0.05, for all). Acknowledging the biases inherent in this retrospective analysis, we found that the dosimetric advantages observed with HT compared to SMLC-IMRT failed to translate into significant improvements in clinical outcome. Prospective studies are needed to further evaluate how HT may affect the therapeutic ratio.

Rotational IMRT delivery using a digital linear accelerator in very high dose rate ‘burst mode’

Recently, there has been a resurgence of interest in arc-based IMRT, through the use of ‘conventional’ multileaf collimator (MLC) systems that can treat large tumor volumes in a single, or very few pass(es) of the gantry. Here we present a novel ‘burst mode’ modulated arc delivery approach, wherein 2000 monitor units per minute (MU min−1) high dose rate bursts of dose are facilitated by a flattening-filter-free treatment beam on a Siemens Artiste (Oncology Care Systems, Siemens Medical Solutions, Concord, CA, USA) digital linear accelerator in a non-clinical configuration. Burst mode delivery differs from continuous mode delivery, used by Elekta's VMAT (Elekta Ltd, Crawley, UK) and Varian's RapidArc (Varian Medical Systems, Palo Alto, CA, USA) implementations, in that dose is not delivered while MLC leaves are moving. Instead, dose is delivered in bursts over very short arc angles and only after an MLC segment shape has been completely formed and verified by the controller. The new system was confirmed to be capable of delivering a wide array of clinically relevant treatment plans, without machine fault or other delivery anomalies. Dosimetric accuracy of the modulated arc platform, as well as the Prowess (Prowess Inc., Concord, CA, USA) prototype treatment planning version utilized here, was quantified and confirmed, and delivery times were measured as significantly brief, even with large hypofractionated doses. The burst mode modulated arc approach evaluated here appears to represent a capable, accurate and efficient delivery approach.

A minimum cost flow formulation for approximated MLC segmentation

Shape matrix decomposition is a subproblem in radiation therapy planning. A given fluence matrix A has to be written as a sum of shape matrices corresponding to homogeneous fields that can be shaped by a multileaf collimator. We solve the problem of finding an approximation B of A satisfying prescribed upper and lower bounds for each entry. The approximation B is determined such that the corresponding fluence can be realized with a prescribed delivery time using a multileaf collimator with an interleaf collision constraint, and under this condition the distance between A and B is minimized. © 2010 Wiley Periodicals, Inc. NETWORKS, Vol. 57(2), 135–140 2011

Potential of Helical Tomotherapy to Reduce Dose to the Ocular Structures for Patients Treated for Unresectable Sinonasal Cancer

To compare intensity-modulated radiotherapy (IMRT) treatment plans generated by segmental multileaf collimator (SMLC) and helical tomotherapy (HT) techniques for patients with unresectable sinonasal cancer. SMLC-IMRT and HT-IMRT plans for 5 patients with cancer of the paranasal sinuses and nasal cavity were independently optimized using the Eclipse treatment planning system (Varian Medial Systems, Palo Alto, CA) and Tomotherapy HI-ART treatment planning system (Tomotherapy, Inc, Madison, WI). The goal was to deliver a prescribed dose of 70 Gy to at least 95% of the planning target volume (PTV) encompassing gross tumor over 35 treatments whereas respecting constraints to organs at risk, notably the ocular structures. HT-IMRT reduced the maximum doses to the optic chiasm, as well as to the ipsilateral optic nerve and retina (P < 0.05, for all). Maximum doses to these structures were reduced by 10%, 16%, and 14%, respectively, using HT-IMRT compared with SMLC-IMRT. Additionally, the mean dose to the ipsilateral lacrimal gland was reduced by 32% using HT-IMRT. With respect to conformality, HT-IMRT improved dose homogeneity by decreasing "hot-spots." The mean percentage of PTV70 receiving greater than 77 Gy (110% of the prescribed dose) was 4.0% for the HT-IMRT plans compared with 17.8% for the SMLC-IMRT plans (P = 0.001). HT-IMRT has the potential to improve dose homogeneity to PTVs whereas reducing dose to the optic structures. Clinical implications are discussed.

The inter‐ and intrafraction reproducibilities of three common IMRT delivery techniques

Intensity modulated radiation therapy (IMRT) treatment delivery requires higher precision than conventional 3D treatment delivery because of the sensitivity of the resulting dose to small geometric misalignment of the modulated beamlets. The chosen treatment delivery technique will affect the treatment precision in different ways, based on the characteristics of the delivery method. Delivery using a multileaf collimator (MLC) can reduce treatment time and therapist workload, but typically requires a greater number of monitor units and the fields are prone to both systematic and random leaf positioning errors. An alternative to MLC-based fields, patient specific brass compensators, do not suffer from these leaf positioning errors. In our study, we set out to investigate which delivery method will provide the highest levels of dosimetric reproducibility and the minimum amount of interfraction variability. In our study, a seven field IMRT plan for a head and neck treatment was created using the Pinnacle3 treatment planning system and the intensity maps for each field were obtained. The intensity maps of the fields were delivered with a Varian 2100C/D linear accelerator, using solid compensators and sliding window (SW) and step-and-shoot (SS) MLC segments. Three fields were selected from the seven-beam IMRT plan for comparison. Analysis was carried out using the MatriXX ion chamber array, radiochromic film, and Varian dynalog files. Our results show that the error in MLC leaf positioning has no gantry angle dependence. The compensator and SW deliveries showed excellent agreement, even when stricter than usual gamma criteria were applied. However, we noted that under these strict conditions, the SS field had at least ten times more pixels out of range than did the compensators. When using step-and-shoot MLC fields, it was observed that the increase in dose rate or the increase of MU/segment degrades the quality of the plan. Analysis of the dynalog files showed that while each individual field had its own propensity for error, all fields showed the same trend: a greater percentage of time the leaves are out of position as dose rate increases, MUs decrease, or both. The compensator-based field and both types of MLC-based fields have MatriXX results that are within the clinically acceptable tolerance of 3% dose difference and 2 mm DTA. However, when the criteria are tightened, it becomes evident that the compensators have a definite advantage over their comparable MLC-based competitors in terms of interfraction reproducibility. Fewer monitor units are required to deliver each portal, potentially improving patient outcomes and reducing unwanted side effects to both patients and therapists. In centers without MLC, compensators represent a simple and cost effective way to offer patients state of the art treatment. Based on the results of this study, compensator-based IMRT is a reliable, viable option for use in clinics both with and without MLC-equipped linacs.

Arc-modulated radiation therapy based on linear models

This paper reports an inverse arc-modulated radiation therapy planning technique based on linear models. It is implemented with a two-step procedure. First, fluence maps for 36 fixed-gantry beams are generated using a linear model-based intensity-modulated radiation therapy (IMRT) optimization algorithm. The 2D fluence maps are decomposed into 1D fluence profiles according to each leaf pair position. Second, a mixed integer linear model is used to construct the leaf motions of an arc delivery that reproduce the 1D fluence profile previously derived from the static gantry IMRT optimization. The multi-leaf collimator (MLC) sequence takes into account the starting and ending leaf positions in between the neighbouring apertures, such that the MLC segments of the entire treatment plan are deliverable in a continuous arc. Since both steps in the algorithm use linear models, implementation is simple and straightforward. Details of the algorithm are presented, and its conceptual correctness is verified with clinical cases representing prostate and head-and-neck treatments.

Comparison of Intensity-Modulated Radiotherapy Using Helical Tomotherapy and Segmental Multileaf Collimator-based Techniques for Nasopharyngeal Carcinoma: Dosimetric Analysis Incorporating Quality Assurance Guidelines from RTOG 0225

Intensity-modulated radiotherapy (IMRT) treatment plans generated by segmental multileaf collimator (SMLC) and helical tomotherapy (HT) techniques for patients with nasopharyngeal carcinoma were compared using standardized criteria proposed by Radiation Therapy Oncology Group (RTOG) protocol 0225. The goal was to deliver a prescribed dose of 70 Gy to at least 95% of the planning target volume (PTV) encompassing gross tumor, and 59.4 Gy and 50.4 Gy, respectively, to areas at high and low risk for microscopic disease, over 33 treatments while respecting constraints to organs at risk (OAR). HT-IMRT significantly reduced dose to the contralateral parotid gland and improved dose homogeneity to the PTVs. Mean doses to the inner and middle ears were also reduced by 18% and 24%, respectively, on the ipsilateral side, and 24%, and 35%, respectively, on the contralateral side using HT-IMRT compared to SMLC-IMRT. Additionally, HT-IMRT reduced mean doses to brainstem (p = 0.02), larynx (p = 0.03), and oral cavity (p = 0.03). These findings suggest that HT-IMRT may be of improve the therapeutic ratio in the radiotherapeutic treatment of nasopharyngeal carcinoma.

SU-GG-T-137: Similarities between Static and Rotational Intensity Modulated Plans

Purpose: To explore similarities between intensity modulated radiotherapy(IMRT) and intensity modulated arc therapy (IMAT) techniques in the context of the number of multi‐leaf collimator(MLC) segments required to achieve plan objectives, the major factor influencing plan quality. Materials and Methods: Three clinical cases with increasing complexity were studied: (a) prostate only, (b) prostate and seminal vesicles, (c) prostate and pelvic lymph nodes. Initial “gold‐standard” plans with the maximum possible organ‐at‐risk sparing were generated for all three cases. For each case, multiple IMRT and IMAT plans were generated with varying intensity levels (IMRT) and arc control points (IMAT), which translates to varying MLC segments in both modalities. The IMAT/IMRT plans were forced to mimic the organ‐at‐risk sparing and target coverage in the gold‐standard plans, thereby only allowing the target dose inhomogeneity to be variable. A higher target dose inhomogeneity (quantified as D5 — dose to the highest 5% of target volume) implies that the plan is less capable of modulation. Results: For each case, given a similar number of MLC segments, both IMRT and IMAT plans exhibit similar target dose inhomogeneity, indicating there is no difference in their ability to provide dose painting. Target dose inhomogeneity remained approximately constant with decreasing segments, but sharply increased below a specific critical number of segments (70, 100, 110 for cases a, b, c, respectively). Conclusions: For the cases studied, IMAT and IMRT plans are similar in their dependence on the number of MLC segments. A minimum critical number of segments is required to ensure adequate plan quality. Future studies are needed to establish the range of minimum critical number of segments for different treatment sites and target‐organ geometries. Acknowledgment: Research supported in part by a master research grant from Varian Medical Systems.

SU‐GG‐T‐162: Using Tumor Motion Trajectory as a Constraint for 4D‐IMRT Treatment Planning

Purpose: To develop a 4D‐IMRT treatment planning technique by adding a tumor motion trajectory (TMTJ) as a constraint in a commercially available Treatment Planning System (TPS). The goal is to create an IMRT plan with a set of MLC segments catering for the motion of the patient anatomy extracted from 4D‐CT images. Method and Materials: 4D‐CT images were obtained from three lung cancer patients. The 4D protocol divided the breathing cycle into ten phases followed by a Maximum Intensity Projection (MIP) volume reconstruction. ITVs were contoured based on the MIP volumes. GTVs were contoured on all ten CT phases to generate a TMTJ, defined as the position of the centroid of the GTV on all phases. A tube shaped contour (TBC) with about 1 cm diameter along the TMTJ and inside the GTV was generated. TBC was added to the objective function as a constraint with minimum dose‐volume histogram (DVH) set to prescribed dose. IMRT plans were generated by using Pinnacle DMPO with and without the TMTJ constraint. Results: The IMRT plans generated with TMTJ constraints optimized the MLC segment shapes and weights for each beam with the TBC being forced/encouraged to inside of the segmented fields. The percentage of total MUs delivered to the largest area segments in each beam increased from 16.4% of the total MUs to 21.5%. The MU weight increased up to 20% by using TMTJ constrain. The DVHs for PTV and ORs, dose distributions and total MUs for plans with/without TMTJ constraints were very similar. Conclusion: The 4D tumor motion trajectory information can be used as a constraint in the Pinnacle DMPO to obtain tempospatially optimized IMRT plans. The method shows the potential to significantly reducing the dosimetric inaccuracies introduced by the interplay between organ motion and multileaf collimator (MLC) motion for IMRT.