Intensity-modulated Arc Therapy Plans Research Articles

Effect of interfractional shoulder motion on low neck nodal targets for patients treated using volume modulated arc therapy (VMAT)

Purpose: To quantify the dosimetric impact of interfractional shoulder motion on targets in the low neck for head and neck patients treated with volume modulated arc therapy (VMAT). Methods : Three patients with head and neck cancer were selected. All three required treatment to nodal regions in the low neck in addition to the primary tumor site. The patients were immobilized during simulation and treatment with a custom thermoplastic mask covering the head and shoulders. One VMAT plan was created for each patient utilizing two full 360° arcs and a second plan was created consisting of two superior VMAT arcs matched to an inferior static AP supraclavicular field. A CT-on-rails alignment verification was performed weekly during each patient’s treatment course. The weekly CT images were registered to the simulation CT and the target contours were deformed and applied to the weekly CT. The two VMAT plans were copied to the weekly CT datasets and recalculated to obtain the dose to the deformed low neck contours. Results: The average observed shoulder position shift in any single dimension relative to simulation was 2.5 mm. The maximum shoulder shift observed in a single dimension was 25.7 mm. Low neck target mean doses, normalized to simulation and averaged across all weekly recalculations were 0.996, 0.991, and 1.033 (Full VMAT plan) and 0.986, 0.995, and 0.990 (Half-Beam VMAT plan) for the three patients, respectively. The maximum observed deviation in target mean dose for any individual weekly recalculation was 6.5%, occurring with the Full VMAT plan for Patient 3. Conclusion : Interfractional variation in dose to low neck nodal regions was quantified for three head and neck patients treated with VMAT. Mean dose was 3.3% higher than planned for one patient using a Full VMAT plan. A Half-Beam technique is likely a safer choice when treating the supraclavicular region with VMAT. ------------------------------------------- Cite this article as: Casey K, Wang P, Tung S, Rosenthal D. Effect of interfractional shoulder motion on low neck nodal targets for patients treated using volume modulated arc therapy (VMAT). Int J Cancer Ther Oncol 2014; 2 (2):020218. DOI : 10.14319/ijcto.0202.18

Dosimetric comparison of 3D conformal, IMRT, and V-MAT techniques for accelerated partial-breast irradiation (APBI)

The purpose is to dosimetrically compare the following 3 delivery techniques: 3-dimensional conformal radiation therapy (3D-CRT), intensity-modulated arc therapy (IMRT), and volumetric-modulated arc therapy (V-MAT) in the treatment of accelerated partial-breast irradiation (APBI). Overall, 16 patients with T1/2N0 breast cancer were treated with 3D-CRT (multiple, noncoplanar photon fields) on the RTOG 0413 partial-breast trial. These cases were subsequently replanned using static gantry IMRT and V-MAT technology to understand dosimetric differences among these 3 techniques. Several dosimetric parameters were used in plan quality evaluation, including dose conformity index (CI) and dose-volume histogram analysis of normal tissue coverage. Quality assurance studies including gamma analysis were performed to compare the measured and calculated dose distributions. The IMRT and V-MAT plans gave more conformal target dose distributions than the 3D-CRT plans (p < 0.05 in CI). The volume of ipsilateral breast receiving 5 and 10Gy was significantly less using the V-MAT technique than with either 3D-CRT or IMRT (p < 0.05). The maximum lung dose and the ipsilateral lung volume receiving 10 (V10) or 20Gy (V20) were significantly less with both V-MAT and IMRT (p < 0.05). The IMRT technique was superior to 3D-CRT and V-MAT of low dose distributions in ipsilateral lung (p < 0.05 in V5 and D5). The total mean monitor units (MUs) for V-MAT (621.0 ± 111.9) were 12.2% less than those for 3D-CRT (707.3 ± 130.9) and 46.5% less than those for IMRT (1161.4 ± 315.6) (p < 0.05). The average machine delivery time was 1.5 ± 0.2 minutes for the V-MAT plans, 7.0 ± 1.6 minutes for the 3D-CRT plans, and 11.5 ± 1.9 minutes for the IMRT plans, demonstrating much less delivery time for V-MAT. Based on this preliminary study, V-MAT and IMRT techniques offer improved dose conformity as compared with 3D-CRT techniques without increasing dose to the ipsilateral lung. In terms of MU and delivery time, V-MAT is significantly more efficient for APBI than for conventional 3D-CRT and static-beam IMRT.

Giant Cell Tumor of Lumbar Spine Treated with RapidArc Intensity Modulated Arc Therapy: Case Report and Review of Literature

Background: Giant cell tumors of the lumbar spine are rare and complete resection without major functional comprise is challenging despite advancements in spine surgery techniques. Radiation therapy has been an option in such cases; however there are high concerns for associated high small bowel toxicity and lack of dose escalation to achieve local control. With advent of intensity modulated radiation therapy (IMRT) it is now possible to give high radiation dose to tumor with minimal toxicity. Herein we present a rare case of giant cell tumor of fourth lumbar (L4) vertebra treated with RapidArc intensity modulated arc therapy (IMAT) with relevant review of literature. Methods: A 30-year-old female had low back pain for 6 months. She underwent surgical decompression at another hospital as having a L4-5 protruded intervertebral disc 2 months previously, but her back pain progressed with weakness of both legs with restricted movement. Radiological and pathological work-up confirmed the diagnosis of giant cell tumor of L4 vertebra. She refused further surgery and was referred to us for radiotherapy. Treatment plans for prescribed radiation dose of 59.4 Gy in 30 fractions were made by 3DCRT and RapidArc IMAT and comparison was made. Student’s unpaired t test was used to determine the significance of the difference between two plans in terms of dose to the tumor and small bowel. A p value of 0.05 was considered statistically significant. Related literature was searched. Results: In RapidArc IMAT and 3DCRT plans mean doses to planning treatment volume (PTV) were 61.24 Gy (55.98 - 66.23) and 60.71 Gy (49.87 - 63.74) respectively (p 0.04) and mean doses to small bowel were found lesser in RapidArc plan [14.78 Gy (range: 0.39 - 53.15)] as compared to 3DCRT plan. Patient was started on RapidArc IMRT and she completed the course without any major sequelae. Conclusion: Lumbar spine giant cell tumors are rare and complete resection is often not possible. RapidArc IMAT is a feasible option for such patients to deliver high dose radiation to achieve good local control with marked symptom relief and without severe toxicity.

The impact of leaf width and plan complexity on DMLC tracking of prostate intensity modulated arc therapy

Intensity modulated arc therapy (IMAT) is commonly used to treat prostate cancer. The purpose of this study was to evaluate the impact of leaf width and plan complexity on dynamic multileaf collimator (DMLC) tracking for prostate motion management during IMAT treatments. Prostate IMAT plans were delivered with either a high-definition MLC (HDMLC) or a Millennium MLC (M-MLC) (0.25 and 0.50 cm central leaf width, respectively), with and without DMLC tracking, to a dosimetric phantom that reproduced four prostate motion traces. The plan complexity was varied by applying leaf position constraints during plan optimization. A subset of the M-MLC plans was converted for delivery with the HDMLC, isolating the effect of the different leaf widths. The gamma index was used for evaluation. Tracking errors caused by target localization, leaf fitting, and leaf adjustment were analyzed. The gamma pass rate was significantly improved with DMLC tracking compared to no tracking (p < 0.001). With DMLC tracking, the average gamma index pass rate was 98.6% (range 94.8%-100%) with the HDMLC and 98.1% (range 95.4%-99.7%) with the M-MLC, using 3%, 3 mm criteria and the planned dose as reference. The corresponding pass rates without tracking were 87.6% (range 76.2%-94.7%) and 91.1% (range 81.4%-97.6%), respectively. Decreased plan complexity improved the pass rate when static target measurements were used as reference, but not with the planned dose as reference. The main cause of tracking errors was leaf fitting errors, which were decreased by 42% by halving the leaf width. DMLC tracking successfully compensated for the prostate motion. The finer leaf width of the HDMLC improved the tracking accuracy compared to the M-MLC. The tracking improvement with limited plan complexity was small and not discernible when using the planned dose as reference.

Comparison study of intensity modulated arc therapy using single or multiple arcs to intensity modulated radiation therapy for high-risk prostate cancer

PurposeIntensity modulated arc therapy (IMAT) is a form of intensity modulated radiation therapy (IMRT) that delivers dose in single or multiple arcs. We compared IMRT plans versus single-arc field (1ARC) and multi-arc fields (3ARC) IMAT plans in high-risk prostate cancer.Materials and MethodsSixteen patients were studied. Prostate (PTVP), right pelvic (PTVRtLN) and left pelvic lymph nodes (PTVLtLN), and organs at risk were contoured. PTVP, PTVRtLN, and PTVLtLN received 50.40 Gy followed by a boost to PTVB of 28.80 Gy. Three plans were per patient generated: IMRT, 1ARC, and 3ARC. We recorded the dose to the PTV, the mean dose (DMEAN) to the organs at risk, and volume covered by the 50% isodose. Efficiency was evaluated by monitor units (MU) and beam on time (BOT). Conformity index (CI), Paddick gradient index, and homogeneity index (HI) were also calculated.ResultsAverage Radiation Therapy Oncology Group CI was 1.17, 1.20, and 1.15 for IMRT, 1ARC, and 3ARC, respectively. The plans' HI were within 1% of each other. The DMEAN of bladder was within 2% of each other. The rectum DMEAN in IMRT plans was 10% lower dose than the arc plans (p < 0.0001). The GI of the 3ARC was superior to IMRT by 27.4% (p = 0.006). The average MU was highest in the IMRT plans (1686) versus 1ARC (575) versus 3ARC (1079). The average BOT was 6 minutes for IMRT compared to 1.3 and 2.9 for 1ARC and 3ARC IMAT (p < 0.05).ConclusionFor high-risk prostate cancer, IMAT may offer a favorable dose gradient profile, conformity, MU and BOT compared to IMRT.

Radiation Therapy for Nasopharyngeal Carcinoma Using Simultaneously Integrated Boost (SIB) Protocol: A Comparison Planning Study between Intensity Modulated Arc Radiotherapy vs. Intensity Modulated Radiotherapy

The aim of this paper is to compare the dosimetric difference between intensity-modulated arc therapy (IMAT) and conventional intensity-modulated radiation therapy (IMRT) for radiotherapy of nasopharyngeal carcinoma (NPC) using simultaneously integrated boost (SIB) protocol. Ten patients with nasopharyngeal carcinoma underwent SIB protocol were retrospectively studied. The plan target volume (PTV) of NPC contained nasopharynx gross target volume and the positive neck lymph nodes, PTV1 contained the high-risk sites of microscopic extension and the whole nasopharynx and PTV2 contained the low-risk sites. The prescription dose of PTV was 66 Gy/30 fractions, and for PTV1 60 Gy/30 fractions and for PTV2 54 Gy/30 fractions. IMAT (two 358° arcs) and IMRT (7 fields) plans were designed for each patients using SIB strategies. The monitor unit (MU), treatment time (T) and dosimetric difference between IMRT and IMAT were compared. IMAT can achieve better conformal index (CI) than IMRT (P < 0.05) for all PTVs, while no significant difference were found in homogeneity index (HI) (P > 0.05). There's no significant difference found in radiation dose of brain stem, lenses and parotids, while the maximum dose of spinal cord of IMAT was higher than IMRT (P < 0.05). The monitor unit of IMRT (1308 ± 213) was more than IMAT (606 ± 96) (P < 0.05), while the treatment time of IMRT (540 ± 160S) was more than IMAT (160 ± 10S). This study shows that IMAT using SIB strategies for NPC radiotherapy can achieve similar target coverage with better conformity with less MU and delivery time comparing to IMRT.

SU-E-T-648: Comparison of VMAT Vs Arc Treatment Plans for Patients Undergoing SBRT of Early-Stage Non-Small Cell Lung Cancer (NSCLC).

To evaluate the dosimetric implications of using VMAT (Volume Modulated Arc Therapy) treatment planning techniques compared to traditional Arc therapy methods for patients undergoing SBRT (Stereotactic Body Radiation Therapy) for early-stage Non-Small Cell Lung Cancer (NSCLC). Ten NSCLC cancer patients are planned with both VMAT and Arc techniques. The SBRT treatment plans comparison was quantified by several Dose-Volume Histogram (DVH) indicators including mean, maximum and minimum doses for GTV, ITV, PTV, OAR (Organs At Risk), and V95 (volume receiving at least 95% of the prescribed dose) for PTV. On average VMAT plans require for treatment delivery 16.6 ± 20.2 % more monitor units (MU) than the traditional Arc plans. The average PTV minimum, maximum and mean doses as a percentage of prescribed dose are 94.5 ± 3.9 %, 114.1 ± 3.3 % and 106.6 ± 1.6 % for VMAT vs 91.6 ± 4.4 %, 119.5 ± 5.3 % and 109.5 ± 2.5 % for the Arc technique. The V95 PTV coverage for VMAT plans range from 99.4 % to 100 % with a mean of 99.7 %, compared with a range of 96.8 % to 100 % with a mean of 99 % for the Arc plans. The maximum dose received by the lungs, spinal cord and chest wall show on average significant increases for Arc plans as opposed to VMAT plans (5.7 ± 6 % increase for lungs, 4.4 ± 9.2 % for cord and 2.4 ± 6.3 % for chest wall). The average mean doses and minimum doses for the OAR are similar for both techniques. The comparison of VMAT vs Arc plans for SBRT of NSCLC patients is subject to many variables, including GTV and PTV volume sizes, shape and their proximity relative to the OAR.

Total-Variation Regularization Based Inverse Planning for Intensity Modulated Arc Therapy

Intensity modulated arc therapy (IMAT) delivers conformal dose distributions through continuous gantry rotation with constant or variable speed while modulating the field aperture shape and weight. The enlarged angular space and machine delivery constraints make inverse planning of IMAT more intractable as compared to its counterpart of fixed gantry IMRT. Currently, IMAT inverse planning is being done using two extreme methods: the first one computes in beamlet domain with a subsequent arc leaf sequencing, and the second proceeds in machine parameter domain with entire emphasis placed on a pre-determined delivery method without exploring potentially better alternative delivery schemes. Towards truly optimizing the IMAT treatment on a patient specific basis, in this work we propose a total-variation based inverse planning framework for IMAT, which takes advantage of the useful features of the above two existing approaches while avoiding their shortcomings. A quadratic optimization algorithm has been implemented to demonstrate the performance and advantage of the proposed approach. Applications of the technique to a prostate case and a head and neck case indicate that the algorithm is capable of generating IMAT plans with patient specific numbers of arcs efficiently. Superior dose distributions and delivery time are achieved with a maximum number of apertures of three for each field. As compared to conventional beamlet-based algorithms, our method regularizes the field modulation complexity during optimization, and permits us to obtain the best possible plan with a pre-set modulation complexity of fluences. As illustrated in both prostate and head-and-neck case studies, the proposed method produces more favorable dose distributions than the segment-based algorithms, by optimally accommodating the clinical need of intensity modulation levels for each individual field. On a more fundamental level, our formulation preserves the convexity of optimization and makes the search of the global optimal solution possible with a deterministic method.

Image-Based Dynamic Multileaf Collimator Tracking of Moving Targets During Intensity-Modulated Arc Therapy

Intensity-modulated arc therapy (IMAT) enables efficient and highly conformal dose delivery. However, intrafraction motion may compromise the delivered target dose distribution. Dynamic multileaf collimator (DMLC) tracking can potentially mitigate the impact of target motion on the dose. The purpose of this study was to use a single kV imager for DMLC tracking during IMAT and to investigate the ability of this tracking to maintain the dose distribution. A motion phantom carrying a two-dimensional (2D) ion chamber array and buildup material with an embedded gold marker reproduced eight representative tumor trajectories (four lung tumors, four prostate). For each trajectory, a low and high IMAT plan were delivered with and without DMLC tracking. The three-dimensional (3D) real-time target position signal for tracking was provided by fluoroscopic kV images acquired immediately before and during treatment. For each image, the 3D position of the embedded marker was estimated from the imaged 2D position by a probability-based method. The MLC leaves were continuously refitted to the estimated 3D position. For lung, prediction was used to compensate for the tracking latency. The delivered 2D dose distributions were measured with the ion chamber array and compared with a reference dose distribution delivered without target motion using a 3%/3 mm γ-test. For lung tumor motion, tracking reduced the mean γ-failure rate from 38% to 0.7% for low-modulation IMAT plans and from 44% to 2.8% for high-modulation plans. For prostate, the γ-failure rate reduction was from 19% to 0% (low modulation) and from 20% to 2.7% (high modulation). The dominant contributor to the residual γ-failures during tracking was target localization errors for most lung cases and leaf fitting errors for most prostate cases. Image-based tracking for IMAT was demonstrated for the first time. The tracking greatly improved the dose distributions to moving targets.

Impact of leaf motion constraints on IMAT plan quality, deliver accuracy, and efficiency

Intensity modulated arc therapy (IMAT) is a radiation therapy delivery technique that combines the efficiency of arc based delivery with the dose painting capabilities of intensity modulated radiation therapy (IMRT). A key challenge in developing robust inverse planning solutions for IMAT is the need to account for the connectivity of the beam shapes as the gantry rotates from one beam angle to the next. To overcome this challenge, inverse planning solutions typically impose a leaf motion constraint that defines the maximum distance a multileaf collimator (MLC) leaf can travel between adjacent control points. The leaf motion constraint ensures the deliverability of the optimized plan, but it also impacts the plan quality, the delivery accuracy, and the delivery efficiency. In this work, the authors have studied leaf motion constraints in detail and have developed recommendations for optimizing the balance between plan quality and delivery efficiency. Two steps were used to generate optimized IMAT treatment plans. The first was the direct machine parameter optimization (DMPO) inverse planning module in the Pinnacle(3) planning system. Then, a home-grown arc sequencer was applied to convert the optimized intensity maps into deliverable IMAT arcs. IMAT leaf motion constraints were imposed using limits of between 1 and 30 mm∕deg. Dose distributions were calculated using the convolution∕superposition algorithm in the Pinnacle(3) planning system. The IMAT plan dose calculation accuracy was examined using a finer sampling calculation and the quality assurance verification. All plans were delivered on an Elekta Synergy with an 80-leaf MLC and were verified using an IBA MatriXX 2D ion chamber array inserted in a MultiCube solid water phantom. The use of a more restrictive leaf motion constraint (less than 1-2 mm∕deg) results in inferior plan quality. A less restrictive leaf motion constraint (greater than 5 mm∕deg) results in improved plan quality but can lead to less accurate dose distribution as evidenced by increasing discrepancies between the planned and the delivered doses. For example, the results from our patient-specific quality assurance measurements demonstrated that the average gamma analysis passing rate decreased from 98% to 80% when the allowable leaf motion increased from 3 to 20 mm∕deg. Larger leaf motion constraints also led to longer treatment delivery times (2 to 4 folds) due to the additional MLC leaf motion. Leaf motion constraints significantly impact IMAT plans in terms of plan quality, delivery accuracy, and delivery efficiency with the impact magnified for more complex cases. Our studies indicate that a leaf motion constraint of 2 to 3 mm∕deg of gantry rotation can provide an optimal balance between plan quality, delivery accuracy, and efficiency.

Sequential Intensity Modulated Arc Therapy (IMAT) Optimization Planning For Improving Dose Homogeneity In Heterogeneous Media

To describe an intensity modulated arc therapy (IMAT) planning technique using sequential optimizations in heterogeneous media for evaluation against conventional IMAT planning. IMAT planning systems often use different algorithms for fluence optimization than for dose calculation which, in heterogeneous media, can lead to a very different dose distribution than what was initially intended. During optimization, dose objectives may appear more achievable than what is reported after final dose calculation. This effect can be reduced by creating a conventional IMAT plan with partial prescription dose as a base plan and sequentially re-optimizing with the initial objectives for the remaining prescription dose. 10 patients were selected with peripheral and central lung tumors undergoing IMAT planning. Conventional IMAT plans were created as baseline and compared with sequentially optimized plans for dose volume histogram (DVH), isodose distribution, and dose conformity index. Using sequential optimizations, the max dose to the internal target volume (ITV) was reduced by 10 - 20% depending on target geometry and location, while maintaining no significant change in mean dose. Dose conformity was improved significantly with magnitudes dependent upon selected index. Sequentially optimized IMAT plans provide a more accurate starting point for a second fluence optimization when planning in low density media. Sequential optimizations can compensate for lateral electronic non-equilibrium with higher success; further study may show this method to be valid for conventional intensity modulated static angle planning as well.

Comparison Between IMAT And IMRT Planning For Stereotactic Body Radiation Therapy (SBRT) Of Hepatocellular Carcinoma (HCC)

To assess the potential of intensity-modulated arc therapy (IMAT) for SBRT planning and delivery for patients with hepatocellular carcinoma (HCC). We compared IMAT plans with our standard static gantry intensity-modulated radiation therapy (IMRT) planning technique.

Two-step intensity modulated arc therapy (2-step IMAT) with segment weight and width optimization

Background2-step intensity modulated arc therapy (IMAT) is a simplified IMAT technique which delivers the treatment over typically two continuous gantry rotations. The aim of this work was to implement the technique into a computerized treatment planning system and to develop an approach to optimize the segment weights and widths.Methods2-step IMAT was implemented into the Prism treatment planning system. A graphical user interface was developed to generate the plan segments automatically based on the anatomy in the beam's-eye-view. The segment weights and widths of 2-step IMAT plans were subsequently determined in Matlab using a dose-volume based optimization process. The implementation was tested on a geometric phantom with a horseshoe shaped target volume and then applied to a clinical paraspinal tumour case.ResultsThe phantom study verified the correctness of the implementation and showed a considerable improvement over a non-modulated arc. Further improvements in the target dose uniformity after the optimization of 2-step IMAT plans were observed for both the phantom and clinical cases. For the clinical case, optimizing the segment weights and widths reduced the maximum dose from 114% of the prescribed dose to 107% and increased the minimum dose from 87% to 97%. This resulted in an improvement in the homogeneity index of the target dose for the clinical case from 1.31 to 1.11. Additionally, the high dose volume V105 was reduced from 57% to 7% while the maximum dose in the organ-at-risk was decreased by 2%.ConclusionsThe intuitive and automatic planning process implemented in this study increases the prospect of the practical use of 2-step IMAT. This work has shown that 2-step IMAT is a viable technique able to achieve highly conformal plans for concave target volumes with the optimization of the segment weights and widths. Future work will include planning comparisons of the 2-step IMAT implementation with fixed gantry intensity modulated radiotherapy (IMRT) and commercial IMAT implementations.

SU-E-T-184: Usefulness of Triple Channel Correction for Gafchromic EBT2 Film on Patient Specific Quality Assurance of IMRT and IMAT

Purpose: Triple channel correction acquisition (TCCA) is a model‐based method for obtaining the characteristic curve of Gafchromic EBT2 film (EBT2). In this study the TCCA method was applied to EBT2 film dosimetry of intensity modulated radiation therapy(IMRT) and intensity modulated arc therapy (IMAT). Method and Materials:EBT2 films were calibrated by a field‐by‐field method in 13 steps. All films were scanned by using flat‐bed scanners at one day after the irradiation. Seven characteristic curves obtained by the TCCA method were compared with those of a single channel acquisition (SCA) method. For clinical applications, the planar dose distributions of IMRT and IMAT plans were compared with dose distributions measured with EBT2 films. The film analyses were performed by applying both TCCA and SCA. We chose two IMRT plans and two Varian IMAT (RapidArc) plans, which were created by the BrainLAB i‐plan and Eclipse Varian treatment planning systems for IMRT and RapidArc, respectively. Results: The constancy of the characteristic curves obtained by TCCA was better than that of SCA. The most significant difference between the characteristic curves of TCCA and SCA was observed in a low dose range between 25 cGy and 100 cGy. The results of clinical cases demonstrated that the dose difference between the measured and planned doses in high dose gradient regions was smaller with TCCA than that with SCA. Conclusions: We showed that characteristic curves obtained by the TCCA method have a good constancy compared with those of SCA. The potential advantage of TCCA was seen in high dose gradient regions in clinically relevant IMRT and IMAT cases. Hence, EBT2 film dosimetry with the TCCA method can be used for high precision patient specific QA.

SU-E-T-556: Interplay Effect Between Dynamic MLC and Moving Target for Lung SBRT with IMAT Technique Delivered by Flattening Filter Free Beam of True Beam Machine

Purpose: To investigate the dosimetric impact of the interplay effect between dynamic MLC and moving target on lungSBRT with intensity‐ modulated arc therapy (IMAT) technique delivered by flattening filter free (FFF) beam of True‐Beam machine. Methods: 6 lungcancer patients with 0.5–1.0 cm tumor motions were investigated in this study. All patients underwent 4DCT scan using audio coaching. For each patient, a 2‐arc IMAT plan was retrospectively generated using Varian RapidArc planning system. Based on the total MU, dose rate and respiratory cycle, the planned MLC control points of IMAT plans were assigned into different respiratory phases. Afterward, 10 new IMAT plans related to different respiratory phases were generated and imported back into Eclipse planning system to calculate the radiationdose on the CTimages of different respiratory phases. In‐house 4D dose calculation program with deformable registration capacity was used to calculate the cumulative doses from all respiratory phases. Following parameters were used to evaluate the dosimetric impacts: dose error (DE), i.e., the difference between the 3D dose of original IMAT plan and the calculated 4‐D dose, and the percentage of volume receiving 100% of the prescribed dose (V100).Results: For all patients, the average maximum DE of PTV and CTV are 32.6% and 4.8%. The DE is larger than 5% for 22% of PTV and 0.3% of CTV volume, and the DE is less than 3% for over 88% of the CTV volume. Compared to the original IMAT plans, the V100 of PTV are lower by 15%. However, the changes on V100 of CTV are less than 1%. Conclusions: For lung IMAT treatment delivered by FFF beam of True‐Beam machine, the interplay effect between dynamic MLC and moving target could change the absolute doses within the target, but its impact on CTV dose coverage is insignificant.

SU‐E‐T‐791: Applications of RapidArc in Cervical Esophageal Cancer Radiation Therapy: A Comparison with Fixed‐Field IMRT in Dosimetry and Implementation

Purpose: Comparing fixed‐field intensity‐modulated radiotherapy (f‐IMRT) with intensity‐modulated arc therapy (IMAT) treatment plans in dosimetry and practical application for cervical esophageal carcinoma.Methods: For ten cervical esophageal carcinoma cases, f‐IMRT plan (7 fixed‐fields) and two IMAT plans, namely RA (co‐planar 360° arcs) and RAx (co‐planar 360° arcs without sectors from 80° to 110°, and 250° to 280°). DVHs were adopted for the statistics of above parameters, as well as conformal index (CI), homogeneity index (HI), dose‐volumetric parameters of normal tissues, total accelerator output MUs, and total treatment time.Results: There were differences between RAx and f‐IMRT as well as RA in PTV parameters such as HI, V95% and V110%, but not in CI. RAx reduced lung V5 from (50.9±9.8)% in f‐IMRT and (51.4±10.8)% in RA to (49.3±10.4)% in RAx (P&lt;0.05). However, lung V30, V40, V50 and MLD increased in RAx. There was no difference in the mean heart dose in three plans. Total MU was reduced from 1174.8±144.6 in f‐IMRT to 803.8±122.2 in RA and 736.2±186.9 in RAx (P&lt;0.05) Conclusions: Compared with fIMRT, IMAT reduced low dose volumes of lung and total MU on the basis of meeting clinical requirements.

Intensity-Modulated Arc Therapy for Pediatric Posterior Fossa Tumors

To compare intensity-modulated arc therapy (IMAT) to noncoplanar intensity-modulated radiation therapy (IMRT) in the treatment of pediatric posterior fossa tumors. Nine pediatric patients with posterior fossa tumors, mean age 9 years (range, 6-15 years), treated using IMRT were chosen for this comparative planning study because of their tumor location. Each patient's treatment was replanned to receive 54 Gy to the planning target volume (PTV) using five different methods: eight-field noncoplanar IMRT, single coplanar IMAT, double coplanar IMAT, single noncoplanar IMAT, and double noncoplanar IMAT. For each method, the dose to 95% of the PTV was held constant, and the doses to surrounding critical structures were minimized. The different plans were compared based on conformity, total linear accelerator dose monitor units, and dose to surrounding normal tissues, including the entire body, whole brain, temporal lobes, brainstem, and cochleae. The doses to the target and critical structures for the various IMAT methods were not statistically different in comparison with the noncoplanar IMRT plan, with the following exceptions: the cochlear doses were higher and whole brain dose was lower for coplanar IMAT plans; the cochleae and temporal lobe doses were lower and conformity increased for noncoplanar IMAT plans. The advantage of the noncoplanar IMAT plan was enhanced by doubling the treatment arc. Noncoplanar IMAT results in superior treatment plans when compared to noncoplanar IMRT for the treatment of posterior fossa tumors. IMAT should be considered alongside IMRT when treatment of this site is indicated.

Optimising Stereotactic Body Radiotherapy for Non-small Cell Lung Cancer with Volumetric Intensity-modulated Arc Therapy—A Planning Study

Aims The potential advantages of stereotactic body radiotherapy (SBRT) for early stage non-small cell lung cancer (NSCLC) over conventional fractionated radiotherapy include a higher biological effective dose, a reduction in accelerated repopulation, greater patient convenience and reduced demand on radiotherapy resources. Before introducing SBRT in our department, a review of planning and delivery was undertaken, starting with an assessment of optimum beam number and arrangement. Materials and methods Radiotherapy planning computed tomography scans for five patients previously treated for T1 peripheral NSCLC were selected. In each the contoured tumour had planning target volume (PTV) margins of 1 cm in all directions. Forward-planned three-field coplanar and non-coplanar plans and a seven-field coplanar plan were produced and optimised. In-house inverse-planning software (AutoBeam) was used to generate three-, five-, seven- and nine-field coplanar and non-coplanar plans and two volumetric intensity-modulated arc therapy (VMAT) plans. The resulting V 20, V 11, PTV 90, PTV 95 and mean lung dose were compared. Results Analysis of variance showed non-coplanar plans to have lower V 11 and higher PTV 90 and PTV 95 than coplanar plans. VMAT showed equivalent V 20 and target coverage when compared with the best non-coplanar plans, but with a faster delivery time (2 min 8 s versus 12 min 40 s). Conclusions Inverse-planned five-field non-coplanar plans and VMAT improve target coverage while minimising the higher dose to normal lung tissue for SBRT of NSCLC compared with coplanar beam arrangements. VMAT is preferable because of significantly shorter treatment delivery times.

Applications of IMAT in cervical esophageal cancer radiotherapy: a comparison with fixed‐field IMRT in dosimetry and implementation

This study aimed to compare fixed‐field, intensity‐modulated radiotherapy (f‐IMRT) with intensity‐modulated arc therapy (IMAT) treatment plans in dosimetry and practical application for cervical esophageal carcinoma. For ten cervical esophageal carcinoma cases, f‐IMRT plan (seven fixed‐fields) and two IMAT plans, namely RA (coplanar 360° arcs) and RAx (coplanar 360° arcs without sectors from 80° to 110°, and 250° to 280°), were generated. DVHs were adopted for the statistics of above parameters, as well as conformal index (CI), homogeneity index (HI), dose‐volumetric parameters of normal tissues, total accelerator output MUs and total treatment time. There were differences between RAx and f‐IMRT, as well as RA in PTV parameters such as HI, V95% and V110%, but not in CI. RAx reduced lung V5 from (50.9%± 9.8% in f‐IMRT and (51.4%± 10.8% in RA to (49.3%± 10.4% in RAx (p<0.05). However, lung V30,V40,V50 and MLD increased in RAx. There was no difference in the mean heart dose in three plans. Total MU was reduced from 1174.8±144.6 in f‐IMRT to 803.8±122.2 in RA and 736.2±186.9 in RAx (p<0.05). Compared with f‐IMRT, IMAT reduced low dose volumes of lung and total MU on the basis of meeting clinical requirements.PACS numbers: 87.55.D, 87.55.dk, 87.55.ne

Helical Tomotherapy Versus Single-Arc Intensity-Modulated Arc Therapy: A Collaborative Dosimetric Comparison Between Two Institutions

Both helical tomotherapy (HT) and single-arc intensity-modulated arc therapy (IMAT) deliver radiation using rotational beams with multileaf collimators. We report a dual-institution study comparing dosimetric aspects of these two modalities. Eight patients each were selected from the University of Maryland (UMM) and the University of Wisconsin Cancer Center Riverview (UWR), for a total of 16 cases. Four cancer sites including brain, head and neck (HN), lung, and prostate were selected. Single-arc IMAT plans were generated at UMM using Varian RapidArc (RA), and HT plans were generated at UWR using Hi-Art II TomoTherapy. All 16 cases were planned based on the identical anatomic contours, prescriptions, and planning objectives. All plans were swapped for analysis at the same time after final approval. Dose indices for targets and critical organs were compared based on dose-volume histograms, the beam-on time, monitor units, and estimated leakage dose. After the disclosure of comparison results, replanning was done for both techniques to minimize diversity in optimization focus from different operators. For the 16 cases compared, the average beam-on time was 1.4 minutes for RA and 4.8 minutes for HT plans. HT provided better target dose homogeneity (7.6% for RA and 4.2% for HT) with a lower maximum dose (110% for RA and 105% for HT). Dose conformation numbers were comparable, with RA being superior to HT (0.67 vs. 0.60). The doses to normal tissues using these two techniques were comparable, with HT showing lower doses for more critical structures. After planning comparison results were exchanged, both techniques demonstrated improvements in dose distributions or treatment delivery times. Both techniques created highly conformal plans that met or exceeded the planning goals. The delivery time and total monitor units were lower in RA than in HT plans, whereas HT provided higher target dose uniformity.