Fluence Smoothing Research Articles

Evaluation of the Minimum Segment Width and Fluence Smoothing Tools for Intensity-modulated Techniques in Monaco Treatment Planning System.

This study aims to minimize monitor units (MUs) of intensity-modulated treatments in the Monaco treatment planning system while preserving plan quality by optimizing the "Minimum Segment Width" (MSW) and "Fluence Smoothing" parameters. We retrospectively analyzed 30 prostate, 30 gynecological, 15 breast cancer, 10 head and neck tumor, 11 radiosurgery, and 10 hypo-fractionated plans. Original prostate plans employed "Fluence Smoothing" = Off and were reoptimized with Low, Medium, and High settings. The remaining pathologies initially used MSW = 0.5 cm and were reoptimized with MSW = 1.0 cm. Plan quality, including total MU, delivery time, and dosimetric constraints, was statistically analyzed with a paired t-test. Prostate plans exhibited the highest MU variation when changing "Fluence Smoothing" from Off to High (average ΔMU = -5.1%; P < 0.001). However, a High setting may increase overall MU when MSW = 0.5 cm. Gynecological plans changed substantially when MSW increased from 0.5 cm to 1.0 cm (average ΔMU = -29%; P < 0.001). Organs at risk sparing and planning target volumes remained within 1.2% differences. Replanning other pathologies with MSW = 1.0 cm affected breast and head and neck tumor plans (average ΔMU = -168.38, average Δt = -11.74 s, and average ΔMU = -256.56, average Δt = -15.05 s, respectively; all with P < 0.004). Radiosurgery and hypofractioned highly modulated plans did not yield statistically significant results. In breast, pelvis, head and neck, and prostate plans, starting with MSW = 1.0 cm optimally reduces MU and treatment time without compromising plan quality. MSW has a greater impact on MU than the "Fluence Smoothing" parameter. Plans with high modulation might present divergent behavior, requiring a case-specific analysis with MSW values higher than 0.5 cm.

Built-in wavelet-induced smoothness to reduce plan complexity in intensity modulated radiation therapy (IMRT)

Objective. Reducing plan complexity in intensity modulated radiation therapy (IMRT) to ensure dosimetric accuracy and delivery efficiency of the radiation treatment plans. We propose a novel approach by representing the beamlet intensities using an incomplete wavelet basis that explicitly excludes fluctuating intensity maps from the decision space (explicit hard constraint). This technique provides a built-in wavelet-induced smoothness that improves both dosimetric plan quality and delivery efficiency. Approach. The beamlet intensity maps need to be especially smooth in the leaf travel direction (referred to as the X-direction). We treat the intensity map of each beam as a 2D image and represent it using the wavelets corresponding to low-frequency changes in the X-direction (i.e. approximation and horizontal). The absence of wavelets corresponding to high-frequency changes (i.e. vertical and diagonal) induces built-in smoothness. We still utilize a regularization term in the objective function to promote smoothness in the Y-direction (perpendicular to the X-direction) and further possible smoothness in the X-direction. This technique has been tested on three patient cases of different disease sites (paraspinal, lung, prostate) and all final evaluations and comparisons have been performed on an FDA-approved commercial treatment planning system (Varian EclipseTM). Main results. Wavelet-induced smoothness reduced monitor units by about 10%, 45%, and 14% for paraspinal, lung, and prostate cases, respectively. It also improved organ at risk sparing, especially on the complex paraspinal case where it resulted in about 7%, 13%, and 14% less mean dose to esophagus, lung, and cord, respectively. Moreover, built-in wavelet-induced smoothness desensitizes the results to changing the weight associated to the regularization term, and thereby mitigates the weight fine-tuning difficulty. Significance. Fluence smoothness is often achieved by smoothing the beamlet intensity maps using a proper regularization term in the objective function aiming at disincentivizing fluctuation in the beamlet intensities (implicit soft constraint). This work reports a novel application of wavelets in imposing an explicit smoothness hard constraint in the search space using an incomplete wavelet basis. This idea has been successfully applied to exclude complex and clinically irrelevant radiation plans from the search space. The code and pertained models along with a sample dataset are released on our LowDimRT GitHub (https://github.com/PortPy-Project/LowDimRT).

Does Fluence Smoothing Reduce the Complexity of the Intensity-Modulated Radiation Therapy Treatment Plan? A Dosimetric Analysis.

Intensity-modulated radiation therapy (IMRT) may have too many peaks and valleys, making the treatment plan undeliverable. When there are too many fluency differences between adjacent pixels in the X or Y directions, the X and Y smoothing factors are utilized as weighting factors to penalize this behavior. Generally, a high degree of complexity is accompanied by many monitor units (MUs), large number of segments, small sized segments, and complex segment shapes. The degree of plan delivery uncertainty can all increase with a higher detailed fluence map. This study aims to evaluate the dosimetric effects of various smoothing levels on the planning target volume (PTV) and organs at risk (OARs) for cervix cancer. IMRT treatment plans were re-optimized by combining several values of the X and Y penalty between 0 and 100. The dose-volume histogram assessed various dosimetric indicators for PTV and OARs. Additionally, gamma passing rates were evaluated and noted as an indicator of the complex treatment plan. At X = 60, Y = 60 fluence map penalty, the conformity index (CI) value reached its highest value of 0.996 ± 0.004. At X = 0, Y = 0, the homogeneity index (HI) was determined to have a maximum value of 0.0628 ± 0.0235. The highest and lowest MU values were 2424.30 ± 471.12 and 1087.80 ± 91.57, respectively, with X = 0, Y = 0 and X = 100, Y = 100. At X = 100, Y = 100, the gamma passing rate reaches its highest value of 99.28% ± 0.44% and minimum value of 85.93% ± 3.87% at X = 0, Y = 0. The CI and HI values showed no discernible fluctuation, and the OAR doses were barely affected as smoothing was increased. When the smoothing factor was raised, the number of MUs sharply dropped, and a decrease in the number of segments and higher gamma passing rates were also seen.

Impact of fluence smoothing on the dosimetry of cervical cancer radiotherapy

Objective To explore the dosimetric differences of radiotherapy plan for cervical cancer with 4 different fluence smoothing (FS) parameters using Monaco treatment planning system (Monaco TPS). Methods Fifteen patients with I B2 stage cervical cancer in our hospital were enrolled in this study. And a 2 Volumetric Modulated Arc Therapy (VMAT) plan for each patient were completed by Monaco 5.11 TPS according to the X-Ray Voxel Monte Carlo (XVMC) method. For each plan was optimized by FS function, with the level of Off, Low, Medium and High. To compare the difference of plan optimization time, conformity index ( CI), Homogeneity index ( HI), D mean, D min, D 2% of PTV, dose to the organ at risk (OAR), the number of Segments# and MU#, estimated total delivery time (ETDT), quantum Efficiency (QE) of the plans, the formation of Segments# with the same angle and verification of inserting 729 two-dimensional matrix into PTW octavius 4D module of different FS function levels, with the precondition of the Prescription isodose curve covering 95% of the target area. The data was analysed by multivariate factor analysis with the application of SPSS, and P < 0.05 was considered as statistically significant. And the Planned revenue score of different FS levels was also calculated. Results Except for the D min of PTV (the lowest value is (32.09 ± 0.26) Gy for the Off group, and the highest value is (35.98 ± 0.42) Gy for the High group), V 40 of the rectum (the lowest value in the Medium group is 55.88% ± 2.02%, and the highest value in the High group was 61.90% ± 2.98%) and bladder (the lowest value was 45.01% ± 2.08% in the Medium group, and the highest value is 50.45% ± 1.98% in the High group), the V 20 (the lowest value High group was 49.05% ± 1.98%, the highest value Off group was 56.52 ± 1.75%) of femoral head ( P < 0.05), there was no significant difference of the dose assessment results for PTV and OARs in 4 different FS function levels. In the High level, the ETDT, QE and MU# were showed better than other groups evidently, however, the number of Segments# showed no significant difference. The plan validation results was increased with the improvement of FS function level, and the level of High was considered to be the optimal. To compare the score of overall benefits of the plan, the level of Medium (−17.18 ± 0.05) got the highest score, and the Low group (−17.58 ± 0.05) and the High group (−17.42 ± 0.06) have similar scores, and Off group (−18.81 ± 0.08) has the lowest score. Conclusion Different FS levels of the Monaco 5.11 TPS can optimize the radiotherapy plan for cervical cancer, but the level of Medium is considered to be the most applicable. 摘要： 目的 研宄 Monaco 计划系统 (Monaco Treatment planning system, Monaco TPS) 应用不同通量平滑度 (Fluence smoothing, FS) 的情况下, 对宫颈癌放射治疗的剂量学影响。 方法 选取 I B2 期宫颈癌患者 15 例进行入组实验, 使用 瑞典医科达公司 Monaco 5.11TPS 的 X 射线体素蒙特卡洛 (X-ray voxel Monte Carlo, XVMC) 算法, 对每例入组病例分 别进行由 Off〜High 4 个不同等级通量优化平滑作用下, 2 个全弧的空间容积调强 (Volumetric modulated arc therapy, VMAT) 计划设计。在处方剂量均覆盖计划|E区 (Planning target volume, PTV) 95% 体积的情况下, 比对不同等级通量 平滑度作用下的计划优化时间, PTV 的适形指数 (Conformity index, CI) 和均句性指数 (Homogeneity index, HI) 以及平 均剂量 D mean、最小剂量 D min、最大剂量 D 2%, 危机器官 (Organ at risk, OAR) 受量, 计划应用子野数量 (Segments#), 计划 实施跳数 (MU#), 计划优化时长 (Estimated total delivery time, ETDT), 光子利用率 (Quantum efficiency, QE) 以及相同 角度子野构成和在 PTW Octavius 4D 模体内插 729 二维矩阵方式下的验证结果差异; 使用 SPSS 进行方差分析, P < 0.05 具有统计学意义;并计算不同 FS 等级设定下计划收益得分。 结果 在不同 FS 功能等级作用下的 4 个实验组的 PTV 和 OARs 剂量评估结果中, 除 PTV 的 D min [最低值为 Off 组 (32.09 ± 0.26) Gy, 最高值为 High 组 (35.98 ± 0.42)Gy]、直肠 (最低值 Medium 组 55.88% ± 2.02%, 最高值 High 组 61.90% ± 2.98%) 和膀胱 (最低值为 Medium 组 45.01 ± 2.08%, 最高值 High 组 50.45 ± 1.98%) 的 V 40、股骨头 (最低值 High 组 49.05% ± 1.98%, 最高值 Off 组 56.52% ± 1.75%) 的 V 20, 差异均有统计学意义 ( P < 0.05); 计划剂量计算时间、光子利用率和计划应用空气跳数上 High 组的结果 明显优于其他 FS 等级, 而计划子野应用数量上 4 个 FS 功能等级上无明显差异;计划验证结果对比中伴随 FS 功能等 级的提高而增加, High 组验证结果最优; 计划整体收益得分中 Medium 组 (−17.18 ± 0.05) 得分最高, Low 组 (−17.58 ± 0.05) 和 High 组 (−17.42 ± 0.06) 得分接近, Off 组 (−18.81 ± 0.08) 为最低。 结论 应用 Monaco 5.11 TPS 进行优化宫颈 癌放射治疗计划时, 不同 FS 功能等级下的计划结果均符合临床要求, 但综合评估结果应用 Medium 是最为适用的。

A Pre-implementation Study of Volumetric Modulated Arc Therapy for Same-day Planning and Treatment of Vertebral Bone Metastases Within a Rapid-access Palliative Radiotherapy Programme

AimsWe aimed to develop a process for same-day contouring, planning, quality assurance and delivery of volumetric modulated arc therapy (VMAT) for vertebral bone metastases within our institution's rapid-access palliative radiotherapy programme. Materials and methodsTwo thoracic (T6–7, T3–7) and two lumbar (L2–3, L1–5) targets were contoured on computed tomography images acquired from an anthropomorphic phantom and five patient scans. Inverse planning aimed to provide coverage of a prescribed dose of 8 Gy with a combined lung V2Gy < 25% and a combined kidney mean dose <2 Gy. Serial plans were created to identify an efficient combination of six main planning variables specific to our treatment planning system: (i) voxel size (3 mm versus 5 mm), (ii) Monte Carlo statistical uncertainty (1% per calculation versus 3% per control point), (iii) fluence smoothing (medium versus high), (iv) number of iterations of segment shape changes during optimisation (1 versus 5), (v) dose calculation algorithm (Monte Carlo versus pencil beam) and (vi) number of arcs (single versus multiple). Contouring, planning, quality assurance and treatment delivery were timed. ResultsThe combination of planning variables deemed efficient and appropriate was: a 3 mm voxel size, statistical uncertainty of 1% per calculation, medium fluence smoothing, five iterations of segment shape changes, Monte Carlo dose calculation and single full arc delivery. Patient scan contouring times ranged from 7 to 9 min (T6–7), 11–13 min (T3–7), 5–7 min (L2–3) and 8–10 min (L1–5) and planning times ranged from 9 to 15 min (T6–7), 13–25 min (T3–7), 18–25 min (L2–3) and 21–31 min (L1–5). Physics quality assurance times ranged from 15 to 21 min and beam-on times ranged from 3 to 6 min. ConclusionsThe combined elements of VMAT for thoracic and lumbar vertebral bone metastases were completed in under 2 h. This new process makes same-day contouring, planning, quality assurance and treatment delivery of VMAT feasible within our rapid-access palliative radiotherapy programme.

Non-Coplanar Many-Isocenter Optimization for Radiotherapy on Robotic Arm Platform

4π non-coplanar radiotherapy significantly improves dosimetry, but its clinical implementation on the widely available C-arm gantry system is limited by the restricted non-colliding beam angles and the complex coordination of gantry and couch motion. Alternatively, a robotic arm platform is more favorable for the non-coplanar and non-isocentric treatment, but the conflict between field size and modulation resolution needs to be managed. This study investigates the dosimetry and delivery efficiency by creating many isocenters to achieve simultaneously large field-of-view (FOV) and high modulation resolution for robotic radiotherapy. The many-isocenter optimization framework includes a least-square dose fidelity objective, a total variation term for regularizing the fluence smoothness, and a group sparsity term for simultaneous beam orientation optimization (BOO) and isocenter selection. A minimal number of candidates isocenters were identified before the optimization for efficient target coverage. The proposed algorithm was evaluated on the intensity-modulated radiotherapy (IMRT) treatment plans of 10 Head and Neck (H&N) cancer patients. Colliding beams excluded, high-resolution small-field non-isocentric 4π IMRT plans with 50cm source-to-isocenter distance (SID-50) were compared with low-resolution large-field isocentric plans with 100cm SID (SID-100). The table reports the planning target volume (PTV) and organs at risk (OARs) statistics, and the corresponding Wilcoxon signed-rank test, showing significant difference (p<0.05) in PTV homogeneity, PTV Dmax, R50, Integral dose, and OAR Dmean and Dmax. With better target coverage, SID-50 substantially improved dose conformality and reduced OAR dose. R50 and integral dose were reduced by 5.3% and 9.6% respectively. The average reduction of [Dmean, Dmax] of SID-50 from SID-100 were [2.09 Gy, 1.19 Gy] for OAR overall, [3.05 Gy, 0.04 Gy] for parotid gland, [3.62 Gy, 5.19 Gy] for larynx, and [3.27 Gy, 1.10 Gy] for mandible. The estimated delivery time of 20-beam [SID-50, SID-100] plans were [19, 18] minutes and [14, 9] minutes, assuming 5 fractions and 30 fractions, respectively. With clinically acceptable delivery efficiency, the proposed non-coplanar many-isocenter optimization is dosimetrically favorable for treating large targets with high modulation resolution.Tabled 1Abstract 2832; Table 2832Wilcoxon signed rank test of PTV and OAR statistics of SID-100 (isocentric) and SID-50 (non-isocentric) for all 10 patients.Statisticsp-valueSigned rankNumber of samplesSID-100SID-50MeanSTDMeanSTDPTV Homogeneity0.00385290.9010.0450.9060.045PTV Dmin0.749202290.9740.0190.9750.016PTV Dmax<.001380291.1270.0621.1180.061R500.00653102.3810.2362.2540.256Integral Dose0.0025510139.37738.481126.0236.139OAR Dmean0.002551017.8223.72715.7293.273OAR Dmax0.002551034.5156.31733.3256.173 Open table in a new tab

Automated planning through robust templates and multicriterial optimization for lung VMAT SBRT of lung lesions

PurposeTo develop and validate a robust template for VMAT SBRT of lung lesions, using the multicriterial optimization (MCO) of a commercial treatment planning system.MethodsThe template was established and refined on 10 lung SBRT patients planned for 55 Gy/5 fr. To improve gradient and conformity a ring structure around the planning target volume (PTV) was set in the list of objectives. Ideal fluence optimization was conducted giving priority to organs at risk (OARs) and using the MCO, which further pushes OARs doses. Segmentation was conducted giving priority to PTV coverage. Two different templates were produced with different degrees of modulation, by setting the Fluence Smoothing parameter to Medium (MFS) and High (HFS).Each template was applied on 20 further patients. Automatic and manual plans were compared in terms of dosimetric parameters, delivery time, and complexity. Statistical significance of differences was evaluated using paired two‐sided Wilcoxon signed‐rank test.ResultsNo statistically significant differences in PTV coverage and maximum dose were observed, while an improvement was observed in gradient and conformity. A general improvement in dose to OARs was seen, which resulted to be significant for chest wall V30 Gy, total lung V20 Gy, and spinal cord D0.1 cc. MFS plans are characterized by a higher modulation and longer delivery time than manual plans. HFS plans have a modulation and a delivery time comparable to manual plans, but still present an advantage in terms of gradient.ConclusionThe automation of the planning process for lung SBRT using robust templates and MCO was demonstrated to be feasible and more efficient.

Many-isocenter optimization for robotic radiotherapy

Despite significant dosimetric gains, clinical implementation of the 4π non-coplanar radiotherapy on the widely available C-arm gantry system is hindered by limited clearance, and the need to perform complex coordinated gantry and couch motion. A robotic radiotherapy platform would be conducive to such treatment but a new conflict between field size and MLC modulation resolution needs to be managed for versatile applications. This study investigates the dosimetry and delivery efficiency of purposefully creating many isocenters to achieve simultaneously high MLC modulation resolution and large tumor coverage. An integrated optimization framework was proposed for simultaneous beam orientation optimization (BOO), isocenter selection, and fluence map optimization (FMO). The framework includes a least-square dose fidelity objective, a total variation term for regularizing the fluence smoothness, and a group sparsity term for beam selection. A minimal number of isocenters were identified for efficient target coverage. Colliding beams excluded, high-resolution small-field 4π intensity-modulated radiotherapy (IMRT) treatment plans with 50 cm source-to-isocenter distance (SID-50) on 10 Head and Neck (H&N) cancer patients were compared with low-resolution large-field plans with 100 cm SID (SID-100). With the same or better target coverage, the average reduction of [Dmean, Dmax] of 20-beam SID-50 plans from 20-beam SID-100 plans were [2.09 Gy, 1.19 Gy] for organs at risk (OARs) overall, [3.05 Gy, 0.04 Gy] for parotid gland, [3.62 Gy, 5.19 Gy] for larynx, and [3.27 Gy, 1.10 Gy] for mandible. R50 and integral dose were reduced by 5.3% and 9.6%, respectively. Wilcoxon signed-rank test showed significant difference (p < 0.05) in planning target volume (PTV) homogeneity, PTV Dmax, R50, Integral dose, and OAR Dmean and Dmax. The estimated delivery time of 20-beam [SID-50, SID-100] plans were [19, 18] min and [14, 9] min, assuming 5 fractions and 30 fractions, respectively. With clinically acceptable delivery efficiency, many-isocenter optimization is dosimetrically desirable for treating large targets with high modulation resolution on the robotic platform.

Single-arc VMAT optimization for dual-layer MLC

Dual-layer multi-leaf collimator (DLMLC) has recently attracted renewed interest due to its good balance among resolution, low leakage, and high fabricability. However, existing progressive sampling based volumetric modulated arc therapy (VMAT) algorithm is ineffective for DLMLC, requiring more arcs to achieve dosimetry comparable to VMAT plans with higher resolution single-layer MLC (SLMLC). In this study, we develop a novel single-arc VMAT optimization framework to take advantage of the unique DLMLC characteristics fully. Direct aperture optimization (DAO) for single-arc DLMLC VMAT was formulated as a least square dose fidelity objective, along with an anisotropic total variation term to regulate the fluence smoothness and a single segment term for forming simple apertures. The DAO was solved through alternating optimization approach. The DLMLC deliverability constraint and the MLC leaf speed constraint were formulated as the optimization constraints and solved using a graph optimization algorithm. Feasibility of the proposed framework was tested on a brain, a lung, and a prostate cancer patient. The framework was further adapted for a simultaneous integrated boost (SIB) case. The single-arc DLMLC-10 mm (leaf width) plan was compared against single-arc SLMLC VMAT plans including SLMLC-5mm, SLMLC-10mm, and SLMLC with 10 mm leaf width and 5 mm leaf step size (SLMLC-10mm-5mm). Compared with the SLMLC-10mm plan and the SLMLC-10mm-5mm plan, with the same target coverage, the DLMLC-10 mm plan reduced R50 by 30.7% and 10.0%, the average max OAR dose by 5.79% and 3.7% of the prescription dose, and the average mean OAR dose by 4.18% and 2.1% of the prescription dose, respectively. The plan quality is comparable to that of the SLMLC-5mm plan. The novel single-arc VMAT optimization framework for DLMLC utilizes two MLC layers to improve the effective modulation resolution and afford more sophisticated modulation. Consequently, DLMLC VMAT achieves superior dosimetry to SLMLC VMAT with the same leaf width.

Feasibility method for reducing the total monitor units in Eclipse TPS

Objective To explore the feasibility of using the optimization parameters modification and optimization processes modification to reduce the total monitor units(MUs) in the Eclipse radiotherapy treatment planning system (TPS). Methods Based on the radiotherapy plan of 10 patients with nasopharyngeal carcinoma, a total of 90 plans were designed for 9 groups using different optimization parameters and processes. The total MUs and the exposure dose of the organs among the different plans were compared. Results There was no significant difference in the doses of the organs at risk (class I) under the premise of target dose requirements (all P>0.05). The increase of the weight of the target area and the organs at risk will increase the total MUs. The increase of the preset limit value of the minimum MUs in the subfield will reduce the total MUs. The increase of the fluence smoothness in the X and Y directions will increase the total MUs. An unreasonable minimum MU value will increase the total MUs. Under the condition that the organ exposure is not changed significantly, the influencing factors of MU are ranked as weight> fluence smoothness >minimum MUs. Conclusions Parameter setting and process planning can reduce the total MUs to a certain extent. However, due to the complexity of the influence of optimization parameters on the plan, the optimization process should be preferred. Especially in the Eclipse TPS, the method of gradual optimization to achieve the final dose distribution requirement and then remove the fluence re-optimization is more convenient and effective for reducing the total MUs. Key words: Eclipse TPS; Nasopharyngeal carcinoma; Fluence; Monitor units

4πVMAT: A Novel Method to Efficiently Deliver Non-Coplanar Treatment

Existing Volumetric Modulated Arc Therapy (VMAT) optimization using one or more arcs, each being coplanar by itself, is a highly efficient way to deliver intensity modulated radiation therapy (IMRT). Static beam IMRT utilizing optimized non-coplanar beam orientation has resulted in superior dosimetry to VMAT but is substantially less efficient. In this study, we developed a novel optimization framework for non-coplanar VMAT, incorporating simultaneous couch and gantry rotation during the arc motion, to maximally utilize the 4π solid angle (4πVMAT) with high efficiency. The 4πVMAT optimization framework includes a Direct Aperture Optimization (DAO) module and a beam trajectory selection (BTS) module. DAO was achieved by adding to the dose fidelity objective a total variation term for regulating the fluence smoothness, a single segment term for forming deliverable fluence map, and a group sparsity term for selecting beam angles. The BTS was formulated as a travelling salesman problem on a hierarchical graph, where the edge and node costs were determined by the maximal gantry rotation speed and the estimated fluence map at the current iteration, respectively. Continuous gantry/couch angle trajectories were selected on the graph using the Dijkstra's algorithm. Candidate beams in the couch-gantry-patient collision space, evaluated on actual machine geometry and a human subject 3D surface, were excluded in both the DAO and BTS modules. The optimization framework was solved by alternating between the DAO module and the BTS. Efficacy of 4πVMAT using one full-arc or two full arcs was tested on a brain, a lung and a prostate cancer patient. The plan was compared against a coplanar VMAT (2πVMAT) plan using more arcs with collimator rotation. The 4πVMAT consisted of two efficient arcs, each taking 160 seconds in estimate to deliver. In the first arc, the couch rotated in either the clockwise or counterclockwise direction, and then reversed the rotation in the second arc. The gantry rotations were coordinated with the couch rotation for optimal dose distribution while avoiding couch-gantry-patient collision. With the same target coverage, 4πVMAT remarkably reduced the max and mean organs-at-risk (OARs) doses compared to 2πVMAT, as shown in Table 1. The dose compactness measured using R50 was reduced by 19.7%. By optimizing feasible and efficient non-coplanar arc trajectories, the novel 4πVMAT optimization framework simultaneously achieves the desirable dosimetric quality of 4π therapy and the delivery efficiency of coplanar 2πVMAT.Abstract TU_35_3241: Table 1OAR dose: 2πVMAT - 4πVMAT (% of the prescription dose)patientDmaxDmeanLargestAverageLargestAveragebrain12.847.529.324.68lung32.6012.7810.583.62prostate10.502.764.300.35OAR mean and maximum dose differences between πVMAT and 2πVMAT for all patients. “Largest” and “Average” represent the largest and average values among all OARs. Open table in a new tab

Effect of fluence smoothening in intensity modulated radiotherapy planning and delivery

Objective: In Intensity Modulated Radiotherapy (IMRT) planning, optimization is a computational problem, potentially susceptible to noise and artifacts (high frequency spatial fluctuations) producing sharp fluence peaks and valleys in millimetric spatial scale. A solution to this problem is to smooth the beam profiles. Methods and Materials: In the Eclipse TPS, fluence smoothing is achieved within the objective function of the inverse treatment planning systems. Plans were developed for a 6 MV photon beam from a Varian Clinac-DHX equipped with a 120 leaves MLC. Total of 160 dose plans were compared and fed into the analysis process. The dose plan quality has been analysed in terms of Statistical computation by means of two-sided paired t-test between two smoothening levels (s25 and s75) in terms of Homogeneity-Index, Conformity-Index, Target dose coverage and OAR dose differences in terms of max, min and mean doses. Results: From our present study on the influence of smoothening of fluence levels in IMRT plans results, there was a reduction in total MU’s with no significant statistical variation in terms of mean differences of HI Index, CI index, PTV coverages, OAR doses. Moreover, the reduction in MU’s will help the less head leakage dose hence the lower whole-body dose, which will help the patient to reduce the chances for secondary malignancies. Conclusion: Hence, we conclude that, higher fluence smoothening levels with acceptable difference in target coverage and minimum variation of OAR should be selected for the execution.

A novel optimization framework for VMAT with dynamic gantry couch rotation

Existing volumetric modulated arc therapy (VMAT) optimization using coplanar arcs is highly efficient but usually dosimetrically inferior to intensity modulated radiation therapy (IMRT) with optimized non-coplanar beams. To achieve both dosimetric quality and delivery efficiency, we proposed in this study, a novel integrated optimization method for non-coplanar VMAT (4πVMAT). 4πVMAT with direct aperture optimization (DAO) was achieved by utilizing a least square dose fidelity objective, along with an anisotropic total variation term for regularizing the fluence smoothness, a single segment term for imposing simple apertures, and a group sparsity term for selecting beam angles. Continuous gantry/couch angle trajectories were selected using the Dijkstra’s algorithm, where the edge and node costs were determined based on the maximal gantry rotation speed and the estimated fluence map at the current iteration, respectively. The couch–gantry–patient collision space was calculated based on actual machine geometry and a human subject 3D surface. Beams leading to collision are excluded from the DAO and beam trajectory selection (BTS). An alternating optimization strategy was implemented to solve the integrated DAO and BTS problem. The feasibility of 4πVMAT using one full-arc or two full-arcs was tested on nine patients with brain, lung, or prostate cancer. The plan was compared against a coplanar VMAT (2πVMAT) plan using one additional arc and collimator rotation. Compared to 2πVMAT, 4πVMAT reduced the average maximum and mean organs-at-risk dose by 9.63% and 3.08% of the prescription dose with the same target coverage. R50 was reduced by 23.0%. Maximum doses to the dose limiting organs, such as the brainstem, the major vessels, and the proximal bronchus, were reduced by 8.1 Gy (64.8%), 16.3 Gy (41.5%), and 19.83 Gy (55.5%), respectively. The novel 4πVMAT approach affords efficient delivery of non-coplanar arc trajectories that lead to dosimetric improvements compared with coplanar VMAT using more arcs.

SU-F-T-343: Dosimetric Investigation for Fluence Smoothing On VMAT Plans in Monaco for Head - Neck Cancers

Purpose: The study investigates the effect of fluence smoothing parameter on VMAT plans for ten head-neck cancer patients using Monaco5.00.04. Methods: VMAT plans were created using Monaco5.00.04 planning system for 10 head-neck patients. Four plans were generated for each patient using available smoothing parameters i.e. high, medium, low and off. The number of monitor units required to deliver 1 cGy was defined as a modulation degree; and was taken as a measure of plan complexity. Routinely used plan quality parameters Conformity index (CI) and Homogeneity index (HI) were used in the study. As a protocol our center, practices “medium” smoothing for clinical implementation. Plans with medium smoothing were opted as reference plans due to the clinical acceptance and dosimetric verifications made on these plans. Plans were generated by varying the smoothing parameter and re-optimization was done. The PTV was evaluated for D98%, D95%, D50%, D1% and prescription isodose volume (PIV). For critical organs; spine and parotids the parameters recorded were D1cc and Dmean respectively. Results: The cohort had the median prescription as 6000 cGy in the range of 6600 cGy - 4500 cGy. The modulation degree was observed to increase up to 6% from reference to the most complex plan. High smoothing had about 11% increase in segments which marginally (0.5 to 1%) increased the homogeneity index while conformity index remains constant. For spine the maximum D1cc was observed in medium smoothing as 4639.8 cGy, this plan was clinically accepted and dosimetrically verified. Similarly for parotids, the Dmean was 2011.9 cGy and 1817.05 cGy. Conclusion: The sensitivity of plan quality in terms of smoothing options (high, medium, low and off) available in Monaco 5.00.04 was resulted in minimal difference in terms of target coverage, conformity index and homogeneity index. Similarly changing smoothing did not result in any enhanced advantage in sparing of critical organs

Effect of fluence smoothing on the quality of intensity-modulated radiation treatment plans.

A fluence-smoothing function applied for reducing the complexity of a treatment plan is an optional requirement in the inverse planning optimization algorithm of intensity-modulated radiation therapy (IMRT). In this study, we investigated the consequences of fluence smoothing on the quality of highly complex and inhomogeneous plans in a treatment-planning system, Eclipse™. The smoothing function was applied both in the direction of leaf travel (X) and perpendicular to leaf travel (Y). Twenty IMRT plans from patients with cancer of the nasopharynx and lung were selected and re-optimized with use of various smoothing combinations from X=0, Y=0 to X=100, Y=100. Total monitor units (MUs), dose-volume histograms, and radiobiological estimates were computed for all plans. The study yielded a significant reduction in the average total MUs from 2079±265.4 to 1107±137.4 (nasopharynx) and from 1556±490.3 to 791±176.8 (lung) while increasing smoothing from X, Y=0 to X, Y=100. Both the tumor control and normal tissue complication probabilities were found to vary, but not significantly so. No appreciable differences in doses to the target and most of the organs at risk (OARs) were noticed. The doses measured with the I'MRT MatriXX 2-D system indicated improvements in deliverability of the plans with higher smoothing values. Hence, it can be concluded that increased smoothing reduced the total MUs exceptionally well without any considerable changes in OAR doses. The observed progress in plan deliverability in terms of the gamma index strongly supports the recommendation of smoothing levels up to X=70 and Y=60, at least for the nasopharynx and lung.

Influence of monte carlo variance with fluence smoothing in VMAT treatment planning with Monaco TPS.

The study aimed to investigate the interplay between Monte Carlo Variance (MCV) and fluence smoothing factor (FSF) in volumetric modulated arc therapy treatment planning by using a sample set of complex treatment planning cases and a X-ray Voxel Monte Carlo-based treatment planning system equipped with tools to tune fluence smoothness as well as MCV. The dosimetric (dose to tumor volume, and organ at risk) and physical characteristic (treatment time, number of segments, and so on) of a set 45 treatment plans for all combinations of 1%, 3%, 5% MCV and 1, 3, 5 FSF were evaluated for five carcinoma esophagus cases under the study. Increase in FSF reduce the treatment time. Variation of MCV and FSF gives a highest planning target volume (PTV), heart and lung dose variation of 3.6%, 12.8% and 4.3%, respectively. The heart dose variation was highest among all organs at risk. Highest variation of spinal cord dose was 0.6 Gy. Variation of MCV and FSF influences the organ at risk (OAR) doses significantly but not PTV coverage and dose homogeneity. Variation in FSF causes difference in dosimetric and physical parameters for the treatment plans but variation of MCV does not. MCV 3% or less do not improve the plan quality significantly (physical and clinical) compared with MCV greater than 3%. The use of MCV between 3% and 5% gives similar results as 1% with lesser calculation time. Minimally detected differences in plan quality suggest that the optimum FSF can be set between 3 and 5.

Sci-Thur AM: Planning - 09: Assessing dynamic IMRT field modulation in prostate plans.

In a previous study, the variogram fractal dimension (FD) method was found to be very accurate at identifying planned head and neck IMRT fields that are overly-modulated. In the current study, the authors used MATLAB® to develop FracMod, a graphical user interface (GUI) and variogram FD analysis tool to assess modulation complexity of dynamic IMRT fields designed for treatments of the prostate alone and prostate plus pelvic nodes. A set of 5 prostate plans (25 fields) and 5 prostate plus pelvic node plans (35 fields) were used to choose FD cut-points that ensure no false positives (100% specificity) in distinguishing between moderate field modulation (typical modulation used clinically at the authors' institution) and high modulation. Field modulation was controlled by adjusting fluence smoothing parameters in the Eclipse™ treatment planning system. The area under the curve (AUC) from receiver operating characteristic (ROC) analysis was used to quantitatively compare the ability of FD and the number of monitor units (MUs) for distinguishing between the moderate and high modulation fields. The variogram FD method gave AUCs of 0.96 (almost perfect classification) and 1.00 (perfect classification) for the prostate alone and the prostate plus pelvic node fields, respectively. The variogram FD method is an accurate metric; performing better than the number of MUs at identifying high modulation IMRT fields planned for the treatment of prostatic carcinoma. Hence, FracMod will enable Radiotherapy Physicists to easily and accurately quantify the degree of modulation of IMRT fields and adjust overly-modulated fields at the treatment planning stage.

Fractal analysis for assessing the level of modulation of IMRT fields

To investigate the potential of three fractal dimension (FD) analysis methods (i.e., the variation, power spectrum, and variogram methods) as metrics for quantifying the degree of modulation in planned intensity modulated radiation therapy (IMRT) treatment fields, and compare the most suitable FD method to the number of monitor units (MUs), the average leaf gap, and the 2D modulation index (2D MI) for assessing modulation. The authors implemented, validated, and compared the variation, power spectrum, and variogram methods for computing the FD. Validation of the methods was done using mathematical fractional Brownian surfaces of known FD that ranged in size from 128 × 128 to 512 × 512. The authors used a test set consisting of seven head and neck carcinoma plans (50 prescribed treatment fields) to choose an FD cut-point that ensures no false positives (100% specificity) in distinguishing between moderate and high degrees of field modulation. The degree of field modulation was controlled by adjusting the fluence smoothing parameters in the Eclipse™ treatment planning system (Varian Medical Systems, Palo Alto, CA). The moderate modulation fields were representative of the degree of modulation used clinically at the authors' institution. The authors performed IMRT quality assurance (QA) on the 50 test fields using the MapCHECK™ device. The FD cut-point was applied to a validation set consisting of four head and neck plans (28 fields). The area under the curve (AUC) from receiver operating characteristic (ROC) analysis was used to compare the ability of FD, number of MUs, average leaf gap, and the 2D MI for distinguishing between the moderate and high modulation fields. The authors found the variogram FD method to be the most suitable for assessing the modulation complexity of IMRT fields for head and neck carcinomas. Pass rates as measured by the gamma criterion for the MapCHECK™ IMRT field measurements were higher for the moderately modulated fields, and a gamma criterion with 1 mm distance-to-agreement and 1% dose difference showed a clear separation between the 94% pass rates of the moderate and high modulation groups. From the ROC analysis of the test set, the authors found the AUC of the variogram FD, number of MUs, average leaf gap, and 2D MI methods to be 0.99 (almost perfect), 0.91 (excellent), 0.91 (excellent), and 0.92 (excellent), respectively. A cut-point of FD > 2.25 correctly identified 92.8% of the high modulation fields and 100% of the moderately modulated fields in the validation set, satisfying the condition of no false positives. Of the three FD methods investigated, the variogram method is the most accurate and precise metric for identifying high modulation treatment fields. It is also more accurate and precise than the number of MUs, the average leaf gap, and the 2D MI. Although MapCHECK™ IMRT QA does a reasonable job at identifying high modulation fields, the variogram FD method provides one with the opportunity to quantitatively and accurately assess modulation and adjust overly modulated fields at the treatment planning stage before they are sent to the treatment machine for QA or patient treatment.

SU‐E‐T‐881: Simulation of a Multileaf Collimator vs. Iris Variable Aperture Collimation for Prostate Robotic Radiosurgery and IMRT

Purpose: To study potential advantages and disadvantages of a multileaf collimator (MLC) relative to iris variable aperture collimation for prostate cancer treatment with a CyberKnife robotic radiosurgery system. Methods: Six treatment plans for three prostate cases are generated using target conformal segments with a hypothetical MLC vs. circular beams with an iris variable aperture collimator. Conformality, homogeneity, number of MUs and segments are used as plan quality indicators. Quasi Newton Gradient descent optimization is applied as optimization strategy. MU reduction during the optimization process is achieved with two‐dimensional median fluence smoothing. The final sequencing is performed at sequencing levels 2 and 3 to reduce the number of segments. Iris baseline plans were generated using the vendorˈs TPS. Treatment plans A and B are conventionally fractionated IMRT with homogeneous PTV coverage. Plans C and D are IMRT with homogeneous PTV coverage (−/+ integrated boosts). Plans E and F are hyperfractionated SBRT with homogeneous PTV coverage (−/+ integrated boosts, and stricter OAR dose constraints). Results: With identical number of CyberKnife nodes for an iris and MLC treatment plan, an improvement in dose conformality is achieved with a MLC for all treatment plans and dose homogeneity is only slightly degraded. In all cases, the total number of MUs with multileaf collimated fields is reduced in comparison to circular collimated iris variable aperture fields (avg. reduction 28%). The number of segments for a comparable MLC treatment plan with an iris plan is also reduced for 4 cases and slightly increased for 2 of them (avg. reduction 17%). Conclusions: This investigation shows that a MLC could provide several benefits for intensity modulated prostate radiosurgery with a CyberKnife robotic radiosurgy system. Multileaf collimated beams for radiosurgery allow a highly conformal radiation. Their target conformal segments in addition reduce the required total number of MUs and segments.

Evaluation of fluence‐smoothing feature for three IMRT planning systems

Commercially available intensity‐modulated radiation therapy (IMRT) inverse treatment planning systems (ITPS) typically include a smoothing function which allows the user to vary the complexity of delivered beam fluence patterns. This study evaluated the behavior of three ITPSs when varying smoothing parameters. We evaluated four cases treated with IMRT in our clinic: sinonasal carcinoma (SNC), glioblastoma multiforme (GBM), base of tongue carcinoma (BOT), and prostate carcinoma (PST). Varian Eclipse v6.5, BrainLAB BrainScan v5.31, and Nomos Corvus v6.2 ITPSs were studied for the SNC, GBM, and PST sites. Only Eclipse and Corvus were studied for BOT due to field size constraints of the BrainLAB MM3 collimator. For each ITPS, plans were first optimized using vendor‐recommended default “smoothing” values. Treatment plans were then reoptimized, exploring various smoothing values. Key metrics recorded included a delivery complexity (DC) metric and the Ian Paddick Conformality Index (IPCI). Results varied widely by vendor with regard to the impact of smoothing on complexity and conformality. Plans run on the Corvus ITPS showed the logically anticipated increase in DC as smoothing was decreased, along with associated improved organ‐at‐risk (OAR) sparing. Both Eclipse and BrainScan experienced an expected trend for increased DC as smoothing was decreased. However, this increase did not typically result in appreciably improved OAR sparing. For Eclipse and Corvus, and to a much lesser extent BrainScan, increases in smoothing decreased DC but eventually caused unacceptable losses in plan conformality. Depending on the ITPS, potential benefits from optimizing fluence smoothing levels can be significant, allowing for increases in either efficiency or conformality. Because of variability in smoothing function behavior by ITPS, it is important that users familiarize themselves with the effects of varying smoothing parameters for their respective ITPS. Based on the experience gained here, we provide recommended workflows for each ITPS to best exploit the fluence‐smoothing features of the system.PACS numbers: 87.56.bd, 87.56.N‐