Intensity modulated radiation therapy with multileaf collimators of different leaf widths: a comparison of achievable dose distributions

Influence of beamlet width on dynamic IMRT plan quality in nasopharyngeal carcinoma

Objective

This study aimed to identify the effects of beamlet width on dynamic intensity-modulated radiation therapy (IMRT) for nasopharyngeal carcinoma (NPC) and determine the optimal parameters for the most effective radiotherapy plan.

Methods

This study evaluated 20 patients with NPC were selected for dynamic IMRT. Only the beamlet width in the optimization parameters was changed (set to 2, 4, 6, 8, and 10 mm that were named BL02, BL04, BL06, BL08, and BL10, respectively) to optimize the results of the five groups of plans. Using the plan quality scoring system, the dose results of the planning target volumes (PTVs) and organs at risks (OARs) were analyzed objectively and comprehensively. The lower the quality score, the better the quality of the plan. The efficiency and accuracy of plan execution were evaluated using monitor units (MUs) and plan delivery time (PDT).

Results

The BL04 mm group had the lowest quality score for the targets and OARs (0.087), while the BL10 mm group had the highest total score (1.249). The BL04 mm group had the highest MUs (837 MUs) and longest PDT (358 s). However, the MUs range of each group plan was below 100 MUs, and the PDT range was within 30 s. In the BL02, BL04, BL06, BL08, and BL10 plans, <5 MUs segments accounted for 33%, 16%, 24%, 33%, and 40% of total segments, respectively, with which the lowest was in the BL04 mm group.

Conclusion

Smaller beamlet widths have not only reduced OARs dose while maintaining high dose coverage to the PTVs, but also lead to more MUs that would produce greater PDT. Considering the quality and efficiency of dynamic IMRT, the beamlet width value of the Monaco treatment planning system set to 4 mm would be optimal for NPC.

Dosimetric sensitivity of leaf width on volumetric modulated arc therapy plan quality: an objective approach

Background

Several authors investigated a dosimetric impact of leaf width on radiotherapy plan quality subjectively, and concluded that thinner leaf-width multileaf collimators (MLC) are beneficial because of their better coverage of clinically relevant structures. Study aimed to investigate the dosimetric effect of MLC leaf width on volumetric modulated arc therapy plan quality by objective approach.

Materials and methods

Twelve of each prostate and head-and-neck patients were planned for volumetric modulated arc therapy (VMAT) treatments for MLC leaf widths of 4 mm and 10 mm. Three different VMAT schemes single-arc, dual-arc and two combined independent single-arcs were optimized. Dose volume histogram and Isodose distribution were used for quantitative and qualitative comparison of the treatment plan, respectively. Dose-volume-indices of the planning target volume, organs at risk and number of delivered monitor units were compared. The 4 mm leaf width being reference over 10 mm and results were noted as statistically significant if p ≤ 0.05 using student t-test.

Results

All VMAT schemes for both tumor sites showed a gain in target coverage, similar organs at risk doses and higher monitor units to be delivered, when changing leaf width from 10 mm to 4 mm. The p-values were significant for majority of head-and-neck dose indices.

Conclusion

The thinner-leaf MLCs, owing to their better spatial resolution, result in an overall gain for target coverage, while maintaining permissible doses to the organs at risk.

Optimization of Minimum Segment Width Parameter in the Intensity-Modulated Radiotherapy Plan for Esophageal Cancer

Purpose

This study was designed to explore the optimal minimum segment width (MSW) in the intensity-modulated radiotherapy (IMRT) plan for esophageal cancer.

Patients and Methods

The imaging data of 20 esophageal cancer patients were selected for this study. Four IMRT plans were designed for each patient with MSWs of 0.5, 1.0, 1.5, and 2.0 cm. The conformity index (CI) and homogeneity index (HI) of the planning target volumes (PTV), organs at risk (OARs), control points (CP), monitor units (MU), plan delivery time (DT), and gamma passing rates (GPR) were collected and compared to appraise the treatment plan quality and delivery efficiency.

Results

Lower-MSW plans had larger CI and smaller HI values, and lower dose parameters of OARs and PTVs. The HI, CI, and dose parameter of OARs in the 0.5 and 1.0 cm MSW groups were similar and much better than those of the 1.5 and 2.0 cm MSW groups. Meanwhile, the plan in the 0.5 cm MSW group had significantly higher MUs, CPs, and DTs, and a significantly lower relative dose of GPR with a 3% dose difference and 3 mm distance to agreement criteria than the other three groups.

Conclusion

The 0.5 and 1 cm MSW groups had better dosimetric parameters and IMRT plan quality than the other groups. However, plans with 0.5 cm MSW had worse delivery accuracy and efficiency than the other three groups. Thus, MSW of 1.0 cm was the optimal choice to ensure good quality, delivery accuracy, and treatment efficiency in IMRT plans for esophageal cancer.

Optimization of Clarkson's Method for Calculating Absorbed Dose under Compensator Filters used in Intensity-modulated Radiation Therapy

Background:

Intensity Modulated Radiation Therapy (IMRT) is extensively used in the treatment of malignancies. Clarkson’s method is one of the leading methods for dose calculation at open points present in irregular fields.

Objective:

The aim of this study is to generalize the Clarkson’s method for dose calculation at points under compensator filters in IMRT method and its application in IMRT quality control as well.

Material and Methods:

In this experimental study, compensator filters were designed in two forms: flat filter and block piled-up compensator. The measurements for the compensator filters and open fields in 5 and10 cm depths at energy levels (6, 10 and 18 MV) and in fields with different dimensions were performed using “Mapcheck2” dosimeter. The aim of performing calculations is to derive the theoretical dose by the generalized Clarkson’s equation and comparing it with data resulted from the measurement for confirming the Clarkson’s equation presented.

Results:

These results demonstrate the data derived from the generalized Clarkson’s method are in good agreement with the data resulted from measurement; the highest error of the proposed equation was 3% for flat filter, and less than 5% for block-piled-up filter. Higher error in the block-piled-up filter compared with the flat filter was due to the presence of leakage between these blocks.

Conclusion:

The results of this study demonstrated that the presented equation is reliable and valid, and the proposed equation can be applied for dose calculation at all points under the compensator filter or the shielded areas.

Impact of Multi-leaf Collimator Parameters on Head and Neck Plan Quality and Delivery: A Comparison between Halcyon™ and Truebeam® Treatment Delivery Systems

Purpose A new dual-layer multi-leaf collimator (MLC) system with several improved characteristics was introduced with the Varian Halcyon™ treatment platform. This study evaluated this new MLC's impact on head and neck plan quality and delivery efficiency. Methods Nine patients were retrospectively studied with Institutional Review Board (IRB) approval. To compare plan quality between the Halcyon dual-layer MLC and Truebeam® MLC, all patients were replanned with the same prescription and target coverage following the institutional clinical protocol for both platforms and using both intensity modulated radiation therapy (IMRT) or volumetrically modulated arc therapy (VMAT) techniques. Organs-at-risk (OAR) dose-volume histogram (DVH) statistics were compared along with total plan monitor units (MU). To evaluate delivery efficiency, actual beam-on time for five patients' plans were recorded and compared. To evaluate the impact of MLC performance parameters on plan quality, virtual MLC models were generated by matching Truebeam MLC's parameters to those of the Halcyon dual-layer MLC both individually and combined. OAR doses were then compared between these virtual MLCs, the Truebeam MLC, and the actual Halcyon MLC. Results Overall the Halcyon dual-layer MLC provided similar plan quality compared to Truebeam MLC for VMAT plans, and improved sparing for majority of the OARs when using IMRT. Paired comparison showed median dose differences in mean doses to the parotids, cochlea, esophagus, and larynx ranged from -0.83 Gy to 0.37 Gy for VMAT, and from -4.79 Gy to -0.04 Gy for IMRT, with negative values indicating improved performance by Halcyon. Despite a slight increase in plan MU, the Halcyon reduced the total beam-on time by 42.8 ± 8.5%. Virtual MLC simulations demonstrated that matching MLC transmission accounted for nearly half of the total dose difference between Halcyon and Truebeam IMRT plans. Conclusion When compared to the Truebeam, the Halcyon's dual-layer MLC achieved similar plan quality using VMAT, and improved OAR sparing using IMRT, while providing nearly twice as fast treatment delivery. Reduction in MLC transmission is the dominating factor contributing to dosimetric differences in OAR sparing.

Impact of MLC properties and IMRT technique in meningioma and head-and-neck treatments

Purpose

The impact of multileaf collimator (MLC) design and IMRT technique on plan quality and delivery improvements for head-and-neck and meningioma patients is compared in a planning study.

Material and methods

Ten previously treated patients (5 head-and-neck, 5 meningioma) were re-planned for step-and-shoot IMRT (ssIMRT), sliding window IMRT (dMLC) and VMAT using the MLCi2 without (−) and with (+) interdigitation and the Agility-MLC attached to an Elekta 6MV linac. This results in nine plans per patient. Consistent patient individual optimization parameters are used. Plans are generated using the research tool Hyperion V2.4 (equivalent to Elekta Monaco 3.2) with hard constraints for critical structures and objectives for target structures. For VMAT plans, the improved segment shape optimization is used.

Critical structures are evaluated based on QUANTEC criteria. PTV coverage is compared by EUD, D_mean, homogeneity and conformity. Additionally, MU/plan, treatment times and number of segments are evaluated.

Results

As constrained optimization is used, all plans fulfill the hard constraints. Doses to critical structures do not differ more than 1Gy between the nine generated plans for each patient. Only larynx, parotids and eyes differ up to 1.5Gy (D_mean or D_max) or 7 % (volume-constraint) due to (1) increased scatter, (2) not avoiding structures when using the full range of gantry rotation and (3) improved leaf sequencing with advanced segment shape optimization for VMAT plans. EUD, D_mean, homogeneity and conformity are improved using the Agility-MLC. However, PTV coverage is more affected by technique. MU increase with the use of dMLC and VMAT, while the MU are reduced by using the Agility-MLC. Fastest treatments are always achieved using Agility-MLC, especially in combination with VMAT.

Conclusion

Fastest treatments with the best PTV coverage are found for VMAT plans with Agility-MLC, achieving the same sparing of healthy tissue compared to the other combinations of ssIMRT, dMLC and VMAT with either MLCi2^−/+ or Agility.

Is 5 mm MMLC suitable for VMAT-based lung SBRT? A dosimetric comparison with 2.5 mm HDMLC using RTOG-0813 treatment planning criteria for both conventional and high-dose flattening filter-free photon beams

The aim of this study is to assess the suitability of 5 mm millennium multileaf collimator (MMLC) for volumetric‐modulated arc therapy (VMAT)‐based lung stereotactic body radiotherapy (SBRT). Thirty lung SBRT patient treatment plans along with their planning target volumes (ranging from 2.01 cc to 150.11 cc) were transferred to an inhomogeneous lung phantom and retrospectively planned using VMAT technique, along with the high definition multileaf collimator (HDMLC) and MMLC systems. The plans were evaluated using Radiation Therapy Oncology Group (RTOG‐0813) treatment planning criteria for target coverage, normal tissue sparing, and treatment efficiency for both the MMLC and HDMLC systems using flat and flattening filter‐free (FFF) photon beams. Irrespective of the target volumes, both the MLC systems were able to satisfy the RTOG‐0813 treatment planning criteria without having any major deviation. Dose conformity was marginally better with HDMLC. The average conformity index (CI) value was found to be 1.069±0.034 and 1.075±0.0380 for HDMLC and MMLC plans, respectively. For the 6 MV FFF beams, the plan was slightly more conformal, with the average CI values of 1.063±0.029 and 1.073±0.033 for the HDMLC and MMLC plans, respectively. The high dose spillage was the maximum for 2 cc volume set (3% for HDMLC and 3.1% for MMLC). In the case of low dose spillage, both the MLCs were within the protocol of no deviation, except for the 2 cc volume set. The results from this study revealed that VMAT‐based lung SBRT using 5 mm MMLC satisfies the RTOG‐0813 treatment planning criteria for the studied target size and shapes.

PACS numbers: 87.53.Ly, 87.53D, 87.56.jk

Dosimetric effect of Elekta Beam modulator micromultileaf in three-dimensional conformal radiotherapy and intensity-modulated radiotherapy for prostate cancer

The purpose of this study is to analyze the dosimetric effect of Elekta Beam Modulator in 3-dimensional conformal radiation therapy (3DCRT) and in intensity-modulated radiation therapy (IMRT) for localized prostate cancer. We compared treatment plans developed with 2 different Elekta multileaf collimators (MLC): Beam Modulator micro-MLC (mMLC) (4-mm leaf width at the isocenter) and standard MLC (10-mm leaf width at the isocenter). The comparison was performed for 15 patients with localized prostate cancer in 3DCRT and IMRT delivery; a total of 60 treatment plans were processed. The dose-volume histograms were used to provide the quantitative comparison between plans. In particular, we analyzed differences between rectum and bladder sparing in terms of a set of appropriate Vx (percentage of organ at risk [OAR] volume receiving the x dose) and differences between target conformity and coverage in terms of coverage factor and conformation number. Our analysis demonstrates that in 3DCRT there is an advantage in the use of Elekta Beam Modulator mMLC in terms of organ sparing; in particular, a significant decrease in rectal V60 and V50 (p = 0.001) and in bladder V70 and V65 (p = 0.007 and 0.002, respectively) was found. Moreover, a better target dose conformity was obtained (p = 0.002). IMRT plans comparison demonstrated no significant differences between the use of the 4 or 10-mm MLCs. Our analysis shows that in 3DCRT the use of the Elekta Beam Modulator mMLC gives a gain in target conformity and in OARs dose sparing whereas in IMRT plans there is no advantage.

Comparison of stereotactic plans for brain tumors with two different multileaf collimating systems

Linac‐based stereotactic radiosurgery (SRS) has been widely used for treating small intracranial lesions. This technique allows conforming the dose distribution to the planning target volume (PTV), providing a steep dose gradient with the surrounding normal tissues. This is realized through dedicated collimation systems. The present study aims to compare SRS plans with two collimating systems: the beam modulator (BM) of the Elekta Synergy linac and the DirexGroup micromultileaf collimator (μMLC). Seventeen patients (25 PTVs) were planned both with BM and μMLC (mounted on an Elekta Precise linac) using the Odyssey (PerMedics) treatment planning system (TPS). Plans were compared in terms of dose‐volume histograms (DVH), minimum dose to the PTV, conformity index (CI), and homogeneity index (HI), as defined by the TPS, and doses to relevant organs at risk (OAR). The mean difference between the μMLC and the BM plans in minimum PTV dose was 5.7%±4.2% in favor of the μMLC plans. No statistically significant difference was found between the distributions of the CI values for the two planning modalities (p=0.54), while the difference between the distributions of the HI values was statistically significant (p=0.018). For both BM and μMLC plans, no differences were observed in CI and HI, depending on lesion size and shape. The PTV homogeneity achieved by BM plans was 15.1%±6.8% compared to 10.4%±6.6% with μMLC. Higher maximum and mean doses to OAR were observed in the BM plans; however, for both plans, dose constraints were respected. The comparison between the two collimating systems showed no substantial differences in terms of PTV coverage or OAR sparing. The improvements obtained by using μMLC are relatively small, and both systems turned out to be adequate for SRS treatments.

PACS numbers: 87.53.Ly, 87.55.dk, 87.56.nk

Impact of MLC leaf width on volumetric-modulated arc therapy planning for head and neck cancers

This dosimetric study investigated the impact of multileaf collimators (MLC) leaf width in volumetric‐modulated arc therapy (VMAT) for head and neck cancers (HNC), either with a “standard” simultaneously integrated boost technique (S‐SIB) or with a “dose painting” SIB technique (DP‐SIB). HNC patients were planned either with an S‐SIB comprising three dose levels, from 56 to 70 Gy (16 patients), or with a DP‐SIB comprising five dose levels, from 56 to 84 Gy (8 patients), in 35 fractions. Two VMAT plans were calculated for each SIB technique using two Elekta MLCs: MLCi2 with 10 mm leaf width and Beam Modulator (BM) with 4 mm leaf width. Dose distributions were evaluated by comparing doses on PTVs, main OARs, and healthy tissue, and by comparing conformation indexes. Treatment efficiencies were evaluated by comparing the number of monitor units and the number of needed arcs. Comparisons of the two MLCs depending on the two SIB techniques showed: i) Regarding PTVs: Dmean and D2% on lower doses PTV decreased respectively by 0.5 Gy (p=0.01) and 0.9 Gy (p=0.01) with BM than with MLCi2 for S‐SIB; no significant difference was found for DP‐SIB; ii) Regarding OARs: for spinal cord and brainstem, D2% decreased respectively by 1.2 Gy (p=0.03) and 4.2 Gy (p=0.04) with BM than with MLCi2 for S‐SIB; for controlateral parotid, D50% decreased by 1.5 Gy (p=0.01) with BM than with MLCi2 for S‐SIB; iii) Regarding treatment efficiency : the number of monitor units was 44% (p=0.00) and 51% (p=0.01) higher with BM for S‐SIB and DP‐SIB, respectively. Two arcs were more frequently needed with BM to reach an acceptable dose distribution. This study demonstrated that Beam Modulator (4 mm leaf width) and MLCi2 (10 mm leaf width) MLCs from Elekta provided satisfactory dose distributions for treatment delivery with VMAT technique for complex HNC cases with standard and dose painting prescriptions. OAR sparing was better with BM, mainly for brainstem and spinal cord. However, delivery efficiency of VMAT plans was better with MLCi2.

PACS numbers: 87.56.N‐, 87.56.nk, 87.55.D‐

Dosimetric comparison of different treatment modalities for stereotactic radiosurgery of arteriovenous malformations and acoustic neuromas

PURPOSE

We investigated the influence of beam modulation on treatment planning by comparing four available stereotactic radiosurgery (SRS) modalities: Gamma-Knife-Perfexion, Novalis-Tx Dynamic-Conformal-Arc (DCA) and Dynamic-Multileaf-Collimation-Intensity-Modulated-radiotherapy (DMLC-IMRT), and Cyberknife.

MATERIAL AND METHODS

Patients with arteriovenous malformation (n = 10) or acoustic neuromas (n = 5) were planned with different treatment modalities. Paddick conformity index (CI), dose heterogeneity (DH), gradient index (GI) and beam-on time were used as dosimetric indices.

RESULTS

Gamma-Knife-Perfexion can achieve high degree of conformity (CI = 0.77 ± 0.04) with limited low-doses (GI = 2.59 ± 0.10) surrounding the inhomogeneous dose distribution (D(H) = 0.84 ± 0.05) at the cost of treatment time (68.1 min ± 27.5). Novalis-Tx-DCA improved this inhomogeneity (D(H) = 0.30 ± 0.03) and treatment time (16.8 min ± 2.2) at the cost of conformity (CI = 0.66 ± 0.04) and Novalis-TX-DMLC-IMRT improved the DCA CI (CI = 0.68 ± 0.04) and inhomogeneity (D(H) = 0.18 ± 0.05) at the cost of low-doses (GI = 3.94 ± 0.92) and treatment time (21.7 min ± 3.4) (p<0.01). Cyberknife achieved comparable conformity (CI = 0.77 ± 0.06) at the cost of low-doses (GI = 3.48 ± 0.47) surrounding the homogeneous (D(H) = 0.22 ± 0.02) dose distribution and treatment time (28.4min±8.1) (p<0.01).

CONCLUSIONS

Gamma-Knife-Perfexion will comply with all SRS constraints (high conformity while minimizing low-dose spread). Multiple focal entries (Gamma-Knife-Perfexion and Cyberknife) will achieve better conformity than High-Definition-MLC of Novalis-Tx at the cost of treatment time. Non-isocentric beams (Cyberknife) or IMRT-beams (Novalis-Tx-DMLC-IMRT) will spread more low-dose than multiple isocenters (Gamma-Knife-Perfexion) or dynamic arcs (Novalis-Tx-DCA). Inverse planning and modulated fluences (Novalis-Tx-DMLC-IMRT and CyberKnife) will deliver the most homogeneous treatment. Furthermore, Linac-based systems (Novalis and Cyberknife) can perform image verification at the time of treatment delivery.

Clinical commissioning and use of the Novalis Tx linear accelerator for SRS and SBRT

The purpose of this study was to perform comprehensive measurements and testing of a Novalis Tx linear accelerator, and to develop technical guidelines for com-missioning from the time of acceptance testing to the first clinical treatment. The Novalis Tx (NTX) linear accelerator is equipped with, among other features, a high-definition MLC (HD120 MLC) with 2.5 mm central leaves, a 6D robotic couch, an optical guidance positioning system, as well as X-ray-based image guidance tools to provide high accuracy radiation delivery for stereotactic radiosurgery and stereotactic body radiation therapy procedures. We have performed extensive tests for each of the components, and analyzed the clinical data collected in our clinic. We present technical guidelines in this report focusing on methods for: (1) efficient and accurate beam data collection for commissioning treatment planning systems, including small field output measurements conducted using a wide range of detectors; (2) commissioning tests for the HD120 MLC; (3) data collection for the baseline characteristics of the on-board imager (OBI) and ExacTrac X-ray (ETX) image guidance systems in conjunction with the 6D robotic couch; and (4) end-to-end testing of the entire clinical process. Established from our clinical experience thus far, recommendations are provided for accurate and efficient use of the OBI and ETX localization systems for intra- and extracranial treatment sites. Four results are presented. (1) Basic beam data measurements: Our measurements confirmed the necessity of using small detectors for small fields. Total scatter factors varied significantly (30% to approximately 62%) for small field measurements among detectors. Unshielded stereotactic field diode (SFD) overestimated dose by ~ 2% for large field sizes. Ion chambers with active diameters of 6 mm suffered from significant volume averaging. The sharpest profile penumbra was observed for the SFD because of its small active diameter (0.6 mm). (2) MLC commissioning: Winston Lutz test, light/radiation field congruence, and Picket Fence tests were performed and were within criteria established by the relevant task group reports. The measured mean MLC transmission and dynamic leaf gap of 6 MV SRS beam were 1.17% and 0.36 mm, respectively. (3) Baseline characteristics of OBI and ETX: The isocenter localization errors in the left/right, posterior/anterior, and superior/inferior directions were, respectively, -0.2 ± 0.2 mm, -0.8 ± 0.2 mm, and -0.8 ± 0.4 mm for ETX, and 0.5 ± 0.7 mm, 0.6 ± 0.5 mm, and 0.0 ± 0.5 mm for OBI cone-beam computed tomography. The registration angular discrepancy was 0.1 ± 0.2°, and the maximum robotic couch error was 0.2°. (4) End-to-end tests: The measured isocenter dose differences from the planned values were 0.8% and 0.4%, measured respectively by an ion chamber and film. The gamma pass rate, measured by EBT2 film, was 95% (3% DD and 1 mm DTA). Through a systematic series of quantitative commissioning experiments and end-to-end tests and our initial clinical experience, described in this report, we demonstrate that the NTX is a robust system, with the image guidance and MLC requirements to treat a wide variety of sites - in particular for highly accurate delivery of SRS and SBRT-based treatments.

[Influence of high-definition multileaf collimator for three-dimensional conformal radiotherapy and intensity-modulated radiotherapy of prostate cancer]

The focus of this work is to evaluate the dosimetric impact of treatment planning for three-dimensional conformal radiotherapy (3DCRT) and intensity-modulated radiotherapy (IMRT) of prostate cancer using Varian/BrainLAB 120-leaf high-definition multileaf collimator (HD120 MLC) with 2.5 mm leaf width and Varian 120-leaf millennium multileaf collimator (M120 MLC) with 5 mm leaf width. We measured the leaf transmission and dosimetric leaf gap (DLG) of two multileaf collimator (MLC) systems using Farmer ionization chamber. The dosimetric impact of treatment planning for 3DCRT and IMRT of prostate cancer for ten clinical cases using two MLC systems was evaluated quantitatively. 3DCRT was divided to 3DCRT(middle) as fitting at middle of leaf tip and 3DCRT(outside) as fitting at outside of leaf tip. The leaf transmission factor and DLG of HD120 MLC for 6 and 10 MV X-ray decreased by 0.2% and 1 mm, respectively, compared to M120 MLC. The mean conformity index of PTV of treatment planning for prostate 3DCRT(middle), 3DCRT(outside) , and IMRT decreased by 0.9%, 6.6%, and 0.9% and the mean homogeneity index increased 2.3%, 13.0%, and 4.2%, respectively. The mean V20, V40, and V65 decreased by 2.4%, 6.6%, and 4.5% for bladder and 3.3%, 6.1%, and 5.9% for rectum, respectively. The results of this work demonstrated that the dose conformity of PTV improved and the dose of bladder and rectum decreased for 3DCRT and IMRT of prostate cancer using HD120 MLC compared to M120 MLC, because of reduction of leaf width, leaf transmission, and rounded leaf end transmission.

The Dosimetric and Delivery Advantages of a New 160-leaf MLC

The purpose of this study was to conduct a measurement and treatment planning study on the dosimetric and delivery advantages of a new 160-leaf multileaf collimator (MLC). Recently, a new 160-leaf multileaf collimator (Siemens 160 MLC™) was introduced. The 160-MLC is a single focused design that consists of 160-leafs (80 pairs), each 95 mm thick with a projected leaf width of 5 mm at the machine isocenter. Compared to its double focused predecessors, the 82-leaf MLC (Siemens OPTIVIEW™ MLC) and 58-leaf MLC (Siemens 3-D MLC™), the 160-MLC has leaf widths of half the size. The most notable difference is the new slanted leaf design that replaced the tongue and groove system and allows for complete interdigitation. A systematic study that compared the dosimetric and delivery differences among the 160-MLC, 58-MLC, and divergent Cerrobend blocks was performed. Dosimetric conformity for each collimator type was determined by conforming each to circular targets of various diameters. The effective penumbra for each collimator type was calculated by conforming each, at various collimator angles, to a square stationary target. The quality of 3D conformal radiotherapy treatment (3D-CRT) plans and the quality intensity modulated radiation treatment (IMRT) plans were respectively compared with each collimator type. The 160-MLC was found to have improved dosimetric conformity over the 58-MLC. The divergent Cerrobend block showed marginal dosimetric conformity improvement over the 160-LMC. Overall, the 160-MLC had a 45% and 29% reduction in the 20/80 and 30/90 effective penumbra over the 58-MLC, respectively, while exhibiting only a slightly larger effective penumbra over the divergent Cerrobend block. Comparing 3D-CRT plans generated for small lesions of the head and neck, the V100 for the PTV of the plans generated with the Cerrobend blocks, the 58-MLC, and the 160-MLC were 97.78%, 92.51%, and 99.18%, respectively, while with regards to the OARs, the three produced similar DVHs. IMRT plans generated with the 160-MLC were found to significantly reduce the total delivered monitor units by up to 14.7% and the number of segments by as much as 10.7% compared to the 58-MLC. The average delivery time for the direct aperture optimized (DAO) IMRT plans generated with the 160-MLC was approximately 5 minutes. Overall, compared to the 58-MLC, the new 160-MLC was found to improve dosimetric conformity and IMRT delivery efficiency.

Impact of the high-definition multileaf collimator on linear accelerator-based intracranial stereotactic radiosurgery

OBJECTIVES

The impact of two multileaf collimator (MLC) systems for linear accelerator-based intracranial stereotactic radiosurgery (SRS) was assessed.

METHODS

68 lesions formed the basis of this study. 2.5 mm leaf width plans served as reference. Comparative plans, with identical planning parameters, were based on a 5 mm leaf width MLC system. Two collimation strategies, with collimation fixed at 0° or 90° and optimised per arc or beam, were also assessed. Dose computation was based on the pencil beam algorithm with allowance for tissue heterogeneity. Plan normalisation was such that 100% of the prescription dose covered 95% of the planning target volume. Plan evaluation was based on target coverage and normal tissue avoidance criteria.

RESULTS

The median conformity index difference between the MLC systems ranged between 0.8% and 14.2%; the 2.5 mm MLC exhibited better dose conformation. The median reduction of normal tissue exposed to ≥100%, ≥50% and ≥25% of the prescription dose ranged from 13.4% to 29.7%, favouring the 2.5 mm MLC system. Dose fall-off was steeper for the 2.5 mm MLC system with an overall median absolute difference ranging from 0.4 to 1.2 mm. The use of collimation optimisation resulted in a decrease in differences between the MLC systems. The results demonstrated the dosimetric merit of the 2.5 mm leaf width MLC system over the 5 mm leaf width system, albeit small, for the investigated range of intracranial SRS targets.

CONCLUSION

The clinical significance of these results warrants further investigation to determine whether the observed dosimetric advantages translate into outcome improvements.

A planning comparison of dynamic IMRT for different collimator leaf thicknesses with helical tomotherapy and RapidArc for prostate and head and neck tumors

PURPOSE

A comparative analysis of the three most advanced intensity-modulated radiotherapy (IMRT) techniques currently commercially available was performed. Treatment plans made in rotational techniques (helical tomotherapy [HT] and RapidArc) were compared with sliding-window IMRT (dIMRT) on a conventional linear accelerator using different leaf thicknesses (2.5 mm, 5 mm, and 10 mm). The influence of the different planning techniques on the coverage of planning volume and sparing of organs at risk (OARs) was investigated.

PATIENTS AND METHODS

Nine patients with localized prostate and nine patients with head and neck cancer were chosen for this study. Treatment planning was performed in Eclipse (Varian) and in Tomotherapy planning software. Treatment plans were compared according to target volume coverage and sparing OARs, as well as by conformity and homogeneity index.

RESULTS

For both investigated tumor sites, the dosimetric effects of leaf widths between 2.5 mm, 5 mm and 10 mm were shown to be small in regard to target coverage. Tomotherapy plans had better target coverage (higher minimum dose). For prostate cancer, better sparing of bladder and rectum was achieved with RapidArc and dIMRT plans. For head and neck cancer, best sparing of parotid glands was achieved in HT plans. There was no significant difference (p > 0.05) in sparing of OARs between the dIMRT plans with different leaf widths neither for prostate cancer nor for head and neck cancer.

CONCLUSION

For prostate and head and neck cases, all investigated IMRT techniques provide highly conformal treatment plans in terms of both target coverage and critical structure sparing.

Dosimetric comparison using different multileaf collimeters in intensity-modulated radiotherapy for upper thoracic esophageal cancer

Purpose

To study the impacts of multileaf collimators (MLC) width [standard MLC width of 10 mm (sMLC) and micro-MLC width of 4 mm (mMLC)] in the intensity-modulated radiotherapy (IMRT) planning for the upper thoracic esophageal cancer (UTEC).

Methods and materials

10 patients with UTEC were retrospectively planned with the sMLC and the mMLC. The monitor unites (MUs) and dose volume histogram-based parameters [conformity index (CI) and homogeneous index (HI)] were compared between the IMRT plans with sMLC and with mMLC.

Results

The IMRT plans with the mMLC were more efficient (average MUs: 703.1 ± 68.3) than plans with the sMLC (average MUs: 833.4 ± 73.8) (p < 0.05). Also, compared to plans with the sMLC, the plans with the mMLC showed advantages in dose coverage of the planning gross tumor volume (Pgtv) (CI 0.706 ± 0.056/HI 1.093 ± 0.021) and the planning target volume (PTV) (CI 0.707 ± 0.029/HI 1.315 ± 0.013) (p < 0.05). In addition, the significant dose sparing in the D₅ (3260.3 ± 374.0 vs 3404.5 ± 374.4)/gEUD (1815.1 ± 281.7 vs 1849.2 ± 297.6) of the spinal cord, the V₁₀ (33.2 ± 6.5 vs 34.0 ± 6.7), V₂₀ (16.0 ± 4.6 vs 16.6 ± 4.7), MLD (866.2 ± 174.1 vs 887.9 ± 172.1) and gEUD (938.6 ± 175.2 vs 956.8 ± 171.0) of the lungs were observed in the plans with the mMLC, respectively (p < 0.05).

Conclusions

Comparing to the sMLC, the mMLC not only demonstrated higher efficiencies and more optimal target coverage, but also considerably improved the dose sparing of OARs in the IMRT planning for UTEC.

New multileaf collimator with a leaf width of 5 mm improves plan quality compared to 10 mm in step-and-shoot IMRT of HNC using integrated boost procedure

PURPOSE

To investigate whether a new multileaf collimator with a leaf width of 5 mm (MLC-5) over the entire field size of 40 x 40 cm(2) improves plan quality compared to a leaf width of 10 mm (MLC-10) in intensity-modulated radiotherapy (IMRT) with integrated boost for head and neck cancer.

PATIENTS AND METHODS

A plan comparison was performed for ten patients with head and neck cancer. For each patient, seven plans were calculated: one plan with MLC-10 and nine beams, four plans with MLC-5 and nine beams (with different intensity levels and two-dimensional median filter sizes [2D-MFS]), and one seven-beam plan with MLC-5 and MLC-10, respectively. Isocenter, beam angles and planning constraints were not changed. Mean values of common plan parameters over all ten patients were estimated, and plan groups of MLC-5 and MLC-10 with nine and seven beams were compared.

RESULTS

The use of MLC-5 led to a significantly higher conformity index and an improvement of the 90% coverage of PTV1 (planning target volume) and PTV2 compared with MLC-10. This was noted in the nine- and seven-beam plans. Within the nine-beam group with MLC-5, a reduction of the segment number by up to 25% at reduced intensity levels and for increased 2D-MFS did not markedly worsen plan quality. Interestingly, a seven-beam IMRT with MLC-5 was inferior to a nine-beam IMRT with MLC-5, but superior to a nine-beam IMRT with MLC-10.

CONCLUSION

The use of an MLC-5 has significant advantages over an MLC-10 with respect to target coverage and protection of normal tissues in step-and-shoot IMRT of head and neck cancer.

Intensity-modulated radiotherapy plan optimisation for skull base lesions: practical class solutions for dose escalation

AIMS

To identify practical intensity-modulated radiotherapy planning solutions when attempting dose escalation in the skull base.

MATERIALS AND METHODS

Twenty cases of skull base meningioma were re-planned using a variation of beam number (three, five, seven and nine), beam arrangement (coplanar vs non-coplanar) and multileaf collimator (MLC) width (2.5 mm vs 10 mm) to 60 Gy/30 fractions. Plan quality and planning target volume coverage was assessed using planning target volume V(95%), equivalent uniform dose (EUD) and integral dose.

RESULTS

Critical structures were maintained below clinical tolerance levels. The 2.5 mm MLC achieved an average improvement in V(95%) by 22.8% (P=0.0003), EUD by 3.7 Gy (P=0.002) and reduced the integral dose by 13.4 Gy (P=0.0001). V(95%) and the integral dose improved with five vs three beams and seven vs five beams, but did not change with nine vs seven beams. There was no effect of beam number on EUD. There was no difference in V(95%) (P=0.54), integral dose (P=0.44) or EUD (P=0.47) for beam arrangement used. Segments per plan increased by a factor of 1.5 with each addition of two beams to a plan, and by a factor of 2.5 for 2.5 mm MLC plans vs 10 mm MLC plans.

CONCLUSIONS

We present evidence-based planning solutions for skull base intensity-modulated radiotherapy, and show that 2.5 mm MLC and five to seven beams can achieve safe dose escalation up to 60 Gy. This must be balanced with an increase in segmentation, which will increase treatment times.

Implications of a high-definition multileaf collimator (HD-MLC) on treatment planning techniques for stereotactic body radiation therapy (SBRT): a planning study

Purpose

To assess the impact of two multileaf collimator (MLC) systems (2.5 and 5 mm leaf widths) on three-dimensional conformal radiotherapy, intensity-modulated radiotherapy, and dynamic conformal arc techniques for stereotactic body radiation therapy (SBRT) of liver and lung lesions.

Methods

Twenty-nine SBRT plans of primary liver (n = 11) and lung (n = 18) tumors were the basis of this study. Five-millimeter leaf width 120-leaf Varian Millennium (M120) MLC-based plans served as reference, and were designed using static conformal beams (3DCRT), sliding-window intensity-modulated beams (IMRT), or dynamic conformal arcs (DCA). Reference plans were either re-optimized or recomputed, with identical planning parameters, for a 2.5-mm width 120-leaf BrainLAB/Varian high-definition (HD120) MLC system. Dose computation was based on the anisotropic analytical algorithm (AAA, Varian Medical Systems) with tissue heterogeneity taken into account. Each plan was normalized such that 100% of the prescription dose covered 95% of the planning target volume (PTV). Isodose distributions and dose-volume histograms (DVHs) were computed and plans were evaluated with respect to target coverage criteria, normal tissue sparing criteria, as well as treatment efficiency.

Results

Dosimetric differences achieved using M120 and the HD120 MLC planning were generally small. Dose conformality improved in 51.7%, 62.1% and 55.2% of the IMRT, 3DCRT and DCA cases, respectively, with use of the HD120 MLC system. Dose heterogeneity increased in 75.9%, 51.7%, and 55.2% of the IMRT, 3DCRT and DCA cases, respectively, with use of the HD120 MLC system. DVH curves demonstrated a decreased volume of normal tissue irradiated to the lower (90%, 50% and 25%) isodose levels with the HD120 MLC.

Conclusion

Data derived from the present comparative assessment suggest dosimetric merit of the high definition MLC system over the millennium MLC system. However, the clinical significance of these results warrants further investigation in order to determine whether the observed dosimetric advantages translate into outcome improvements.

Dosimetric comparisons of helical tomotherapy and step-and-shoot intensity-modulated radiotherapy in nasopharyngeal carcinoma

PURPOSE

The study evaluates and quantifies the potential dosimetric gains of helical tomotherapy (HT) versus step-and-shoot intensity-modulated radiotherapy (SaS-IMRT) for nasopharyngeal carcinoma (NPC).

MATERIALS AND METHODS

Twenty consecutive NPC patients curatively treated by HT were examined. Each case was planned by HT and SaS-IMRT (ADAC Pinnacle(3)) planning system, respectively. Dose plans were compared using dose volume histograms (DVH), conformity index (CI), homogeneity index (HI), and minimal dose to 1cc (D(min_1cc)) of the planned target volume (PTV) and a comprehensive quality index (CQI) of ten organs at risk (OARs). The prescribed dose/fractionation was 72Gy to the PTV, 64.8Gy to the elective PTV, and 54Gy to the clinically negative neck region. The plan of 54Gy to the PTV (PTV(54)) was used to evaluate the CI and HI in the target. The cumulative doses of the three PTV plans to the OARs were calculated.

RESULTS

We observed the HT plans significantly improved the CI (improvement ratio: 11.9+/-5.5%) and HI (improvement ratio: 8.8+/-1.5%) of the PTV(54) compared with SaS-IMRT plans. In addition, the mean/maximal dose of most of the OARs except chiasm was significantly reduced in HT plans, with the CQI of 0.92+/-0.08. A negative result of HT in chiasm was observed but only significantly revealed in cases without skull base infiltration.

CONCLUSIONS

A dosimetric gain in CI and HI of PTV and sparing of OARs was significantly obtained in HT versus SaS-IMRT plans in NPC patients. Whether such dosimetric superiority in HT could transfer into clinical advantages needs further investigation.

Influence of the linac design on intensity-modulated radiotherapy of head-and-neck plans

In this study, we quantify the impact of linac/MLC design parameters on IMRT treatment plans. The investigated parameters were leaf width in the MLC, leaf transmission, related to the thickness of the leaves, and penumbra related primarily to the source size. Seven head-and-neck patients with stage T1-T3N0-N2cM0 oropharyngeal cancer were studied. For each patient nine plans were made with a different set of linac/MLC parameters. The plans were optimized in Pinnacle(3) v7.6c and PLATO RTS v2.6.4, ITP v1.1.8. A hypothetical ideal linac/MLC was introduced to investigate the influence of one parameter at a time without interaction of other parameters. When any of the three parameters was increased from the ideal set-up values (leaf width 2.5 mm, transmission 0%, penumbra 3 mm), the mean dose to the parotid glands increased, given the same tumour coverage. The largest increase was found for increasing leaf transmission. The investigation showed that by changing more than one parameter of the ideal linac/MLC set-up, the increase in the mean dose was smaller than the sum of dose increments for each parameter separately. As a reference to clinical practice, we also optimized the plans of the seven patients with the clinically used Elekta SLi 15, equipped with a standard MLC with a leaf width of 10 mm. As compared to the ideal linac, this resulted in an increase of the average dose to the parotid glands of 5.8 Gy.

Intensity-modulated radiation therapy (IMRT) dosimetry of the head and neck: a comparison of treatment plans using linear accelerator-based IMRT and helical tomotherapy

PURPOSE

To date, most intensity-modulated radiation therapy (IMRT) delivery has occurred using linear accelerators (linacs), although helical tomotherapy has become commercially available. To quantify the dosimetric difference, we compared linac-based and helical tomotherapy-based treatment plans for IMRT of the oropharynx.

METHODS AND MATERIALS

We compared the dosimetry findings of 10 patients who had oropharyngeal carcinoma. Five patients each had cancers in the base of the tongue and tonsil. Each plan was independently optimized using either the CORVUS planning system (Nomos Corporation, Sewickly, PA), commissioned for a Varian 2300 CD linear accelerator (Varian Medical Systems, Palo Alto, CA) with 1-cm multileaf collimator leaves, or helical tomotherapy. The resulting treatment plans were evaluated by comparing the dose-volume histograms, equivalent uniform dose (EUD), dose uniformity, and normal tissue complication probabilities.

RESULTS

Helical tomotherapy plans showed improvement of critical structure avoidance and target dose uniformity for all patients. The average equivalent uniform dose reduction for organs at risk (OARs) surrounding the base of tongue and the tonsil were 17.4% and 27.14% respectively. An 80% reduction in normal tissue complication probabilities for the parotid glands was observed in the tomotherapy plans relative to the linac-based plans. The standard deviation of the planning target volume dose was reduced by 71%. In our clinic, we use the combined dose-volume histograms for each class of plans as a reference goal for helical tomotherapy treatment planning optimization.

CONCLUSIONS

Helical tomotherapy provides improved dose homogeneity and normal structure dose compared with linac-based IMRT in the treatment of oropharyngeal carcinoma resulting in a reduced risk for complications from focal hotspots within the planning target volume and for the adjacent parotid glands.

IMRT-application with an add-on MMLC

In order to provide automatic IMRT dose delivery with an add‐on MMLC a technical integration of a MMLC system with a linear accelerator was realized. The principle of this integration and the changes and enhancements of the existing hard‐and software are briefly described. The system was tested by the automatic delivery of an IMRT plan designed for a head and neck phantom. A verification of dose delivery was performed with film dosimetry. The plan consisting of 78 “step and shoot” segments could be delivered within 17 minutes. A high spatial accuracy of the fluence pattern at the isocentre was reached by a resolution of 2.75×2.75 mm². The measured dose profiles were within 3% of the maximum dose of the calculated profiles.

PACS number(s): 87.53.–j, 87.90.+y