Dosimetric effects of multileaf collimator leaf width on intensity-modulated radiotherapy for head and neck cancer

Key changes in the future clinical application of ultra-high dose rate radiotherapy

Ultra-high dose rate radiotherapy (FLASH-RT) is an external beam radiotherapy strategy that uses an extremely high dose rate (≥40 Gy/s). Compared with conventional dose rate radiotherapy (≤0.1 Gy/s), the main advantage of FLASH-RT is that it can reduce damage of organs at risk surrounding the cancer and retain the anti-tumor effect. An important feature of FLASH-RT is that an extremely high dose rate leads to an extremely short treatment time; therefore, in clinical applications, the steps of radiotherapy may need to be adjusted. In this review, we discuss the selection of indications, simulations, target delineation, selection of radiotherapy technologies, and treatment plan evaluation for FLASH-RT to provide a theoretical basis for future research.

Evaluation of the dataset quality in gamma passing rate predictions using machine learning methods

OBJECTIVE

Gamma passing rate (GPR) predictions using machine learning methods have been explored for treatment verification of radiotherapy plans. However, these methods presented datasets with unbalanced number of plans having different treatment conditions (heterogeneous datasets), such as anatomical sites or dose per fractions, leading to lower model interpretability and prediction performance.

METHODS

We investigated the impact of the dataset composition on GPR binary classification (pass/fail) using random forest (RF), XG-boost, and neural network (NN) models. 945 plans were used to create one reference dataset (randomly assembled) and 24 customized datasets that considered four heterogeneity factors independently (anatomical region, number of arcs, dose per fraction, and treatment unit). 309 predictor features were extracted and calculated from plan parameters, modulation complexity metrics, and radiomic analysis (leave-trajectory maps, 3D dose distributions, and portal dosimetry images). The models' performances were measured using the area under the curve from the receiver operating characteristic (ROC-AUC).

RESULTS

Radiomics features for reference models increased ROC-AUC values up to 13%, 15%, and 5% for RF, XG-Boost, and NN, respectively. The datasets with higher heterogeneous conditions presented the lower ROC-AUC values (RF: 0.72 ± 0.11, XG-Boost: 0.67 ± 0.1, NN: 0.89 ± 0.05) compared to models with less heterogeneous treatment conditions (RF: 0.88 ± 0.06, XG-Boost: 0.89 ± 0.07, NN: 0.98 ± 0.01). The ten most important features for each heterogeneity dataset group demonstrated their correlation with the treatments' physical aspects and GPR prediction.

CONCLUSION

Improvements in data generalization and model performances can be associated with datasets having similar treatment conditions. This analysis might be implemented to evaluate the dataset quality and model consistency of further ML applications in radiotherapy.

ADVANCES IN KNOWLEDGE

Dataset heterogeneities decrease ML model performance and reliability.

Dosimetric evaluation study of IMRT and VMAT techniques for prostate cancer based on different multileaf collimator designs

The hypofractionated radiotherapy modality was established to reduce treatment durations and enhance therapeutic efficiency, as compared to conventional fractionation treatment. However, this modality is challenging because of rigid dosimetric constraints. This study aimed to assess the impact of multi-leaf collimator (MLC) widths (10 mm and 5 mm) on plan quality during the treatment of prostate cancer. Additionally, this study aimed to investigate the impact of the MLC mode of energy on the Agility flattening filter (FF), MLC Agility-free flattening filter (FFF), and MLCi2 for patients receiving hypofractionated radiotherapy. Two radiotherapy techniques; Intensity Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Radiotherapy (VMAT), were used in this research. In the present study, computed tomography simulations of ten patients (six plans per patient) with localized prostate adenocarcinoma were analyzed. Various dosimetric parameters were assessed, including monitor units, treatment delivery times, conformity, and homogeneity indices. To evaluate the plan quality, dose-volume histograms (DVHs) were estimated for each technique. The results demonstrated that the determined dosimetric parameters of planning target volume (PTV)p (such as D mean, conformity, and homogeneity index) showed greater improvement with MLC Agility FF and MLC Agility FFF than with MLCi2. Additionally, the treatment delivery time was reduced in the MLC Agility FF (by 31%) and MLC Agility FFF (by 10.8%) groups compared to the MLCi2 group. It is concluded that for both the VMAT and IMRT techniques, the smaller width (5 mm) MLCs revealed better planning target volume coverage, improved the dosimetric parameters for PTV, reduced the treatment time, and met the constraints for OARs. It is therefore recommended to use 5 mm MLCs for hypofractionated prostate cancer treatment due to better target coverage and better protection of OARs.

Dosimetric comparison between VMAT plans using the fast-rotating O-ring linac with dual-layer stacked MLC and helical tomotherapy for nasopharyngeal carcinoma

BACKGROUND

To compare the dosimetric profiles of volumetric modulated arc therapy (VMAT) plans using the fast-rotating O-ring linac (the Halcyon system) based on a dual-layer stacked multi-leaf collimator and helical tomotherapy (HT) for nasopharyngeal cancer (NPCa).

METHODS

For 30 NPCa patients, three sets of RT plans were generated, under the same policy of contouring and dose constraints: HT plan; Halcyon VMAT plan with two arcs (HL_2arc); and Halcyon VMAT plan with four arcs (HL_4arc), respectively. The intended dose schedule was to deliver 67.2 Gy to the planning gross target volume (P-GTV) and 56.0 Gy to the planning clinical target volume (P-CTV) in 28 fractions using the simultaneously integrated boost concept. Target volumes and organ at risks dose metrics were evaluated for all plans. Normal tissue complication probabilities (NTCP) for esophagus, parotid glands, spinal cord, and brain stem were compared.

RESULTS

The HT plan achieved the best dose homogeneity index for both P_GTV and P_CTV, followed by the HL_4arc and L_2arc plans. No significant difference in the dose conformity index (CI) for P_GTV was observed between the HT plan (0.80) and either the HL_2arc plan (0.79) or the HL_4arc plan (0.83). The HL_4arc plan showed the best CI for P_CTV (0.88), followed by the HL_2arc plan (0.83) and the HT plan (0.80). The HL_4arc plan (median, interquartile rage (Q1, Q3): 25.36 (22.22, 26.89) Gy) showed the lowest D_mean in the parotid glands, followed by the HT (25.88 (23.87, 27.87) Gy) and HL_2arc plans (28.00 (23.24, 33.99) Gy). In the oral cavity (OC) dose comparison, the HT (22.03 (19.79, 24.85) Gy) plan showed the lowest D_mean compared to the HL_2arc (23.96 (20.84, 28.02) Gy) and HL_4arc (24.14 (20.17, 27.53) Gy) plans. Intermediate and low dose regions (40-65% of the prescribed dose) were well fit to the target volume in HL_4arc, compared to the HT and HL_2arc plans. All plans met the dose constraints for the other OARs with sufficient dose margins. The between-group differences in the median NTCP values for the parotid glands and OC were < 3.47% and < 1.7% points, respectively.

CONCLUSIONS

The dosimetric profiles of Halcyon VMAT plans were comparable to that of HT, and HL_4arc showed better dosimetric profiles than HL_2arc for NPCa.

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.

Effect of the simulated half leaf width of a multileaf collimator on volumetric modulated arc therapy plan quality in hippocampal avoidance whole-brain radiotherapy

Purpose

Whole‐brain radiotherapy (WBRT) is commonly used in patients with multiple brain metastases. Compared with conventional WBRT, hippocampal avoidance WBRT (HA‐WBRT) more favorably preserves cognitive function and the quality of life. The hippocampal volume is considerably small (approximately 3.3 cm³). Therefore, downsizing the leaf width of a multileaf collimator (MLC) may provide higher spatial resolution and better plan quality. Volumetric modulated arc therapy (VMAT) could simulate the half MLC leaf width through couch shifting between arcs. This study investigated changes in VMAT quality for HA‐WBRT with a simulated fine MLC leaf width.

Methods

We included 18 patients with brain metastasis. All target and avoidance structures were contoured by an experienced radiation oncologist. The prescribed dose was 30 Gy in 10 fractions. For each patient, three different treatment plans were generated for comparison: VMAT with couch‐shift, VMAT without couch‐shift, and TomoTherapy. All treatment plans fulfilled Radiation Therapy Oncology Group (RTOG) 0933 criteria for HA‐WBRT. The Wilcoxon paired signed‐rank test was used to compare different treatment plans.

Results

VMAT with couch‐shift had the better average conformity index (0.823) with statistically significant difference compared to VMAT without couch‐shift (0.810). VMAT with couch‐shift (0.219) had a more favorable average homogeneity index (HI) than did VMAT without couch‐shift (0.230), although the difference was not significant. TomoTherapy had an optimal average HI of 0.070. In terms of the hippocampus, all three treatment plans met the RTOG 0933 criteria. VMAT with couch‐shift had a lower average D_max (15.2 Gy) than did VMAT without couch‐shift (15.3 Gy, p = 0.071) and TomoTherapy (15.5 Gy, p = 0.133). The average D_100% of hippocampus was the same for both VMAT with and without couch‐shift (8.5 Gy); however, TomoTherapy had a lower average D_100% value of 7.9 Gy. The treatment delivery time was similar between VMAT with and without couch‐shift (average, 375.0 and 369.6 s, respectively). TomoTherapy required a long average delivery time of 1489.9 s.

Conclusion

The plan quality of VMAT for HA‐WBRT was improved by using the couch‐shift technique to simulate the half MLC leaf width. However, the improvement was not statistically significant except conformity index. The downsizing effect decreased with the use of the sophisticated grade of VMAT. TomoTherapy offered superior plan quality but required the longest delivery time.

Reducing variability among treatment machines using knowledge-based planning for head and neck, pancreatic, and rectal cancer

Purpose

This study aimed to assess dosimetric indices of RapidPlan model‐based plans for different energies (6, 8, 10, and 15 MV; 6‐ and 10‐MV flattening filter‐free), multileaf collimator (MLC) types (Millennium 120, High Definition 120, dual‐layer MLC), and disease sites (head and neck, pancreatic, and rectal cancer) and compare these parameters with those of clinical plans.

Methods

RapidPlan models in the Eclipse version 15.6 were used with the data of 28, 42, and 20 patients with head and neck, pancreatic, and rectal cancer, respectively. RapidPlan models of head and neck, pancreatic, and rectal cancer were created for TrueBeam STx (High Definition 120) with 6 MV, TrueBeam STx with 10‐MV flattening filter‐free, and Clinac iX (Millennium 120) with 15 MV, respectively. The models were used to create volumetric‐modulated arc therapy plans for a 10‐patient test dataset using all energy and MLC types at all disease sites. The Holm test was used to compare multiple dosimetric indices in different treatment machines and energy types.

Results

The dosimetric indices for planning target volume and organs at risk in RapidPlan model‐based plans were comparable to those in the clinical plan. Furthermore, no dose difference was observed among the RapidPlan models. The variability among RapidPlan models was consistent regardless of the treatment machines, MLC types, and energy.

Conclusions

Dosimetric indices of RapidPlan model‐based plans appear to be comparable to the ones based on clinical plans regardless of energies, MLC types, and disease sites. The results suggest that the RapidPlan model can generate treatment plans independent of the type of treatment machine.

Comparison of Intensity Modulated Radiotherapy Treatment Plans Between 1.5T MR-Linac and Conventional Linac

In this study, we assess the dosimetric qualities and usability of planning for 1.5 T MR-Linac based intensity modulated radiotherapy (MRL-IMRT) for various clinical sites in comparison with IMRT plans using a conventional linac. In total of 30 patients with disease sites in the brain, esophagus, lung, rectum and vertebra were re-planned retrospectively for simulated MRL-IMRT using the Elekta Unity dedicated treatment planning system (TPS) Monaco (v5.40.01). Currently, the step-and-shoot (ss) is the only delivery technique for IMRT available on Unity. All patients were treated on an Elekta Versa HD^TM with IMRT using the dynamic multileaf collimator (dMLC) technique, and the plans were designed using Monaco v5.11. For comparison, the same dMLC-IMRT plan was recalculated with the same machine and TPS but only changing the technique to step-and-shoot. The dosimetric qualities of the MRL-IMRT plans, to be evaluated by the Dose Volume Histograms (DVH) metrics, Homogeneity Index and Conformality Index, were compared with the clinical plans. The planning usability was measured by the optimization time and the number of Monitor Units (MUs). Comparing MRL-IMRT with conventional linac based plans, all created plans were clinically equivalent to current clinical practice. However, MRL-IMRT plans had higher dose to skin and larger low dose region of normal tissues. Furthermore, MRL-IMRT plans had significantly reduced optimization time by comparing conventional linac based plans. The number of MUs of MRL-IMRT was increased by 23% compared with ss-IMRT, and no difference from dMLC-IMRT. In conclusion, clinically acceptable plans can be achieved with 1.5 T MR-Linac system for multiple tumor sites. Given the differences in machine characteristics, some minor differences in plan quality were found between MR-Linac plans and current clinical practice and this should be considered in clinical practice.

Technological quality requirements for stereotactic radiotherapy : Expert review group consensus from the DGMP Working Group for Physics and Technology in Stereotactic Radiotherapy

This review details and discusses the technological quality requirements to ensure the desired quality for stereotactic radiotherapy using photon external beam radiotherapy as defined by the DEGRO Working Group Radiosurgery and Stereotactic Radiotherapy and the DGMP Working Group for Physics and Technology in Stereotactic Radiotherapy. The covered aspects of this review are 1) imaging for target volume definition, 2) patient positioning and target volume localization, 3) motion management, 4) collimation of the irradiation and beam directions, 5) dose calculation, 6) treatment unit accuracy, and 7) dedicated quality assurance measures. For each part, an expert review for current state-of-the-art techniques and their particular technological quality requirement to reach the necessary accuracy for stereotactic radiotherapy divided into intracranial stereotactic radiosurgery in one single fraction (SRS), intracranial fractionated stereotactic radiotherapy (FSRT), and extracranial stereotactic body radiotherapy (SBRT) is presented. All recommendations and suggestions for all mentioned aspects of stereotactic radiotherapy are formulated and related uncertainties and potential sources of error discussed. Additionally, further research and development needs in terms of insufficient data and unsolved problems for stereotactic radiotherapy are identified, which will serve as a basis for the future assignments of the DGMP Working Group for Physics and Technology in Stereotactic Radiotherapy. The review was group peer-reviewed, and consensus was obtained through multiple working group meetings.

Monte Carlo simulation of a 2D dynamic multileaf collimator to improve the plan quality in radiotherapy plan: a proof-of-concept study

The leaf width of a multileaf collimator (MLC) determines the dose conformity to the target volume. The objective of this study was to investigate the feasibility of a two-dimensional dynamic MLC (2DDMLC) to improve the treatment plan quality with a fixed leaf width. The treatment head of the Clinac^™ linear accelerator with the Millennium 120^™ MLC was modelled with the Geant4 (for GEometry ANd Tracking) tollkit using the Monte Carlo (MC) method. The 2DDMLC produces a beam aperture by moving the MLC bank vertically to the leaf movement. Thus, the effect of the 2DDMLC motion on beam divergence and beam fluence resolution was evaluated by comparing the dose distributions between the conventional MLC motion and the 2DDMLC. Finally, the 2DDMLC was employed for dynamic conformal arc therapy for 13 brain cancer patients. The dose-volumetric parameters, including the dose delivered to 98% of the target volume (D _98%), percent volume given 20% of the prescribed dose (V _20%), and conformity index (CI) were compared with those of the conventional MLC. For the 6 MV beam of the MC model, the depth dose and lateral dose distribution differed by less than 2% between the simulation and measurement. The 2DDMLC did not significantly influence beam divergence and sharpened the beam. In clinical use, the dose delivered to the target was almost identical between the 2DDMLC and conventional MLC (D _98%  =  29.74 Gy versus 29.71 Gy, p   =  0.18). The CI was improved with the use of the 2DDMLC (CI  =  1.49 versus 1.47, p   =  0.14). Moreover, irradiation of normal tissue was reduced with the 2DDMLC compared with conventional MLC (V _20%  =  17.22% versus 17.45%, p   <  0.001). The 2DDMLC improved the dose conformity to the target volume and reduced the irradiation of the normal tissue compared with the conventional MLC.

Is High Definition MLC Dosimetrically Superior to Standard Definition MLC for SIB-SBRT for Carcinoma Prostate

OBJECTIVE

The study was conducted to quantitatively evaluate the dosimetric effects of high definition (2.5 mm) and standard definition (5.0 mm) MLC on the quality of SBRT plans using SIB-IMRT and SIB-VMAT technique for carcinoma prostate and also to evaluate the dosimetric advantage of one technique over the other.

MATERIALS AND METHODS

Seventeen annonymized planning CT data sets were used to generate plans for both VMAT and IMRT techniques using 2.5 mm and 5.0 mm MLC.The prescription to the nodule was 45Gy in 5 fractions and to the prostate was 35Gy in 5 fractions.CI, GI, D2%, D98%, D50% and V95% for target; D2%, Dmean, V80%, V20% for OAR's; V5% of the irradiated volume, and delivered MU's were analyzed.An independent t-test was used to compare the plans. Patient specific QA for all plans were also performed and analyzed.

RESULTS

Minor difference in dosimetric indices was observed between 2.5mm and 5mm MLC VMAT plans, except D2% (PTV35) and D98% (GTV45) were better in 2.5mm MLC plans (p.

Feasibility of 5-mm vs 2.5-mm width multileaf collimator in noncoplanar volumetric modulated arc stereotactic radiotherapy for multiple brain metastases

The aim of this study was to examine the feasibility of noncoplanar volumetric modulated arc stereotactic radiotherapy (VMAT-SRT) using a 5-mm multileaf collimator (MLC) for multiple brain metastases. We identified 34 multiple-target cases (3 to 19 targets in each case) with a total of 257 of targets and constructed noncoplanar VMAT-SRT plans using 5-mm and 2.5-mm MLCs with 4-arc. The prescribed dose was 36 Gy/6 fr. Plans were evaluated using the Paddick conformity indices (PCI), Paddick gradient index (PGI), and normal brain dose (NBD, equal to the mean brain dose minus gross tumor volume). There were no significant differences in PCI (median [range]: 5 mm, 0.88 [0.78 to 0.94]; 2.5 mm, 0.89 [0.78 to 0.94]; p= 0.691), PGI (median [range]: 5 mm, 3.96 [2.21 to 6.63]; 2.5 mm, 3.96 [2.24 to 6.45]; p= 0.358), or NBD (median [range]: 5 mm, 7.5 Gy [2.5 to 12.4]; 2.5 mm, 7.5 Gy [2.5 to 12.5]; p= 0.675). The performance of the 5-mm MLC was not inferior to the 2.5-mm MLC in applications of noncoplanar VMAT-SRT for multiple brain metastases with regards to dose conformity, gradient, and NBD. This study provides the necessary background for generalizing noncoplanar VMAT-SRT approaches in treating multiple brain lesions.

Treatment plan quality and delivery accuracy assessments on 3 IMRT delivery methods of stereotactic body radiotherapy for spine tumors

Stereotactic body radiotherapy (SBRT) for spine tumors has demonstrated clinical effectiveness. The treatment planning and delivery techniques have evolved from dynamic conformal arc therapy, to fixed gantry angle intensity modulated radiotherapy (IMRT), and most recently to volumetric modulated arc therapy (VMAT). A hybrid-arc (HARC) planning and delivery method combining dynamic conformal arc therapy delivery with a number of equally spaced IMRT beams is proposed. In this study we investigated plan quality, delivery accuracy, and efficiency of 3 delivery techniques: IMRT, HARC, and VMAT. Patients who underwent spine SBRT treatments were randomly selected from an Institutional Review Board-approved registry. For each patient, the prescription dose was 14 to 16 Gy in a single fraction to cover >90% of the tumor (without planning margin) while constraining V_10Gy ≤ 10% of the spinal cord and the maximum point dose (MPD) of the spinal cord ≤ 14 Gy. All cases were clinically treated with fixed gantry step-shoot IMRT plans and then re-planned with VMAT using Pinnacle 9.0 and with HARC using Brainlab iPlan 4.5. Student t-test was used to compare the dosimetric end points, including V_16Gy to the planning target volume, homogeneity index, MPD_PTV, the conformity index, V_10Gy of the spinal cord, and MPD_cord. To compare the accuracy of delivery, we delivered all plans on a phantom and conducted gamma index (GI) comparisons with 3 mm/3% and 2 mm/2% criteria. All plans met our clinical requirements. Among 3 techniques, there were no differences on dose coverage to the tumor volume, maximum dose to the spinal cord, and plan homogeneity index (p > 0.05). The average V_10Gy of the spinal cord was 6.66 ± 0.03%, 5.49 ± 0.03%, and 4.76 ± 0.02% for IMRT, HARC, and VMAT plans, respectively. Accordingly, the conformity indices were 1.30 ± 0.11 and 1.29 ± 0.20, 1.53 ± 0.29, respectively. VMAT plans were significantly (p < 0.05) less conformal but significantly (p < 0.05) lower V_10Gy of the spinal cord than those from HARC and IMRT plans. With delivery accuracy measured by GIs, the average GIs of 3%/3 mm were 92.6 ± 1.1%, 96.5 ± 2.7%, 99.0 ± 1.1% for IMRT, HARC, and VMAT plans, respectively. The differences were significant (p < 0.05). Accordingly, the average monitor units were 9238 ± 2242, 9853 ± 2548 and 5091 ± 910. The plan quality created from the 3 planning techniques can meet the clinical requirement. Adding arc beams in delivery such as in HARC and VMAT plans improves the delivery accuracy. VMAT is the most efficient delivery method.

Improvement of VMAT plan quality for head and neck cancer with high resolution fluences generated by couch shift between arcs

PURPOSE

To investigate the changes in quality of the volumetric modulated arc therapy (VMAT) plans with couch-shift between arcs by half of a multi-leaf collimator (MLC) leaf width.

METHODS

A total of 22 patients with head-and-neck cancer were retrospectively selected. Since the smallest MLC leaf width was 5 mm in this study, the couch was shifted by 2.5 mm in the longitudinal-direction between arcs to increase the resolution of fluence map. A total of three types of VMAT plans were generated for each patient; the three types of plans were a two-full-arc plan without couch-shift (NS plan), a two-half-arc-pair plan with couch-shift (HAS plan), and a two-full-arc pair plan with couch-shift (FAS plan). Changes in the dose-volumetric parameters were investigated.

RESULTS

The FAS plan showed the best plan quality for the target volumes and organs at risk compared to the NS and HAS plans. However, the magnitudes of differences among the three types of plans were minimal, and every plan was clinically acceptable. The average integral doses of the NS, HAS, and FAS plans were 160,549 ± 37,600 Gy-cc, 147,828 ± 33,343 Gy-cc, and 156,030 ± 36,263 Gy-cc, respectively. The average monitor unit of the NS, HAS, and FAS plans were 717 ± 120 MU, 648 ± 100 MU, and 763 ± 158 MU, respectively.

CONCLUSIONS

The HAS plan was better than the others in terms of normal tissue sparing and plan efficiency. By shifting the couch by half of the MLC leaf width in the longitudinal direction between arcs, the VMAT plan quality could be improved.

Normal lung sparing Tomotherapy technique in stage III lung cancer

PURPOSE

Radiation pneumonitis (RP) has been a challenging obstacle in treating stage III lung cancer patients. Beam angle optimization (BAO) technique for Tomotherapy was developed to reduce the normal lung dose for stage III non-small cell lung cancer (NSCLC). Comparative analyses on plan quality by 3 different Intensity-modulated radiation therapy (IMRT) methods with BAO were done.

MATERIALS AND METHODS

Ten consecutive stage IIIB NSCLC patients receiving linac-based static IMRT (L-IMRT) with total 66 Gy in 33 fractions to the PTV were selected. Two additional Tomotherapy-based IMRT plans (helical beam (TH-IMRT) and static beam (TD-IMRT)) were generated on each patient. To reduce the normal lung dose, Beam angles were optimized by using complete and directional block functions in Tomotherapy based on knowledge based statistical analysis. Plan quality was compared with target coverage, normal organ sparing capability, and normal tissue complication probability (NTCP). Actual beam delivery times and risk of RP related with planning target volume (PTV) were also evaluated.

RESULTS

The best PTV coverage measured by conformity index and homogeneity index was achievable by TH-IMRT (0.82 and 1.06), followed by TD-IMRT (0.81 and 1.07) and L-IMRT (0.75 and 1.08). Mean lung dose was the lowest in TH-IMRT plan followed by TD-IMRT and L-IMRT, all of which were ≤20 Gy. TH-IMRT plan could significantly lower the lung volumes receiving low to medium dose levels: V5~30 when compared to L-IMRT plan; and V5~20 when compared to TD-IMRT plan, respectively. TD-IMRT plan was significantly better than L-IMRT with respects to V20 and V30 and there was no significant difference with respect to V40 among three plans. The NTCP of the lung was the lowest in TH-IMRT plan, followed by TD-IMRT and L-IMRT (6.42% vs. 6.53% vs. 8.11%). Beam delivery time was the shortest in TD-IMRT plan followed by L-IMRT. As PTV length increased, NTCP and Mean lung dose proportionally increased significantly in all three plans.

CONCLUSION

Advantageous profiles by TH-IMRT could be achieved by BAO by complete and directional block functions. Current observation could help radiation oncologists to make wise selection of IMRT method for stage IIIB NSCLC.

Plan quality and delivery time comparisons between volumetric modulated arc therapy and intensity modulated radiation therapy for scalp angiosarcoma: A planning study

Introduction

Due to its spherical surface, scalp angiosarcoma requires careful consideration for radiation therapy planning and dose delivery. Herein, we investigated whether volumetric modulated arc therapy (VMAT) is superior to intensity modulated radiation therapy (IMRT) in terms of the plan quality and delivery time.

Methods

Three different coplanar treatment plans were created for four patients, comprising a two‐arc VMAT plan as well as 5‐field and 9‐field IMRT plans with 6 MV beams. The X‐ray Voxel Monte Carlo algorithm was employed for dose calculation. A radiation therapy dose of 60 Gy was prescribed to the planning target volume (PTV) in 30 fractions. The homogeneity indexes (HIs) and conformity indexes (CIs) of the PTV, organs at risk (OARs) doses and delivery times were calculated and compared.

Results

For the VMAT, 5‐field and 9‐field IMRT plans, the mean HIs were 0.14, 0.16 and 0.15; CIs_100% were 0.63, 0.61 and 0.64; CIs_98% were 0.72, 0.66 and 0.70 and CIs_95% were 0.74, 0.67 and 0.71 respectively. All mean dose parameters of the VMAT and 9‐field IMRT plans for the brain were equal to or lower than those of the 5‐field IMRT plan. For the 5‐field IMRT plan, the dose constraints for the left lens were not satisfied in two patients. The mean delivery times were 3.3, 11.1 and 14.7 min for the VMAT, 5‐field and 9‐field IMRT plans respectively.

Conclusion

The VMAT plan quality is comparable to that of 9‐field IMRT, with a reduced delivery time. Therefore, VMAT represents a valuable, sophisticated irradiation technique for treating scalp angiosarcoma.

A comparative planning study for lung SABR between tri-Co-60 magnetic resonance image guided radiation therapy system and volumetric modulated arc therapy

BACKGROUND AND PURPOSE

To compare the plan quality of tri-(60)Co magnetic-resonance image-guided radiation therapy (IGRT) to that of volumetric modulated arc therapy (VMAT) for lung stereotactic ablative radiotherapy (SABR).

MATERIALS AND METHODS

A total of 22 patients with lung tumors located in the lower lobe were selected retrospectively. For each patient, VMAT plans with linac and intensity-modulated radiation therapy (IMRT) plans with the tri-(60)Co system were generated with prescription doses of 60Gy (daily dose=15Gy). For both plan types, identical CT image sets and structures were used, with the exception of planning target volumes (PTV). The PTV for VMAT was generated from the internal target volume (ITV) while the PTV for the tri-(60)Co system was generated from the gross tumor volume (GTV). Clinically relevant dose-volumetric parameters were calculated and analyzed.

RESULTS

The average PTV volumes of tri-(60)Co plans and VMAT plans were 10.5±12.3cc vs. 27.2±23.5cc, respectively (p<0.001). The maximum and mean doses to PTVs were 64.0±2.6Gy vs. 62.5±0.9Gy (p=0.005) and 61.4±1.7Gy vs. 60.0±0.5Gy (p<0.001), respectively. The conformity and homogeneity indices were 1.89±0.38 vs. 1.01±0.40 (p<0.001) and 0.06±0.02 vs. 0.04±0.00 (p<0.001), respectively. No considerable differences for organs at risk (OARs) were observed between tri-(60)Co plans and VMAT plans. In terms of target conformity, integral dose and lung mean dose, the plan quality of tri-(60)Co plans was inferior to that of VMAT plans when the PTV volumes of tri-(60)Co plans were less than 10cc. However, all treatment plans of tri-(60)Co system were clinically acceptable.

CONCLUSION

For lung SABR, the quality of ITV-based VMAT plans was better than that of GTV-based tri-(60)Co plans especially when the PTV volumes of the tri-(60)Co plans were less than 10cc. If the breathing pattern of a patient is reproducible, VMAT is considered the optimal option for lung SABR, otherwise the tri-(60)Co IGRT should be considered due to the ability to monitor tumor motion during treatment.

The effect of extremely narrow MLC leaf width on the plan quality of VMAT for prostate cancer

BACKGROUND

To investigate the effect of multi-leaf collimators (MLCs) with leaf width of 1.25 mm on the plan quality of volumetric modulated arc therapy (VMAT) for prostate cancer.

METHODS

A total of 20 patients with prostate cancer were retrospectively selected. Using a high definition MLC (HD MLC), primary and boost VMAT plans with two full arcs were generated for each patient (original plan). After that, by shifting the isocenter position of the 2nd arc by 1.25 mm in the cranio-caudal direction, we simulated fluences made with MLCs with leaf width of 1.25 mm. After shifting, primary and boost plans were generated for each patient (shifted plan). A sum plan was generated by summation of the primary and boost plan for each patient. Dose-volumetric parameters were calculated and compared.

RESULTS

Both the homogeneity index (HI) and conformity index (CI) of the shifted plans were better than those of the original plans in primary plans (HI = 0.065 vs. 0.059 with p < 0.001 and CI = 1.056 vs. 1.044 with p = 0.006). Similarly, the shifted plans for the boost target volume showed better homogeneity and conformity than did the original plans (HI = 0.060 vs. 0.053 with p < 0.001 and CI = 1.015 vs. 1.009 with p < 0.001). The target mean dose of the original plans was closer to the prescription dose than that of the shifted plans in the case of sum plans (81.45 Gy vs. 81.12 Gy with p = 0.001).

CONCLUSIONS

Use of extremely narrow MLCs could increase dose homogeneity and conformity of the target volume for prostate VMAT.

Capability of leaf interdigitation with different inverse planning strategies in Monaco: an investigation of representative tumour sites

Purpose

The aim of this study was to experimentally assess the dosimetric impact of leaf interdigitation using different inverse treatment strategies for representative tumour sites and to identify the situations in which leaf interdigitation can benefit these tumour sites.

Material and methods

Sixty previously treated patients (15 nasopharyngeal carcinoma (NPC), 15 multiple brain metastasis (MBM), 15 cervical cancer and 15 prostate cancer) were re-planned for volumetric modulated arc therapy (VMAT), sliding window IMRT (dMLC) and step-and-shoot IMRT (ssIMRT) with and without leaf interdigitation. Various dosimetric variables, such as PTV coverage, OARs sparing, delivery efficiency and planning time, were evaluated for each plan. In addition, a protocol developed by our group was applied to identify the situations in which leaf interdigitation can achieve benefits in clinical practice.

Results

Leaf interdigitation produced few benefits in PTV homogeneity for the MBM VMAT plans and NPC ssIMRT plans. For OARs, sparing was equivalent with and without leaf interdigitation. Leaf interdigitation showed an increase in MUs for dMLC plans and a decrease in MUs for ssIMRT plans. Leaf interdigitation resulted in an increase in segments for dMLC plans and a decrease in segments for NPC and MBM ssIMRT plans. For beam on time, leaf interdigitation showed an increase in MBM dMLC, NPC ssIMRT and prostate ssIMRT plans. In addition, leaf interdigitation saved planning time for VMAT and dMLC plans but increased planning time for ssIMRT plans.

Conclusion

Leaf interdigitation does not improve plan quality when performing inverse treatment strategies, regardless of whether the target is simple or complex. However, it influences the delivery efficiency and planning time. Based on these observations, our study suggests that leaf interdigitation should be utilized when performing MBM VMAT plans and NPC ssIMRT plans.

Electronic supplementary material

The online version of this article (doi:10.1186/s13014-016-0655-1) contains supplementary material, which is available to authorized users.

Clinical effect of multileaf collimator width on the incidence of late rectal bleeding after high-dose intensity-modulated radiotherapy for localized prostate carcinoma

BACKGROUND

Several studies have confirmed a dosimetric advantage associated with use of a smaller leaf in intensity-modulated radiation therapy (IMRT). However, no studies have identified any clinical benefits. We investigated the effect of a smaller multileaf collimator (MLC) width on the onset of late rectal bleeding after high-dose prostate IMRT.

MATERIALS AND METHODS

Two hundred and five prostate cancer patients were treated with a total dose of 78 Gy in 39 fractions by use of a dynamic MLC technique; however, two different MLC were used: a 10-mm-wide device and a 5-mm-wide device. Gastrointestinal toxicity and several clinical factors were assessed.

RESULTS

The 5-year actuarial risk of grade 2 or higher rectal bleeding was 6.9 % for the 10-mm-wide group (n = 132) and 1.8 % for the 5-mm-wide group (n = 73) (p = 0.04). The median estimated rectal doses for the two groups were 55.1 and 50.6 Gy (p < 0.001), respectively. Univariate analysis showed that acute toxicity, rectal V30-60, median rectal dose, normal tissue complication probability (NTCP), and MLC type were significant predictive factors for late rectal toxicity. In multivariate analysis, acute toxicity and NTCP remained significant.

CONCLUSION

In our planning approach for prostate IMRT, a decrease in MLC width from 10 to 5 mm contributed to further rectal dose reduction, which was the most important predictor of late rectal toxicity.

A treatment planning study comparing Elekta VMAT and fixed field IMRT using the varian treatment planning system eclipse

Background

The newest release of the Eclipse (Varian) treatment planning system (TPS) includes an optimizing engine for Elekta volumetric-modulated arc therapy (VMAT) planning. The purpose of this study was to evaluate this new algorithm and to compare it to intensity-modulated radiation therapy (IMRT) for various disease sites by creating single- and double-arc VMAT plans.

Methods

A total of 162 plans were evaluated in this study, including 38 endometrial, 57 head and neck, 12 brain, 10 breast and 45 prostate cancer cases. The real-life IMRT plans were developed during routine clinical cases using the TPS Eclipse. VMAT plans were generated using a preclinical version of Eclipse with tumor-region-specific optimizing templates without interference of the operator: with one full arc (1A) and with two full arcs (2A), and with partial arcs for breast and prostate with hip implant cases. All plans were evaluated based on target coverage, homogeneity and conformity. The organs at risk (OARs) were analyzed according to plan objectives, such as the mean and maximum doses. If one or more objectives were exceeded, the plan was considered clinically unacceptable, and a second VMAT plan was created by adapting the optimization penalties once.

Results

Compared to IMRT, single- and double-arc VMAT plans showed comparable or better results concerning the target coverage: the maximum dose in the target for 1A is the same as that for IMRT; for 2A, an average reduction of 1.3% over all plans was observed. The conformity showed a statistically significant improvement for both 1A (+3%) and 2A (+6%). The mean total body dose was statistically significant lower for the considered arc techniques (IMRT: 16.0 Gy, VMAT: 15.3 Gy, p < 0.001). However, the sparing of OARs shows individual behavior that depends strongly on the different tumor regions. A clear difference is found in the number of monitor units (MUs) per plan: VMAT shows a reduction of 31%.

Conclusion

These findings demonstrate that based on optimizing templates with minimal interaction of the operator, the Eclipse TPS is able to achieve a plan quality for the Elekta VMAT delivery technique that is comparable to that of fixed-field IMRT. Plans with two arcs show better dose distributions than plans with one arc.