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

Comparative dosimetric analysis of volumetric modulated arc therapy based craniospinal irradiation plans between Halcyon ring gantry and TrueBeam C-arm linear accelerator

This study evaluates the volumetric modulated arc therapy (VMAT) dosimetric comparison between Halcyon ring gantry and TrueBeam c-arm linear accelerators for craniospinal irradiation (CSI) of the neuro-axis. 25 patients, who received treatment for medulloblastoma and primitive neuro-ectodermal tumors between 2018 and 2021, were planned for VMAT in True Beam (TB), and Halcyon (HAL) linear accelerators using 6 MV unflattened (FFF) photon beams (HALFFF and TBFFF). Dose-volume statistics for the target and organs at risk (OARs) and the total number of monitoring units (MUs) in the treatment plans were compared which included dose received by 95% PTV volume (V95%), volume receiving ≥ 107% dose, homogeneity index (HI), conformity index (PI), MU and dose spillage (D10%, D30%, D50%, D70%, D90%). In all 26 OARs were considered of which five were serial and the remaining were parallel structures. For the former, the dose received by 0.2 cm³, volume = D0.2 cm³) were evaluated and for the latter mean dose were evaluated. Both arms were statistically compared with paired sample t-test with a significant value of ≤ 0.05. 11 patients received treatment with the Halcyon and the rest 14 in the TrueBeam C-arm linear accelerator. Patients in the low- and intermediate-risk category (n = 13) received 23.4 Gy in 13 fractions. The remaining patients were in the high-risk category and received 35 Gy in 21 fractions or 36 Gy in 20 fractions. For HALFFF and TBFFF, PTVV95% were 97.5 ± 0.8% and 97.4 ± 0.9% respectively (p = 0.371) while the V107% were 0.6 ± 0.4% and 0.5 ± 0.5 respectively (p = 0.504). However, the number of monitoring units showed statistical significance (p < 0.001) with values of 1331.9 ± 243.4 MU and 1089 ± 206.7 MU respectively for the HAL and TB plans. The differences in spillage dose were also statistically significant, favouring HAL plans at D30% (p = 0.002), D50% (p < 0.001), D70% (p = 0.039), and D90% (p = 0.01) level except for D10% (p = 0.090). Conformity index also showed statistical significance with PI_HAL = 0.9 ± 0.02 and PI_TB = 0.89 ± 0.03 (p = 0.029). For 10 of the 21 parallel structures, the mean dose differences were statistically significant in favouring of HAL plans. Halcyon based VMAT CSI plans are dosimetrically superior in terms of organ dose, especially for the large organs, and offer lower spillage doses than the TrueBeam plans. Plans generated by both linear accelerators are suitable for the patients' treatments.

Craniospinal Irradiation: A Dosimetric Comparison Between O-Ring Linac and Conventional C-arm Linac

Purpose

The aim of this study was to perform a dosimetric evaluation between craniospinal irradiation volumetric modulated arc therapy plans designed for an O-Ring and a conventional C-arm Linac.

Methods and Materials

Two adult patients were selected for this study. Two plans were designed one for a TrueBeam Edge and one for Halcyon O-ring Linac for each patient. The evaluation of the plans was conducted in terms of dose volume histogram analysis of the target volume and organs at risk (OARs) along with total plan monitor units and beam-on time. Paired sample t test was performed to compare D_max and D_mean of OARs for each plan's comparison. The delivery of volumetric modulated arc therapy plans was evaluated using Octavius 4D phantom.

Results

All plans demonstrated dose distributions with sufficient planned target volume conformity and homogeneity. The Homogeneity Index and Conformity Index for all plans were found comparable. The paired sample t test did not demonstrate significant difference in terms of D_max and D_mean of OARs between plans for both patients. All plans met the threshold of 90%, with Halcyon plans having higher gamma passing rates.

Conclusions

Craniospinal irradiation plans generated for Halcyon and Edge are equivalent in terms of plan quality and dose sparing at OARs. The small variations may have originated from the differences in beam profile or mean energy, the degree of the modulation for each plan and characteristics of multi leaf collimators for each treatment unit. Halcyon is able to deliver a distinctly faster treatment, but Edge provides an automatic rotational correction for patient positioning.

Offline Quality Assurance for Intensity Modulated Radiation Therapy Treatment Plans for NRG-HN001 Head and Neck Clinical Trial Using Knowledge-Based Planning

PURPOSE

This study aimed to investigate whether a disease site-specific, multi-institutional knowledge based-planning (KBP) model can improve the quality of intensity modulated radiation therapy treatment planning for patients enrolled in the head and neck NRG-HN001clinical trial and to establish a threshold of improvements of treatment plans submitted to the clinical trial.

METHODS AND MATERIALS

Fifty treatment plans for patients enrolled in the NRG-HN001 clinical trial were used to build a KBP model; the model was then used to reoptimize 50 other plans. We compared the dosimetric parameters of the submitted and KBP reoptimized plans. We compared differences between KBP and submitted plans for single- and multi-institutional treatment plans.

RESULTS

Mean values for the dose received by 95% of the planning target volume (PTV_6996) and for the maximum dose (D0.03cc) of PTV_6996 were 0.5 Gy and 2.1 Gy higher in KBP plans than in the submitted plans, respectively. Mean values for D0.03cc to the brain stem, spinal cord, optic nerve_R, optic nerve_L, and chiasm were 2.5 Gy, 1.9 Gy, 6.4 Gy, 6.6 Gy, and 5.7 Gy lower in the KBP plans than in the submitted plans. Mean values for Dmean to parotid_R and parotid_L glands were 2.2 Gy and 3.8 Gy lower in KBP plans, respectively. In 33 out of 50 KBP plans, we observed improvements in sparing of at least 7 organs at risk (OARs) (brain stem, spinal cord, optic nerves (R & L), chiasm, and parotid glands [R & L]). A threshold of improvement of OARs sparing of 5% of the prescription dose was established for providing the quality assurance results back to the treating institution.

CONCLUSIONS

A disease site-specific, multi-institutional, clinical trial-based KBP model improved sparing of OARs in a large number of reoptimized plans submitted to the NRG-HN001 clinical trial, and the model is being used as an offline quality assurance tool.

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.

Dosimetric characteristics of LinaTech DMLC H multi leaf collimator: Monte Carlo simulation and experimental study

This study evaluated the basic dosimetric characteristics of a Dynamic Multi Leaf Collimator (DMLC) using a diode detector and film measurements for Intensity Modulated Radiation Therapy Quality Assurance (IMRT QA). The EGSnrc Monte Carlo (MC) simulation system was used for the determination of MLC characteristics. Radiation transmission and abutting leaf leakage relevant to the LinaTech DMLC H were measured using an EDGE detector and EBT3 film. In this study, the BEAMnrc simulation code was used for modeling. The head of Siemens PRIMUS linac (6 MV) with external DMLC H was entered into a BEAMnrc Monte Carlo model using practical dosimetry data. Leaf material density, as well as interleaf and abutting air gaps were determined according to the computed and measured dose profiles. The IMRT QA field was used to evaluate the dose distribution of the simulated DMLC H. According to measurements taken with the EDGE detector and film, the total average measured leakage was 1.60 ± 0.03% and 1.57 ± 0.05%, respectively. For these measurements, abutting leaf transmission was 54.35 ± 1.85% and 53.08 ± 2.05%, respectively. To adapt the simulated leaf dose profiles with measurements, leaf material density, interleaf and abutting air gaps were adjusted to 18 g/cm³, 0.008 cm and 0.108 cm, respectively. Thus, the total average leakage was estimated to be about 1.59 ± 0.02%. The step‐and‐shoot IMRT was implemented and 94% agreement was achieved between the film and MC, using 3%‐3 mm gamma criteria. The results of this study showed that the dosimetric characteristics of DMLC H satisfied international standards.

Determination of MLC model parameters for Monaco using commercial diode arrays

Multileaf collimators (MLCs) need to be characterized accurately in treatment planning systems to facilitate accurate intensity‐modulated radiation therapy (IMRT) and volumetric‐modulated arc therapy (VMAT). The aim of this study was to examine the use of MapCHECK 2 and ArcCHECK diode arrays for optimizing MLC parameters in Monaco X‐ray voxel Monte Carlo (XVMC) dose calculation algorithm. A series of radiation test beams designed to evaluate MLC model parameters were delivered to MapCHECK 2, ArcCHECK, and EBT3 Gafchromic film for comparison. Initial comparison of the calculated and ArcCHECK‐measured dose distributions revealed it was unclear how to change the MLC parameters to gain agreement. This ambiguity arose due to an insufficient sampling of the test field dose distributions and unexpected discrepancies in the open parts of some test fields. Consequently, the XVMC MLC parameters were optimized based on MapCHECK 2 measurements. Gafchromic EBT3 film was used to verify the accuracy of MapCHECK 2 measured dose distributions. It was found that adjustment of the MLC parameters from their default values resulted in improved global gamma analysis pass rates for MapCHECK 2 measurements versus calculated dose. The lowest pass rate of any MLC‐modulated test beam improved from 68.5% to 93.5% with 3% and 2 mm gamma criteria. Given the close agreement of the optimized model to both MapCHECK 2 and film, the optimized model was used as a benchmark to highlight the relatively large discrepancies in some of the test field dose distributions found with ArcCHECK. Comparison between the optimized model‐calculated dose and ArcCHECK‐measured dose resulted in global gamma pass rates which ranged from 70.0%–97.9% for gamma criteria of 3% and 2 mm. The simple square fields yielded high pass rates. The lower gamma pass rates were attributed to the ArcCHECK overestimating the dose in‐field for the rectangular test fields whose long axis was parallel to the long axis of the ArcCHECK. Considering ArcCHECK measurement issues and the lower gamma pass rates for the MLC‐modulated test beams, it was concluded that MapCHECK 2 was a more suitable detector than ArcCHECK for the optimization process.

PACS number(s): 87.55.Qr

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.

Modeling and dosimetric performance evaluation of the RayStation treatment planning system

The physics modeling, dose calculation accuracy and plan quality assessment of the RayStation (v3.5) treatment planning system (TPS) is presented in this study, with appropriate comparisons to the more established Pinnacle (v9.2) TPS. Modeling and validation for the Elekta MLCi and Agility beam models resulted in a good match to treatment machine-measured data based on tolerances of 3% for in-field and out-of-field regions, 10% for buildup and penumbral regions, and a gamma 2%/2mm dose/distance acceptance criteria. TPS commissioning using a wide range of appropriately selected dosimetry equipment, and following published guidelines, established the MLC modeling and dose calculation accuracy to be within standard tolerances for all tests performed. In both homogeneous and heterogeneous mediums, central axis calculations agreed with measurements within 2% for open fields and 3% for wedged fields, and within 4% off-axis. Treatment plan comparisons for identical clinical goals were made to Pinnacle for the following complex clinical cases: hypofractionated non-small cell lung carcinoma, head and neck, stereotactic spine, as well as for several standard clinical cases comprising of prostate, brain, and breast plans. DVHs, target, and critical organ doses, as well as measured point doses and gamma indices, applying both local and global (Van Dyk) normalization at 2%/2 mm and 3%/3 mm (10% lower threshold) acceptance criteria for these composite plans were assessed. In addition 3DVH was used to compare the perturbed dose distributions to the TPS 3D dose distributions. For all 32 cases, the patients QA checks showed &gt; 95% of pixels passing 3% global/3mm gamma.

Beam modeling and VMAT performance with the Agility 160-leaf multileaf collimator

The Agility multileaf collimator (Elekta AB, Stockholm, Sweden) has 160 leaves of projected width 0.5 cm at the isocenter, with maximum leaf speed 3.5cms−1. These characteristics promise to facilitate fast and accurate delivery of radiotherapy, particularly volumetric‐modulated arc therapy (VMAT). The aim of this study is therefore to create a beam model for the Pinnacle3 treatment planning system (Philips Radiation Oncology Systems, Fitchburg, WI), and to use this beam model to explore the performance of the Agility MLC in delivery of VMAT. A 6 MV beam model was created and verified by measuring doses under irregularly shaped fields. VMAT treatment plans for five typical head‐and‐neck patients were created using the beam model and delivered using both binned and continuously variable dose rate (CVDR). Results were compared with those for an MLCi unit without CVDR. The beam model has similar parameters to those of an MLCi model, with interleaf leakage of only 0.2%. The verification of irregular fields shows a mean agreement between measured and planned dose of 1.3% (planned dose higher). The Agility VMAT head‐and‐neck plans show equivalent plan quality and delivery accuracy to those for an MLCi unit, with 95% of verification measurements within 3% and 3 mm of planned dose. Mean delivery time is 133 s with the Agility head and CVDR, 171 s without CVDR, and 282 s with an MLCi unit. Pinnacle3 has therefore been shown to model the Agility MLC accurately, and to provide accurate VMAT treatment plans which can be delivered significantly faster with Agility than with an MLCi.

PACS number: 87.55kd, 87.55km, 87.56bd, 87.56jk

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.

An analytical approach for optimizing the leaf design of a multi-leaf collimator in a linear accelerator

In this study, we present an analytical approach for optimizing the leaf design of a multi-leaf collimator (MLC) in a linear accelerator. Because leaf designs vary between vendors, our goal is to characterize and quantify the effects of different compromises which have to be made between performance parameters. Subsequently, an optimal leaf design for an earlier proposed six-bank MLC which combines a high-resolution field-shaping ability with a large field size is determined. To this end a model of the linac is created that includes the following parameters: the source size, the maximum field size, the distance between source and isocenter, and the leaf's design parameters. First, the optimal radius of the leaf tip was found. This optimum was defined by the requirement that the fluence intensity should fall from 80% of the maximum value to 20% in a minimal distance, defining the width of the fluence penumbra. A second requirement was that this penumbra width should be constant when a leaf moves from one side of the field to the other. The geometric, transmission and total penumbra width (80-20%) were calculated depending on the design parameters. The analytical model is in agreement with Elekta, Varian and Siemens collimator designs. For leaves thinner than 4 cm, the transmission penumbra becomes dominant, and for leaves close to the source the geometric penumbra plays a role. Finally, by choosing the leaf thickness of 3.5 cm, 4 cm and 5 cm from the lowest to the highest bank, respectively, an optimal leaf design for a six-bank MLC is achieved.

Radiothérapie conformationnelle avec modulation d'intensité pour les cancers des voies aérodigestives supérieures

Intensity-modulated radiotherapy (IMRT) is a new high precision radiotherapy technique, allowing to protect some normal tissues without compromising the tumour cure probability. This is a more complex and more costly technique, compared to conventional radiotherapy, which needs increased quality assurance programs. The early results obtained in head-and-neck carcinomas have been very promising but would need to be confirmed in randomised trials to determine whether these results can be transposed to the majority of these cancers and especially to locally advanced cases, which are the most frequently occurring forms of these cancers.

A virtual photon source model of an Elekta linear accelerator with integrated mini MLC for Monte Carlo based IMRT dose calculation

A dedicated, efficient Monte Carlo (MC) accelerator head model for intensity modulated stereotactic radiosurgery treatment planning is needed to afford a highly accurate simulation of tiny IMRT fields. A virtual source model (VSM) of a mini multi-leaf collimator (MLC) (the Elekta Beam Modulator (EBM)) is presented, allowing efficient generation of particles even for small fields. The VSM of the EBM is based on a previously published virtual photon energy fluence model (VEF) (Fippel et al 2003 Med. Phys. 30 301) commissioned with large field measurements in air and in water. The original commissioning procedure of the VEF, based on large field measurements only, leads to inaccuracies for small fields. In order to improve the VSM, it was necessary to change the VEF model by developing (1) a method to determine the primary photon source diameter, relevant for output factor calculations, (2) a model of the influence of the flattening filter on the secondary photon spectrum and (3) a more realistic primary photon spectrum. The VSM model is used to generate the source phase space data above the mini-MLC. Later the particles are transmitted through the mini-MLC by a passive filter function which significantly speeds up the time of generation of the phase space data after the mini-MLC, used for calculation of the dose distribution in the patient. The improved VSM model was commissioned for 6 and 15 MV beams. The results of MC simulation are in very good agreement with measurements. Less than 2% of local difference between the MC simulation and the diamond detector measurement of the output factors in water was achieved. The X, Y and Z profiles measured in water with an ion chamber (V = 0.125 cm(3)) and a diamond detector were used to validate the models. An overall agreement of 2%/2 mm for high dose regions and 3%/2 mm in low dose regions between measurement and MC simulation for field sizes from 0.8 x 0.8 cm(2) to 16 x 21 cm(2) was achieved. An IMRT plan film verification was performed for two cases: 6 MV head&neck and 15 MV prostate. The simulation is in agreement with film measurements within 2%/2 mm in the high dose regions (> or = 0.1 Gy = 5% D(max)) and 5%/2 mm in low dose regions (<0.1 Gy).