Stereotactic Volumetric Modulated Arc Therapy Research Articles

Clinical application of real-time tumor-tracking for stereotactic volumetric modulated arc therapy for liver tumors

Real-time tumor-tracking volumetric modulated arc therapy (RT-VMAT) enabling beam-gating based on continuous X-ray tracking of the three-dimensional position of internal markers is relevant for moving tumors. Dose-volume characteristics and treatment time were evaluated in ten consecutive patients who underwent liver stereotactic body radiation therapy with RT-VMAT. Target dose conformity and sparing of the stomach and the intestine were improved comparing RT-VMAT with RT-3D conformal radiotherapy. The mean treatment time for each fraction was less than 10 min. RT-VMAT could be effective, especially for targets located adjacent to organs at risk.

Treatment plan complexity quantification for predicting gamma passing rates in patient-specific quality assurance for stereotactic volumetric modulated arc therapy.

To investigate the beam complexity of stereotactic Volumetric Modulated Arc Therapy (VMAT) plans quantitively and predict gamma passing rates (GPRs) using machine learning. The entire dataset is exclusively made of stereotactic VMAT plans (301 plans with 594 beams) from Varian Edge LINAC. The GPRs were analyzed using Varian's portal dosimetry with 2%/2mm criteria. A total of 27 metrics were calculated to investigate the correlation between metrics and GPRs. Random forest and gradient boosting models were developed and trained to predict the GPRs based on the extracted complexity features. The threshold values of complexity metric were obtained to predict a given beam to pass or fail from ROC curve analysis. The three moderately significant values of Spearman's rank correlation to GPRs were 0.508 (p < 0.001), 0.445 (p < 0.001), and -0.416 (p < 0.001) for proposed metric LAAM, the ratio of the average aperture area over jaw area (AAJA) and index of modulation, respectively. The random forest method achieved 98.74% prediction accuracy with mean absolute error of 1.23% using five-fold cross-validation, and 98.71% with 1.25% for gradient boosting regressor method, respectively. LAAM, leaf travelling distance (LT), AAJA, LT modulation complexity score (LTMCS) and index of modulation, were the top five most important complexity features. The LAAM metric showed the best performance with AUC value of 0.801, and threshold value of 0.365. The calculated metrics were effective in quantifying the complexity of stereotactic VMAT plans. We have demonstrated that the GPRs could be accurately predicted using machine learning methods based on extracted complexity metrics. The quantification of complexity and machine learning methods have the potential to improve stereotactic treatment planning and identify the failure of QA results promptly.

Dosimetric impact of mechanical movements of the Linac gantry during treatments with small fields.

Current accepted linac Quality Assurance (QA) guidelines used for Volumetric Modulated Arc Therapy (VMAT) suggest a mechanical isocentre tolerance level of 1mm. However, this tolerance level has not been well-established for the specific case of small field stereotactic VMAT. This study aims to evaluate the clinical impact of mechanical uncertainty on this treatment modality by modelling systematic gantry sag derived isocentre variance in the Treatment Planning System (TPS). A previously reported dataset of gantry sag values in the literature served as a starting point for this study. Using an in-house developed VMAT arc splitting algorithm, isocentre shifts were applied at a Control Point (CP) level to DICOM-RT treatment plans. Dose distributions for varying isocentre shift magnitudes were calculated for a set of 29 stereotactic VMAT plans using the Eclipse Acuros XB dose algorithm. These plans had a range of Planning Target Volume (PTV) sizes. A quantitative comparison of each plan was conducted by evaluating five Dose Volume Histogram (DVH)-derived plan quality metrics. All metrics exhibited a deterioration in plan quality with increasing magnitudes of isocentre shift. At small PTV sizes, these effects were amplified, exhibiting significant changes at 1mm of average shift when typical targets and tolerances were considered. For plans with PTVs between 0 and 5 cm3, a 1mm shift reduced PTV coverage by 6.6 ± 2.2% and caused a 12.1 ± 3.8% deterioration in the conformity index. Based on the results of this study, the prevalent tolerance of 1mm may not be suitable for treatments of small PTVs with small fields. In contrast to commonly accepted values, an absolute mechanical isocentre of 0.5mm with action level at 0.75mm is recommended for stereotactic VMAT of PTV sizes below 10 cm3.

EPID-based 3D dosimetry for pre-treatment FFF VMAT stereotactic body radiotherapy plan verification using dosimetry CheckTM.

The software Dosimetry Check (DC) reconstructs the 3D dose distribution on CT images data set by using EPID measured signal. This study aimed to evaluate DC for stereotactic body radiotherapy (SBRT) with unflattened photon beams (FFF) for dosimetric independent plan verification in pre-treatment modality. DC v.4.1 was configured for Varian TrueBeam STx FFF beams equipped with EPID aS-1200. The DC FFF models were tested using arc open fields (from 1×1cm2 to 15×15cm2) and VMAT (Volumetric Modulated Arc Therapy) SBRT plans on phantom and patient CTs. DC dose distributions (DDC) were compared with that calculated by Eclipse with Acuros XB algorithm (DAXB) and one measured by Octavius 1000 SRS detector (DOCT). All differences were quantified in terms of the local 3D gamma passing rate (%GP), DVH and point dose differences. DC was configured for FFF VMAT using an appropriate correction procedure. %GP2%2mm (mean±standard deviation) of DOCT-DDC was 96.3±2.7% for open fields whereas it was 90.1±5.9% for plans on homogeneous phantom CT. However, average %GP3%3mm of DAXB-DDC was 95.0±4.1 for treatments on patient CT. The fraction of plans passing the %GP3%3mm DQA tolerance level [10% (50%) of maximum dose threshold] were 20/20 (14/20) and 18/20 (16/20) for OCT on phantom CT and DC on patient CT, respectively. DC characterization for FFF beams was performed. For stereotactic VMAT plan verifications DC showed good agreement with TPS whereas underlined discrepancies with Octavius in the high dose regions. A customized tolerance level is required for EPID-based VMAT FFF pre-treatment verification when DC system is applied.

Optimized stereotactic volumetric modulated arc therapy as an alternative to brachytherapy for vaginal cuff boost. A dosimetric study

We evaluate the role of stereotactic body radiotherapy using volumetric modulated arc therapy (VMAT) technique as an alternative to high-dose rate brachytherapy (HDR-BT) in the treatment of vaginal cuff in postoperative endometrial cancer. CT scans of 8 patients were used in this study. The clinical target volume (CTV) was defined as the 0.5 cm tissue around the applicator (then subtracting the applicator). Total dose was 30Gy delivered in 5 fractions. In HDR-BT, dose was prescribed at a distance of 0.5 cm from the surface applicator. For VMAT irradiation, a planning target volume (PTV) was obtained from CTV by an expansion of 3 mm. Two VMAT plans were generated using a full arc rotation. The first plan was optimized with an anatomy-based optimization module (PO-VMAT) using a 1mm multileaf collimator beam margin to enhance dose heterogeneity and dose fallout outside the target. The second plan was generated with a full-inverse planning module (FI-VMAT). Conformity (CI100, CI50, CI25), gradient (GI) indexes, and integral doses were calculated. To account for various dose heterogeneity distributions we calculated the equivalent uniform dose (EUD) using the Niemerko model. A Kruskal-Wallis analysis of variance followed by Dunn’s-type multiple comparisons was performed. Dose distributions were more heterogeneous with HDR-BT: Dmean was 144.2% of prescription dose for CTV in HDR-BT and 118.5 and 108.6% for PTV in PO-VMAT and FI-VMAT, respectively. The mean values of EUD for CTV were 136.9%, 130.0 %, and 111.0% of prescription dose in HDR-BT, PO-VMAT, and FI-VMAT plans, respectively. GI indexes were 2.81, 3.41, and 4.14 for HDR-BT, PO-VMAT, and FI-VMAT, respectively. Near-maximal doses (D0.1cc) for rectum and bladder were significantly higher in HDR-BT plans compared to PO-VMAT and FI-VMAT plans (rectum: 131.2% vs112.8% vs 112.0%, respectively; bladder: 129.2% vs 108.7%, and 109.8%, respectively). PO-VMAT plans were able to mimic the HDR-BT dose distribution, showing a successful capability of highly conformal dose distribution, EUD values similar to HDR-BT, and steep dose-gradient outside PTV, then providing a reasonable alternative to brachytherapy.

Impact of plan parameters and modulation indices on patient-specific QA results for standard and stereotactic VMAT

PurposeTo demonstrate the impact of modulation indices and plan parameters on the gamma passing rates (GPR) of patient-specific quality assurance of standard and stereotactic volumetric modulated arc therapy (VMAT) plans. MethodsA total of 758 patients’ QA plans were utilized, including standard VMAT plans with Trilogy (n = 87, group A) and TreuBeam STx (n = 332, group B), and 339 stereotactic VMAT plans with TrueBeam STx (group C). Modulation indices were obtained considering the speed and acceleration of the multileaf collimator (MLC) (MIs, MIa), and MLC, gantry speed, and dose rate changes (MIt). The mean aperture size (MA), monitor unit (MU), and amount of jaw tracking (%JT) were acquired. Gamma analysis was performed with 2 mm/2% and 1 mm/2% for the standard and stereotactic VMAT plans, respectively. Statistical analyses were performed to investigate the correlation between modulation index/plan parameters and GPR. ResultsSpearman’s rank correlation to GPRs with MIs, MIa, and MIt, were −0.44, −0.45, and −0.46 for group A; −0.39, −0.37, and −0.38 for group B; and −0.04, −0.11, and −0.10 for group C, respectively. While MU and MA showed significant correlations in all groups, %JT showed a significant correlation only with stereotactic VMAT plans. The most influential parameter combinations were MU-MA (rs = 0.50), MIs-%JT (rs = 0.43), and MU-%JT (rs = 0.38) for groups A, B, and C, respectively. ConclusionsMLC modulation mostly affected the GPR in the delivery of standard VMAT plans, while MU and %JT showed more importance in stereotactic VMAT plans.

Compass model-based quality assurance for stereotactic VMAT treatment plans

PurposeTo use Compass as a model-based quality assurance (QA) tool for stereotactic body radiation therapy (SBRT) and stereotactic radiation therapy (SRT) volumetric modulated arc therapy (VMAT) treatment plans calculated with Eclipse treatment planning system (TPS). Materials and methodsTwenty clinical stereotactic VMAT SBRT and SRT treatment plans were blindly selected for evaluation. Those plans included four different treatment sites: prostate, brain, lung and body. The plans were evaluated against dose-volume histogram (DVH) parameters and 2D and 3D gamma analysis. The dose calculated with Eclipse treatment planning system (TPS) was compared to Compass calculated dose (CCD) and Compass reconstructed dose (CRD). ResultsThe maximum differences in mean dose of planning target volume (PTV) were 2.7 ± 1.0% between AAA and Acuros XB calculation algorithm TPS dose, −7.6 ± 3.5% between Eclipse TPS dose and CCD dose and −5.9 ± 3.7% between Eclipse TPS dose and CRD dose for both Eclipse calculation algorithms, respectively. 2D gamma analysis was not able to identify all the cases that 3D gamma analysis specified for further verification. ConclusionsCompass is suitable for QA of SBRT and SRT treatment plans. However, the QA process should include wide set of DVH-based dose parameters and 3D gamma analysis should be the preferred method when performing clinical patient QA. The results suggest that the Compass should not be used for smaller field sizes than 3 × 3 cm2 or the beam model should be adjusted separately for both small (FS ≤ 3 cm) and large (FS > 3 cm) field sizes.

Parenchymal lung changes on computed tomography after stereotactic radiotherapy using high dose rate flattening filter free beams

Flattening filter free (FFF) beams allow fast delivery of stereotactic radiotherapy. To evaluate biological effects of FFF in lung, we compared parenchymal changes after FFF and non-FFF stereotactic volumetric modulated arc therapy. Standardized multi-observer consensus evaluation of follow-up CT scans revealed no major differences between FFF and non-FFF.

Kilovoltage intrafraction motion monitoring and target dose reconstruction for stereotactic volumetric modulated arc therapy of tumors in the liver

PurposeTo use intrafraction kilovoltage (kV) imaging during liver stereotactic body radiotherapy (SBRT) delivered by volumetric modulated arc therapy (VMAT) to estimate the intra-treatment target motion and to reconstruct the delivered target dose. MethodsSix liver SBRT patients with 2–3 implanted gold markers received SBRT in three fractions of 18.75Gy or 25Gy. CTV-to-PTV margins of 5mm in the axial plane and 10mm in the cranio-caudal directions were applied. A VMAT plan was designed to give minimum target doses of 95% (CTV) and 67% (PTV). At each fraction, the 3D marker trajectory was estimated by fluoroscopic kV imaging throughout treatment delivery and used to reconstruct the actually delivered CTV dose. The reduction in D95 (minimum dose to 95% of the CTV) relative to the planned D95 was calculated. ResultsThe kV position estimation had mean root-mean-square errors of 0.36mm and 0.47mm parallel and perpendicular to the kV imager, respectively. Intrafraction motion caused a mean 3D target position error of 2.9mm and a mean D95 reduction of 6.0%. The D95 reduction correlated with the mean 3D target position error during a fraction. ConclusionsKilovoltage imaging for detailed motion monitoring with dose reconstruction of VMAT-based liver SBRT was demonstrated for the first time showing large dosimetric impact of intrafraction tumor motion.

Impact of flattening-filter-free techniques on delivery time for lung stereotactic volumetric modulated arc therapy and image quality of concurrent kilovoltage cone-beam computed tomography: a preliminary phantom study

To the Editor, Volumetric modulated arc therapy (VMAT) serves as a means for stereotactic hypofractionated treatment of lung tumors [1]. The present authors proposed an efficient VMAT sequence by restricting leaf speeds of a multi-leaf collimator per gantry rotation angle to below 1 mm/degree, thereby reducing the dose delivery time down to 210 s for a D95 prescription dose of 50 Gy in four fractions [2, 3]. The authors also suggested further reduction of the dose delivery time to below 100 s using flattening-filter-free (FFF) techniques [3]. Recently we evaluated a non-clinical FFF research configuration by adding an FFF filter to our linac system, a Synergy with an MLCi multi-leaf collimator (Elekta AB, Stockholm, Sweden). The FFF filter is made of stainless steel and has a constant thickness of 2 mm for 6- and 10-MV beams. Furthermore, we also tested a non-clinical on-board cone-beam CT (CBCT) system, XVI 5.0 research version, that allows concurrent 4D CBCT imaging during VMAT delivery. It is anticipated that the reduced VMAT delivery time may degrade CBCT image quality due to a much lower number of projection images. The purposes of this study were: (i) to quantify the stereotactic lung VMAT delivery time with FFF, and (ii) to visually inspect CBCT image quality with FFF.

4D registration and 4D verification of lung tumor position for stereotactic volumetric modulated arc therapy using respiratory-correlated cone-beam CT

We propose a clinical workflow of stereotactic volumetric modulated arc therapy (VMAT) for a lung tumor from planning to tumor position verification using 4D planning computed tomography (CT) and 4D cone-beam CT (CBCT). A 4D CT scanner, an Anzai belt and a BodyFix were employed to obtain 10-phase respiratory-correlated CT data for a lung patient under constrained breathing conditions. A planning target volume (PTV) was defined by adding a 5-mm margin to an internal target volume created from 10 clinical target volumes, each of which was delineated on each of the 10-phase planning CT data. A single-arc VMAT plan was created with a D95 prescription dose of 50 Gy in four fractions on the maximum exhalation phase CT images. The PTV contours were exported to a kilovoltage CBCT X-ray Volume Imaging (XVI) equipped with a linear accelerator (linac). Immediately before treatment, 10-phase 4D CBCT images were reconstructed leading to animated lung tumor imaging. Initial bone matching was performed between frame-averaged 4D planning CT and frame-averaged 4D CBCT datasets. Subsequently, the imported PTV contours and the animated moving tumor were simultaneously displayed on the XVI monitor, and a manual 4D registration was interactively performed on the monitor until the moving tumor was symmetrically positioned inside the PTV. A VMAT beam was delivered to the patient and during the delivery further 4D CBCT projection data were acquired to verify the tumor position. The entire process was repeated for each fraction. It was confirmed that the moving tumor was positioned inside the PTV during the VMAT delivery.