Fast Arc Delivery for Stereotactic Body Radiotherapy of Vertebral and Lung Tumors

Efficacy of flattening filter-free beams with the acuros XB algorithm in thoracic spine stereotactic body radiation therapy

This study aimed to determine the dosimetric value of flattening filter-free (FFF) beams compared to flattening filter (FF) beams using different algorithms in the treatment planning of thoracic spine stereotactic body radiation therapy (SBRT). A total of 120 plans were created for 15 patients using the Anisotropic Analytical Algorithm (AAA) and the Acuros External Beam (AXB) algorithm with FF and FFF beams at 6 MV and 10 MV energies. Various dosimetric parameters were evaluated, including target coverage, dose spillage, and organs-at-risk sparing of the spinal cord and esophagus. Treatment delivery parameters, such as the monitor units (MUs), modulation factors (MFs), beam-on time (BOT), and dose calculation time (DCT), were also collected. Significant differences were observed in the dosimetric parameters when AXB was used for all energies (P < 0.05). 6 XFFF energy was the best option for target coverage, dose spillage, and organs-at-risk sparing. In contrast, dosimetric parameters had no significant difference when using the AAA. The AAA and AXB calculations showed that the 6 XFFF beam had the shortest DCT. The treatment delivery parameters indicated that 10 XFF beam required the fewest MUs and MFs. In addition, the 10 XFFF beam demonstrated the shortest BOT. For effective treatment of the thoracic spine using SBRT, it is recommended to use the 10 XFFF beam because of the short BOT. Moreover, the AXB algorithm should be used because of its accurate dose calculation in regions with tissue heterogeneity.

Deep learning-based markerless lung tumor tracking in stereotactic radiotherapy using Siamese networks

BACKGROUND

Radiotherapy (RT) is involved in about 50% of all cancer patients, making it a very important treatment modality. The most common type of RT is external beam RT, which consists of delivering the radiation to the tumor from outside the body. One novel treatment delivery method is volumetric modulated arc therapy (VMAT), where the gantry continuously rotates around the patient during the radiation delivery.

PURPOSE

Accurate tumor position monitoring during stereotactic body radiotherapy (SBRT) for lung tumors can help to ensure that the tumor is only irradiated when it is inside the planning target volume. This can maximize tumor control and reduce uncertainty margins, lowering organ-at-risk dose. Conventional tracking methods are prone to errors, or have a low tracking rate, especially for small tumors that are in close vicinity to bony structures.

METHODS

We investigated patient-specific deep Siamese networks for real-time tumor tracking, during VMAT. Due to lack of ground truth tumor locations in the kilovoltage (kV) images, each patient-specific model was trained on synthetic data (DRRs), generated from the 4D planning CT scans, and evaluated on clinical data (x-rays). Since there are no annotated datasets with kV images, we evaluated the model on a 3D printed anthropomorphic phantom but also on six patients by computing the correlation coefficient with the breathing-related vertical displacement of the surface-mounted marker (RPM). For each patient/phantom, we used 80% of DRRs for training and 20% for validation.

RESULTS

The proposed Siamese model outperformed the conventional benchmark template matching-based method (RTR): (1) when evaluating both methods on the 3D phantom, the Siamese model obtained a 0.57-0.79-mm mean absolute distance to the ground truth tumor locations, compared to 1.04-1.56 mm obtained by RTR; (2) on patient data, the Siamese-determined longitudinal tumor position had a correlation coefficient of 0.71-0.98 with the RPM, compared to 0.07-0.85 for RTR; (3) the Siamese model had a 100% tracking rate, compared to 62%-82% for RTR.

CONCLUSIONS

Based on these results, we argue that Siamese-based real-time 2D markerless tumor tracking during radiation delivery is possible. Further investigation and development of 3D tracking is warranted.

Target volume size effect on comparison of dynamic arc treatment plans computed using flattened and unflattened 6MV beams

INTRODUCTION/BACKGROUND

In conventional linear accelerators, to obtain flat profiles leading to uniform dose distribution in homogeneous medium, the flattening filter is usually applied on the beam path. In recent years, to obtain higher dose rates, there have been the options of flattening filter free (FFF) beams and it has been noticed that these have many advantages. The aim of this study was to clearly underline the advantages and the drawbacks of flattened filter free (FFF) beams in comparison with the flattening filter (FF) beams for different clinical contexts (planning target volumes locations).

METHODS

Two groups (planned with auto-planning VMAT, full and partial arcs) of eight patients each were analyzed: Group I (small planning target volume PTV, with average volume 48.9 ±44.4 cm³), Group II (large PTV, with average volume 532.4 ±368.8 cm³). Both beam modalities 6MV and 6MVFFF were compared in terms of Dmax, D95%, D1cc, D2cc, homogeneity index (HI), number of monitor units (MU), treatment delivery time.

RESULTS

Using the 6MVFFF, the treatment delivery time was significantly reduced (p<0.05). For larger PTVs, the number of MU increased by more than twice, and the p-value shown a significant difference (p= 0.008). The value of Dmax increased by 4%. On the contrary, for small volumes, the results were quite similar from 6MVFFF to 6MV except some differences in terms of MU.

CONCLUSION

It is recommended to use 6MVFFF beam with small PTV volumes. Dose distributions are almost the same as with 6MV and there is a significant reduction of the treatment delivery time up to 57%. Due to the dose profile shape in FFF mode, the dose is lowered beyond the central axis for the FFF beams, and the additional MU allows the dose to be delivered away from the beam axis.

Markerless 3D tumor tracking during single-fraction free-breathing 10MV flattening-filter-free stereotactic lung radiotherapy

BACKGROUND AND PURPOSE

Positional verification during single fraction lung SBRT could increase confidence and reduce the chance of geographic miss. As planar 2DkV imaging during VMAT irradiation is already available on current linear accelerators, markerless tracking based on these images could offer widely available and low-cost verification. We evaluated treatment delivery data and template matching and triangulation for 3D-positional verification during free-breathing, single fraction (34 Gy), 10 MV flattening-filter-free VMAT lung SBRT.

METHODS AND MATERIALS

Tumor tracking based on kV imaging at 7 frames/second was performed during irradiation in 6 consecutive patients (7 lesions). Tumor characteristics, tracking ability, comparison of tracking displacements with CBCT-based shifts, tumor position relative to the PTV margin, and treatment times are reported.

RESULTS

For all 7 lesions combined, 3D tumor position could be determined for, on average, 71% (51-84%) of the total irradiation time. Visually estimated tracked and automated match +/- manually-corrected CBCT-derived displacements generally agreed within 1 mm. During the tracked period, the longitudinal, lateral and vertical position of the tumor was within a 5 mm/3 mm PTV margin 95.5/85.3% of the time. The PTV was derived from the ITV including all tumor motion. The total time from first set-up imaging to end of the last arc was 18.3-31.4 min (mean = 23.4, SD = 4.1).

CONCLUSION

3D positional verification during irradiation of small lung targets with limited motion, was feasible. However, tumor position could not be determined for on average 29% of the time. Improvements are needed. Margin reduction may be feasible. Imaging and delivery of a single 34 Gy fraction was fast.

Bringing FLASH to the Clinic: Treatment Planning Considerations for Ultrahigh Dose-Rate Proton Beams

PURPOSE

Preclinical research into ultrahigh dose rate (eg, ≥40 Gy/s) "FLASH"-radiation therapy suggests a decrease in side effects compared with conventional irradiation while maintaining tumor control. When FLASH is delivered using a scanning proton beam, tissue becomes subject to a spatially dependent range of dose rates. This study systematically investigates dose rate distributions and delivery times for proton FLASH plans using stereotactic lung irradiation as the paradigm.

METHODS AND MATERIALS

Stereotactic lung radiation therapy FLASH-plans, using 244 MeV scanning proton transmission beams, with the Bragg peak behind the body, were made for 7 patients. Evaluated parameters were dose rate distribution within a beam, overall irradiation time, number of times tissue is irradiated, and quality of the FLASH-plans compared with the clinical volumetric-modulated arc therapy (VMAT) plans.

RESULTS

Sparing of lungs, thoracic wall, and heart in the FLASH-plans was equal to or better than that in the VMAT-plans. For a spot peak dose rate (SPDR, the dose rate in the middle of the spot) of 100 Gy/s, ∼40% of dose is delivered at FLASH dose rates, and for SPDR = 360 Gy/s this increased to ∼75%. One-hundred percent FLASH dose rate cannot be achieved owing to small contributions from distant spots with lower dose rates. The total irradiation time varied between 300 to 730 ms, and around 85% of the dose-receiving body volume was irradiated by either 1 or 2 beams.

CONCLUSIONS

Clinical implementation of FLASH using scanning proton beams requires multiple treatment planning considerations: dosimetric, temporal, and spatial parameters all seem important. The FLASH efficiency of a scanning proton beam increases with SPDR. The methodology proposed in this proof-of-principle study provides a framework for evaluating the FLASH characteristics of scanning proton beam plans and can be adapted as FLASH parameters are better defined. It currently seems logical to optimize plans for the shortest delivery time, maximum amount of high dose rate coverage, and maximum amount of single beam and continuous irradiation.

Evaluation of organ-at-risk dose reduction with jaw tracking technique in flattening filter-free beams in lung stereotactic body radiation therapy

PURPOSE

(i) to investigate the capability of organ-at-risk (OAR) dose reduction with the jaw tracking (JT) technique in flattening filter-free (FFF) beams in lung stereotactic body radiation therapy (SBRT), (ii) to propose a novel metric to quantify the jaw movements during JT, and (iii) to examine the relationships between the quantified jaw movements and reduction rate of OAR doses.

METHODS

The individual SBRT plans with volumetric modulated arc therapy using the JT technique (JT-VMAT) and VMAT plans with a fixed jaw (FJ-VMAT) were created for 15 patients, and dosimetric parameters were compared. A jaw tracking complexity score (JTCS) was defined and compared with the multi-leaf collimator (MLC) modulation complexity score (MCS). The correlations between the JTCS and reduction rate of OAR doses were examined.

RESULTS

The decrease of OARs doses was statistically significant in the JT-VMAT plans (1.2% in V20 of the lung and <1% in all other OARs). The correlations between the JTCS and MCS were not significant. There were significant correlations between the JTCS and the reduction rates in V₂₀, V_2.5, and D_mean of the lung, D_1% of the spinal cord, and D_90% of the body.

CONCLUSIONS

A significant decrease of dosimetric parameters of OARs was found with JT-VMAT in FFF beams. This reduction is very small and probably not clinically relevant. JTCS, a novel metric to quantify the jaw movements during JT, was proposed, and the complexity of jaw movements did not correlate with that of the movements of MLC leaves. There were significant correlations between the JTCS and some dosimetric parameters of OARs.

Improvement of off-axis SABR plan verification results by using adapted dose reconstruction algorithms for the Octavius 4D system

PURPOSE

Stereotactic ablative body radiotherapy (SABR) for lung patients can be performed with volumetric-modulated arc therapy (VMAT) plans using off-axis target geometry to allow treatment in their CBCT verified position. For patient-specific quality assurance measurements using the PTW Octavius 4D phantom (PTW, Freiburg, Germany) (OCT4D) in conjunction with an Octavius 1000SRS array (OCT1000) (PTW, Freiburg, Germany), repositioning the phantom off-axis is required to ensure the measurement area coincides with the tumor. The aim of this work is to quantify delivery errors using an array repositioned off-axis and evaluate new software which incorporates corrections for off-axis phantom measurements.

METHODS

Dynamic conformal arcs and 25 lung SABR plans were created with the isocenter at the patient midline and the target volume off-axis. Measurements were acquired with an OCT4D phantom in conjunction with a 729 array (PTW, Freiburg, Germany) (OCT729) placed at isocenter. These plans were recalculated and delivered to both the OCT729 and OCT1000 arrays repositioned so that the high-dose region was at the center of the phantom. Comparisons were made using VeriSoft v7.0 (PTW, Freiburg, Germany) and the newly implemented version 7.1 with 2%/2 mm gamma criterion (10% threshold) and results correlated with off-axis distance to the tumor.

RESULTS

Average pass rates for VeriSoft v7.0 significantly reduced from 92.7 ± 2.4% to 84.9 ± 4.1% when the phantom was repositioned compared to the isocenter setup for the OCT729. The gamma pass rates significantly decreased the further the phantom was moved off-axis. Significantly higher pass rates were observed for the OCT1000 of 95.7 ± 3.6% and a significant decrease in gamma pass rate with off-axis phantom distance was again observed. In contrast, even with phantom repositioning, the pass rates for analysis with VeriSoft v7.1 were 93.7 ± 2.1% and 99.4 ± 1.1% for OCT729 and OCT1000, respectively. No significant difference in gamma pass rate was observed with off-axis phantom position irrespective of array type with the new software.

CONCLUSION

The errors in QA phantom measurements due to dose reconstruction at off-axis target geometry have been demonstrated for conformal arcs and clinical VMAT SABR plans. A novel software solution implemented by the vendor to allow accurate pass rates has been tested. This solution enables high-resolution arrays with small active detection areas to be used for quality assurance of SABR treatment plans in the off-axis treatment position.

[Evaluation of a 2D Diode Array Corresponding to Flattening Filter Free X-ray Beams]

Recently, a medical linear accelerator with a flattening filter free (FFF) mode has led to the use of FFF X-ray beams at clinical sites. The usefulness of FFF X-ray beams in high-precision radiation therapy has been reported. Therefore, the quality assurance and quality control for FFF X-ray beams have become necessary. In this study, the characteristics of the detectors of a newly developed 2-D diode array (MapCHECK2, Sun Nuclear Corporation) for FFF X-ray beams, i.e., dose reproducibility, dose rate dependence, dose linearity, and output factor, were evaluated. For the measurements, 6 and 10 MV FFF beams were used. The results showed that the coefficient of variation for dose reproducibility was within 0.08%, the dose rate dependence was less than 1.0%, the coefficient of determination of dose linearity was found to be R²=1.0, which was high, and the output factor agreed within 2.5% as compared with the farmer ion chamber, diode E, and pinpoint ion chamber for field sizes greater than 2×2 cm². The results suggested that MapCHECK2 could be a useful tool for quality assurance and quality control for FFF X-ray beams.

Improvement of conformal arc plans by using deformable margin delineation method for stereotactic lung radiotherapy

PURPOSE

Stereotactic body radiotherapy (SBRT) is an established treatment technique in the management of medically inoperable early stage non-small cell lung cancer (NSCLC). Different techniques such as volumetric modulated arc (VMAT) and three-dimensional conformal arc (DCA) can be used in SBRT. Previously, it has been shown that VMAT is superior to DCA technique in terms of plan evaluation parameters. However, DCA technique has several advantages such as ease of use and considerable shortening of the treatment time. DCA technique usually results in worse conformity which is not possible to ameliorate by inverse optimization. In this study, we aimed to analyze whether a simple method - deformable margin delineation (DMD) - improves the quality of the DCA technique, reaching similar results to VMAT in terms of plan evaluation parameters.

METHODS

Twenty stage I-II (T1-2, N0, M0) NSCLC patients were included in this retrospective dosimetric study. Noncoplanar VMAT and conventional DCA plans were generated using 6 MV and 10 MV with flattening filter free (FFF) photon energies. The DCA plan with 6FFF was calculated and 95% of the PTV was covered by the prescription isodose line. Hot dose regions (receiving dose over 100% of prescription dose) outside PTV and cold dose regions (receiving dose under 100% of prescription dose) inside PTV were identified. A new PTV (PTV-DMD) was delineated by deforming PTV margin with respect to hot and cold spot regions obtained from conventional DCA plans. Dynamic multileaf collimators (MLC) were set to PTV-DMD beam eye view (BEV) positions and the new DCA plans (DCA-DMD) with 6FFF were generated. Three-dimensional (3D) dose calculations were computed for PTV-DMD volume. However, the prescription isodose was specified and normalized to cover 95% volume of original PTV. Several conformity indices and lung doses were compared for different treatment techniques.

RESULTS

DCA-DMD method significantly achieved a superior conformity index (CI), conformity number (CIPaddick ), gradient index (R50% ), isodose at 2 cm (D2 cm ) and external index (CΔ) with respect to VMAT and conventional DCA plans (P < 0.05 for all comparisons). CI ranged between 1.00-1.07 (Mean: 1.02); 1.00-1.18 (Mean: 1.06); 1.01-1.23 (Mean 1.08); 1.03-1.29 (Mean: 1.15); 1.04-1.29 (Mean: 1.18) for DCA-DMD-6FFF, VMAT-6FFF, VMAT-10FFF DCA-6FFF and DCA-10FFF respectively. DCA-DMD-6FFF technique resulted significantly better CI compared to others (P = 0.002; < 0.001; < 0.001; < 0.001). R50% ranged between 3.22-4.74 (Mean: 3.99); 3.24-5.92 (Mean: 4.15) for DCA-DMD-6FFF, VMAT-6FFF, respectively. DCA-DMD-6FFF technique resulted lower intermediate dose spillage compared to VMAT-6FFF, though the difference was statistically insignificant (P = 0.32). D2 cm ranged between 35.7% and 67.0% (Mean: 53.2%); 42.1%-79.2% (Mean: 57.8%) for DCA-DMD-6FFF, VMAT-6FFF respectively. DCA-DMD-6FFF have significantly better and sharp falloff gradient 2 cm away from PTV compared to VMAT-6FFF (P = 0.009). CΔ ranged between 0.052 and 0.140 (Mean: 0.085); 0,056-0,311 (Mean: 0.120) for DCA-DMD, VMAT-6FFF, respectively. DCA-DMD-6FFF have significantly improved CΔ (P = 0.002). VMAT- V20 Gy , V2.5 Gy and mean lung dose (MLD) indices are calculated to be 4.03%, 23.83%, 3.42 Gy and 4.19%, 27.88%,3.72 Gy, for DCA-DMD-6FFF and DCA techniques, respectively. DCA-DMD-6FFF achieved superior lung sparing compared to DCA technique. DCA-DMD-6FFF method reduced MUs 44% and 33% with respect to VMAT-6FFF and 10FFF, respectively, without sacrificing dose conformity (P < 0.001; P < 0.001).

CONCLUSIONS

Our results demonstrated that DCA plan evaluation parameters can be ameliorated by using the DMD method. This new method improves DCA plan quality and reaches similar results with VMAT in terms of dosimetric parameters. We believe that DCA-DMD is a simple and effective technique for SBRT and can be preferred due to shorter treatment and planning time.

European Organization for Research and Treatment of Cancer (EORTC) recommendations for planning and delivery of high-dose, high precision radiotherapy for lung cancer

PURPOSE

To update literature-based recommendations for techniques used in high-precision thoracic radiotherapy for lung cancer, in both routine practice and clinical trials.

METHODS

A literature search was performed to identify published articles that were considered clinically relevant and practical to use. Recommendations were categorised under the following headings: patient positioning and immobilisation, Tumour and nodal changes, CT and FDG-PET imaging, target volumes definition, radiotherapy treatment planning and treatment delivery. An adapted grading of evidence from the Infectious Disease Society of America, and for models the TRIPOD criteria, were used.

RESULTS

Recommendations were identified for each of the above categories.

CONCLUSION

Recommendations for the clinical implementation of high-precision conformal radiotherapy and stereotactic body radiotherapy for lung tumours were identified from the literature. Techniques that were considered investigational at present are highlighted.

A comprehensive dosimetric study on switching from a Type-B to a Type-C dose algorithm for modern lung SBRT

BACKGROUND

Type-C dose algorithms provide more accurate dosimetry for lung SBRT treatment planning. However, because current dosimetric protocols were developed based on conventional algorithms, its applicability for the new generation algorithms needs to be determined. Previous studies on this issue used small sample sizes and reached discordant conclusions. Our study assessed dose calculation of a Type-C algorithm with current dosimetric protocols in a large patient cohort, in order to demonstrate the dosimetric impacts and necessary treatment planning steps of switching from a Type-B to a Type-C dose algorithm for lung SBRT planning.

METHODS

Fifty-two lung SBRT patients were included, each planned using coplanar VMAT arcs, normalized to D95% = prescription dose using a Type-B algorithm. These were compared against three Type-C plans: re-calculated plans (identical plan parameters), re-normalized plans (D95% = prescription dose), and re-optimized plans. Dosimetric endpoints were extracted and compared among the four plans, including RTOG dosimetric criteria: (R100%, R50%, D2cm, V105%, and lung V20), PTV Dmin, Dmax, Dmean, V% and D90%, PTV coverage (V100%), homogeneity index (HI), and Paddick conformity index (PCI).

RESULTS

Re-calculated Type-C plans resulted in decreased PTV Dmin with a mean difference of 5.2% and increased Dmax with a mean difference of 3.1%, similar or improved RTOG dose compliance, but compromised PTV coverage (mean D95% and V100% reduction of 2.5 and 8.1%, respectively). Seven plans had >5% D95% reduction (maximum reduction = 16.7%), and 18 plans had >5% V100% reduction (maximum reduction = 60.0%). Re-normalized Type-C plans restored target coverage, but yielded degraded plan conformity (average PCI reduction 4.0%), and RTOG dosimetric criteria deviation worsened in 11 plans, in R50%, D2cm, and R100%. Except for one case, re-optimized Type-C plans restored RTOG compliance achieved by the original Type-B plans, resulting in similar dosimetric values but slightly higher target dose heterogeneity (mean HI increase = 13.2%).

CONCLUSIONS

Type-B SBRT lung plans considerably overestimate target coverage for some patients, necessitating Type-C re-normalization or re-optimization. Current RTOG dosimetric criteria appear to remain appropriate.

Volumetric-modulated Arc Therapy Lung Stereotactic Body Radiation Therapy Dosimetric Quality Assurance: A Comparison between Radiochromic Film and Chamber Array

Introduction:

The aim of this work is to verify the use of radiochromic film in the quality assurance (QA) of volumetric-modulated arc therapy (VMAT) lung stereotactic body radiation therapy (SBRT) plans and compare the results with those obtained using an ion chamber array.

Materials and Methods:

QA was performed for 14 plans using a two-dimensional-array seven29 and EBT3 film. Dose values per session ranged between 7.5 Gy and 18 Gy. The multichannel method was used to obtain a dose map for film.

Results:

The results obtained were compared with treatment planning system calculated profiles through gamma analysis. Passing criteria were 3%/3 mm, 2%/2 mm and 3%/1.5 mm with maximum and local dose (LD) normalization. Mean gamma passing rate (GPR) (percentage of points presenting a gamma function value of <1) was obtained and compared. Calibration curves were obtained for each color channel within the dose range 0–16 Gy. Mean GPR values for film were >98.9% for all criteria when normalizing per maximum dose. When using LD, normalization was >92.7%. GPR values for the array were lower for all criteria; this difference being statistically significant when normalizing at LD, reaching 12% for the 3%/1.5 mm criterion.

Conclusion:

Both detectors provide satisfactory results for the QA of plans for VMAT lung SBRT. The film provided greater mean GPR values, afforded greater spatial resolution and was more efficient overall.

Establishing stereotactic body radiotherapy with flattening filter free techniques in the treatment of pulmonary lesions - initial experiences from a single institution

BACKGROUND

Stereotactic body radiotherapy (SBRT) using flattening filter free (FFF)-techniques has been increasingly applied during the last years. However, clinical studies investigating this emerging technique are still rare. Hence, we analyzed toxicity and clinical outcome of pulmonary SBRT with FFF-techniques and performed dosimetric comparison to conventional techniques using flattening filters (FF).

MATERIALS AND METHODS

Between 05/2014 and 06/2015, 56 consecutive patients with 61 pulmonary lesions were treated with SBRT in FFF-mode. Central lesions received 8 × 7.5 Gy delivered to the conformally enclosing 80 %-isodose, while peripheral lesions were treated with 3 × 15 Gy, prescribed to the 65 %-isodose. Early and late toxicity (after 6 months) as well as initial clinical outcomes were evaluated. Furthermore, [deleted] plan quality and efficiency were evaluated by analyzing conformity, beam- on and total treatment delivery times in comparison to plans with FF-dose application.

RESULTS

Median follow-up time was 9.3 months (range 1.5-18.0 months). Early toxicity was low with only 5 patients (8.9 %) reporting CTCAE 2° or higher side-effects. Only one patient (1.8 %) was diagnosed with radiation-induced pneumonitis CTCAE 3°, while 2 (3.6 %) patients suffered from pneumonitis CTCAE 2°. After 6 months, no toxicity greater than CTCAE 2° was reported. 1-year local progression-free survival, distant progression-free survival and overall survival were 92.8 %, 78.0 %, and 94.4 %, respectively. While plan quality was similar for FFF- and FF-plans in respect to conformity (p = 0.275), median beam-on time as well as total treatment time were significantly reduced for SBRT in FFF-mode compared to FF-mode (p ≤ 0.001, p ≤ 0.001).

CONCLUSIONS

Patient treatment with SBRT using FFF-techniques is safe and provides promising clinical results with only modest toxicity at significantly increased dose delivery speed.

Dose calculation for hypofractionated volumetric-modulated arc therapy: approximating continuous arc delivery and tongue-and-groove modeling

Hypofractionated treatments generally increase the complexity of a treatment plan due to the more stringent constraints of normal tissues and target coverage. As a result, treatment plans contain more modulated MLC motions that may require extra efforts for accurate dose calculation. This study explores methods to minimize the differences between in-house dose calculation and actual delivery of hypofractionated volumetric-modulated arc therapy (VMAT), by focusing on arc approximation and tongue-and-groove (TG) modeling. For dose calculation, the continuous delivery arc is typically approximated by a series of static beams with an angular spacing of 2°. This causes significant error when there is large MLC movement from one beam to the next. While increasing the number of beams will minimize the dose error, calculation time will increase significantly. We propose a solution by inserting two additional apertures at each of the beam angle for dose calculation. These additional apertures were interpolated at two-thirds' degree before and after each beam. Effectively, there were a total of three MLC apertures at each beam angle, and the weighted average fluence from the three apertures was used for calculation. Because the number of beams was kept the same, calculation time was only increased by about 6%-8%. For a lung plan, areas of high local dose differences (> 4%) between film measurement and calculation with one aperture were significantly reduced in calculation with three apertures. Ion chamber measurement also showed similar results, where improvements were seen with calculations using additional apertures. Dose calculation accuracy was further improved for TG modeling by developing a sampling method for beam fluence matrix. Single element point sampling for fluence transmitted through MLC was used for our fluence matrix with 1 mm resolution. For Varian HDMLC, grid alignment can cause fluence sampling error. To correct this, transmission volume averaging was applied. For three paraspinal HDMLC cases, the average dose difference was greatly reduced in film and calculation comparisons with our new approach. The gamma (3%, 3 mm) pass rates have improved significantly from 74.1%, 90.0%, and 90.4% to 99.2%, 97.9%, and 97.3% for three cases, for calculation without volume averaging and calculation with volume averaging, respectively. Our results indicate that more accurate MLC leaf position and transmission sampling can improve accuracy and agreement between calculation and measurement, and are particularly important for hypofractionated VMAT that consists of large MLC movement.

Re-irradiating spinal column metastases using IMRT and VMAT with and without flattening filter - a treatment planning study

Background

The aim of this study was to investigate the potential of the flattening filter free (FFF) mode of a linear accelerator for intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) for patients with in-field recurrence of vertebral metastases.

Methods

An Elekta Synergy Linac with Agility™ head is used to simulate the treatment of ten patients with locally recurrent spinal column metastases. Four plans were generated for each patient treating the vertebrae sparing the spinal cord: Dual arc VMAT and nine field step and shoot IMRT each with and without flattening filter. Plan quality was assessed considering target coverage and sparing of the spinal cord and normal tissue. All plans were verified by a 2D-ionisation-chamber-array, peripheral doses were measured and compared to calculations. Delivery times were measured and compared. The Wilcoxon test was used for statistical analysis with a significance level of 0.05.

Results

Target coverage, homogeneity index and conformity index were comparable for both flat and flattening filter free beams. The volume of the spinal cord receiving the allowed maximum dose to keep the risk of radiation myelopathy at 0 % was at the same time significantly reduced to below the clinically relevant 1 ccm using FFF mode. In addition the mean dose deposited in the surrounding healthy tissue was significantly reduced in the FFF mode. All four techniques showed equally good gamma scores for plan verification. FFF plans required considerably more MU per fraction dose. Regardless of the large number of MU, out-of-field point dose was significantly lower for FFF plans, with an average reduction of 33 % and mean delivery time was significantly reduced by 22 % using FFF beams. When compared to IMRT FF, VMAT FFF offered even a reduction of 71 % in delivery time and 45 % in peripheral dose.

Conclusions

FFF plans showed a significant improvement in sparing of normal tissue and the spinal cord, keeping target coverage and homogeneity comparable. In addition, delivery times were significantly reduced for FFF treatments, minimizing intrafractional motion as well as strain for the patient. Shortest delivery times were achieved using VMAT FFF. For radiotherapy of spinal column metastases VMAT FFF may therefore be considered the preferable treatment option for the combination of Elekta Synergy Linacs and Oncentra® External Beam v4.5 treatment planning system.

Monitor unit optimization in stereotactic body radiotherapy for small peripheral non-small cell lung cancer patients

The increasingly attractive stereotactic body radiotherapy (SBRT) treatment for stage I lung cancer is concomitant with a large amount of monitor units (MU), leading to excessive out-of-field dose and prolonged beam-on time. The study aims to reduce the MU number and shorten the beam-on time by optimizing the planning parameters. Clinically acceptable treatment plans from fourteen patients suffered from peripheral stage I non-small cell lung cancer (NSCLC) were created in the study. Priority for the upper objective of the target (PUOT), strength and Max MU setting in the MU objective function (MUOF) were adjusted respectively to investigate their effect on MU number, organs at risk (OARs) sparing and beam-on time. We found that the planning parameters influenced the MU number in a PUOT, strength and Max MU dependent manner. Combined with high priority for the UOT (HPUOT) and MUOF, the MU number was reduced from 443 ± 25 to 228 ± 22 MU/Gy without compromising the target coverage and OARs sparing. We also found beam-on time was proportional to MU number and it could be shortened from 7.9 ± 0.5 to 4.1 ± 0.4 minutes.

Monte Carlo validation of the TrueBeam 10XFFF phase-space files for applications in lung SABR

PURPOSE

To establish the clinical acceptability of universal Monte Carlo phase-space data for the 10XFFF (flattening filter free) photon beam on the Varian TrueBeam Linac, including previously unreported data for small fields, output factors, and inhomogeneous media. The study was particularly aimed at confirming the suitability for use in simulations of lung stereotactic ablative radiotherapy treatment plans.

METHODS

Monte Carlo calculated percent depth doses (PDDs), transverse profiles, and output factors for the TrueBeam 10 MV FFF beam using generic phase-space data that have been released by the Varian MC research team were compared with in-house measurements and published data from multiple institutions (ten Linacs from eight different institutions). BEAMnrc was used to create field size specific phase-spaces located underneath the jaws. Doses were calculated with DOSXYZnrc in a water phantom for fields ranging from 1 × 1 to 40 × 40 cm(2). Particular attention was paid to small fields (down to 1 × 1 cm(2)) and dose per pulse effects on dosimeter response for high dose rate 10XFFF beams. Ion chamber measurements were corrected for changes in ion collection efficiency (P(ion)) with increasing dose per pulse. MC and ECLIPSE ANISOTROPIC ANALYTICAL ALGORITHM (AAA) calculated PDDs were compared to Gafchromic film measurement in inhomogeneous media (water, bone, lung).

RESULTS

Measured data from all machines agreed with Monte Carlo simulations within 1.0% and 1.5% for PDDs and in-field transverse profiles, respectively, for field sizes >1 × 1 cm(2) in a homogeneous water phantom. Agreements in the 80%-20% penumbra widths were better than 2 mm for all the fields that were compared. For all the field sizes considered, the agreement between their measured and calculated output factors was within 1.1%. Monte Carlo results for dose to water at water/bone, bone/lung, and lung/water interfaces as well as within lung agree with film measurements to within 2.8% for 10 × 10 and 3 × 3 cm(2) field sizes. This represents a significant improvement over the performance of the ECLIPSE AAA.

CONCLUSIONS

The 10XFFF phase-space data offered by the Varian Monte Carlo research team have been validated for clinical use using measured, interinstitutional beam data in water and with film dosimetry in inhomogeneous media.

Optimizing the flattening filter free beam selection in RapidArc®-based stereotactic body radiotherapy for Stage I lung cancer

OBJECTIVE

To optimize the flattening filter-free (FFF) beam selection in stereotactic body radiotherapy (SBRT) treatment for Stage I lung cancer in different fraction schemes.

METHODS

Treatment plans from 12 patients suffering from Stage I lung cancer were designed using the 6XFFF and 10XFFF beams in different fraction schemes of 4 × 12, 3 × 18 and 1 × 34 Gy. Plans were evaluated mainly in terms of organs at risk (OARs) sparing, normal tissue complication probability (NTCP) estimation and treatment efficiency.

RESULTS

Compared with the 10XFFF beam, 6XFFF beam showed statistically significant lower dose to all the OARs investigated. The percentage of NTCP reduction for both lung and chest wall was about 10% in the fraction schemes of 4 × 12 and 3 × 18 Gy, whereas only 7.4% and 2.6% was obtained in the 1 × 34 Gy scheme. For oesophagus, heart and spinal cord, the reduction was greater with the 6XFFF beam, but their absolute estimates were <10(-6)%. The mean beam-on time for 6XFFF and 10XFFF beams at 4 × 12, 3 × 18 and 1 × 34 Gy schemes were 2.2 ± 0.2 vs 1.5 ± 0.1, 3.3 ± 0.9 vs 2.0 ± 0.5 and 6.3 ± 0.9 vs 3.5 ± 0.4 min, respectively.

CONCLUSION

The 6XFFF beam obtains better OARs sparing and lower incidence of NTCP in SBRT treatment of Stage I lung cancer, whereas the 10XFFF beam improves the treatment efficiency. To balance the OARs sparing and intrafractional variation owing to the prolonged treatment time, the authors recommend using the 6XFFF beam in the 4 × 12 and 3 × 18 Gy schemes but the 10XFFF beam in the 1 × 34 Gy scheme.

ADVANCES IN KNOWLEDGE

This study optimizes the FFF beam selection in different fraction schemes in SBRT treatment of Stage I lung cancer.

A review of stereotactic body radiotherapy - is volumetric modulated arc therapy the answer?

Stereotactic body radiotherapy (SBRT) is a high precision radiotherapy technique used for the treatment of small to moderate extra-cranial tumours. Early studies utilising SBRT have shown favourable outcomes. However, major disadvantages of static field SBRT include long treatment times and toxicity complications. Volumetric modulated arc therapy (VMAT) and intensity modulated radiotherapy (IMRT) may potentially mitigate these disadvantages. This review aims to assess the feasibility of emerging VMAT and IMRT-based SBRT treatment techniques and qualify which offers the best outcome for patients, whilst identifying any emerging and advantageous SBRT planning trends. A review and synthesis of data from current literature up to September 2013 was conducted on EMBASE, Medline, PubMed, Science Direct, Proquest central, Google Scholar and the Cochrane Database of Systematic reviews. Only full text papers comparing VMAT and or IMRT and or Static SBRT were included. Ten papers were identified that evaluated the results of VMAT/IMRT SBRT. Five related to medically inoperable stage 1 and 2 non-small-cell lung cancer (NSCLC), three to spinal metastasis, one related to abdominal lymph node malignancies, with the final one looking at pancreatic adenocarcinoma. Overall treatment times with VMAT were reduced by 66–70% for lung, 46–58% for spine, 42% and 21% for lymph node and pancreatic metastasis respectively, planning constraints were met with several studies showing improved organs at risk sparing with IMRT/VMAT to static SBRT. Both IMRT and VMAT were able to meet all planning constraints in the studies reviewed, with VMAT offering the greatest treatment efficiency. Early clinical outcomes with VMAT and IMRT SBRT have demonstrated excellent local control and favourable survival outcomes.

Sub-millimeter spine position monitoring for stereotactic body radiotherapy using offline digital tomosynthesis

PURPOSE

Spine stereotactic radiotherapy (SBRT) requires intrafraction motion <1-2mm. We evaluated the accuracy and precision of digital tomosynthesis (DTS) in combination with triangulation for spine position tracking.

MATERIALS/METHODS

Single-slice DTS images were generated from kV cone beam CT (CBCT) projection images. They were registered to reference DTS images reconstructed from the planning CT-scan to determine 2D shifts between actual patient position and treatment plan position. 3D spine position was obtained by triangulation of each registration with a previous registration, for every 1° of data. For 7 patients who underwent spine SBRT, the standard deviation (SD) of DTS+triangulation over one entire dataset was evaluated for different DTS angles (2-16°) and triangulation angles (1-46°). For 32 CBCT datasets, acquired before or after treatment of the 7 patients, using 4° DTS and 18° triangulation angle, SDs were determined and average positions were compared to clinically performed CBCT registrations.

RESULTS

Mean SDs were 0.29±0.10mm for lateral (range 0.1-0.55mm), 0.14±0.08 for longitudinal (0.05-0.39) and 0.24±0.10 for the vertical direction (0.10-0.57). Lateral and vertical SDs for thoracic spine were higher than for lumbar spine. Differences between clinical CBCT registration and DTS+triangulation were 0.1±0.26, 0.02±0.33 and -0.07±0.21mm.

CONCLUSION

The combination of DTS and triangulation allows for monitoring spine position with sub-mm accuracy and precision.

Dose calculation of Acuros XB and Anisotropic Analytical Algorithm in lung stereotactic body radiotherapy treatment with flattening filter free beams and the potential role of calculation grid size

BACKGROUND

The study aimed to appraise the dose differences between Acuros XB (AXB) and Anisotropic Analytical Algorithm (AAA) in stereotactic body radiotherapy (SBRT) treatment for lung cancer with flattening filter free (FFF) beams. Additionally, the potential role of the calculation grid size (CGS) on the dose differences between the two algorithms was also investigated.

METHODS

SBRT plans with 6X and 10X FFF beams produced from the CT scan data of 10 patients suffering from stage I lung cancer were enrolled in this study. Clinically acceptable treatment plans with AAA were recalculated using AXB with the same monitor units (MU) and identical multileaf collimator (MLC) settings. Furthermore, different CGS (2.5 mm and 1 mm) in the two algorithms was also employed to investigate their dosimetric impact. Dose to planning target volumes (PTV) and organs at risk (OARs) between the two algorithms were compared. PTV was separated into PTV_soft (density in soft-tissue range) and PTV_lung (density in lung range) for comparison.

RESULTS

The dose to PTV_lung predicted by AXB was found to be 1.33 ± 1.12% (6XFFF beam with 2.5 mm CGS), 2.33 ± 1.37% (6XFFF beam with 1 mm CGS), 2.81 ± 2.33% (10XFFF beam with 2.5 mm CGS) and 3.34 ± 1.76% (10XFFF beam with 1 mm CGS) lower compared with that by AAA, respectively. However, the dose directed to PTV_soft was comparable. For OARs, AXB predicted a slightly lower dose to the aorta, chest wall, spinal cord and esophagus, regardless of whether the 6XFFF or 10XFFF beam was utilized. Exceptionally, dose to the ipsilateral lung was significantly higher with AXB.

CONCLUSIONS

AXB principally predicts lower dose to PTV_lung compared to AAA and the CGS contributes to the relative dose difference between the two algorithms.

Comparison of Two RapidArc Delivery Strategies in Stereotactic Body Radiotherapy of Peripheral Lung Cancer with Flattening Filter Free Beams

PURPOSE

To investigate the performance of using partial arc (PA) and full arc with avoidance sectors (FAAS) in stereotactic body radiotherapy (SBRT) of peripheral lung cancer with flattening filter free (FFF) beams.

METHODS

Eighteen patients with primary (T1 or T2) non-small-cell lung cancer (NSCLC) or lung metastatic were selected for this study. Nine patients with a gross tumor volume (GTV) <= 10 cc were designated as the small tumor group. The other nine patients with a GTV between 10 cc and 44 cc were assigned to the large tumor group. The treatment plans were generated in eighteen patients using PA and FAAS techniques, respectively, and delivered with a Varian TrueBeam Linac. Dosimetry of the target and organs at risk (OARs), monitor unit (MU), out-of-field dose, and delivery time were statistically analyzed. Delta4 and portal dosimetry were employed to evaluate the delivery accuracy.

RESULTS

For the small tumor group, compared with the PA plans, the FAAS plans significantly achieved a lower MU/fraction, out-of-field dose and a shorter treatment time (p<0.05), but the target dose was slightly higher than that delivered by PA plans (p<0.05). For the large tumor group, the PA plans significantly attained a shorter treatment time (p<0.05), whereas MU/fraction, out-of-field dose and dose to OARs were comparable between the two plans (p>0.05). Furthermore, all plans generated from the eighteen patients achieved a high pass rate in patient-specific quality assurance, with all the gamma indices greater than 97% at the Γ3mm, 3% threshold.

CONCLUSION

This study suggests that the FAAS technique is more beneficial for the small tumor patients undergoing lung SBRT with FFF beams because of its higher treatment efficiency and MU reduction. However, for the large tumor patients, the PA technique is recommended due to its higher treatment efficiency.

Improving the intra-fraction update efficiency of a correlation model used for internal motion estimation during real-time tumor tracking for SBRT patients: fast update or no update?

BACKGROUND AND PURPOSE

For tumor tracking, a correlation model is used to estimate internal tumor position based on external surrogate motion. When patients experience an internal/external surrogate drift, an update of the correlation model is required to continue tumor tracking. In this study, the accuracy of the internal tumor position estimation for both the clinical available update at discrete points in time (rebuild) and an in-house developed non-clinical online update approach was investigated.

METHODS

A dynamic phantom with superimposed baseline drifts and 14 SBRT patients, treated with real-time tumor tracking (RTTT) on the Vero system, were retrospectively simulated for three update scenarios, respectively no update, clinical rebuild and 0.5 Hz automated online update of the correlation model. By comparing the target positions based on 0.5 Hz verification X-ray images with the estimated internal tumor positions regarding all three update scenarios, 95th percentile modeling errors (ME95), incidences of full geometrical coverage of the CTV by a 5 mm extended PTV (P₅mm) and population-based PTV margins were calculated. Further, the treatment time reduction was estimated when switching from the clinical rebuild approach to the online correlation model update.

RESULTS

For dynamic phantom motion with baseline drifts up to 0.4 mm/min, a 0.5 Hz intra-fraction update showed a similar accuracy in terms of ME95 and P5 mm compared to clinical rebuild. For SBRT patients treated on Vero with RTTT, accuracy was improved by 0.5 Hz online update compared to the clinical rebuild protocol, yielding smaller PTV margins (from 3.2 mm to 2.7 mm), reduced ME95,3D (from 4.1 mm to 3.4 mm) and an increased 5th percentile P5 mm (from 90.7% to 96.1%) for the entire patient group. Further, 80% of treatment sessions were reduced in time with on average 5.5 ± 4.1(1 SD)min.

CONCLUSION

With a fast (0.5 Hz) automated online update of the correlation model, an efficient RTTT workflow with improved geometrical accuracy was obtained.

Use of jaw tracking in intensity modulated and volumetric modulated arc radiation therapy for spine stereotactic radiosurgery

PURPOSE

This study was conducted to evaluate the advantages of jaw tracking for intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) in spine radiosurgery.

METHODS AND MATERIALS

VMAT and IMRT plans were retrospectively generated for 10 RTOG 0631 spine radiosurgery protocol patients. A total of 8 plans for each patient were created for a Varian TrueBeam equipped with a Millennium 120 multileaf collimator. Plans were created to compare IMRT and VMAT plans with and without jaw tracking, as well as with different flattening-filter-free energies: 6 MV unflattened (6U) and 10 MV unflattened (10U). The plans were prescribed to the 90% isodose line to either 16 or 18 Gy in 1 fraction. Planning target volume coverage, conformity index, dose to the spinal cord, and distance to falloff from the 90% to 50% isodose line were evaluated. Ion chamber and film measurements were performed to verify calculated dose distributions.

RESULTS

Jaw tracking decreased spinal cord dose for both IMRT and VMAT plans, but a larger decrease was seen with the IMRT plans (P = .004 vs P = .04). The average D(10%) for the spinal cord (dose that covered 10% of the spinal cord) was least for the 6U IMRT plan with jaw tracking and was greatest for the 10U IMRT plan without jaw tracking. Measurements showed greater than 98.5% agreement for planar dose gamma analysis and less than 2.5% for point dose analysis.

CONCLUSIONS

The addition of jaw tracking to IMRT and VMAT can decrease spinal cord dose without a change in calculation accuracy. A lower dose to the spinal cord was achieved with 6U than with 10U, although in some cases, 10U may be justified.

A dosimetric comparison of three-dimensional conformal radiotherapy, volumetric-modulated arc therapy, and dynamic conformal arc therapy in the treatment of non-small cell lung cancer using stereotactic body radiotherapy

This study evaluates three‐dimensional conformal radiotherapy (3D CRT), volumetric‐ modulated arc therapy (VMAT), and dynamic conformal arc therapy (DCAT) planning techniques using dosimetric indices from Radiation Therapy Oncology Group (RTOG) protocols 0236, 0813, and 0915 for the treatment of early‐stage non‐small cell lung cancer (NSCLC) using stereotactic body radiotherapy (SBRT). Twenty‐five clinical patients, five per lung lobe, previously treated for NSCLC using 3D CRT SBRT under respective RTOG protocols were replanned with VMAT and DCAT techniques. All plans were compared using respective RTOG dosimetric indices. High‐ and low‐dose spillage improved for VMAT and DCAT plans, though only VMAT was able to improve dose to all organs at risk (OARs). DCAT was only able to provide a minimal improvement in dose to the heart and ipsilateral brachial plexus. Mean bilateral, contralateral, and V20 (percentage of bilateral lung receiving at least 20 Gy dose) doses were reduced with VMAT in comparison with respective 3D CRT clinical plans. Though some of the DCAT plans had values for the above indices slightly higher than their respective 3D CRT plans, they still were able to meet the RTOG constraints. VMAT and DCAT were able to offer improved skin dose by 1.1% and 11%, respectively. Monitor units required for treatment delivery increased with VMAT by 41%, but decreased with DCAT by 26%. VMAT and DCAT provided improved dose distributions to the PTV, but only VMAT was consistently superior in sparing dose to OARs in all the five lobes. DCAT should still remain an alternative to 3D CRT in facilities that do not have VMAT or intensity‐modulated radiotherapy (IMRT) capabilities.

PACS numbers: 87.53.Ly, 87.55.dk, 87.55.D‐

IMRT and RapidArc commissioning of a TrueBeam linear accelerator using TG-119 protocol cases

The purpose of this study is to evaluate the overall accuracy of intensity‐modulated radiation therapy (IMRT) and RapidArc delivery using both flattening filter (FF) and flattening filter‐free (FFF) modalities based on test cases developed by AAPM Task Group 119. Institutional confidence limits (CLs) were established as the baseline for patient specific treatment plan quality assurance (QA). The effects of gantry range, gantry speed, leaf speed, dose rate, as well as the capability to capture intentional errors, were evaluated by measuring a series of Picket Fence (PF) tests using the electronic portal imaging device (EPID) and EBT3 films. Both IMRT and RapidArc plans were created in a Solid Water phantom (30 × 30 × 15 cm3) for the TG‐119 test cases representative of normal clinical treatment sites for all five photon energies (6X, 10X, 15X, 6X‐FFF, 10X‐FFF) and the Exact IGRT couch was included in the dose calculation. One high‐dose point in the PTV and one low‐dose point in the avoidance structure were measured with an ion chamber in each case for each energy. Similarly, two GAFCHROMIC EBT3 films were placed in the coronal planes to measure planar dose distributions in both high‐ and low‐dose regions. The confidence limit was set to have 95% of the measured data fall within the tolerance. The mean of the absolute dose deviation for variable dose rate and gantry speed during RapidArc delivery was within 0.5% for all energies. The corresponding results for leaf speed tests were all within 0.4%. The combinations of dynamic leaf gap (DLG) and MLC transmission factor were optimized based on the ion chamber measurement results of RapidArc delivery for each energy. The average 95% CLs for the high‐dose point in the PTV were 0.030 ± 0.007 (range, 0.022–0.038) for the IMRT plans and 0.029 ± 0.011 (range, 0.016–0.043) for the RapidArc plans. For low‐point dose in the avoidance structures, the CLs were 0.029 ± 0.006 (range, 0.024–0.039) for the IMRT plans and 0.027 ± 0.013 (range, 0.017–0.047) for the RapidArc plans. The average 95% CLs using 3%/3 mm gamma criteria in the high‐dose region were 5.9 ± 2.7 (range, 1.4–8.6) and 3.9 ± 2.9 (range, 1.5–8.8) for IMRT and RapidArc plans, respectively. The average 95% CLs in the low‐dose region were 5.3 ± 2.6 (range, 1.2–7.4) and 3.7 ± 2.8 (range, 1.8–8.3) for IMRT and RapidArc plans, respectively. Based on ion chamber, as well as film measurements, we have established CLs values to ensure the high precision of IMRT and RapidArc delivery for both FF and FFF modalities.

PACS number: 87

On the robustness of VMAT-SABR treatment plans against isocentre positioning uncertainties

Background

To appraise the robustness of VMAT dose distributions against uncertainties in the positioning of the patients when single fraction SABRT treatments are planned.

Methods

A set of 18 patients (8 lung, 5 brain, 5 spinal or para-spinal) treated with VMAT in a single fraction of 24Gy were retrospectively analyzed. All approved plans were re-calculated by applying shifts to the isocentre of ±0.5, ±1, ±1.5, ±2 and ±3 mm along the primary X, Y and Z axes. Dose calculations were performed with the AAA and the Acuros engines. Quantitative analysis of DVH was performed on a total of 36 references (18 patients with AAA, 18 with Acuros) and 1080 re-calculated plans to measure the potential degree of deterioration of the plans according to the simulated errors.

Results

The dose to the CTV was essentially not affected by the isocenter shifts in all cases. Concerning PTV, The main impact was observed on the near-to-minimum dose D_99%. No relevant impact was observed on organs at risk in the case of lung patients. In the case of patients treated in the spinal or para-spinal region, the near-to-maximum dose to the spine showed, in the worst scenario, referring to Acuros calculation, a potential average increase of 0.3Gy with a maximum of 1.9Gy (from 10.3 to 12.2 Gy) or 18%. This was partially mitigated to 12% with 1 mm and to 5% with 0.5 mm shifts.

Conclusions

The study showed that shifts in the position of the isocenter as large as 3 mm tend to have modest impacts on the quality of the VMAT plans, scored by means of conventional DVH parameters. From the data shown, the VMAT approach should be considered adequately robust for single fraction SABR.

A treatment planning and delivery comparison of volumetric modulated arc therapy with or without flattening filter for gliomas, brain metastases, prostate, head/neck and early stage lung cancer

BACKGROUND

Flattening filter-free (FFF) beams are an emerging technology that has not yet been widely implemented as standard practice in radiotherapy centers. To facilitate the clinical implementation of FFF, we attempted to elucidate the difference in plan quality and treatment delivery time compared to flattening filter beams (i.e. standard, STD) for several patient groups. We hypothesize that the treatment plan quality is comparable while the treatment delivery time of volumetric modulated arc therapy (VMAT) is considerably shorter using FFF beams, especially for stereotactic treatments.

METHODS

A total of 120 patients treated for head and neck (H&N) tumors, high-grade glioma, prostate cancer, early stage lung cancer and intra-cranial metastatic disease (both single and multiple metastases) were included in the study. For each cohort, 20 consecutive patients were selected. The plans were generated using STD- and FFF-VMAT for both 6 MV and 10 MV, and were compared with respect to plan quality, monitor units and delivery time using Wilcoxon signed rank tests.

RESULTS

For H&N and high-grade gliomas, there was a significant difference in homogeneity index in favor for STD-VMAT (p < 0.001). For the stereotactic sites there were no differences in plan conformity. Stereotactic FFF-VMAT plans required significantly shorter delivery time compared to STD-VMAT plans (p < 0.001) for higher dose per fraction, on average 54.5% for 6 MV and 71.4% for 10 MV. FFF-VMAT generally required a higher number of MU/Gy (p < 0.001), on average 7.0% for 6 MV and 8.4% for 10 MV.

CONCLUSION

It was generally possible to produce FFF-VMAT plans with the same target dose coverage and doses to organs at risk as STD-VMAT plans. Target dose homogeneity tended to be somewhat inferior for FFF-VMAT for the larger targets investigated. For stereotactic radiotherapy, FFF-VMAT resulted in a considerable time gain while maintaining similar plan quality compared to STD beams.

Treating patients with real-time tumor tracking using the Vero gimbaled linac system: implementation and first review

PURPOSE

To report on the first clinical application of a real-time tumor tracking (RTTT) solution based on the Vero SBRT gimbaled linac system for treatment of moving tumors.

METHODS AND MATERIALS

A first group of 10 SBRT patients diagnosed with NSCLC or oligometastatic disease in lung or liver was treated with the RTTT technique. The PTV volumes and OAR exposure were benchmarked against the widely used ITV approach. Based on data acquired during execution of RTTT treatments, a first review was performed of the process.

RESULTS

The 35% PTV volume reduction with RTTT of the studied single lesions SBRT irradiations of small target volumes is expected to result in a small (<1%) reduction of lung or liver NTCP. A GTV-PTV margin of 5.0mm was applied for treatment planning of RTTT. From patient data on residual geometric uncertainties, a CTV-PTV margin of 3.2mm was calculated. Reduction of the GTV-PTV margin below 5.0mm without better understanding of biological definition of tumor boundaries was discouraged. Total treatment times were reduced to 34.4 min on average.

CONCLUSION

A considerable PTV volume reduction was achieved applying RTTT and time efficiency for respiratory correlated SBRT was reestablished with Vero RTTT.

The effect of arm position on the dosimetry of thoracic stereotactic ablative radiation therapy using volumetric modulated arc therapy

PURPOSE

Patient comfort and positioning stability may be improved in the arms down (AD) compared with the typical arms up (AU) position in thoracic stereotactic ablative radiation therapy (SABR). We compared plan quality for AD vs AU when using volumetric modulated arc therapy (VMAT), and evaluated the sensitivity of AD plans to arm positioning variability.

METHODS AND MATERIALS

We took plans of 14 patients with 17 lung tumors treated with thoracic SABR using VMAT in the AD position and simulated the same treatments in the AU position by re-optimizing after digitally removing the ipsilateral arm. To evaluate the sensitivity of AD plans to arm positioning variability, all plans were recalculated without re-optimization after assigning water density to the ipsilateral arm (AD-W) and then digitally shifting the arm 2.5 cm anterolaterally (AD-WS).

RESULTS

Between AD and AU plans, statistically significant but clinically insignificant (all original planning constraints met) differences were found for the following parameters: mean planning target volume maximum dose, difference of 2.3% of prescription dose (P = .049); mean intermediate dose conformity index, difference of 0.27 (P = .012); median percent lung volume receiving a minimum of 10, 20, and 30 Gy (V10, V20, and V30), differences of 0.5%, 0.2%, and 0.1%, respectively (P = .040, .007, and .001); and median spinal cord maximum dose, difference of 33.5 cGy (P = .017). Similarly, between AD-W and AD-WS plans, statistically significant but clinically insignificant differences were found for median lung V20 and V30, difference of 0.0% for both (P = .034 and .016, by matched pair analysis).

CONCLUSIONS

Our exploratory planning study suggests that when using VMAT for lung tumor SABR, AD and AU positioning achieve clinically equivalent plan quality, and AD plans are insensitive to relatively large variability in arm position.

Digital tomosynthesis (DTS) for verification of target position in early stage lung cancer patients

PURPOSE

The ability to verify intrafraction tumor position is clinically useful for hypofractionated treatments. Short arc kV digital tomosynthesis (DTS) could facilitate more frequent target verification. The authors used DTS combined with triangulation to determine the mean temporal position of small-volume lung tumor targets treated with stereotactic radiotherapy. DTS registration results were benchmarked against online clinical localization using registration between free-breathing cone-beam computed tomography (CBCT) and the average intensity projection (AvIP) of the planning 4DCT.

METHODS

In this retrospective study, 76 sets of kV-projection images from online CBCT scans of 13 patients were used to generate DTS image slices (CB-DTS) with nonclinical research software (DTS Toolkit, Varian Medical Systems). Three-dimensional tumor motion was 1.3-4 mm in six patients and 6.1-25.4 mm in seven patients on 4DCT (significant difference in the mean of the groups, P < 0.01). The 4DCT AvIP was used to digitally reconstruct the Reference-DTS. DTS registration and DTS registration combined with triangulation were investigated. Progressive shortening of total DTS arc lengths from 95° to 35° around 0° gantry position was evaluated for different scenarios: DTS registration using the entire arc; DTS registration plus triangulation using two nonoverlapping arcs; and for 55° and 45° total gantry rotation, DTS registration plus triangulation using two overlapping arcs. Finally, DTS registration plus triangulation performed at eight gantry angles, each separated by 45° was evaluated using full fan kV projection data for one patient with an immobile tumor and five patients with mobile tumors.

RESULTS

For DTS registration alone, shortening arc length did not influence accuracy in X- and Y-directions, but in Z-direction, mean deviations from online CBCT localization systematically increased for shorter arc length (P < 0.05). For example, using a 95° arc mean DTS-CBCT difference was 0.8 mm (1 SD = 0.6 mm) and for a 35° arc the mean was 2.4 mm (1 SD = 1.7 mm). DTS plus triangulation using nonoverlapping-arcs increased accuracy in Z-direction for tested arc lengths ≤55° (P < 0.01). Overlapping arcs increased accuracy in Y-direction for tumors with motion >4 mm (P < 0.02) but increased Z-direction accuracy was only observed with 55° total gantry rotation. The 95th percentile deviations with this overlapping technique in X-, Y-, and Z-directions were 1.3, 2.0, and 2.5 mm, respectively. For the five patients with mobile tumors where DTS + triangulation was performed with 45° intervals, the pooled deviation from online CBCT correction showed, for X-, Y-, and Z-directions, mean of 1.1 mm, standard deviations (SD) of 0.9, 1.0, and 0.9 mm, respectively. The mean + 2 SD was <3 mm for each direction.

CONCLUSIONS

Short-arc DTS verification of time averaged lung tumor position is feasible using free-breathing kV projection data and the AvIP of the 4DCT as a reference. Observed differences between DTS and online CBCT registration with AvIP were ≤3 mm (mean + 2 SD), however, the increased temporal resolution of DTS + triangulation also identified short period deviations from the average target position on the CBCT. Short-arc DTS appears promising for intrafraction tumor position monitoring during stereotactic lung radiotherapy delivered with a rotational technique.