Volumetric Modulated Arc Therapy Fields Research Articles

Deep learning-based prediction of the dose-volume histograms for volumetric modulated arc therapy of left-sided breast cancer.

The advancements in artificial intelligence and computational power have made deep learning an attractive tool for radiotherapy treatment planning. Deep learning has the potential to significantly simplify the trial-and-error process involved in inverse planning required by modern treatment techniques such as volumetric modulated arc therapy (VMAT). In this study, we explore the ability of deep learning to predict organ-at-risk (OAR) dose-volume histograms (DVHs) of left-sided breast cancer patients undergoing VMAT treatment based solely on their anatomical characteristics. The predicted DVHs could be used to derive patient-specific dose constraints and dose objectives, streamlining the treatment planning process, standardizing the quality of the plans, and personalizing the treatment planning. This study aimed to develop a deep learning-based framework for the prediction of organ-specific dose-volume histograms (DVH) based on structures delineated for left-sided breast cancer treatment. We used a dataset of 249 left-sided breast cancer patients treated with tangential VMAT fields. We extracted delineated structures and dose distributions for each patient and derived slice-by-slice DVHs for planning target volume (PTV) and organs-at-risk. The patients were divided into training (70%, n=174), validation (10%, n=24), and test (20%, n=51) sets. Collected data were used to train a deep learning model for the prediction of the DVHs based on the delineated structures. The developed deep learning model comprised a modified DenseNet architecture followed by a recurrent neural network. In the independent test set (n=51), the point-wise differences in the slice-by-slice DVHs between the clinical and predicted DVHs were small; the mean squared errors were 3.53, 1.58, 2.28, 3.37, and 1.44 [×10-4] for PTV, heart, ipsilateral lung, contralateral lung, and contralateral breast, respectively. With the derived cumulative DVHs, the mean absolute difference±standard deviation of mean doses between the clinical and the predicted DVH were 0.08± 0.04Gy, 0.24± 0.22Gy, 0.73± 0.46Gy, 0.07± 0.06Gy, and 0.14± 0.14Gy for PTV, heart, ipsilateral lung, contralateral lung, and contralateral breast, respectively. The deep learning-based approach enabled automatic and reliable prediction of the DVH based on delineated structures. The predicted DVHs could potentially serve as patient-specific clinical goals used to aid treatment planning and avoid suboptimal plans or to derive optimization objectives and constraints for automated treatment planning.

Comparison of photon energies and field techniques for ovarian dose reduction in volumetric modulated arc therapy

For fertility preservation in young female patients undergoing pelvic radiotherapy for cervical cancer, lateral ovarian transposition (LOT) is a procedure that moves the ovaries away from the radiation field. However, it is still challenging to fully protect the ovaries from radiation damage in volumetric modulated arc therapy (VMAT). In this study, we optimized VMAT techniques and simulated the effect of the LOT position on the ovarian dose. Ten cervical cancer patients who received pelvic radiotherapy after LOT were retrospectively analyzed. A restricted arc plus split field technique, which incorporated beam avoidance sectors, split field, fixed jaw, and orthogonal collimator orientation with 10 MV energy (rasf-10MV-VMAT), was designed to minimize the dose to the ovaries. This technique was compared with dual-arc conventional VMAT (dac-6MV-VMAT) and restricted arc and field VMAT (raf-6MV-VMAT) techniques with 6 MV energy. Ovarian dose, target coverage, and organs at risk (OAR) sparing were compared. The location of the ovaries in the anterior-posterior and superior-inferior directions was changed to simulate the effect of the LOT position on the ovarian dose. Compared to dac-6MV-VMAT and raf-6MV-VMAT, rasf-10MV-VMAT decreased the ovarian dose significantly more, especially for a low-dose volume. Both ovaries, received a 44.0–52.1 % lower volume of >3 Gy dose, and (0.5–1.9 Gy) lower maximum and mean doses with rasf-10MV-VMAT (p < 0.05). Rasf-10MV-VMAT also improved target conformity and coverage more than raf-6MV-VMAT (p < 0.05) and reduced the bladder mean dose more than dac-6MV-VMAT (p < 0.05). The ovarian dose decreased with the distance from the target contour, and placing the ovaries near the mid-frontal plane or as high as possible after the LOT lowered ovarian exposure. This technique, using 10 MV photon energy combined with field technique optimization, reduced the ovarian dose, particularly by decreasing the low-dose exposure of ovarian tissue and improving the target conformity simultaneously. The ovarian dose could be minimized by a proper LOT position. This optimized technique could enhance ovarian protection and fertility preservation for young female survivors of cervical cancer.

Dosimetric Comparison of Commonly Used Volumetric Modulated Arc Therapy Field Arrangements Based on Flattening Filter-Free Beams for Synchronous Bilateral Breast Carcinoma Radiation Therapy

Dosimetric Comparison of Commonly Used Volumetric Modulated Arc Therapy Field Arrangements Based on Flattening Filter-Free Beams for Synchronous Bilateral Breast Carcinoma Radiation Therapy

Novel hybrid treatment planning approach for irradiation a pediatric craniospinal axis

This study presents a new treatment planning approach merging 3D-CRT and VMAT fields into a hybrid treatment plan (HybTP), in order to achieve an optimum dose coverage of the planning target volume (PTV) and protection of OAR. Craniospinal axis irradiation (CSI) treated with 3D conformal radiotherapy (3D-CRT) is associated with high doses to the heart and eye lenses but provides better sparing of lungs and kidneys compared to volumetric modulated arc therapy (VMAT). VMAT treatment spares eye lenses and the heart, but lungs and kidneys are not as effective as 3D-CRT. Thus, a combination of both techniques (HybTP) may be optimal in sparing all these organs at risk (OAR). The results of HybTP are compared with helical tomotherapy (HT), intensity modulated radio therapy (IMRT), VMAT, and 3D-CRT plans. Hybrid, HT, VMAT, IMRT, and 3D-CRT treatment plans for a male child (age 6 years) with medulloblastoma were created and compared. A total dose of 35.2 Gy (PTV) with a dose per fraction of 1.6 Gy was prescribed. The following dose acceptance criteria were defined:(1)Mean PTV dose should be 100% of the prescribed dose, while the maximum dose should not exceed 107%.(2)At least 98% of the PTV should receive 95% of the prescribed dose.(3)The cribriform plate should be covered by the 95% isodose line.(4)The acceptance criteria for the OARs were: lenses Dmax<10 Gy, lungs Dmean<7 Gy, kidneys Dmean<15 Gy, heart Dmean<26 Gy, and heart V25<10%.The plans were compared regarding dose homogeneity index (HI) and conformity index (CI), PTV coverage, (particularly at cribriform plate) and doses at OARs. Best conformity was achieved with HT (CI = 0.98) followed by VMAT (CI = 0.96), IMRT (CI = 0.91), HybTP (CI = 0.86), and 3D-CRT (CI = 0.83). The homogeneity index varied marginally. For both HT and IMRT the HI was 0.07, and for 3D-CRT, VMAT and HybTP the HI was between 0.13 and 0.15. The cribriform plate was sufficiently covered by HybTP, VMAT, and 3D-CRT. The dose acceptance criteria for OARs were met by HT and HybTP. VMAT did not meet the criteria for lung (Dmean = right 10.4 Gy/left 10.2 Gy), 3D-CRT did not meet the criteria for eye lenses (Dmax = right 32.3 Gy/left 33.1), and heart (V25≈44%) and IMRT did not meet the criteria for lung (Dmean = right 11.1 Gy/left 11.2 Gy) and eye lenses (Dmax = right 12.2 Gy/left 13.1). HybTP meets all defined acceptance criteria and has proved to be a reasonable alternative for CSI. With HybTP that combines VMAT at the brain and heart with 3D-CRT posterior spinal fields (to spare lungs and kidneys), both appropriate coverage of the PTV and sparing of OAR can be achieved.

Dosimetric Study of Total Marrow and Lymphoid Irradiation on a Ring Gantry-Based Medical Linac with a Two-Layer Multi-Leaf Collimator

In this study, we aimed to evaluate dosimetric quality of total marrow and lymphoid irradiation (TMLI) plans for a ring gantry-based medical Linac with a two-layer multi-leaf collimator. We retrospectively retrieved treatment planning CT images, structure sets, and plan dose for four adult patients, two male and two female, who previously received TMLI treatments on helical tomotherapy (HT) at our institution. TMLI plans were optimized for a ring gantry-based medical Linac with a two-layer multi-leaf collimator (Halcyon, Varian Medical Systems, Inc., Palo Alto, CA). A prescription dose of 12 Gy in 8 fractions was prescribed to the skeletal bones from the skull to mid-thigh, spleen, spinal canal, and lymphoid volume. Five or six isocenters were placed with equal spacing along the patient's longitudinal direction in each TMLI plan with two 6-MV flattening filter-free volumetric modulated arc therapy (VMAT) fields at each isocenter. Isocenter separation ranged from 15 cm to 16.5 cm. Each VMAT field has a field size of 28 cm to 28 cm with the collimator at 90° and a full gantry rotation. The nominal dose rate was 800 MU/minute, and the maximum gantry rotation speed was 24°/sec. Institutional dosimetric constraints were used for optimization including a mean lung dose limit of less than 8 Gy. All the plans were normalized so that 85% the primary planning target volume received the prescription dose. The average mean doses to the target volumes ranged from 12.2 to 12.6 Gy in the Halcyon TMLI plans, while they ranged from 12.1 to 12.5 Gy in the HT TMLI plans. Relative to the prescription dose, the average mean dose for normal organs ranged from 21.3% to 56.6% in the Halcyon TMLI plans, while it ranged from 10.1% to 68.4% in the clinical HT plans. The difference in the average mean dose to normal organs was less than 0.5 Gy except two organs between the Halcyon and HT TMLI plans. The average median dose for normal organs ranged from 18.2% to 48.8% relative to the prescription dose in the Halcyon TMLI plans. The mean lung dose (MLD) in the Halcyon TMLI plans met the institutional limit with an average dose of 6.75±0.42 Gy (range: 6.44 - 7.36 Gy), while the average MLD was 6.54±0.77 Gy (range: 6.24 - 7.22 Gy) in the HT plans (p-value = 0.71 in the paired t-test). The average total monitor unit in the Halcyon TMLI plans was 4,425±906 MU (range: 3,470 - 5,575 MU) with an average beam-on time of 5.1±1.3 minutes (range: 4.1 - 7.0 minutes), which excludes isocenter setup time, while the average beam-on time was 22.2±3.2 minutes (range: 19.6 - 26.1 minutes) with the HT plans. Halcyon TMLI plans met our institutional dosimetric constraints with adequate normal organ sparing and target dose coverage. The beam-on time with the Halcyon plans was significantly shorter than that with the HT plans, which could lead to shorter treatment time and increased patient comfort. This study showed the feasibility of TMLI treatments on the Halcyon machine. The same method could be used for total body irradiation on Halcyon.

Target Coverage and Normal Organ Sparing in Dose-Escalated Total Marrow and Lymphatic Irradiation: A Single-Institution Experience.

Purpose/ObjectivesThe aim of this study is to report historical treatment planning experience at our institution for patients receiving total marrow and lymphatic irradiation (TMLI) as part of the conditioning regimen prior to hematopoietic stem cell transplant.Materials/MethodsBased on a review of all historical clinical TMLI treatments plans, we retrieved a 12-Gy cohort of 108 patients with a prescription dose of 12 Gy to the skeletal bones, lymph nodes, spleen, and spinal canal, and retrieved a 20-Gy cohort of 120 patients with an escalated prescription dose of 20 Gy to the skeletal bones, lymph nodes, spleen, and spinal cord, and 12 Gy to the brain and liver. Representative dosimetric parameters including mean and median dose, D80, and D10 (dose covering 80% and 10% of the structure volume, respectively) for targets and normal organs were extracted and compared between the two groups of patients.ResultsFor the 12-Gy cohort, the average mean dose for normal organs ranged from 18.3% to 78.3% of 12 Gy, and the average median dose ranged from 18.3% to 77.5% of 12 Gy. For the 20-Gy cohort, the average mean dose for normal organs ranged from 13.0% to 76.0% of 20 Gy, and the average median dose ranged from 12.5% to 75.0% of 20 Gy. Compared to the mean dose to normal organs in the 12-Gy cohort, the average mean dose to normal organs increased from 0.0% to 73.1%, with only four normal organs showing a >50% increase. Normal organ dose in TMLI plans using volumetric modulated arc therapy fields fell within the dose range in historical TMLI plans.ConclusionDosimetric data in historical TMLI plans at our institution are summarized at prescription dose levels of 12 Gy and 20 Gy, respectively. Compared to the normal organ dose with a prescription dose of 12 Gy, the mean and median dose to most normal organs at an escalated prescription dose of 20 Gy had an increase less than prescription dose scaling. Dosimetric results from this study can be used as reference data to facilitate clinical implementation of TMLI at other institutions.

Virtual patient-specific QA with DVH-based metrics.

We demonstrate a virtual pretreatment patient-specific QA (PSQA) procedure that is capable of quantifying dosimetric effect on patient anatomy for both intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT). A machine learning prediction model was developed to use linear accelerator parameters derived from the DICOM-RT plan to predict delivery discrepancies at treatment delivery (defined as the difference between trajectory log file and DICOM-RT) and was coupled with an independent Monte Carlo dose calculation algorithm for dosimetric analysis. Machine learning models for IMRT and VMAT were trained and validated using 120 IMRT and 206 VMAT fields of prior patients, with 80% assigned for iterative training and testing, and 20% for post-training validation. Various prediction models were trained and validated, with the final models selected for clinical implementation being a boosted tree and bagged tree for IMRT and VMAT, respectively. After validation, these models were then applied clinically to predict the machine parameters at treatment delivery for 7 IMRT plans from various sites (61 fields) and 10 VMAT multi-target intracranial radiosurgery plans (35 arcs) and compared to the dosimetric effect calculated directly from trajectory log files. Dose indices tracked for targets and organs at risk included dose received by 99%, 95%, and 1% of the volume, mean dose, percent of volume receiving 25%-100% of the prescription dose. The average coefficient of determination (r2 ) when comparing intra-field predicted and actual delivery error was 0.987±0.012 for IMRT and 0.895±0.095 for VMAT, whereas r2 when comparing inter-field predicted versus actual delivery error was 0.982 for IMRT and 0.989 for VMAT. Regarding dosimetric analysis, r2 when comparing predicted versus actual dosimetric changes for all dose indices was 0.966 for IMRT and 0.907 for VMAT. Prediction models can be used to anticipate the dosimetric effect calculated from trajectory files and have potential as a "delivery-free" pretreatment analysis to enhance PSQA.

Evaluation of Two Intensity Modulated Radiotherapy Techniques for Total Body Irradiation: Initial Clinical Experience From an Ongoing Pilot Study

A multi-institutional pilot study is initiated to compare two IMRT delivery techniques (volumetric modulated arc therapy (VMAT) and helical tomotherapy (HT)) for total body irradiation as conditioning regimen in patients undergoing hematopoietic stem cell transplant with an updated lung dose limit based on recently published clinical data on lung toxicities. We report comparison and initial clinical experience in dosimetric plan quality, patient setup correction techniques, and dose delivery efficiency.We reviewed treatment planning and delivery record for the initial seven patients that were prospectively enrolled in this study. TBI was given to each patient in 8 fractions at 165 cGy per fraction. Patient height ranged from 154.9 to 193.5 cm. The planning target volume (PTV) is the whole body excluding the lung volume. The total lung and 15 additional normal organs were included in plan optimization and dosimetric evaluation. In planning, mean lung dose (MLD) limit was 8 Gy and maximum dose (Dmax) to normal organs was required to be < 130% of the prescribed dose with a recommended goal of < 115%. Four patients received TBI with HT and the other three on a conventional Linac. Patients treated with HT had an upper body HT plan with a rotating gantry using a 5-cm jaw and a lower extremity HT plan using static AP/PA HT fields. Patients treated on a conventional Linac had an upper body VMAT plan using 9 to 10 VMAT fields and a lower body plan with either VMAT fields or static AP/PA fields. The virtual bolus technique was used in VMAT plans to reduce dose variation due to setup uncertainty. The VMAT fields used a 6 MV photon beam with a 90° collimator angle and had an overlap of at least 2 cm between adjacent fields along the longitudinal direction. All the plans were normalized so that 85% of the PTV received at least 13.2 Gy in total dose.In the upper body HT plans the average PTV dose was 13.6 Gy (range: 13.5 - 13.7 Gy), the MLD averaged at 7.71 Gy (range: 7.21 - 7.95 Gy), and Dmax was < 115% of prescribed dose for at least 14 normal organs. In the upper-body VMAT plans the average PTV dose was 13.95 Gy (range: 13.86 - 14.02 Gy), the MLD averaged at 7.12 Gy (range: 6.85 - 7.63 Gy), and Dmax was < 115% of prescribed dose with 1 to 9 normal organs. The Dmax limit of 130% of prescribed dose was achieved in all the plans. In HT treatments, an MVCT scan was used for daily patient setup corrections in both the upper and lower body treatments and the overall treatment time per fraction was 1.23 ± 0.21 hours (range: 0.90 - 1.88 hours). In VMAT treatments, both cone beam CT scans and orthogonal kilovoltage images were used for daily patient setup on a conventional Linac and the overall treatment time per fraction was 1.39 ± 0.20 hours.Both the HT and VMAT plans achieved adequate lung sparing under our new institutional lung dose limit in TBI. VMAT TBI is clinically feasible and could provide access to more patients for TBI treatments. The two IMRT techniques also allow selective sparing of additional organs in TBI treatments.

Identification of a potential source of error for 6FFF beams delivered on an AgilityTM multileaf collimator.

PurposeThe performance of the AgilityTM multileaf collimator was investigated with a focus on dynamic, small fields for flattening filter free (FFF) beams.MethodsIn this study we have developed a simple tool to test the robustness of the control mechanisms during dynamic beam delivery for Elekta’s VersaHD linear accelerator with Integrity 4.0.4 control software. We have programed the planning system to calculate dose for delivery of sweeping gaps. These sweeping gaps have a constant speed, constant size, and are delivered at a constant dose rate. Therefore they specifically identify delivery problems in dynamic mode.ResultsThe Elekta AgilityTM control mechanism fails to maintain accurate delivery for small, dynamic sweeping gaps. For small gap sizes, the AgilityTM control mechanism delivers a field that is more than four times the size of the planned field width without generating an interlock. This has dosimetric implications: The discrepancy between calculated and measured doses increases with decreasing gap size and exceeds 10% and 60% at isocenter for a 3.5 mm and 1 mm gap size, respectively.ConclusionA deficiency of the AgilityTM control system was identified in this study. This deficiency is a potential source of error for volumetric modulated arc therapy fields and could therefore contribute to relatively high failure rates in quality assurance measurements, especially for FFF beams.

Improved Volumetric Modulated Arc Therapy Field Junctions Using In Silico Base Plans: Application to Craniospinal Irradiation

IntroductionField junctions present a major challenge for planning craniospinal irradiation (CSI) using volumetric modulated arc therapy (VMAT). In this study, the feasibility of using in silico base dose distributions for planning junctioned VMAT fields for CSI is assessed. MethodsAn in-house computer program was created to generate strategic base plans with controlled linear dose gradients across the junction. The algorithm was generalized to allow user-defined parameters such as number of junctions and junction length. In silico base plans were used to optimize junctioned VMAT CSI plans for a pediatric case and an adult case. Throughout optimization, dose to the eyes, kidneys, lungs, heart, and liver were minimized. Final plan quality was evaluated using the percent of planning target volume receiving at least 95% prescription dose (V95%), homogeneity index, and conformity number. Final plan robustness to setup error was evaluated using changes in near-minimum, median, and near-maximum doses defined as the most exposed 98%, 50%, and 2% of the planning target volume (D98%, D50%, D2%) within the junction region before and after setup errors of ±3, ±5, and ±7 mm in the craniocaudal direction. ResultsThe program generated ideal in silico dose distributions that were compatible with a commercial treatment planning system for use as base doses during VMAT optimization. VMAT plans, that were optimized with the in silico base plans, had complementary linear dose profiles across the junction. Final pediatric and adult VMAT CSI plans both had V95% ≥98.1% and 98.1%, homogeneity index: 0.09 and 0.10, and conformity number: 0.86, 0.84, respectively. In addition, dose to surrounding organs at risk was acceptably low for both cases. For ±3 mm setup errors, small variations in the junction dose were recorded with ΔD98% ≤2.3%, ΔD50% ≤2.3%, and ΔD2% ≤2.8%. ConclusionsThis is the first demonstration of junctioned VMAT field optimization with a controlled linear dose gradient across the junction without the use of any extra planning contours. Planning junctioned VMAT using in silico base plans is feasible and capable of generating high-quality plans that are robust to clinically expected setup variations.

Jaw Tracking Impacts on Treatment Plan Quality in Small Volumetric Modulated Arc Therapy Fields: A Dosimetric Verification

Jaw Tracking Impacts on Treatment Plan Quality in Small Volumetric Modulated Arc Therapy Fields: A Dosimetric Verification

SU‐F‐T‐273: Using a Diode Array to Explore the Weakness of TPS DoseCalculation Algorithm for VMAT and Sliding Window Techniques

Purpose:To identify the weakness of dose calculation algorithm in a treatment planning system for volumetric modulated arc therapy (VMAT) and sliding window (SW) techniques using a two‐dimensional diode array.Methods:The VMAT quality assurance(QA) was implemented with a diode array using multiple partial arcs that divided from a VMAT plan; each partial arc has the same segments and the original monitor units. Arc angles were less than ± 30°. Multiple arcs delivered through consecutive and repetitive gantry operating clockwise and counterclockwise. The source‐toaxis distance setup with the effective depths of 10 and 20 cm were used for a diode array. To figure out dose errors caused in delivery of VMAT fields, the numerous fields having the same segments with the VMAT field irradiated using different delivery techniques of static and step‐and‐shoot. The dose distributions of the SW technique were evaluated by creating split fields having fine moving steps of multi‐leaf collimator leaves. Calculated doses using the adaptive convolution algorithm were analyzed with measured ones with distance‐to‐agreement and dose difference of 3 mm and 3%..Results:While the beam delivery through static and step‐and‐shoot techniques showed the passing rate of 97 ± 2%, partial arc delivery of the VMAT fields brought out passing rate of 85%. However, when leaf motion was restricted less than 4.6 mm/°, passing rate was improved up to 95 ± 2%. Similar passing rate were obtained for both 10 and 20 cm effective depth setup. The calculated doses using the SW technique showed the dose difference over 7% at the final arrival point of moving leaves.Conclusion:Error components in dynamic delivery of modulated beams were distinguished by using the suggested QA method. This partial arc method can be used for routine VMAT QA. Improved SW calculation algorithm is required to provide accurate estimated doses.

TH‐EF‐BRB‐11: Volumetric Modulated Arc Therapy for Total Body Irradiation

Purpose:To develop a modern, patient‐comfortable total body irradiation (TBI) technique suitable for standard‐sized linac vaults.Methods:An indexed rotatable immobilization system (IRIS) was developed to make possible total‐body CT imaging and radiation delivery on conventional couches. Treatment consists of multi‐isocentric volumetric modulated arc therapy (VMAT) to the upper body and parallel‐opposed fields to the lower body. Each isocenter is indexed to the couch and includes a 180° IRIS rotation between the upper and lower body fields. VMAT fields are optimized to satisfy lung dose objectives while achieving a uniform therapeutic dose to the torso. End‐to‐end tests with a rando phantom were used to verify dosimetric characteristics. Treatment plan robustness regarding setup uncertainty was assessed by simulating global and regional isocenter setup shifts on patient data sets. Dosimetric comparisons were made with conventional extended distance, standing TBI (cTBI) plans using a Monte Carlo‐based calculation. Treatment efficiency was assessed for eight courses of patient treatment.Results:The IRIS system is level and orthogonal to the scanned CT image plane, with lateral shifts &lt;2mm following rotation. End‐to‐end tests showed surface doses within ±10% of the prescription dose, field junction doses within ±15% of prescription dose. Plan robustness tests showed &lt;15% changes in dose with global setup errors up to 5mm in each direction. Local 5mm relative setup errors in the chest resulted in &lt; 5% dose changes. Local 5mm shift errors in the pelvic and upper leg junction resulted in &lt;10% dose changes while a 10mm shift error causes dose changes up to 25%. Dosimetric comparison with cTBI showed VMAT‐TBI has advantages in preserving chest wall dose with flexibility in leveraging the PTV‐body and PTV‐lung dose.Conclusion:VMAT‐TBI with the IRIS system was shown clinically feasible as a cost‐effective approach to TBI for standard‐sized linac vaults.

SU‐F‐T‐260: Using Portal Image Device for Pre‐Treatment QA in Volumetric Modulated Arc Plans with Flattening Filter Free (FFF) Beams

Purpose:To implement and validate a method of using electronic portal image device (EPID) for pre‐treatment quality assurance (QA) of volumetric modulated arc therapy (VMAT) plans using flattering filter free (FFF) beams for stereotactic body radiotherapy (SBRT).Methods:On Varian Edge with 6MV FFF beam, open field (from 2×2 cm to 20×20 cm) EPID images were acquired with 200 monitor unit (MU) at the image device to radiation source distance of 150cm. With 10×10 open field and calibration unit (CU) provided by vendor to EPID image pixel, a dose conversion factor was determined by dividing the center dose calculated from the treatment planning system (TPS) to the corresponding CU readout on the image. Water phantom measured beam profile and the output factors for various field sizes were further correlated to those of EPID images. The dose conversion factor and correction factors were then used for converting the portal images to the planner dose distributions of clinical fields. A total of 28 VMAT fields of 14 SBRT plans (8 lung, 2 prostate, 2 liver and 2 spine) were measured. With 10% low threshold cutoff, the delivered dose distributions were compared to the reference doses calculated in water phantom from the TPS. A gamma index analysis was performed for the comparison in percentage dose difference/distance‐to‐agreement specifications.Results:The EPID device has a linear response to the open fields with increasing MU. For the clinical fields, the gamma indices between the converted EPID dose distributions and the TPS calculated 2D dose distributions were 98.7%±1.1%, 94.0%±3.4% and 70.3%±7.7% for the criteria of 3%/3mm, 2%/2mm and 1%/1mm, respectively.Conclusion:Using a portal image device, a high resolution and high accuracy portal dosimerty was achieved for pre‐treatment QA verification for SBRT VMAT plans with FFF beams.

SU-E-T-61: Capabilities and Considerations for the Clinical Use of Flattening Filter Free (FFF) Beams for Volumetric Modulated Arc Therapy (VMAT)

Purpose: Machines with FFF beams are becoming more prevalent. As technology evolves, guidelines for its use need to be developed. FFF beams have the potential for much greater dose rates and lower out-of-field dose. The use of VMAT with FFF adds additional complexity. This work attempts to define the capabilities and limitations of FFF when applied to VMAT fields and characterize the situations which this technology is best suited. Methods: The total monitor units of a VMAT delivery is modified to determine the minimum deliverable dose rate for FFF and non-FFF beams on an Elekta VersaHD linear accelerator. The minimum dose rate limits the minimum dose per segments, thus reducing the available modulation. Time is assessed by recording the beam-on time for FFF and non-FFF beams. For 10 plans with standard fraction sizes (180–212 cGy/Fx) maximum dose rate per control point and number of control points exceeding 400 MU/min are recorded. Results: VMAT minimum dose rate is related to the pulse rate. The minimum deliverable dose rate is ∼36 MU/min (0.12 MU/deg) for 6x-FFF (max 1400 MU/min) and ∼75 MU/min (0.27 MU/deg) for 10x-FFF (max 2400 MU/min) beams. Delivery time is independent of dose rate for dose rates below 1500 MU/min as it is limited by gantry speed. The maximum dose rate across in all plans according to Pinnacle is 1170 MU/min for FFF plans and 800 MU/min for non-FFF plans. The percentage of control points exceeding 400 MU/min is 13.3% for FFF and 9% for non-FFF beams. Conclusion: FFF does not improve delivery time of VMAT fields for conventional fraction sizes, since the dominant factor is gantry speed instead of dose rate. However, there may be potential for speed improvement for very large fraction sizes. Due to pulse rate dependency, FFF beams require a higher minimum dose rate.

Comparison of the performance between portal dosimetry and a commercial two-dimensional array system on pretreatment quality assurance for volumetric-modulated arc and intensity-modulated radiation therapy

The aim of this study was to compare the dosimetric performance and to evaluate the pretreatment quality assurance (QA) of a portal dosimetry and a commercial two-dimensional (2-D) array system. In the characteristics comparison study, the measured values for the dose linearity, dose rate response, reproducibility, and field size dependence for 6-MV photon beams were analyzed for both detector systems. To perform the qualitative evaluations of the 10 IMRT and the 10 VMAT plans, we used the Gamma index for quantifying the agreement between calculations and measurements. The performance estimates for both systems show that overall, minimal differences in the dosimetric characteristics exist between the Electron portal imaging device (EPID) and 2-D array system. In the qualitative analysis for pretreatment quality assurance, the EPID and 2-D array system yield similar passing rate results for the majority of clinical Intensity-modulated radiation therapy (IMRT) and Volumetric-modulated arc therapy (VMAT) cases. These results were satisfactory for IMRT and VMAT fields and were within the acceptable criteria of γ%≤1, γavg<0.5. The EPDI and the 2-D array systems showed comparable dosimetric results. In this study, the results revealed both systems to be suitable for patient-specific QA measurements for IMRT and VMAT. We conclude that, depending on the status of clinic, both systems can be used interchangeably for routine pretreatment QA.

Clinical evaluation of an anatomy‐based patient specific quality assurance system

The Delta4DVH Anatomy 3D quality assurance (QA) system (ScandiDos), which converts the measured detector dose into the dose distribution in the patient geometry was evaluated. It allows a direct comparison of the calculated 3D dose with the measured back‐projected dose. In total, 16 static and 16 volumetric‐modulated arc therapy (VMAT) fields were planned using four different energies. Isocenter dose was measured with a pinpoint chamber in homogeneous phantoms to investigate the dose prediction by the Delta4DVH Anatomy algorithm for static fields. Dose distributions of VMAT fields were measured using GAFCHROMIC film. Gravitational gantry errors up to 10° were introduced into all VMAT plans to study the potential of detecting errors. Additionally, 20 clinical treatment plans were verified. For static fields, the Delta4DVH Anatomy predicted the isocenter dose accurately, with a deviation to the measured phantom dose of 1.1%±0.6%. For VMAT fields the predicted Delta4DVH Anatomy dose in the isocenter plane corresponded to the measured dose in the phantom, with an average gamma agreement index (GAI) (3 mm/3%) of 96.9±0.4%. The Delta4DVH Anatomy detected the induced systematic gantry error of 10° with a relative GAI (3 mm/3%) change of 5.8%±1.6%. The conventional Delta4PT QA system detected a GAI change of 4.2%±2.0%. The conventional Delta4PT GAI (3 mm/3%) was 99.8%±0.4% for the clinical treatment plans. The mean body and PTV‐GAI (3 mm/5%) for the Delta4DVH Anatomy were 96.4%±2.0% and 97.7%±1.8%; however, this dropped to 90.8%±3.4% and 87.1%±4.1% for passing criteria of 3 mm/3%. The anatomy‐based patient specific quality assurance system predicts the dose distribution correctly for a homogeneous case. The limiting factor for the error detection is the large variability in the error‐free plans. The dose calculation algorithm is inferior to that used in the TPS (Eclipse).PACS numbers: 87.56.Fc, 87.56.‐v

Results of 1 year of clinical experience with independent dose calculation software for VMAT fields.

It is widely accepted that a redundant independent dose calculation (RIDC) must be included in any treatment planning verification procedure. Specifically, volumetric modulated arc therapy (VMAT) technique implies a comprehensive quality assurance (QA) program in which RIDC should be included. In this paper, the results obtained in 1 year of clinical experience are presented. Eclipse from Varian is the treatment planning system (TPS), here in use. RIDC were performed with the commercial software; Diamond® (PTW) which is capable of calculating VMAT fields. Once the plan is clinically accepted, it is exported via Digital Imaging and Communications in Medicine (DICOM) to RIDC, together with the body contour, and then a point dose calculation is performed, usually at the isocenter. A total of 459 plans were evaluated. The total average deviation was -0.3 ± 1.8% (one standard deviation (1SD)). For higher clearance the plans were grouped by location in: Prostate, pelvis, abdomen, chest, head and neck, brain, stereotactic radiosurgery, lung stereotactic body radiation therapy, and miscellaneous. The highest absolute deviation was -0.8 ± 1.5% corresponding to the prostate. A linear fit between doses calculated by RIDC and by TPS produced a correlation coefficient of 0.9991 and a slope of 1.0023. These results are very close to those obtained in the validation process. This agreement led us to consider this RIDC software as a valuable tool for QA in VMAT plans.

Time-resolved dose reconstruction by motion encoding of volumetric modulated arc therapy fields delivered with and without dynamic multi-leaf collimator tracking

Background. Organ motion during treatment delivery in radiotherapy (RT) may lead to deterioration of the planned dose, but can be mitigated by dynamic multi-leaf collimator (DMLC) tracking. The purpose of this study was to implement and experimentally validate a method for time-resolved motion including dose reconstruction for volumetric modulated arc therapy (VMAT) treatments delivered with and without DMLC tracking. Material and methods. Tracking experiments were carried out on a linear accelerator (Trilogy, Varian) with a prototype DMLC tracking system. A motion stage carrying a biplanar dosimeter phantom (Delta4PT, Scandidos) reproduced eight representative clinical tumor trajectories (four lung, four prostate). For each trajectory, two single-arc 6 MV VMAT treatments with low and high modulation were delivered to the moving phantom with and without DMLC tracking. An existing in-house developed program that adds target motion to treatment plans was extended with the ability to split an arc plan into any number of sub-arcs, allowing the calculated dose for different parts of the treatment to be examined individually. For each VMAT sub-arc, reconstructed and measured doses were compared using dose differences and 3%/3 mm γ-tests. Results. For VMAT sub-arcs the reconstructed dose distributions had a mean root-mean-square (rms) dose difference of 2.1% and mean γ failure rate of 2.0% when compared with the measured doses. For final accumulated doses the mean rms dose difference was 1.6% and the γ failure rate was 0.7%. Conclusion. The time-resolved motion including dose reconstruction was experimentally validated for complex tracking and non-tracking treatments with patient-measured tumor motion trajectories. The reconstructed dose will be of high value for evaluation of treatment plan robustness facing organ motion and adaptive RT.

A Comparison of Lung SBRT Using Volumetric Modulated Arc Therapy With Static Field IMRT

Volumetric modulated arc therapy allows for fast delivery of stereotactic body radiation therapy (SBRT) delivery in lung cancer treatment. At our institution, SBRT lung patients are planned using either volumetric modulated arc therapy (VMAT) or Static Field IMRT treatment. The decision for either approach is determined based on treatment machine availability. The objective of this study is to compare and evaluate the plan quality between VMAT and Static Field IMRT treatments.