MV Flattening Filter Free Photon Research Articles

Adapting a practical EPR dosimetry protocol to measure output factors in small fields with alanine.

Modern radiotherapy techniques often deliver small radiation fields. In this work, a practical Electron Paramagnetic Resonance (EPR) dosimetry protocol is adapted and applied to measure output factors (OF) in small fields of a 6 MV radiotherapy system. Correction factors and uncertainties are presented and OFs are compared to the values obtained by following TRS-483 using an ionization chamber (IC). Irradiations were performed at 10cm depth inside a water phantom positioned at 90cm source to surface distance with a 6 MV flattening filter free photon beam of a Halcyon radiotherapy system. OFs for different nominal field sizes (1×1, 2×2, 3×3, 4×4, normalized to 10×10 cm2 ) were determined with a PinPoint 3D (PTW 31022) IC following TRS-483 as well as with alanine pellets with a diameter of 4mm and a height of 2.4mm. EPR readout was performed with a benchtop X-band spectrometer. Correction factors due to volume averaging and due to positional uncertainties were derived from 2D film measurements. OFs obtained from both dosimeter types agreed within 0.7% after applying corrections for the volume averaging effect. For the used alanine pellets, volume averaging correction factors of 1.030(2) for the 1×1 cm2 field and <1.002 for the larger field sizes were determined. The correction factor for positional uncertainties of 1mm was in the order of 1.018 for the 1×1 cm2 field. Combined relative standard uncertainties uc for the OFs resulting from alanine measurements were estimated to be below 1.5% for all field sizes. For IC measurements, uc was estimated to be below 1.0%. A practical EPR dosimetry protocol is adaptable for precisely measuring OFs in small fields down to 1×1cm2 . It is recommended to consider the effect of positional uncertainties for field sizes <2×2cm2 .

Evaluation of a High-Resolution Large-Area Two-Dimensional Detector Array for Pre-Treatment Verification of Multiple-Target SRS Plans Using a Single Isocenter

<h3>Purpose/Objective(s)</h3> Pre-treatment verification of multiple-target SRS plans using a single isocenter is challenging because of small target sizes and multiple off-isocenter targets. Targets located further away from the isocenter are more sensitive to rotational setup variations, increasing the risk of geometrical miss. It demands high-resolution detector for small target sizes and large detector area for measuring multiple targets at once. The purpose of this study is to evaluate a high-resolution large-area two-dimensional (2D) detector array for pre-treatment verification of multiple-target SRS plans using a single isocenter. <h3>Materials/Methods</h3> A 2D detector array was evaluated for this purpose. It is based on a silicon complementary metal-oxide-semiconductor platform, which enables a compact design, fast read-out, and high pixel density with each pixel comprising a radiation-sensitive photodiode, a capacitor and three transistors. Its spatial resolution is 0.4 mm, with 105,000 pixels across a large active area of 120 × 140 mm². Ten multiple-target SRS plans were selected such that each plan has at least 2 targets appear on a longitudinal plane cross the isocenter. All plans were created in a treatment planning system using 6 MV flattening filter free photon beam. The typical plan consisted of one or two coplanar arcs, and two to three non-coplanar arcs. All plans were delivered in patient geometry with the detector center aligned to the plan isocenter. The detector plane was rotated to the corresponding desired longitudinal plane to measure the composite dose in a single measurement. The smallest target measured was 0.12 cc. The largest center-to-center target separation measured was 12 cm. Six of the plans in which the target separations were relatively short were able to be cross verified with a patient-specific quality assurance (PSQA) tool, whose detector area is of 77 × 77 mm². The measured 2D dose distributions in true composite were compared with the calculated dose distributions. Gamma index analysis was performed using 3%/1 mm criteria with a 10% dose threshold. <h3>Results</h3> With the 2D detector array, the gamma passing rates are all greater than 95% except one, which is 93%. The average gamma-passing rate is 97.7%. For the 6 plans cross verified, the average gamma-passing rates are 98.2% and 96.1% with the 2D detector array and PSQA tool, respectively. <h3>Conclusion</h3> The 2D detector array was demonstrated to be suitable and offers an efficient way for pre-treatment verification of multiple-target SRS plans using a single isocenter. The advantages of this 2D array include: 1) High-resolution of 0.4 mm avoids dose interpolations for plans calculated with 0.5 -1.0 mm dose grid and potentially avoids "false positive" results due to more measured points; 2) Large active area of 120 × 140 mm² allows simultaneous verification of multiple targets in true composite in patient geometry.

A Comprehensive Evaluation of Single Isocenter Multiple Target SRS Plans and the Analytical Relationship between Plan Quality Indices with the Number and Volume of Targets.

To study the relationship of plan quality indices with the number and volume of target for 5 to 10 brain metastases treated with LINAC-based Single Isocenter Multiple Target (SIMT) Stereotactic radiosurgery (SRS) planning and to determine the maximum volume of spherical targets treated without exceeding the normal tissue tolerances. Spherical targets of 5 to 10 numbers per plan, with individual target volumes ranging from 0.025 cc to 11.5 cc, were simulated with randomly drawn planning target volumes (PTVs) within the brain. SIMT SRS plans were generated for the 21 Gy prescription dose with a 6 MV Flattening Filter Free photon beam. Target coverage, organ at risk sparing, plan quality indices, R50%, and gradient measure were studied. Mean brain dose, V12 for Brain minus PTV (BmP), V10, V12, V15, V18, V20, and V24 for brain volume were evaluated. Equations relating the gradient index, mean brain dose, and V12 (BmP) to the given number and volume of the targets were constructed. PTV coverage D98 was 98.77 ± 1.37 %. The mean CIRTOG, QRTOG, HIRTOG, CIP, GI, and R50% of the individual targets were 1.02 ± 0.08, 0.94 ± 0.02, 1.49 ± 0.11, 0.91 ± 0.06, 4.74 ± 2.3, and 4.95 ± 2.67, respectively. The gradient measure achieved was in the range of 0.49 to 1.35 cm. The mean brain dose was in the range of 1.62 to 6.69 Gy. The mean V12 (BmP) per target obtained was 3.85 ± 2.83 cc. Equations relating the number and volume of targets to the gradient measure, mean brain dose, V12 (BmP), and V10-24 can serve as a baseline for multiple brain metastases SIMT planning. The target volume for 5, 6, 7, 8, 9, and 10 targets that can be treated without exceeding V12 (BmP) is 6, 5, 4.7, 4, 3.7, and 3.4 cc, respectively, for the 21 Gy prescription dose.

Evaluation of dosimetric parameters of small fields of 6 MV flattening filter free photon beam measured using various detectors against Monte Carlo simulation – CORRIGENDUM

Evaluation of dosimetric parameters of small fields of 6 MV flattening filter free photon beam measured using various detectors against Monte Carlo simulation – CORRIGENDUM

Evaluation of dosimetric parameters of small fields of 6 MV flattening filter free photon beam measured using various detectors against Monte Carlo simulation

AbstractPurpose:This study aims to evaluate dosimetric parameters like percentage depth dose, dosimetric field size, depth of maximum dose surface dose, penumbra and output factors measured using IBA CC01 pinpoint chamber, IBA stereotactic field diode (SFD), PTW microDiamond against Monte Carlo (MC) simulation for 6 MV flattening filter-free small fields.Materials and Methods:The linear accelerator used in the study was a Varian TrueBeam® STx. All field sizes were defined by jaws. The required shift to effective point of measurement was given for CC01, SFD and microdiamond for depth dose measurements. The output factor of a given field size was taken as the ratio of meter readings normalised to 10 × 10 cm2 reference field size without applying any correction to account for changes in detector response. MC simulation was performed using PRIMO (PENELOPE-based program). The phase space files for MC simulation were adopted from the MyVarian Website.Results and Discussion:Variations were seen between the detectors and MC, especially for fields smaller than 2 × 2 cm2 where the lateral charge particle equilibrium was not satisfied. Diamond detector was seen as most suitable for all measurements above 1 × 1 cm2. SFD was seen very close to MC results except for under-response in output factor measurements. CC01 was observed to be suitable for field sizes above 2 × 2 cm2. Volume averaging effect for penumbra measurements in CC01 was observed. No detector was found suitable for surface dose measurement as surface ionisation was different from surface dose due to the effect of perturbation of fluence. Some discrepancies in measurements and MC values were observed which may suggest effects of source occlusion, shift in focal point or mismatch between real accelerator geometry and simulation geometry.Conclusion:For output factor measurement, TRS483 suggested correction factor needs to be applied to account for the difference in detector response. CC01 can be used for field sizes above 2 × 2 cm2 and microdiamond detector is suitable for above 1 × 1 cm2. Below these field sizes, perturbation corrections and volume averaging corrections need to be applied.

Clinical experience with lung-specific electromagnetic transponders for real-time tumor tracking in lung stereotactic body radiotherapy.

Motion management is crucial for optimal stereotactic body radiotherapy (SBRT) of moving targets. We aimed to describe our clinical experience with real-time tracking of lung-specific electromagnetic transponders (EMTs) for SBRT of early stage non-small cell lung cancer in free-breathing (FB) or deep inspiration breath-hold (DIBH). Seven patients were implanted with EMTs. Simulation for SBRT was performed in FB and in DIBH. We prescribed 60Gy in 3, 5 or 8 fractions to the tumor and delivered SBRT with volumetric modulated arcs and a 6MV flattening filter free photon beam. Patients' setup at the linac was performed using EMT positions and cone-beam CT (CBCT) verification. Four patients were treated in DIBH because of a dosimetric benefit. We analysed patient alignment and treatment delivery parameters using DIBH or FB and EMT real-time tracking. There were no complications from the EMT implantation. Visual inspection of CBCT before and/or after SBRT revealed good alignment of structures and EMTs. The median setup time was 9.8min (range: 4.6-34.1min)and the median session time was 14.7min (range: 7.3-36.5min). EMT positions in lungs remained stable during overall treatment and allowed real-time tracking both in FB and in DIBH SBRT. The treatment beam was gated when EMT centroid position exceeded tolerance thresholds ensuring correct delivery of radiation to the tumor. Using EMTs for real-time tracking of tumor motion during lung SBRT proved to be safe, accurate and easy to integrate clinically for treatments in FB or DIBH.

Effects of metal implants and a metal artifact reduction tool on calculation accuracy of AAA and Acuros XB algorithms in small fields.

In this study, the dosimetric accuracy of analytical anisotropic algorithm (AAA) and Acuros XB (AXB) dose calculation algorithms (Varian Medical Systems, Palo Alto, CA) was investigated for small radiation fields incident on phantoms of various metals that include stainless steel grade 316L (SS316L) and titanium alloy grade 5 (Ti5) implants. In addition, the effects of using metal artifact reduction for orthopedic implants (O-MAR, Philips Healthcare, Cleveland, OH) were evaluated. The evaluations of AAA and AXB were performed by comparing the crossline profiles calculated by AAA and AXB with GafChromicTM EBT3 film measurements at the phantom-implant interfaces and in close vicinity of implant materials for small field sizes (1×1cm2 , 2×2cm2 , 3×3cm2 , and 4×4cm2 ) of a 6MV flattening filter free photon beam. O-MAR corrected and uncorrected (UC) computed tomography (CT) images were used for dose calculations. The values of average and standard deviations (SD) of Hounsfield unit (HU) for selected regions of each case were evaluated. The differences in average dose percentages in defined regions were calculated to quantify the relative dosimetric changes between doses calculated on UC and O-MAR corrected CT images. Compared to UC images, the values of SD were reduced, and the average HU became closer to its reference value in the O-MAR images. There was some discrepancy in average dose percentage differences between calculations using UC and O-MAR images at 1cm above the SS316L implant (average dose percentage differences were AXB/UC=5.9% and AXB/O-MAR=-1.2%; AAA/UC=2.2%, and AAA/O-MAR=-0.8%). Neither AAA nor AXB algorithms predict increase in dose at upper phantom-implant interface (4.9%, 9.9%. 13.5%, and 13.8% for the fields from 1×1cm2 to 4×4cm2 , respectively). At the side of the SS316L implant (where dark streak artifacts exist), dose difference averages were estimated as-1.1% and 22.3% when AXB/O-MAR and AXB/UC calculations are compared with EBT3 measurements, respectively. Dose predictions at 1cm below the SS316L implant were underestimated by AXB/O-MAR (average -0.5%) and AXB/UC (average 2.0%). The O-MAR tool was shown to have a favorable dosimetric effect or no effect on the calculations in the upper proximity of the implant materials. The dose differences between EBT3 film measurements and calculations at upper phantom-implant interfaces were smaller when they were calculated using O-MAR images. However, the dose differences increased when O-MAR corrected images were used for AAA calculations at lower phantom-implant interfaces. Use of O-MAR enabled closer agreement for the AXB algorithm, especially in the dark streak artifact regions. The O-MAR algorithm should be used when the dose is calculated with the AXB algorithm in cases of patients with the metal implants. The estimations using AAA and AXB algorithms, in phantom setups, with Ti5 implant material were found to be closer to the EBT3 film measurements, when compared with the same estimations using SS316L implant material.

Response characterization of EBT-XD radiochromic films in megavoltage photon and electron beams.

The purpose of this study was to investigate the beam quality dependence of the EBT-XD radiochromic films for megavoltage photon and electron beam irradiations by studying their spectral response. Dose, dose rate, and interbatch dependencies were investigated as well. EBT-XD and EBT3 radiochromic films were cut into 5cm2 ×5cm2 pieces, placed between solid water phantoms, and irradiated with 6 and 15MV photon beams, 6 and 10MV flattening filter free photon beams, and 6 and 20MeV electron beams at dose levels between 0.5 and 50Gy. In order to measure the spectral response, the films were illuminated by a deuterium and tungsten-halogen lamp and net absorption was measured in 400-800nm spectral range using a fiber-coupled optical spectrometer. Film samples were then analyzed using a flatbed scanner in the red, green, and blue channel to obtain the optical density of the films. The net absorption spectra of the EBT-XD and EBT3 films showed two absorption bands centered at 635 and 585nm, characteristic of the EBT model. However, for a given dose, the EBT-XD films showed lower net absorbance compared to the EBT3 films. At a given dose level, the net absorption spectra and dose-response curves for EBT-XD films showed only slight variations across various beam qualities. When a calibration curve obtained from a given beam quality is used to measure dose from films irradiated with other beam qualities, the maximum uncertainty in the calculated dose, in ranges 5-40 and 8-50Gy for red and green channels, respectively, were ~3.1% and ~2.4%. The response of the EBT-XD films is dose rate independent but batch dependent. No significant beam quality dependence was observed in EBT-XD films in the dose range up to 50Gy.

Sci‐Thur AM: YIS – 02: Imaging dose distributions through the detection of radiation‐induced acoustic waves

Purpose:X‐ray acoustic computed tomography (XACT) is an emerging technique that images the dose deposited within an object following linac irradiation by detecting acoustic waves induced via the photoacoustic effect. This work shows that XACT images can be formed in soft‐tissue equivalent material and that dosimetric information can be extracted from such images.Methods:Acoustic waves induced in a water tank following irradiation by a 10 MV flattening filter free photon beam were detected with an immersion ultrasound transducer at 60 angles surrounding the radiation field. A back‐projection algorithm was used to reconstruct an XACT image from the detected transducer signals. Profiles extracted from XACT images were compared to profiles measured with ion chambers as per the current clinical protocol.Results:XACT images were successfully formed of simple 4 cm × 4 cm and 6 cm × 3 cm fields, as well as of more complicated multi‐leaf collimator defined fields. For the 6 cm × 3 cm field, 74% and 87% of the XACT profile points in the 6 cm and 3 cm dimensions, respectively, passed a 7% / 4 mm gamma test when compared to ion chamber measurements. In a complicated puzzle piece shaped field, 86% of the pixels in an extracted profile passed a 7% / 4 mm gamma test.Conclusions:XACT is capable of imaging the dose distribution delivered by a variety of field sizes and shapes in water, and is a viable technique for both water tank and in vivo dosimetry.

Determination of small field output factors in 6 and 10 MV flattening filter free photon beams using various detectors

The study aimed to determine appropriate detectors for output factor measurement of small fields in 6 and 10 MV flattening filter free photon beams using five different detectors. Field sizes were varied between 0.6 × 0.6 and 4.0 × 4.0 cm2. An indirect method (daisy-chaining) was applied to normalize the output factors. For the smallest field size, the variations of output factors compared among the detectors were 13%. Exradin A16 had the lowest output factor and increasing in sequence with CC01, microDiamond, microLion and EDGE detectors, respectively, for both energies. The similarity between CC01 and microDiamond output factor values were within 1.6% and 1% for all field sizes of 6 and 10 MV FFF, respectively. EDGE and microLion presented the highest values while ExradinA16 gave lowest values. In conclusion, IBACC01, Exradin A16, microLion, microDiamond and EDGE detectors seem to be the detectors of choices for small field output factor measurement of FFF beams down to 1.6 × 1.6 cm2. However, we could not guarantee which detector is the most suitable for output factor measurement in small field less than 1.6 × 1.6 cm2 of FFF beams. Further studies are required to provide reference information for validation purposes.

Poster — Thur Eve — 37: Respiratory gating with an Elekta flattening filter free photon beam

In cases where surgery is not possible for lung cancer treatment, stereotactic body radiation therapy (SBRT) may be an option. One problem when treating this type of cancer is the motion of the lungs caused by the patient's respiration. It is possible to reduce the impact of this movement with the use of respiratory gating. By combining respiratory gating with a flattening filter free (FFF) photon beam linac, the increased treatment time caused by a reduced beam‐on time of respiratory gating methods can be compensated by the inherent increased dose rate of FFF beams.This project's aim is to create hardware and software interfaces allowing free respiration gating on an Elekta Synergy‐S linac specially modified to deliver 6 MV FFF photon beams.First, a printed circuit board was created for reading the signal from a Bellows Belt from Philips (a respiration monitor belt) and transmitting an On/Off signal to the accelerator. A software was also developed to visualize patient respiration.Secondly, a FFF model was created with the Pinnacle treatment planning system from Philips. Gamma (Γ) analysis (2%, 2 mm) was used to evaluate model. For fields going from 5.6 × 5.6 to 12 × 12 cm2, central axis depth dose model fitting shows an average gamma value of 0.2 and 100% of gamma values remain under the Γ = 1 limit. For smaller fields (0.8 × 0.8 and 1.6 × 1.6 cm2), Pinnacle has more trouble trying to fit the measurements, overestimating dose in penumbra and buildup regions.