Flattening Filter Free Research Articles

81 Application of the TRS-483 correction factors on output ratios measurements for 8 detectors in stereotactic radiosurgery with cones

Introduction. The purpose of this work is to assess correction factors performances from TRS-483 [ [1] International Atomic Energy Agency. Dosimetrie of small static fields used in external beam radiotherapy. Rapport n483, 211 pages, Vienne; 2017. Google Scholar ] for output ratio (OR) determination in clinical 6 MV with flattening filter (WFF) Clinac and 6 MV flattening filter free (FFF) Truebeam stereotactic radiosurgery mode for cone irradiation using EBT3 films as reference [ [2] Girardi A. et al. Small field correction factors determination for the IBA razor nano chamber and the IBA razor chamber. Phys Med. 2018; 52: 151-152 Abstract Full Text PDF Google Scholar ].

82 Output ratios measurements in stereotactic radiosurgery using cones with a 6 MV FFF beam: comparison between 8 detectors

Introduction. The purpose of this work is to assess 8 detectors performances for output ratio (OR) determination, in clinical 6 MV flattening filter free (FFF) beam of the Truebeam STx Novalis stereotactic radiosurgery linear accelerator for cone irradiation using EBT3 radiochromic films (Ashland) as reference 1 Morales JE. et al. A comparison of surface doses for very small field size x-ray beams: Monte Carlo calculations and radiochromic film measurements. Australas. Phys Eng Sci Med 2014;37,303–9. Google Scholar , 2 Garnier N, Amblard R, Villeneuve R. et al. Detectors assessment for stereotactic radiosurgery with Cones. J Appl Clin Med Phys 2018;19(6), 88–98. Google Scholar , 3 Girardi A. et al. Small field correction factors determination for the IBA razor nano chamber and the IBA razor chamber. Phys Med 2018;52,151–2. Google Scholar .

Impact of detector selections on inter-institutional variability of flattening filter-free beam data for TrueBeam™ linear accelerators.

This study evaluates the type of detector influencing the inter‐institutional variability in flattening filter‐free (FFF) beam‐specific parameters for TrueBeam™ linear accelerators (Varian Medical Systems,Palo Alto, CA, USA). Twenty‐four beam data sets, including the percent depth dose (PDD), off‐center ratio (OCR), and output factor (OPF) for modeling within the Eclipse (Varian Medical Systems) treatment planning system, were collected from 19 institutions. Although many institutions collected the data using CC13 (IBA Dosimetry, Schwarzenbruck, Germany) or PTW31010 semiflex (PTW Freiburg, Freiburg, Germany) ionization chambers, some institutions used diode detectors, diamond detectors, and ionization chambers with smaller cavities. The OCR data included penumbra width, full width at half maximum (FWHM), and FFF beam‐specific parameters, including unflatness and slope. The data measured by CC13/PTW31010 ionization chambers were compared with those measured by all other detectors. PDD data demonstrated the variations within ±1% at the dose fall‐off region deeper than peak depth. The penumbra widths of the OCR measured with the CC13/PTW31010 detectors were significantly larger than those measured with all other detectors (P < 0.05). Especially the EDGE detector (Sun Nuclear Corp., Melbourne, FL, USA) and the microDiamond detectors (model 60019; PTW Freiburg) demonstrated much smaller penumbra values compared to those of the CC13/PTW31010 detectors for the 30 × 30 mm2 field. There was no difference in the FWHM, unflatness, and slope parameters between the values for the CC13/PTW31010 detectors and all other detectors. OPF curves demonstrated small variations, and the relative difference from the mean value of each data point was almost within 1% for all field sizes. Although the penumbra region exhibited detector‐dependent variations, all other parameters showed tiny interunit variations regardless of the detector type.

Determination of inflection points of CyberKnife dose profiles within acceptability criteria of deviations in measurements.

The aim of this study was to determine the Inflection Points (IPs) of flattening filter free (FFF) CyberKnife dose profiles for cone-based streotactic radiotherapy. In addition, dosimetric field sizes were determined. The increased need for treatment in the early stages of cancer necessitated the treatment of smaller tumors. However, efforts in that direction required the modeling accuracy of the beam. Removal of the flattening filter (FF) from the path of x-ray beam has provided the solution to those efforts, but required a different normalization approach for the beam to ensure the delivery of the dose accurately. As a solution, researchers proposed a normalization factor based on IPs. Measurements using microDiamond (PTW 60019), Diode SRS (PTW 60018) and Monte Carlo (MC) calculations of dose profiles were completed at SAD 80cm and 5cm depth for 15-60mm cones. Performance analysis of detectors with respect to MC calculation was carried out. Gamma evaluation method was used to determine achievable acceptability criteria for FFF CyberKnife beams. Acceptability within (3%-0.5mm) was found to be anachievable criterion for all dose profile measurements of the cone beams used in this study. To determine the IP, the first and second derivatives of the dose profile were determined via the cubic spline interpolation technique. Derivatives of the interpolated profiles showed that locations of IPs and 50% isodose points coincide.

Intrafractional motion in stereotactic body radiotherapy of spinal metastases utilizing cone beam computed tomography image guidance.

Spine stereotactic body radiotherapy (SBRT) requires a high degree of accuracy due to steep dose gradients close to the spinal cord. This study aimed to (1) evaluate intrafractional motion in spine SBRT utilizing flattening filter free (FFF) beam delivery and cone beam computed tomography (CBCT) image guidance and (2) evaluate if adding another CBCT acquisition and corrections prior to treatment improves the overall position accuracy. Intrafractional motion was retrospectively analyzed for 78 fractions in 54 patients. All patients were immobilized with an evacuated cushion. Before treatment, a CBCT was acquired, a bony fusion with the planning CT was performed and translational and rotational errors were corrected. For 30 of the patients (39 fractions) acquisition of another CBCT and corrections were performed before treatment. A post treatment CBCT was acquired for all patients, and translational and rotational errors measured by fusion of the post treatment CBCT with the planning CT were recorded to calculate means and standard deviations (SDs). The positional errors were significantly smaller in 4 out of 6 error values in the patient group treated with verification CBCT. In this group, translational and rotational SDs ranged from 0.5 to 0.6 mm and 0.3°, respectively. Corresponding values in the group treated without verification CBCT were 0.7-1.0 mm and 0.4-0.7°. With proper CBCT image guidance, patient immobilization and FFF-beam delivery, one can obtain very high patient position accuracy in spine SBRT. Inclusion of a verification CBCT prior to treatment increases the overall position accuracy.

Modeling gold nanoparticle radiosensitization using a clustering algorithm to quantitate DNA double-strand breaks with mixed-physics Monte Carlo simulation.

The radiosensitization properties of gold nanoparticles (GNPs) are investigated using a simple Geant4 cell model considering a realistic cell geometry and a clustering algorithm to characterize the number of DNA double-strand breaks (DSBs). A mixed-physics approach is taken for accurate modeling of low-energy photon interactions in the different regions of the model using Geant4-DNA physics within the cell, and Livermore physics within gold. Density-based spatial clustering of applications with noise (DBSCAN), a clustering algorithm, is used to directly quantitate DNA DSBs after irradiation. The simulation was run using different sizes of GNPs, different distances of GNPs from the cell nucleus, and several combinations of these two conditions. Four types of radiation were simulated in the work: 80-keV monoenergetic photons, 100-keV monoenergetic photons, a 250-kVp photon spectrum, and a 6-MV flattening filter free (FFF) photon spectrum. A variable enhancement in DSB yield, nucleus dose, and cell dose was observed when there are GNPs in the cell cytoplasm, and increases with larger GNPs and proximity to the nucleus. The distance of the GNPs from the nucleus has a large impact on the DSB yield and nucleus dose, but little to no effect on the cell dose. The cell dose enhancement factor of 80keV photons varies from 1.037-1.125 at 0.2µm for 30-100nm GNPs to 1.040-1.127 at 4μm. The DSB enhancement factor varies from 1.050 to 1.174 at 0.2µm to a marginal effect of <1.01 at 4μm. For 100keV, the dose enhancement factor is from 1.142-1.470 at 0.2µm to 1.106-1.371 at 4μm. The DSB enhancement factor varies from 1.249-1.813 at 0.2µm to almost no effect at 4μm. For 250kVp, the dose enhancement factor is from 1.117-1.393 at 0.2μm to 1.110-1.342 at 4μm. The DSB enhancement factor varies from 1.183-1.600 at 0.2μm to a marginal effect of ~1.03 at 4μm. A 6-MV FFF shows a dose enhancement factor of 1.061-1.103 at 0.2μm and 1.053-1.107 at 4μm. The DSB yield varies from 1.070-1.143 at 0.2μm to a marginal effect at 4μm. The stark difference in behavior for DSB yield when compared to cell dose highlights the importance of evaluating more complex radiobiological quantities rather than dose alone when evaluating the radiosensitization properties from metallic nanomaterials. The nucleus dose showed similar characteristics to the DSB yield demonstrating the ability of the method to predict DNA damage and its relationship with nuclear dose. The proposed method provides a way to explore the radiobiological mechanisms of radiation-induced DNA damages, and it aids to evaluate the physical radiosensitization properties of GNP-aided radiotherapy, which can be easily combined with radiochemical DSB quantitation in order to better understand the intricate DNA damage induction mechanisms that are involved in GNP-aided radiotherapy.

Feasibility of a GATE Monte Carlo platform in a clinical pretreatment QA system for VMAT treatment plans using TrueBeam with an HD120 multileaf collimator.

PurposeTo evaluate the quality of patient‐specific complicated treatment plans, including commercialized treatment planning systems (TPS) and commissioned beam data, we developed a process of quality assurance (QA) using a Monte Carlo (MC) platform. Specifically, we constructed an interface system that automatically converts treatment plan and dose matrix data in digital imaging and communications in medicine to an MC dose‐calculation engine. The clinical feasibility of the system was evaluated.Materials and MethodsA dose‐calculation engine based on GATE v8.1 was embedded in our QA system and in a parallel computing system to significantly reduce the computation time. The QA system automatically converts parameters in volumetric‐modulated arc therapy (VMAT) plans to files for dose calculation using GATE. The system then calculates dose maps. Energies of 6 MV, 10 MV, 6 MV flattening filter free (FFF), and 10 MV FFF from a TrueBeam with HD120 were modeled and commissioned. To evaluate the beam models, percentage depth dose (PDD) values, MC calculation profiles, and measured beam data were compared at various depths (Dmax, 5 cm, 10 cm, and 20 cm), field sizes, and energies. To evaluate the feasibility of the QA system for clinical use, doses measured for clinical VMAT plans using films were compared to dose maps calculated using our MC‐based QA system.ResultsA LINAC QA system was analyzed by PDD and profile according to the secondary collimator and multileaf collimator (MLC). Values for MC calculations and TPS beam data obtained using CC13 ion chamber (IBA Dosimetry, Germany) were consistent within 1.0%. Clinical validation using a gamma index was performed for VMAT treatment plans using a solid water phantom and arbitrary patient data. The gamma evaluation results (with criteria of 3%/3 mm) were 98.1%, 99.1%, 99.2%, and 97.1% for energies of 6 MV, 10 MV, 6 MV FFF, and 10 MV FFF, respectively.ConclusionsWe constructed an MC‐based QA system for evaluating patient treatment plans and evaluated its feasibility in clinical practice. We observed robust agreement between dose calculations from our QA system and measurements for VMAT plans. Our QA system could be useful in other clinical settings, such as small‐field SRS procedures or analyses of secondary cancer risk, for which dose calculations using TPS are difficult to verify.

Radiobiological Difference Between Flattening Filter Free (FFF) Vs. Flattening Filter (FF) X-Ray Beams in Lung Cancer Cells

Stereotactic Body Radiotherapy (SBRT) is a well-established therapeutic option for early lung cancer, and many clinical trials indicate that SBRT can provide excellent local control and survival with limited toxicity. Compared with conventional flattening filter (FF) beams, flattening filter free (FFF) beams are characterized by higher average dose rate with higher dose per pulse and inhomogeneous dose distribution with reduced peripheral dose. These benefits make FFF beams an attractive alternative for SBRT. Therefore, it is crucially to determine whether there are any differences in biological effects of FFF beams compared with FF beams. However, there are still conflicts on them between two beams. Therefore, we investigated the radiobiological effect between FF and FFF beam in human lung cancer cell lines. Three human lung cancer cell lines with different genetic characteristics (A549, NCI-H1299, NCI-H1650) were used. Irradiation was performed with a 10 MV linear accelerator (Versa HD, Elekta AB). The dose rate and dose per pulse for FF and FFF beams were 5 and 22 Gy/min, and 0.042 cGy and 0.171 cGy, respectively. To investigate the effect by different dose rate, but with same dose per pulse, FFF beam of 22 Gy/ min was modified to 5 Gy/min by increasing pulse interval. The radiobiologic effect was studied with three different conditions: FF (5 Gy/min, 0.042 cGy/pulse), FFF (5 Gy/min, and 0.171 cGy/pulse) and FFF (22 Gy/min, 0.171 cGy/pulse). Cell survival was measured using clonogenic assay and protein expression was measured using Western blot analysis. No significant differences in cell survival were observed upon three kinds of irradiation for all the three cell lines. No significant differences were observed in the expression of γ-H2AX and repair proteins (DNA-PKcs, Rad 51) after irradiation in all the three cell lines. There is no radiobiological difference (cell survival, DNA damage and repair) between FF and FFF beams in lung cancer cells.

Single-Isocenter/Multi-Lesion Synchronous Lung Stereotactic Body Radiation Therapy (SBRT): Plan Quality, Treatment Efficiency and Clinical Outcomes

Patients with multiple primary or metastatic lung lesions may not well tolerate relatively longer SBRT treatments due to distress or shortness of breath, with traditional treatment planning and delivery with individual isocenter per lesion. Treating lesions synchronously with a single-isocenter volumetric modulated arc therapy (VMAT) plan with flattening filter free (FFF) beam will decrease treatment time and improve patient comfort. We report dosimetry, treatment efficiency and early clinical outcomes of oligometastases (< 5 lesions) lung cancer patients who underwent synchronous single-isocenter/multiple-lesions VMAT lung SBRT. Fourteen patients with multiple peripherally located non-small-cell-lung cancer (NSCLC) lesions (two n = 12, three n = 1, five n = 1, total 32 lesions) were treated synchronously with highly conformal single-isocenter non-coplanar VMAT-SBRT plan (bilateral lesions n = 6, right lesions n = 5, left lesions n = 3). Single-isocenter was placed between or in the middle of all lesions. Doses were 54 Gy/50 Gy in 3/5 fractions prescribed to 70-80% isodose line. Plans were generated with Acuros-based algorithm and 6MV-FFF beam following RTOG criteria. Plan quality and delivery efficiency were reported. Outcomes evaluated include local control rates, radiation pneumonitis and rib toxicity. Kaplan-Meier (KM) estimates of time-to-local failure were investigated. Median follow-up time was 5 ± 5 months (range 1 to 15 months). Mean isocenter to tumor distance was 5.6 ± 1.9 cm. Mean combined planning target volume (PTV) was 38.7 ± 22.7 cc. All plans were accepted by treating physicians and met RTOG criteria for target coverage and organs at risk (OAR) dose. Average values of conformity index, gradient index, maximum dose at 2 cm away from the PTV and normal lung V20 were 1.05 ± 0.1, 5.1 ± 0.8, 50.1 ± 5.2% and 6.4 ± 3.1%, respectively. All OAR doses were well below RTOG criteria. Additionally, normal lung V10, V5 and mean lung dose were 18.2%, 31.0 % and 5.4 Gy, on average, respectively. Average beam on time was 3.4 ± 1.4 min. One patient died before follow up and one was lost to follow up. Of twelve patients, eleven achieved complete response to treatment with one lesion of one patient progressing. KM estimated 1-year actuarial tumor local control rate was 100%; however, 50% of the patients developed distant metastases. During the follow-up period, no patient had adverse pulmonary effects, developed grade+2 pneumonitis or had chest wall pain/rib fracture. However, four patients developed radiographic changes that were asymptomatic. Synchronous SBRT treatment to oligometastases lung lesions was fast, effective and safe, with promising early clinical outcomes in terms of local control rates and no adverse side effects to lung or ribs. Although six patients had distant failure, the local control of treated lesions could preserve the patient’s quality of life or delay chemotherapy treatment. Longer follow up of these patients is warranted.

The comparison of spatial resolution of ion chamber, diode, and EBT3 film based on the dose profile

Background: It is essential for the physicist to quantify the dose of the radiation prior and during cancer treatment to ensure proper execution of the treatment plan. The most common way to measure the dose is utilizing Farmer chamber. The limitation of Farmer chamber is that it only provides dose of a specific point, while the beam dose profile is needed for the planning system. The Gafchromic EBT3 film can provide a quantitative dose profile in a 2D space. The EBT3 film has low energy dependence, similar effective atomic number to water, and self-developing; making it an ideal candidate for dose profile measurement. Methods: Dose profiles of EBT3 was measured, analyzed and compared to semiflex chamber and EDGE detector under same conditions. The EBT3 films were placed at the center of RW3 water phantom (30×30×30 cm 3 ) under flattening filter free (FFF) Mode of Linac. The EBT3 films were calibrated by additional RW3 with a Farmer chamber under the stacked RW3 water phantom. Percentage depth dose (PDD) and Matlab were used to fit the dose curve of EBT3 and the results are presented. Results: The penumbra of the measured dose profile through EBT3 film demonstrated little differences when comparing with that of the EDGE detector and noticeable differences when compared to that of the semiflex chamber. For a field size of 2×2 cm 2 , (b=0.08, depth of 5 cm, beam off central axis 0.8 cm), the measured difference of dose profile between EBT3 and EDGE detector is –2.24%. Under the same conditions, the dose profile difference between EBT3 and semiflex chamber is 10.59%. Conclusions: This study shows that EDGE detector has the highest spatial resolution to the dose profile. EBT3 film in combination with solid phantom can quickly sample dose profile in 2D. EBT3 film can be considered to be an excellent quality assurance (QA) tool with accurate dosimetry, a good spatial resolution and tolerable dose uncertainty.

A Phase II Study of Stereotactic Body Radiotherapy with Hydrogel Spacer for Prostate Cancer; Dosimetric Comparison, Acute Toxicity and Quality of Life

Volumetric modulated arc therapy (VMAT) stereotactic body radiotherapy (SBRT) with flattening filter-free (FFF) beam can achieve faster dose delivery and shorter treatment time for prostate cancer. A hydrogel spacer is used to reduce the rectal toxicity, but the clinical data of SBRT with hydrogel spacer are limited. The aim of this prospective single-center phase II study was to determine the efficacy and safety of FFF-VMAT SBRT combined with hydrogel spacer for localized prostate cancer. Men with localized prostate cancer were eligible for the study. A hydrogel spacer was inserted into the perirectal space between prostate and rectum before the initiation of SBRT. All patients underwent the planning CT scans before and after the spacer placement. Patients received 36.25 Gy to 95% of the PTV in 5 fractions every other day excluding weekends with 6MV single arc FFF-VMAT. The primary endpoint was acute toxicities during and within 3 months after the completion of SBRT. Toxicities were assessed using Common Terminology Criteria for Adverse Events v4.03. The secondary endpoints were International Prostate Symptom Score (IPSS) and quality of life (QOL) assessed by the Expanded Prostate Cancer Index Composite (EPIC) questionnaire. Target and normal tissue dosimetry were compared before and after the hydrogel spacer insertion. Between February 2017 and July 2018, forty patients with localized prostate cancer were enrolled. The median age was 70 years (55-79). Three, 25, 6, and 6 patients had low-, intermediate-, high-, and very high-risk prostate cancer according to the National Comprehensive Cancer Network (NCCN) risk classification. Twenty-three patients (58%) received concomitant androgen deprivation therapy. The hydrogel spacer placement was successful in 39 cases (98%). Seven (18%) and eighteen patients (45%) developed grade 2 acute gastrointestinal and genitourinary toxicities, respectively. No grade 3 acute toxicities were observed. Median IPSS temporarily increased at 2 weeks and 1 month after RT when compared to the pretreatment baseline (P < 0.05, 0.05, respectively), and returned to the baseline value in 3 months. EPIC urinary and bowel scores significantly declined at 2 weeks and 1 month after RT (P <0.05, 0.05, respectively), and no difference was observed at 3 months post-RT. Spacer use was associated with lower rectum dose for mean dose, maximum dose, and V100% to V50% (all P < 0.05). FFF-VMAT SBRT with a hydrogel spacer can be safely administered with acceptable acute toxicity. Hydrogel spacer insertion significantly reduced the dose to the rectum.

Consequence of Patient Motion during Lung SBRT While Treating with Flattening-Filter Free (FFF) Beams or Flattened Beams (FB)

Consequence of Patient Motion during Lung SBRT While Treating with Flattening-Filter Free (FFF) Beams or Flattened Beams (FB)

Increase in Superficial Dose in Whole-Breast Irradiation With Halcyon Straight-Through Linac Compared With Traditional C-arm Linac With Flattening Filter: In vivo Dosimetry and Planning Study

PurposeSuperficial dose is an important parameter in breast cancer radiation therapy. When treated with conventional linacs, bolus is commonly applied to improve target coverage near the surface while also managing the risk of severe skin reactions and negative cosmesis. With the introduction of modern linacs with 6X flattening filter free (FFF) photon beams, the effect on superficial dose and the need for bolus must be evaluated. Methods and MaterialsIn vivo measurements of superficial dose were made with optically stimulated luminescence dosimeters on 11 breast cancer patients treated with the Halcyon 6X FFF linac (Varian Medical Systems, Palo Alto, CA). Additionally, measurements were made with the Halycon 6X FFF beam and a 6X beam with flattening filter (FF) delivered to an anthropomorphic phantom. A planning study was carried out in which 14 patients treated on the Halcyon were replanned with a conventional linac to determine the difference in superficial dose predicted by the treatment planning system. Measures were taken to increase the accuracy of the treatment planning system superficial dose. ResultsThe use of the Halcyon 6X FFF beam led to higher superficial dose compared with 6X FF beams. The in vivo measurements show an average superficial dose of 83.8% ± 0.6%, which is an increase of approximately 10% compared with published measurements for a 6X FF linac. Comparison of superficial dose for 6X FF and 6X FFF beams in the phantom measurements show an increase from 70% ± 1.3% to 84% ± 1.3%, which is consistent with the in vivo measurements. The planning comparison shows an increase in V70%Rx from 62% ± 4.4% to 81% ± 2.2% for the superficial breast tissue for the Halcyon 6X FFF beam compared with a standard C-arm linac with FF. ConclusionsThe use of the Halcyon 6X FFF beam was associated with higher superficial dose which may obviate the use of bolus.

Determination of an inflection point for a dosimetric analysis of unflattened beam using the first principle of derivatives by python code programming.

Practice of Unflattened or Flattening filter free (FFF) beam has become the high dose standard in radiotherapy (RT), such as stereotactic radio-surgery (SRS) and stereotactic radiotherapy (SRT). The removal of a flattening filter (FF) from the path of a photon beam alters the characteristics of FFF beam. Since the conventional route for dosimetric analysis of FF beam cannot be applied to FFF beam, the procedure of analyzing beam characteristics for FFF beam based on inflection points (IPs) is used. IP is a point where the concavity change observed corresponds to its change in sign (±) of the second derivative. The objective of the study is to determine IPs for dosimetric analysis of the FFF beam profile. In this study, IPs are determined through the python code programming based on the mathematical first principle of the derivative. They are compared with IPs estimated by the conventional graphical manual method using Microsoft Excel (MS). IPs and their dependent dosimetric parameters determined by both mathematical and graphical manual methods are compared. Percentage differences between the IPs determined by both methods, for 6MVFFF inline and crossline beam profile are found to be 2.7% and 0.8% respectively. Similarly, the average penumbra differences for 6MVFFF inline and crossline beam profile are found to be 0.15mm and 0.9mm, respectively. However, differences in the field width between both methods are found insignificant. Graphical manual method is very time-consuming, tedious and user dependent. However, the mathematical method through python code programming is more precise, faster and independent of individual users.

Clinical advantages of using unflattened 6-MV and 10-MV photon beams generated by the medical accelerator Elekta Versa HD based on their dosimetric parameters in comparison to conventional beams

Abstract Photon beams with wide energy ranges from 4 MV to 25 MV are commonly used in radiotherapy nowadays. In recent years, there has been a strong interest in a certain modification of a radiotherapeutic apparatus by the application of the so-called flattening filter-free (FFF) beam. Several advantages of FFF beams over standard flattening filter (FF) beams are noticed, and this technical solution has aroused great interest among radiotherapeutic facilities. The goal of the present study is to investigate the differences between the conventional FF and unflattened FFF 6-MV and 10-MV photon beams in some basic dosimetric parameters and their influence on the whole radiotherapeutic treatment. The data provided here include the detailed characteristics as follows: percent depth dose (PDD), beam profile, edge of a half-profile, total scatter correction factor (TSCF) and head scatter correction factor (HSCF) for FF and FFF 6-MV and 10-MV photon beams from the Elekta Versa HD accelerator in the Katowice Oncology Center in Poland.

A relative comparison of neutron production between conventional and energy-matched flattening-filter-free (FFF) 10MV modes for an elekta linear accelerator

Flattening Filter Free (FFF) Intensity Modulated Radiotherapy (IMRT) has become a widespread modality in radiotherapy treatments. Linac vendors have taken different approaches to implementing this technique. Elekta match the depth dose characteristics of FFF beams to their Conventional Flattening Filter (cFF) beams, increasing the maximum photon energy . At higher energies, greater than 10MV, there is the potential for increased unwanted neutron production . This study investigated the neutron production from an Elekta linac for 10MV FFF and cFF beams for various machine geometries.

An in vitro study for the dosimetric and radiobiological validation of respiratory gating in conventional and hypofractionated radiotherapy of the lung: effect of dose, dose rate, and breathing pattern

Stereotactic body radiotherapy (SBRT) of the lung has become a standard of care for early-stage inoperable non-small cell lung cancer (NSCLC). A common strategy to manage respiratory motion is gating, which inevitably results in an increase in treatment time, especially in irregularly-breathing patients. Flattening-filter free (FFF) beams allow for delivery of the treatment at a higher dose rate, therefore counteracting the lengthened treatment time due to frequent interruption of the beam during gated radiotherapy. In this study, we perform our in vitro evaluation of the dosimetric and radiobiological effect of gated lung SBRT with simultaneous integrated boost (SIB) using both flattened and FFF beams. A moving thorax-shaped phantom with inserts and applicators was used for simulation, planning, gated treatment delivery measurements and in vitro tests. The effects of gating window, dose rate, and breathing pattern were evaluated. Planned doses represented a typical conventional fractionation, 200 cGy per fraction with SIB to 240 cGy, flattened beam only, and SBRT, 800 cGy with SIB to 900 cGy, flattened and FFF beams. Ideal, as well as regular and irregular patient-specific breathing patterns with and without gating were used. A survival assay for lung adenocarcinoma A549 cell line was performed. Delivered dose was within 6% for locations planned to receive 200 and 800 cGy and within 4% for SIB locations. Time between first beam-on and last beam-off varied from approximately 1.5 min for conventional fractionation, 200/240 cGy, to 10.5 min for gated SBRT, 800/900 cGy doses, flattened beam and irregular breathing motion pattern. With FFF beams dose delivery time was shorter by a factor of 2–3, depending on the gating window and breathing pattern. We have found that, for the most part, survival depended on dose and not on dose rate, gating window, or breathing regularity.

Characterization of the Megavoltage Cone-Beam Computed Tomography (MV-CBCT) System on HalcyonTM for IGRT: Image Quality Benchmark, Clinical Performance, and Organ Doses.

Purpose: The Varian Halcyon includes an ultrafast 6 MV flattening filter free (FFF) cone-beam computed tomography (MV-CBCT). Although a kV-CBCT add-on is available, in the basic configuration MV is used for image guided radiotherapy (IGRT). We characterized the MV-CBCT imager in terms of reproducibility, linearity, field of view (FOV) dependence, detectability of soft-tissue, and the effect of metal implants. The performance of the MV-CBCT in the clinic, including resulting dose to organs, is also discussed herein.Methods: A Gammex phantom was scanned using a Halcyon MV-CBCT and a 120 kVp Siemens Definition Edge CT. Mean and standard deviation of Hounsfield Units (HUs) for different electron density relative to water () inserts were extracted. Doses to clinical patients due to MV-CBCT are calculated within Eclipse during treatment planning.Results: A stable and near-linear HU-to- curve was obtained using the MV-CBCT. As the scan length increased from 10 to 28cm, the linearity of curve improved while the mean HUs decreased by 30%. All soft tissue inserts in the Gammex phantom were distinguishable. A crescent artifact affected HU measurements by up to 40 HUs. Soft-tissue contrast was sufficient for clinical online image-guidance in the low dose (5 MU) mode. Mean doses per fraction to organs-at-risk (OARs) were as high as 6 cGy for head and neck, 5 cGy for breast, and 4 cGy for pelvis patients. Metal rods did not affect HU values or introduce noticeable artifacts.Conclusions: Halcyon's MV-CBCT has sufficient soft tissue contrast for IGRT and lacks metal-induced artifacts. Even though the absolute HU values vary with phantom size and scanning length, the HU-to- conversions are linear and stable day-to-day. In clinical cases, highest tissue doses from MV-CBCT ranged from 2-7cGy per fraction for various treatment sites, which could be significant for some organs at risk. Dose to out-of-treatment-field organs can be limited by reducing the scan length definition during planning and using the low dose mode. The high quality imaging mode did not provide material advantages over the low dose mode. Adequate IGRT was successfully delivered to multiple tumor sites using MV-CBCT.

The impact of flattening filter free (FFF) photon beams to ion recombination correction factor

This study aimed to evaluate ion recombination correction for ionization chamber in flattening filter free (FFF) photon beams and compare it to the flattened (with-flattening-filter) photon beams. The evaluation of ion recombination correction factor was performed for FC65-G, SNC600c, and CC13 ionization chambers. The measurements of three ionization chambers were performed using the water phantom and the Varian Trilogy linac with FFF capability. The ion recombination correction factor values for the three ionization chambers were obtained from the calculation using the simple two-voltage method and Jaffe plot curve fitting. The ion recombination correction factor value obtained from all three ionization chambers were higher for unflattened (FFF) photon beams than the flattened (WFF) photon beams with discrepancy less than 3%. The ion recombination correction value obtained from the linear Jaffe plot curve fitting had the highest discrepancy at about 7.67% when compared to the two-voltage method. On the contrary, the ion recombination correction value obtained from the Jaffe plot with quadratic and exponential quadratic curve fitting had discrepancies less than 2% when compared to the two-voltage method.

Study of detectors response on the scatter radiation of 6 MV flattening filter free beams

Small field dosimetry which related to the use of Flattening Filter Free (FFF) beams is a great challenge because of the perturbation effects caused by the size of the detector’s active volume or detector’s materials. This phenomenon encourages researchers to do a further study about detector response. In this study, scatter and primary radiation from 6 MV FFF photon beams were studied to evaluate the detector response in small fields for a better understanding of the contribution of every phenomenon. Ion chamber and Gafchromic films were used in three measurement configurations representing open field measurement and approximations of both primary and scatter part of the beam, described as follows: (i) detectors positioned under steel block, exposing the detectors only to scatter part of radiation field, (ii) detectors positioned inside mini phantom, approximating the detector response to primary radiation with minimum scatter, (iii) detectors positioned in the standard open field, which was the superposition of the primary and scatter radiation. The results show that detector responses were heavily depended on its design (i.e. active volume) especially in small beams. The detector response in the open field could be reproduced from the blocked and primary beam set-up with a discrepancy ranging from 1.0% to 36%. Moreover, the volume averaging and other phenomenon affecting the detector response could be observed in the blocked beam.