Properties of unflattened photon beams shaped by a multileaf collimator

The purpose of this study is to characterize the dosimetric properties and accuracy of a novel treatment platform (Edge radiosurgery system) for localizing and treating patients with frameless, image‐guided stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Initial measurements of various components of the system, such as a comprehensive assessment of the dosimetric properties of the flattening filter‐free (FFF) beams for both high definition (HD120) MLC and conical cone‐based treatment, positioning accuracy and beam attenuation of a six degree of freedom (6DoF) couch, treatment head leakage test, and integrated end‐to‐end accuracy tests, have been performed. The end‐to‐end test of the system was performed by CT imaging a phantom and registering hidden targets on the treatment couch to determine the localization accuracy of the optical surface monitoring system (OSMS), cone‐beam CT (CBCT), and MV imaging systems, as well as the radiation isocenter targeting accuracy. The deviations between the percent depth‐dose curves acquired on the new linac‐based system (Edge), and the previously published machine with FFF beams (TrueBeam) beyond Dmax were within 1.0% for both energies. The maximum deviation of output factors between the Edge and TrueBeam was 1.6%. The optimized dosimetric leaf gap values, which were fitted using Eclipse dose calculations and measurements based on representative spine radiosurgery plans, were 0.700 mm and 1.000 mm, respectively. For the conical cones, 6X FFF has sharper penumbra ranging from 1.2−1.8 mm (80%‐20%) and 1.9−3.8 mm (90%‐10%) relative to 10X FFF, which has 1.2−2.2 mm and 2.3−5.1 mm, respectively. The relative attenuation measurements of the couch for PA, PA (rails‐in), oblique, oblique (rails‐out), oblique (rails‐in) were: −2.0%, −2.5%, −15.6%, −2.5%, −5.0% for 6X FFF and −1.4%, −1.5%, −12.2%, −2.5%, −5.0% for 10X FFF, respectively, with a slight decrease in attenuation versus field size. The systematic deviation between the OSMS and CBCT was −0.4±0.2 mm, 0.1±0.3 mm, and 0.0±0.1 mm in the vertical, longitudinal, and lateral directions. The mean values and standard deviations of the average deviation and maximum deviation of the daily Winston‐Lutz tests over three months are 0.20±0.03 mm and 0.66±0.18 mm, respectively. Initial testing of this novel system demonstrates the technology to be highly accurate and suitable for frameless, linac‐based SRS and SBRT treatment.PACS number: 87.56.J‐

2 \times 2 {cm}^{2}

and

10 \times 10 {cm}^{2}

. The profiles between the Edge and the TrueBeam were practically the same, with slightly sharper penumbra obtained on the Edge at all the depths.…”

Section: Resultsmentioning

confidence: 99%

Characteristics of a novel treatment system for linear accelerator–based stereotactic radiosurgery

Song

Chin-Snyder

et al. 2015

“…In addition, the MLC transmission and dosimetric leaf gap for 6 MV FFF beam were 1% and 1.1 mm as compared to 1.5% and 2 mm for 6 MV flat beam, respectively. Moreover, the comparison of penumbra of 6 MV flat beam with 6 MV FFF beam for MMLC in Table 8 revealed the softer photon beam spectrum and missing scatter from flattening filter resulting in a smaller penumbra in 6 MV FFF beam (22) . All the above parameters (MLC transmission, dosimetric leaf gap, lower energy spectra, and penumbra) had a positive impact on reducing the

{normalV}_{20 Gy}

{normalV}_{13.5 Gy}

{normalV}_{12.5 Gy}

, and

{normalV}_{5 Gy}

with 6 MV FFF beam.…”

Section: Discussionmentioning

confidence: 93%

Is 5 mm MMLC suitable for VMAT‐based lung SBRT? A dosimetric comparison with 2.5 mm HDMLC using RTOG‐0813 treatment planning criteria for both conventional and high‐dose flattening filter‐free photon beams

Subramanian

Subramani

Swamy

et al. 2015

The aim of this study is to assess the suitability of 5 mm millennium multileaf collimator (MMLC) for volumetric‐modulated arc therapy (VMAT)‐based lung stereotactic body radiotherapy (SBRT). Thirty lung SBRT patient treatment plans along with their planning target volumes (ranging from 2.01 cc to 150.11 cc) were transferred to an inhomogeneous lung phantom and retrospectively planned using VMAT technique, along with the high definition multileaf collimator (HDMLC) and MMLC systems. The plans were evaluated using Radiation Therapy Oncology Group (RTOG‐0813) treatment planning criteria for target coverage, normal tissue sparing, and treatment efficiency for both the MMLC and HDMLC systems using flat and flattening filter‐free (FFF) photon beams. Irrespective of the target volumes, both the MLC systems were able to satisfy the RTOG‐0813 treatment planning criteria without having any major deviation. Dose conformity was marginally better with HDMLC. The average conformity index (CI) value was found to be 1.069±0.034 and 1.075±0.0380 for HDMLC and MMLC plans, respectively. For the 6 MV FFF beams, the plan was slightly more conformal, with the average CI values of 1.063±0.029 and 1.073±0.033 for the HDMLC and MMLC plans, respectively. The high dose spillage was the maximum for 2 cc volume set (3% for HDMLC and 3.1% for MMLC). In the case of low dose spillage, both the MLCs were within the protocol of no deviation, except for the 2 cc volume set. The results from this study revealed that VMAT‐based lung SBRT using 5 mm MMLC satisfies the RTOG‐0813 treatment planning criteria for the studied target size and shapes.PACS numbers: 87.53.Ly, 87.53D, 87.56.jk

“…Various studies have reported the characteristics of FFF beams based on measurements 1 , 2 , 3 and Monte Carlo simulation 4 , 5 from Varian linacs. The properties of FFF beams have also been summarized for Elekta 6 , 7 , 8 , 9 and Siemens (10) linacs.…”

Section: Introductionmentioning

confidence: 99%

“…The removal of the flattening filter alone, without any further modifications in the beam control parameters, results in a higher dose rate and reduced beam quality 1 , 2 , 3 , 6 , 7 , 11 , 12 . The beam quality can be increased to that of the corresponding flattened beam by matching the percentage depth dose (PDD) at 10 cm depth.…”

Section: Introductionmentioning

confidence: 99%

“…Kragl et al (6) studied both 6 MV and 10 MV unmatched FFF beams and also investigated only the 6 MV matched FFF beam from an Elekta Precise linac. In this paper, we will follow the designs of the previous studies, 1 , 2 , 3 , 6 , 7 , 8 , 9 , 10 , 11 , 13 , 14 particularly the Huang et al (10) study on the quality matched 6MVFFF beam provided by a Siemens linac, to provide medical physics community with our evaluation on the dosimetric differences between conventional flattened and equivalent‐quality FFF beams of lower and higher energies from Elekta Versa HD linac.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Dosimetric properties of equivalent‐quality flattening filter‐free (FFF) and flattened photon beams of Versa HD linear accelerator

Meshram

Pramanik

Ranjith

et al. 2016

This study presents the basic dosimetric properties of photon beams of a Versa HD linear accelerator (linac), which is capable of delivering flattening filter‐free (FFF) beams with a beam quality equivalent to the corresponding flattened beams based on comprehensive beam data measurement. The analyzed data included the PDDs, profiles, penumbra, out‐of‐field doses, surface doses, output factors, head and phantom scatter factors, and MLC transmissions for both FFF and flattened beams of 6 MV and 10 MV energy from an Elekta Versa HD linac. The 6MVFFF and 10MVFFF beams had an equivalent mean energy to the flattened beams and showed less PDD variations with the field sizes. Compared with their corresponding flattened beams, Dmax was deeper for FFF beams for all field sizes; the ionization ratio variations with the field size were lower for FFF beams; the out‐of‐field doses were lower and the penumbras were sharper for the FFF beams; the off‐axis profile variations with the depths were lesser for the FFF beams. Further, the 6MVFFF and 10MVFFF beams had 35.7% and 40.9% less variations in output factor with the field size, respectively. The collimator exchange effect was reduced in the FFF mode. The head scatter factor showed 59.1% and 73.6% less variations, on average, for the 6MVFFF and 10MVFFF beams, respectively; the variations in the phantom scatter factor were also smaller. The surface doses for all beams increased linearly with the field size. The 6MVFFF and 10MVFFF beams had higher surface doses than the corresponding flattened beams for field sizes of up to 10×10cm2 but had lower surface doses for larger fields. Both FFF beams had lower average MLC transmissions than the flattened beams. The finding that the FFF beams were of equivalent quality to the corresponding flattened beams indicates a significant difference from the data on unmatched FFF beams.PACS number(s): 87.56.bd, 87.55.Qr