Jan Hrbacek

University of Zurich, Zürich, Zurich, Switzerland

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Publications (18)40.29 Total impact

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    ABSTRACT: To compare contribution and accuracy of delivery for two flattening filter free (FFF) beams of the nominal energy 6 and 10 MV and a 6 MV flattened beam for early stage lung cancer. For each of 11 patients with stage I nonsmall cell lung cancer three volumetric modulated arc therapy plans were prepared utilizing a 6 MV flattened photon beam (X6FF) and two nonflattened beams of nominal energy 6 and 10 MV (X6FFF, X10FFF). Optimization constraints were set to produce dose distributions that meet the criteria of the RTOG-0915 protocol. The radiation schedule used for plan comparison in all patients was 50 Gy in five fractions. Dosimetric parameters of planning target volume (PTV) and organs-at-risk and delivery times were assessed and compared. All plans were subject to verification using Delta(4) unit (Scandidos, Sweden) and absolutely calibrated gafchromic films in a thorax phantom. All plans had a qualitatively comparable outcome. Obtained dose distributions were conformal (CI < 1.17) and exhibited a steep dose fall-off outside the PTV. The ratio of monitor units for FFF versus FF plans in the authors' study ranged from 0.95 to 1.21 and from 0.93 to 1.25 for X6FFF/X6FF and X10FFF/X6FF comparisons, respectively. The ratio systematically increased with increasing size of the PTV (up to +25% for 150 cm(3) PTV). Yet the integral dose to healthy tissue did not follow this trend. Comparison of cumulative dose volume histograms for a patient's body showed that X6FFF plans exhibit improved conformity and reduced the volume of tissue that received more than 50% of the prescription dose. Parameters related to dose gradient showed statistically significant improvement. CI50%, CI60%, CI80%, and CI100% were on average reduced by 4.6% (p < 0.001), 4.6% (p = 0.002), 3.1% (p = 0.002), and 1.2% (p = 0.039), respectively. Gradient measure was on average reduced by 4.2% (p < 0.001). Due to dose reduction in the surrounding lung tissue, the V20 Gy and V12.5 Gy were reduced by 5.5% (p = 0.002) and 4.5% (p < 0.001). These dosimetric improvements in the fall-off were not observed for the X10FFF plans. Differences in sparing of normal tissues were not found to be statistically significant for either of the two FFF beams. Mean beam-on times were 111 s (2SD = 11 s) for X10FFF, 128 s (2SD = 19 s) for X6FFF, and X6FF plans required on average 269 s (2SD = 71 s). While the mean dose rate was 1555 ± 264 and 1368 ± 63 MU/min, for X10FFF and X6FFF, plans using the conventional X6FF were delivered with the constant maximum dose rate of 600 MU/min. Verification of all plans showed acceptable and comparable results for all plans in homogeneous as well as heterogeneous phantoms. Mean GS (3%, 2 mm) using the Delta(4) phantom were 98.9% (2SD = 3.2%), 99.2% (2SD = 2.3%), and 99.2% (2SD = 2.3%) for X6FFF, X6FF, and X10FFF modalities. Verification using a thorax phantom showed GS > 98% in all cases. The use of FFF beams for stereotactic radiation therapy of nonsmall cell lung cancer patients yielded dose distributions qualitatively comparable to flattened beams and significantly reduced treatment delivery time. Utilizing the X6FFF beam improved conformity of dose distribution. On the other hand, X10FFF beam offered a slight improvement in treatment efficiency, and lower skin and peripheral dose. All effects were relatively small.
    Medical Physics 03/2014; 41(3):031709. · 2.91 Impact Factor
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    ABSTRACT: PURPOSE: To investigate the clinical application of flattening filter free (FFF) beams at maximum dose rate for stereotactic body radiotherapy (SBRT). METHODS AND MATERIALS: Patients with tumors in the lung or abdomen were subjected to SBRT using 6MV FFF or 10MV FFF beams. For each patient, three plans were calculated using 6MV flattened, 6MV FFF, and 10MV FFF beams. Treatment times were recorded and analyzed, and tumor displacements were assessed by pre- and post-treatment cone beam computed tomography (CBCT). RESULTS: Altogether, 26 patients (16 lung, 10 abdominal tumors) were treated. The average dose rate per patient ranged from 442 to 1860MU/min. Beam-on time was on average 1.6min (1SD=0.6min), with the total treatment times recorded at 18.5min (1SD=3.5min). The time advantage of using FFF beams was dose-dependent and started at 4Gy for 6MV FFF and at 10Gy for 10MV FFF beams. The average of the tumor displacements during treatment was 2.0mm (1SD=1.0mm). CONCLUSIONS: SBRT using FFF beams is time efficient and associated with excellent patient stability. According to Van Herk's formula, ITV-PTV margins of 6mm are sufficient in our patient cohort. Further studies are necessary to assess clinical outcome and toxicity.
    Radiotherapy and Oncology 02/2013; · 4.52 Impact Factor
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    ABSTRACT: To evaluate the potential usage of flattening filter-free (FFF) photon beams in the treatment of prostate cancer. Volumetric-modulated arc therapy (VMAT) treatment planning was performed for 7 patients using TrueBeam(®) linear accelerator and photon beams with (X6, X10) and without (X6FFF, X10FFF) flattening filter. Prescribed dose was 19 × 3 Gy = 57 Gy. One or two 360° arcs with dose rate of 600 MU/min for flattened beams, and 1,200 MU/min for FFF beams were used. No difference was detected between the four beams in PTV coverage, conformity, and homogeneity. Mean body dose and body volume receiving 50% of the prescribed dose decreased with increasing mean energy (r(2) = 0.8275, p < 0.01). X6FFF delivered 3.6% more dose compared with the X6 (p < 0.01). X10FFF delivered 3.0% (p < 0.01), and the X10 5.8% (p < 0.01) less mean body dose compared with X6. There was a significant increase in the mean dose to the rectum for the X10 compared with X6 (2.6%, p < 0.01). Mean dose to the bladder increased by 1.3% for X6FFF and decreased by 2.3% for X10FFF. Using a single arc and FFF, treatment time was reduced by 35% (2 SD = 10%). FFF beams resulted in dose distributions similar to flattened beams. X10FFF beam provided the best solution, sparing rectum and bladder and minimizing whole-body dose. FFF beams lead to a time efficient treatment delivery, particularly when combined with hypofractionated VMAT.
    International journal of radiation oncology, biology, physics 05/2012; 83(5):1655-60. · 4.59 Impact Factor
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    ABSTRACT: As mechanical stability of radiation therapy treatment devices has gone beyond sub-millimeter levels, there is a rising demand for simple yet highly accurate measurement techniques to support the routine quality control of these devices. A combination of using high-resolution radiosensitive film and computer-aided analysis could provide an answer. One generally known technique is the acquisition of star shot films to determine the mechanical stability of rotations of gantries and the therapeutic beam. With computer-aided analysis, mechanical performance can be quantified as a radiation isocenter radius size. In this work, computer-aided analysis of star shot film is further refined by applying an analytical solution for the smallest intersecting circle problem, in contrast to the gradient optimization approaches used until today. An algorithm is presented and subjected to a performance test using two different types of radiosensitive film, the Kodak EDR2 radiographic film and the ISP EBT2 radiochromic film. Artificial star shots with a priori known radiation isocenter size are used to determine the systematic errors introduced by the digitization of the film and the computer analysis. The estimated uncertainty on the isocenter size measurement with the presented technique was 0.04 mm (2σ) and 0.06 mm (2σ) for radiographic and radiochromic films, respectively. As an application of the technique, a study was conducted to compare the mechanical stability of O-ring gantry systems with C-arm-based gantries. In total ten systems of five different institutions were included in this study and star shots were acquired for gantry, collimator, ring, couch rotations and gantry wobble. It was not possible to draw general conclusions about differences in mechanical performance between O-ring and C-arm gantry systems, mainly due to differences in the beam-MLC alignment procedure accuracy. Nevertheless, the best performing O-ring system in this study, a BrainLab/MHI Vero system, and the best performing C-arm system, a Varian Truebeam system, showed comparable mechanical performance: gantry isocenter radius of 0.12 and 0.09 mm, respectively, ring/couch rotation of below 0.10 mm for both systems and a wobble of 0.06 and 0.18 mm, respectively. The methodology described in this work can be used to monitor mechanical performance constancy of high-accuracy treatment devices, with means available in a clinical radiation therapy environment.
    Physics in Medicine and Biology 04/2012; 57(10):2997-3011. · 2.70 Impact Factor
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    ABSTRACT: Recently, there has been an increased interest in flattening-filter-free (FFF) linear accelerators. Removal of the filter results in available dose rates up to 24 Gy min(-1) (for nominal energy 10 MV in depth of maximum dose, a source-surface distance of 100 cm and a field size of 10×10 cm2). To guarantee accurate relative and reference dosimetry for the FFF beams, we investigated the charge collection efficiency of multiple air-vented and one liquid ionization chamber for dose rates up to 31.9 Gy min(-1). For flattened beams, the ion-collection efficiency of all air-vented ionization chambers (except for the PinPoint chamber) was above 0.995. By removing the flattening filter, we found a reduction in collection efficiency of approximately 0.5-0.9% for a 10 MV beam. For FFF beams, the Markus chamber showed the largest collection efficiency of 0.994. The observed collection efficiencies were dependent on dose per pulse, but independent of the pulse repetition frequency. Using the liquid ionization chamber, the ion-collection efficiency for flattened beams was above 0.990 for all dose rates. However, this chamber showed a low collection efficiency of 0.940 for the FFF 10 MV beam at a dose rate of 31.9 Gy min(-1). All investigated air-vented ionization chambers can be reliably used for relative dosimetry of FFF beams. The order of correction for reference dosimetry is given in the manuscript. Due to their increased saturation in high dose rate FFF beams, liquid ionization chambers appear to be unsuitable for dosimetry within these contexts.
    Physics in Medicine and Biology 04/2012; 57(9):2819-27. · 2.70 Impact Factor
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    ABSTRACT: Pretreatment quality assurance data from four centers, members of the European TrueBeam council were analyzed with different verification devices to assess reliability of flattening filter free beam delivery for intensity modulated radiotherapy (IMRT) and RapidArc (RA) techniques. TrueBeam(®) (Varian Medical System) is a new linear accelerator designed for delivering flattened, as well as flattening filter free beams. Pretreatment dosimetric validation of plan delivery was performed with different verification devices and responses to high dose rates were tested. Treatment planning was done in Eclipse planning system (PRO 8.9, AAA 8.9). γ evaluation was performed with (dose difference) = 3% and (distance to agreement) = 3 mm scoring the gamma agreement index (GAI, % of field area passing the test). Two hundred and twenty-four patients with 1-6 lesions in various anatomical regions and dose per fraction ranging from 1.8 Gy to 25 Gy were included in the study; 88 were treated with 6 MV flattening filter free (X6FFF) beam energy and 136 with 10 MV flattening filter free (X10FFF) beam. Gafchromic films in solid water, delta(4), arccheck, and matrixx phantom were used to verify the dose distributions. Additionally, point measurements were performed using a PinPoint chamber and a Farmer chamber. Dose calculation as well as dose delivery was equally accurate for IMRT and RA delivery (IMRT: GAI = 99.3% (±1.1); RA: GAI = 98.8% (±1.1) as well as for the two beams evaluated (X6FFF: GAI = 99.1% (±1.0); X10FFF: GAI = 98.8% (±1.2). Only small differences were found for the four verification devices. A point dose verification was performed on 52 cases, obtaining a dose deviation of 0.34%. The GAI variations with number of monitor units were statistically significant. The TrueBeam FFF modality, analyzed with a variety of verification devices and planned with Eclipse planning system is dosimetrically accurate (within the specified limits 3 mm/3%) for both X6FFF and X10FFF beam energy.
    Medical Physics 03/2012; 39(3):1351-6. · 2.91 Impact Factor
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    ABSTRACT: To investigate if there is a statistically significant difference in cancer cell survival using a high dose per pulse flattening filter-free (FFF) beam compared to a standard flattened beam. To validate the radiobiological effect of the flattened and FFF beam, two glioblastoma cell lines were treated with either 5 or 10 Gy using different dose rates. Dose verification was performed and colony formation assays were carried out. To compare the predictability of our data, radiobiological models were included. The results presented here demonstrate that irradiation of glioblastoma cell lines using the FFF beam is more efficient in reducing clonogenic cell survival than the standard flattened beam, an effect which becomes more significant the higher the single dose. Interestingly, in our experimental setting, the radiobiological effect of the FFF beam is dependent on dose per pulse rather than on delivery time. The used radiobiological models are able to describe the observed dose rate dependency between 6 and 24 Gy/min. The results presented here show that dose per pulse might become a crucial factor which influences cancer cell survival. Using high dose rates, currently used radiobiological models as well as molecular mechanisms involved urgently need to be re-examined.
    Radiotherapy and Oncology 07/2011; 101(1):226-32. · 4.52 Impact Factor
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    ABSTRACT: Quality assurance data from five centres were analysed to assess the reliability of RapidArc radiotherapy delivery in terms of machine and dosimetric performance. A large group of patients was treated with RapidArc radiotherapy and treatment data recorded. Machine quality assurance was performed according to Ling et al (Int J Radiat Oncol Biol Phys 2008;72:575-81). In addition, treatment to a typical clinical case was delivered biweekly as a constancy check. Pre-treatment dosimetric validation of plan delivery was performed for each patient. All measurements and computations were performed at the depth of the maximum dose in water according to the GLAaS method using electronic portal imaging device measurements. Evaluation was carried out according to a gamma agreement index (GAI, the percentage of field area passing the test); the threshold dose difference was 3% and the threshold distance to agreement was 3 mm. A total of 275 patients (395 arcs) were included in the study. Mean delivery parameters were 31.0±20.0° (collimator angle), 4.7±0.5° s(-1) (gantry speed), 343±134 MU min(-1) (dose rate) and 1.6±1.4 min (beam-on time) for prescription doses ranging from 1.8 to 16.7 Gy/fraction. Mean deviations from the baseline dose rate and gantry speed ranged from -0.61% to 1.75%. Mean deviations from the baseline for leaf speed variation ranged from -0.73% to 0.41%. The mean GAI of repeated clinical fields was 99.2±0.2%. GAI varied from 84.7% to 100%; the mean across all patients was 97.1±2.4%. RapidArc can provide a reliable and accurate delivery of radiotherapy for a variety of clinical conditions.
    The British journal of radiology 06/2011; 84(1002):534-45. · 2.11 Impact Factor
  • Radiotherapy and Oncology - RADIOTHER ONCOL. 01/2011; 99.
  • European Journal of Cancer - EUR J CANCER. 01/2011; 47.
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    Radiotherapy and Oncology - RADIOTHER ONCOL. 01/2011; 99.
  • Fuel and Energy Abstracts 01/2011; 81(2).
  • Radiotherapy and Oncology - RADIOTHER ONCOL. 01/2011; 99.
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    ABSTRACT: To investigate dosimetric characteristics of a new linear accelerator designed to deliver flattened, as well as flattening filter-free (FFF), beams. To evaluate the accuracy of beam modeling under physical conditions using an anisotropic analytical algorithm. Dosimetric data including depth dose curves, profiles, surface dose, penumbra, out-of-field dose, output, total and scatter factors were examined for four beams (X6, X6FFF, X10, and X10FFF) of Varian's TrueBeam machine. Beams modeled by anisotropic analytical algorithm were compared with measured dataset. FFF beams have lower mean energy (tissue-phantom ratio at the depths of 20 and 10 cm (TPR 20/10): X6, 0.667; X6FFF, 0.631; X10, 0.738; X10FFF, 0.692); maximum dose is located closer to the surface; and surface dose increases by 10%. FFF profiles have sharper but faster diverging penumbra. For small fields and shallow depths, dose outside a field is lower for FFF beams; however, the advantage fades with increasing phantom scatter. Output increases 2.26 times for X6FFF and 4.03 times for X10FFF and is less variable with field size; collimator exchange effect is reduced. A good agreement between modeled and measured data is observed. Criteria of 2% depth-dose and 2-mm distance-to-agreement are always met. Reference dosimetric characteristics of TrueBeam photon bundles were obtained, and successful modeling of the beams was achieved.
    International journal of radiation oncology, biology, physics 12/2010; 80(4):1228-37. · 4.59 Impact Factor
  • Medical Physics 01/2010; 37(6). · 2.91 Impact Factor
  • Medical Physics 01/2008; 35(6). · 2.91 Impact Factor
  • Medical Physics - MED PHYS. 01/2008; 35(6).
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    ABSTRACT: This article provides a quantitative evaluation of Varian Medical Systems' beam matching procedure. A one-dimensional y analysis is employed to investigate the level of agreement of matched beams. A customized concept of one-dimensional gamma evaluation was designed. Our algorithm first performs a "local" fit of the reference and the evaluated datasets. For a particular point on the fitted evaluated curve, the y is derived as the shortest distance between the point and the fitted reference curve. This approach removes variations of the obtained y value related to the discrete character and noise in the original datasets. Criteria of 1 mm distance-to-agreement and 1% dose difference were used to evaluate the level of agreement of according profiles. Relative point and profile measurements were performed for all photon and electron beams of two Varian Clinacs 2100C/D. Matched beams show a good level of agreement. 70% of profiles completely pass the chosen criteria. The analysis of remaining 30% of the profiles demonstrates that measurement error becomes a limiting factor in achieving a better score. The highest obtained y value was 1.70. The quality of beam matching allowed us to treat according beams of both treatment units as "identical" and to use the reference beam data for the new unit. Nevertheless, the vendor's acceptance criteria of beam matching are much more benevolent. It might happen that the acceptance criteria are met, however, resulting quality of beam matching does not allow full interchangeability of beams.
    Medical Physics 08/2007; 34(7):2917-27. · 2.91 Impact Factor