Yvonne Dzierma

Beam properties and stability of a flattening-filter free 7 MV beam-an overview.

PURPOSE Several works have recently focused on flattening-filter-free (FFF) beams of linear accelerators of various companies (in particular, Varian and Elekta), but no overview as yet exists for the flattening-filter free 7XU beam (Siemens Artiste). METHODS Dosimetric properties of the 7XU beam were measured in May and September 2011. We present depth dose curves and beam profiles, output factors, and MLC transmission and assess the stability of the measurements. The 7XU beam was commissioned in the Pinnacle[superscript three] treatment planning system (TPS), and modeling results including the spectrum are presented. RESULTS The percent depth dose curve of the 7XU beam is similar to the flat 6X beam line, with a slightly smaller surface dose. The beam profiles show the characteristic shape of flattening-filter free beams, with deviations between measurements of generally less than 1%. The output factors of the 7XU beam decrease more slowly than for the 6X beam. The MLC transmission is comparable but slightly less for the 7XU beam. The 7XU beam can be adequately modeled by the Pinnacle[superscript three] TPS, with successful dosimetric verification. The spectrum of the 7XU beam has lower photon fluence up to approximately 2.5 MeV and higher fluence beyond, with a slightly higher mean energy. CONCLUSIONS The 7XU beam has been commissioned for clinical use after successful modeling, stability checks, and dosimetric verification.

Set-up errors and planning margins in planar and CBCT image-guided radiotherapy using three different imaging systems: A clinical study for prostate and head-and-neck cancer.

PURPOSE The purpose of this work is to compare the positioning accuracy achieved by three different imaging techniques and planar vs. CBCT imaging for two common IGRT indications. METHODS A collective of prostate cancer and head-and-neck cancer patients treated at our institution during the year 2013 was retrospectively analyzed. For all treatment fractions (3078 in total), the kind of acquired set-up image and the performed couch shift before treatment were assessed. The distribution of couch corrections was compared for three different imaging systems available at our institution: the treatment beam line operating at 6 MV, a dedicated imaging beam line of nominally 1 MV, and the kVision system at 70-121 kV. Shifts were analyzed for planar and cone-beam CT images. Based on the set-up corrections, CTV to PTV expansion margins were calculated. RESULTS The difference in set-up corrections performed for the three energies and both techniques (planar vs. CBCT) was not significant for head-and-neck cancer patients. For prostate cancer all shifts had equal variance. Averages ranged from -0.7 to +0.7 mm. The set-up margins calculated on the basis of the observed shifts are 4.0 mm (AP) and 3.8 mm (SI, LR) for the head-and-neck PTV and 6.6 mm (SI), 6.7 mm (AP) and 7.9 mm (LR) for the prostate cancer patients. CONCLUSIONS For three different linac-based imaging energies and planar/CBCT imaging, no relevant differences in set-up shifts were observed. The suggested set-up margins for these indications are of the order of 4 mm for head-and-neck and 6-8 mm for prostate treatment.

Dosimetry of an in-line kilovoltage imaging system and implementation in treatment planning.

PURPOSE To present the beam properties of the Siemens 70-kV and 121-kV linear accelerator-mounted imaging modalities and commissioning of the 121-kV beam in the Philips Pinnacle treatment planning system (TPS); measurements in an Alderson phantom were performed for verification of the model and to estimate the cone-beam CT (CBCT) imaging dose in the head and neck, thorax, and pelvis. METHODS AND MATERIALS The beam profiles and depth-dose curve were measured in an acrylic phantom using thermoluminescent dosimeters and a soft x-ray ionization chamber. Measurements were imported into the TPS, modeled, and verified by phantom measurements. RESULTS Modeling of the profiles and the depth-dose curve can be achieved with good quality. Comparison with the measurements in the Alderson phantom is generally good; only very close to bony structures is the dose underestimated by the TPS. For a 200° arc CBCT of the head and neck, a maximum dose of 7 mGy is measured; the thorax and pelvis 360° CBCTs give doses of 4-10 mGy and 7-15 mGy, respectively. CONCLUSIONS Dosimetric characteristics of the Siemens kVision imaging modalities are presented and modeled in the Pinnacle TPS. Thermoluminescent dosimeter measurements in the Alderson phantom agree well with the calculated TPS dose, validating the model and providing an estimate of the imaging dose for different protocols.

mARC prostate treatment planning with Varian Eclipse for flat vs. FFF beams

PURPOSE The modulated arc (mARC) technique as an alternative to VMAT is a rotational IMRT irradiation with burst mode delivery. Varian has recently implemented an option for mARC-planning into the Eclipse treatment planning system (TPS) and so far mARC-planning with this TPS has not been evaluated systematically. Therefore, for prostate treatment with Eclipse we compare mARC with IMRT using flat (6MV) and flattening-filter-free (FFF, 7MV) beam energies. METHODS For ten prostate cancer patients standardized re-contouring and re-planning was performed with a prescription of 76Gy to the complete planning-target-volume (PTV). IMRT and mARC plans (6MV vs. FFF 7MV) were compared pairwise considering indices for plan quality. All plans were delivered on an anthromorphic phantom equipped with thermoluminescent dosimeters to measure out-of-field dose and treatment times. RESULTS Regarding PTV coverage, there was no marked preference for either technique or energy. The evaluation of organs at risk showed improved bladder sparing of the mARC plans up to about 75Gy; above this dose the IMRT plans achieved significant better sparing. The use of the FFF-beam-energy and mARC-technique resulted in a significant decrease in out-of-field dose. This combination also led to a drastic reduction of treatment time by factor of three in comparison with 6MV IMRT. CONCLUSION While highly conformal treatment plans could be created by the use of all modalities, the combination of the high dose rate with mARC appears to be the preferable option as it benefits from a marked decrease in treatment time and out-of-field dose.

Planning study and dose measurements of intracranial stereotactic radiation surgery with a flattening filter-free linac

PURPOSE Flattening filter-free (FFF) beams have recently become available for radiation therapy, offering much higher dose rates but complicating treatment owing to the nonflat profile. Stereotactic treatment is one of the most evident scenarios to investigate the use of FFF beams. METHODS AND MATERIALS We present a planning study of a FFF 7-MV beam for the treatment of brain metastases using multiple noncoplanar arcs. Plan differences as compared with flat 6 MV photon fields are estimated using different measures of quality. Absolute dosimetry and fluence distribution are verified and the out-of-field dose is measured. RESULTS The FFF 7-MV plans are slightly better than the flat 6-MV plans as evaluated by a number of quality indices, dose to organs at risk, and out-of-field dose, although differences may not be clinically relevant. Verification does not pose any problems. CONCLUSIONS The FFF 7-MV treatment plans are marginally superior to the flat-beam 6-MV plans in almost all cases, with greatly reduced treatment times (almost 50%).

Visualisation of Respiratory Tumour Motion and Co-Moving Isodose Lines in the Context of Respiratory Gating, IMRT and Flattening-Filter-Free Beams

Respiratory motion during percutaneous radiotherapy can be considered based on respiration-correlated computed tomography (4DCT). However, most treatment planning systems perform the dose calculation based on a single primary CT data set, even though cine mode displays may allow for a visualisation of the complete breathing cycle. This might create the mistaken impression that the dose distribution were independent of tumour motion. We present a movie visualisation technique with the aim to direct attention to the fact that the dose distribution migrates to some degree with the tumour and discuss consequences for gated treatment, IMRT plans and flattening-filter-free beams. This is a feasibility test for a visualisation of tumour and isodose motion. Ten respiratory phases are distinguished on the CT, and the dose distribution from a stationary IMRT plan is calculated on each phase, to be integrated into a movie of tumour and dose motion during breathing. For one example patient out of the sample of five lesions, the plan is compared with a gated treatment plan with respect to tumour coverage and lung sparing. The interplay-effect for small segments in the IMRT plan is estimated. While the high dose rate, together with the cone-shaped beam profile, makes the use of flattening-filter-free beams more problematic for conformal and IMRT treatment, it can be the option of choice if gated treatment is preferred. The different effects of respiratory motion, dose build-up and beam properties (segments and flatness) for gated vs. un-gated treatment can best be considered if planning is performed on the full 4DCT data set, which may be an incentive for future developments of treatment planning systems.

Comparative Planning of Flattening-Filter-Free and Flat Beam IMRT for Hypopharynx Cancer as a Function of Beam and Segment Number

Although highly conformal dose distributions can be achieved by IMRT planning, this often requires a large number of segments or beams, resulting in increased treatment times. While flattening-filter-free beams offer a higher dose rate, even more segments may be required to create homogeneous target coverage. Therefore, it is worthwhile to systematically investigate the dependence of plan quality on gantry angles and number of segments for flat vs. FFF beams in IMRT planning. For the practical example of hypopharynx cancer, we present a planning study of flat vs. FFF beams using three different configurations of gantry angles and different segment numbers. The two beams are very similar in physical properties, and are hence well-suited for comparative planning. Starting with a set of plans of equal quality for flat and FFF beams, we assess how far the number of segments can be reduced before the plan quality is markedly compromised, and compare monitor units and treatment times for the resulting plans. As long as a sufficiently large number of segments is permitted, all planning scenarios give good results, independently of gantry angles and flat or FFF beams. For smaller numbers of segments, plan quality decreases both for flat and FFF energies; this effect is stronger for fewer gantry angles and for FFF beams. For low segment numbers, FFF plans are generally worse than the corresponding flat beam plans, but they are less sensitive to a decrease in segment number if many gantry angles are used (18 beams); in this case the quality of flat and FFF plans remains comparable even for few segments.

Shear-wave velocity structure of the Chilean subduction zone (39–40°S) based on Rayleigh wave dispersion: evidence of fluid release and melts in the mantle beneath the Villarrica volcano

Abstract A seismic network operated from December 2008 to November 2009 in south-central Chile covering the Chile subduction zone from c. 39°S to 40°S. This segment of the subduction zone includes the highly active Villarrica volcano and the maximum slip area of the 1960 Mw 9.5 earthquake. We applied surface wave dispersion analysis to data from a linear array of broadband stations and to records of four areal sub-arrays. Fifty regional and teleseismic events were used to produce dispersion curves of Rayleigh waves. From the dispersion curves, we determined depth functions of the shear-wave velocity for 4 subregions of the subduction zone: the Coastal Range, the Central Valley, the Volcanic Arc and the Back-arc Region in Argentina. The resulting models reveal the structure of the crust and the depth of the Moho discontinuity. Below the volcanic arc, the shear-wave velocities of the continental mantle are reduced by c. 7% with respect to a background value of 4.3 km s−1. This low-velocity zone coincides with a zone of reduced electrical resistivity that was previously determined from magnetotelluric measurements. The combined occurrences of minima in the S-wave velocity and resistivity can be interpreted as an indicator of partial melts.

Abdominal imaging dose in radiology and radiotherapy – Phantom point dose measurements, effective dose and secondary cancer risk

PURPOSE To compare abdominal imaging dose from 3D imaging in radiology (standard/low-dose/dual-energy CT) and radiotherapy (planning CT, kV cone-beam CT (CBCT)). METHODS Dose was measured by thermoluminescent dosimeters (TLDs) placed at 86 positions in an anthropomorphic phantom. Point, organ and effective dose were assessed, and secondary cancer risk from imaging was estimated. RESULTS Overall dose and mean organ dose comparisons yield significantly lower dose for the optimized radiology protocols (dual-source and care kV), with an average dose of 0.34±0.01 mGy and 0.54±0.01 mGy (average ± standard deviation), respectively. Standard abdominal CT and planning CT involve considerably higher dose (13.58 ± 0.18 mGy and 18.78±0.27 mGy, respectively). The CBCT dose show a dose fall-off near the field edges. On average, dose is reduced as compared with the planning or standard CT (3.79 ± 0.21 mGy for 220° rotation and 7.76 ± 0.37 mGy for 360°), unless the high-quality setting is chosen (20.30 ± 0.96 mGy). The mean organ doses show a similar behavior, which translates to the estimated secondary cancer risk. The modelled risk is in the range between 0.4 cases per million patient years (PY) for the radiological scans dual-energy and care kV, and 300 cases per million PY for the high-quality CBCT setting. CONCLUSIONS Modern radiotherapy imaging techniques (while much lower in dose than radiotherapy), involve considerably more dose to the patient than modern radiology techniques. Given the frequency of radiotherapy imaging, a further reduction in radiotherapy imaging dose appears to be both desirable and technically feasible.

Image guidance in clinical practice – Influence of positioning inaccuracy on the dose distribution for prostate cancer

BACKGROUND In order to consider potential positioning errors there are different recipes for safety-margins for CTV-to-PTV expansion. The aim of this study is to simulate the effect of positioning inaccuracy with clinically realistic patient treatment plans. METHODS For a collective of 40 prostate patients, the isocenter was shifted back appropriately to the applied table shifts after positioning verification, simulating that no positioning correction had been performed and the treatment plans were recalculated. All the treatment fractions with the appropriate isocenter-shifts were added to yield a new plan considering two scenarios:Afterwards all plans were analysed and compared with each other regarding target coverage, sparing of organs at risk (OAR) and normal tissue complication probability (NTCP). RESULTS Dose distributions and especially DVH show a deterioration of the target-coverage caused by the positioning inaccuracy. Deviations in dose at a single point can reach values of over 10 Gy. In single cases minimum plan agreement only achieved 66% pass within 3% local dose for the realistic case. Organs at risk and NTCP analysis result in a slightly better sparing of the rectum. Measures of quality like homogeneity and conformity differ just minimally regarding the different scenarios. CONCLUSION PTV-coverage suffers markedly by the positioning uncertainties, the shifted plans are in large parts clinically not acceptable. Surprisingly sparing of the OAR is not negatively affected by potential positioning errors for this prostate collective.