Peter Winkler

Dose-response characteristics of an amorphous silicon EPID.

Electronic portal imaging devices(EPIDs) were originally developed for the purpose of patient setup verification. Nowadays, they are increasingly used as dosimeters (e.g., for IMRT verification and linac-specific QA). A prerequisite for any clinical dosimetric application is a detailed understanding of the detector’sdose-response behavior. The aim of this study is to investigate the dosimetric properties of an amorphous siliconEPID (Elekta IVIEWGT) with respect to three photon beam qualities: 6, 10, and 25 MV . The EPID showed an excellent temporal stability on short term as well as on long term scales. The stability throughout the day was strongly influenced by warming up, which took several hours and affected EPID response by 2.5%. Ghosting effects increased the sensitivity of the EPID. They became more pronounced with decreasing time intervals between two exposures as well as with increasing dose. Due to ghosting, changes in pixel sensitivity amounted up to 16% (locally) for the 25 MV photon beam. It was observed that the response characteristics of our EPID depended on dose as well as on dose rate. Doubling the dose rate increased the EPID sensitivity by 1.5%. This behavior was successfully attributed to a dose per frame effect, i.e., a nonlinear relationship between the EPID signal and the dose which was delivered to the panel between two successive readouts. The sensitivity was found to vary up to 10% in the range of 1 to 1000 monitor units. This variation was governed by two independent effects. For low doses, the EPID signal was reduced due to the linac’s changing dose rate during startup. Furthermore, the detector reading was influenced by intrabeam variations of EPID sensitivity, namely, an increase of detector response during uniform exposure. For the beam qualities which were used, the response characteristics of the EPID did not depend on energy. Differences in relative dose-response curves resulted from energy dependent temporal output characteristics of the accelerator. If ghosting is prevented from affecting the results and all dose-response effects are properly corrected for, the EPID signal becomes independent of dose rate, dose, and exposure time.

Advanced kernel methods vs. Monte Carlo-based dose calculation for high energy photon beams.

PURPOSE The aim of this study was to compare the dose calculation accuracy of advanced kernel-based methods and Monte Carlo algorithms in commercially available treatment planning systems. MATERIALS AND METHODS Following dose calculation algorithms and treatment planning (TPS) systems were compared: the collapsed cone (CC) convolution algorithm available in Oncentra Masterplan, the XVMC Monte Carlo algorithm implemented in iPlan and Monaco, and the analytical anisotropic algorithm (AAA) implemented in Eclipse. Measurements were performed with a calibrated ionization chamber and radiochromic EBT type films in a homogenous polystyrene phantom and in heterogeneous lung phantoms. Single beam tests, conformal treatment plans and IMRT plans were validated. Dosimetric evaluations included absolute dose measurements, 1D gamma-evaluation of depth-dose curves and profiles using 2mm and 2% dose difference criteria for single beam tests, and gamma-evaluation of axial planes for composite treatment plans applying 3mm and 3% dose difference criteria. RESULTS Absolute dosimetry revealed no large differences between MC and advanced kernel dose calculations. 1D gamma-evaluation showed significant discrepancies between depth-dose curves in different phantom geometries. For the CC algorithm gamma(mean) values were 0.90+/-0.74 vs. 0.43+/-0.41 in heterogeneous vs. homogeneous conditions and for the AAA gamma(mean) values were 1.13+/-0.91 vs. 0.41+/-0.28, respectively. In general, 1D gamma results obtained with both MC TPS were similar in both phantoms and on average equal to 0.5 both for profiles and depth-dose curves. The results obtained with the CC algorithm in heterogeneous phantoms were slightly better in comparison to the AAA algorithm. The 2D gamma-evaluation results of IMRT plans and four-field plans showed smaller mean gamma-values for MC dose calculations compared to the advanced kernel algorithms (gamma(mean) for four-field plan and IMRT obtained with Monaco MC were 0.28 and 0.5, respectively, vs. 0.40 and 0.54 for the AAA). CONCLUSION All TPS investigated in this study demonstrated accurate dose calculation in homogenous and heterogeneous phantoms. Commercially available TPS with Monte Carlo option performed best in heterogeneous phantoms. However, the difference between the CC and the MC algorithms was found to be small.

The impact of photon dose calculation algorithms on expected dose distributions in lungs under different respiratory phases

A planning study was carried out on a cohort of CT datasets from breast patients scanned during different respiratory phases. The aim of the study was to investigate the influence of different air filling in lungs on the calculation accuracy of photon dose algorithms and to identify potential patterns of failure with clinical implications. Selected respiratory phases were free breathing (FB), representative of typical end expiration, and deep inspiration breath hold (DIBH), a typical condition for clinical treatment with respiratory gating. Algorithms investigated were the pencil beam (PBC), the anisotropic analytical algorithm (AAA) and the collapsed cone (CC) from the Varian Eclipse or Philips Pinnacle planning system. Reference benchmark calculations were performed with the Voxel Monte Carlo (VMC++). An analysis was performed in terms of physical quantities inspecting either dose-volume or dose-mass histograms and in terms of an extension to three dimensions of the gamma index of Low. Results were stratified according to a breathing phase and algorithm. Collectives acquired in FB or DIBH showed well-separated average lung density distributions with mean densities of 0.27 +/- 0.04 and 0.16 +/- 0.02 g cm(-3), respectively, and average peak densities of 0.17 +/- 0.03 and 0.09 +/- 0.02 g cm(-3). Analysis of volume-dose or mass-dose histograms proved the expected deviations on PBC results due to the missing lateral transport of electrons with underestimations in the low dose region and overestimations in the high dose region. From the gamma analysis, it resulted that PBC is systematically defective compared to VMC++ over the entire range of lung densities and dose levels with severe violations in both respiratory phases. The fraction of lung voxels with gamma > 1 for PBC reached 25% in DIBH and about 15% in FB. CC and AAA performed, in contrast, similarly and with fractions of lung voxels with gamma > 1 in average inferior to 2% in FB and 4-5% (AAA) or 6-8% (CC) in DIBH. In summary, PBC proved to be severely defective in calculations involving lungs and particularly for cases where specific respiratory phases (e.g. DIBH) are assumed for treatment. In contrast, CC and AAA manifested a high degree of consistency against the Monte Carlo method and provided stable results over the entire range of clinically relevant densities.

Inverse Planning – a Comparative Intersystem and Interpatient Constraint Study

Purpose:To compare commercial treatment-planning systems (TPS) for inverse planning (IP) and to assess constraint variations for specific IMRT indications.Material and Methods:For IP, OTP, XiO and BrainSCAN were used and step-and-shoot intensity-modulated radiotherapy (IMRT) delivery was assumed. Based on identical constraints, IP was performed for a prostate, head and neck, brain, and gynecologic case. IMRT plans were compared in terms of conformity/homogeneity, dose-volume histograms (DVHs), and delivery efficiency. For ten patients each of a class of indications, constraint variations were evaluated.Results:IMRT plans were comparable concerning minimum target dose, homogeneity, conformity, and maximum doses to organs at risk. Larger differences were seen in dose gradients outside the target, monitor units, and segment number. Using help structures proved efficient to shape isodoses and to reduce segmentation workload. For IMRT class solutions, IP constraint variations depended on anatomic site.Conclusion:IP systems requiring doses as input and having objective functions based on physical parameters had a very similar performance. Constraint templates can be established for a class of IMRT indications.Ziel:Die Funktionalität der inversen Planung (IP) von kommerziellen Bestrahlungsplanungssystemen (TPS) sowie die Unterschiede von Dosiszielgrößen bei typischen IMRT-Indikationen (intensitätsmodulierte Strahlentherapie) wurden untersucht.Material und Methodik:Für die IP fanden OTP, XiO und BrainSCAN Verwendung, und eine „Step-and-shoot“-IMRT wurde angenommen. Basierend auf gleichen Zielgrößenvorgaben wurde für je einen Fall mit Prostatakarzinom, Hirntumor, HNO-Tumor sowie gynäkologischem Tumor eine IP durchgeführt. Die IMRT-Pläne wurden anhand von Dosis-Volumen-Histogrammen (DVHs), Konformität, Homogenität und Bestrahlungseffizienz bewertet. Für je zehn Patienten mit bestimmten Indikationen wurde die Schwankung der IP-Zielgrößen untersucht.Ergebnisse:Mit allen drei TPS konnten ähnliche IMRT-Pläne mit vergleichbarer Zielgebietsauslastung, Dosishomogenität, Konformität und maximaler Dosisbelastung der Risikoorgane erstellt werden. Größere Unterschiede wurden hinsichtlich des Dosisgradienten außerhalb des Zielgebiets, der Monitoreinheiten sowie der Segmentzahlen beobachtet. Die Verwendung von Hilfsstrukturen erwies sich als zeitsparend. Für eine IMRT-Indikation schwanken die Zielgrößenvorgaben in Abhängigkeit von der Patientenanatomie.Schlussfolgerung:Mit TPS, deren Zielgrößen und Zielfunktionen auf physikalischen Dosen beruhen, konnten ähnliche IMRT-Pläne erzielt werden. Für IMRT-Konzepte lassen sich Standardzielvorgaben festlegen.

Normalized sensitometric curves for the verification of hybrid IMRT treatment plans with multiple energies

With the clinical implementation of time-variable dose patterns and intensity modulated radiotherapy (IMRT) film dosimetry has regained popularity. Films are currently the most frequently used dosimetric means for patient specific quality assurance in IMRT. A common method is to verify a so-called hybrid IMRT plan, which is the patient specific treatment plan with unmodified fluence patterns recalculated in a dedicated phantom. For such applications the sensitometric curve, i.e., the relation between optical density (OD) and absorbed dose, should not depend critically on beam energy, field size and depth, or film orientation. In order to minimize the influence of all these variables a normalization of sensitometric curves is performed at various photon beam energies (6 MV, 10 MV, 25 MV). By doing so one unique sensitometric curve can be used for these three beam qualities. This holds for both film types investigated: Kodak X-Omat V films and EDR-2 films. Additionally, the influence of field size, depth and film orientation on a normalized sensitometric curve is determined for both film types. For doses smaller than 0.8 Gy for X-Omat V and doses smaller than 3 Gy for EDR-2 films the field size variation of normalized sensitometric curves is much smaller than 3% for fields up to 20 x 20 cm2. For X-Omat V films all differences between sensitometric curves determined at depths of 5, 10, and 15 cm are smaller than 3%. For EDR-2 films deviations larger than 3% are only observed at low net OD smaller than 0.25. The dependence of film orientation (parallel versus perpendicular) on a normalized sensitometric curve is found to be not critical. However, processing conditions have the largest influence and can result in differences up to 20% for sensitometric curves derived from films of the same batch but using different film processors. When normalizing sensitometric curves to the dose value necessary to obtain a net OD=1 for that respective geometry and energy the large energy dependence of sensitometric curves can be almost eliminated. This becomes especially important for the verification of hybrid IMRT plans with multiple energies. Additionally, such a normalization minimizes other influences such as field size, depth, and film orientation. This method is generally applicable to both Kodak X-Omat V and EDR-2 films. In order to achieve the highest accuracy level an upper dose limit of 0.8 Gy for X-Omat V films and 3 Gy for EDR-2 films should be taken into account. However, these dose limits may vary with film reading instrument and film processor.

Introducing a system for automated control of rotation axes, collimator and laser adjustment for a medical linear accelerator

Mechanical stability and precise adjustment of rotation axes, collimator and room lasers are essential for the success of radiotherapy and particularly stereotactic radiosurgery with a linear accelerator. Quality assurance procedures, at present mainly based on visual tests and radiographic film evaluations, should desirably be little time consuming and highly accurate. We present a method based on segmentation and analysis of digital images acquired with an electronic portal imaging device (EPID) that meets these objectives. The method can be employed for routine quality assurance with a square field formed by the built-in collimator jaws as well as with a circular field using an external drill hole collimator. A number of tests, performed to evaluate accuracy and reproducibility of the algorithm, yielded very satisfying results. Studies performed over a period of 18 months prove the applicability of the inspected accelerator for stereotactic radiosurgery.

An intercomparison of 11 amorphous silicon EPIDs of the same type : implications for portal dosimetry

The use of electronic portal imaging devices (EPIDs) for portal dosimetry requires knowledge of their dosimetric properties. The pixel value response of amorphous silicon EPIDs of type Elekta iViewGT is known to be nonlinear with dose. However, it is not clear whether these nonlinearities vary with time and from one detector to another, respectively. In the present study, the dose-response characteristics of 11 iViewGT EPIDs were investigated with respect to dose rate, total dose and field size. It was found that each detector needs to be individually calibrated, not only in terms of absolute sensitivity but also with respect to its relative response variations with exposure parameters. Doubling the dose rate typically increased the EPID signal between 1.4% and 2.8%. Changing the number of monitor units from 30 to 500 was accompanied by an increase in detector sensitivity between 1.7% and 2.8%. The EPID scatter factors were always within +/-1%. It was observed that the dose-response behaviour was not stable with respect to time. Particularly within the first weeks of operation, detector ageing caused variations in both absolute sensitivity and relative response curves. It is recommended to establish a quality assurance programme if the amorphous silicon EPIDs are intended to be used for clinical portal dosimetry.

Implementation and validation of portal dosimetry with an amorphous silicon EPID in the energy range from 6 to 25 MV

The purpose of this study was to develop, implement and validate a method for portal dosimetry with an amorphous silicon EPID for a wide energy range. Analytic functions were applied in order to correct for nonlinearities in detector response with dose rate, irradiation time and total dose. EPID scattering processes were corrected for by means of empirically determined convolution kernels. For a variety of rectangular and irregularly shaped fields, head scatter factors determined from central axis portal dose values and those measured with an ionization chamber showed a maximum deviation of 0.5%. The accuracy of our method was further investigated for pretreatment IMRT verification (i.e. without absorbers in the beam). The agreement between EPID and film dosimetry was quantified using gamma (gamma) evaluation, with 2% dose and 2 mm distance-to-agreement criteria. All gamma-distributions showed a gamma(mean) < 0.5, a 99th percentile <1.5 and a fraction of pixels with gamma > 1 smaller than 7%. The number of monitor units delivered by single segments of the IMRT fields could be extracted from the portal images with high accuracy. Measured and delivered doses were within +/-3% for more than 98% of data points. Ghosting effects were found to have limited effects on dosimetric IMRT verification.

Ruthenium-106 plaque brachytherapy for uveal melanoma

Background To report on local tumour control, eye preservation and visual outcome after ruthenium-106 brachytherapy for uveal melanoma. Methods Medical records of 143 eyes with uveal melanoma, treated by ruthenium-106 brachytherapy between 1997 and 2012 at one single centre, were included. Primary outcome measures were local tumour control, eye preservation and visual outcome. The influence of patient, tumour and treatment parameters on outcome was analysed by time to event analysis and competing risk regression. Results The median overall follow-up was 37.9 months. Tumour control: recurrent tumour growth was observed in 17 patients. The estimated local tumour recurrence rate at 12, 24 and 48 months after irradiation was 3%, 8.4% and 14.7%, respectively. The only significant risk factors for tumour recurrence were age (p=0.046) and reduced initial visual acuity (VA, p=0.045). No significant difference could be shown for tumour size or tumour category (T1–T2 vs T3–T4), and for any other tumour or treatment parameters (including combined transpupillary thermo-therapy (TTT)). Eye preservation: The likelihood of keeping the eye 12, 24 and 48 months after irradiation was 97.7%, 94.7% and 91.8%, respectively. Most significant risk factors for secondary enucleation were initial VA (p<0.001), tumour height (p=0.002) and tumour category (p=0.015). Vision The chances of keeping VA of 20/200 or better at 1, 2 and 5 years after treatment were 86.4%, 80.8% and 61.7%, respectively. Patients receiving sandwich-TTT showed significantly worse visual outcomes. Conclusions Ruthenium-106 brachytherapy appears to be a useful treatment regarding tumour control, eye preservation and visual function. Adjunct sandwich therapy resulted in worse visual outcome.

Untersuchung von Bildqualität und Genauigkeit der Dosisberechnung bei cone-beam-CT-Aufnahmen mit reduziertem Projektionsdatensatz (half scan, half fan) im Hinblick auf die Verwendbarkeit für die adaptive Bestrahlungsplanung

UNLABELLED Subject of this study is the question of whether cone beam CT (CBCT) images with reduced projection data are suitable for use in adaptive radiation therapy (ART) treatment planning. For this purpose image quality and dose calculation accuracy depending on imaging modality were analysed. In this context, two CBCT-methods will be indicated having reduced projection data sets: Scans acquired with 200° rotation angle in order to accelerate the CBCT process (half scan), or scans with an asymmetric cone beam and detector offset, used to enlarge the field-of-view (half fan). METHODS For three different CBCT-modes (On-Board-Imaging, Varian Medical Systems), two of them based on reduced projection data, and a conventional multidetector CT system, the main image quality parameters were studied. Treatment plans for two phantoms were transferred to all datasets and re-computed to analyse dose calculation accuracy. Furthermore imaging dose was measured for all modalities. RESULTS All three CBCT-modes showed similar results with regard to image quality. It was found, that a reduction in projection data does not necessarily involve deterioration in image quality parameters. For dose calculation based on CBCT images, a good agreement with the reference plan was found, with a maximum deviation for the mean dose in regions of interest of 1.1%. Imaging dose was found to be 2.5 cGy and 2.9 cGy for the large-FOV mode and the partial rotation mode, respectively, and 5.4 cGy for the 360°-full fan mode.