Otto A. Sauer

Measurement of output factors for small photon beams

A variety of detectors and procedures for the measurement of small field output factors are discussed in the current literature. Different detectors with or without corrections are recommended. Correction factors are often derived by Monte Carlo methods, where the bias due to approximations in the model is difficult to judge. Over that, results appear to be contradictory in some cases. In this work, output factors were measured for field sizes from 4 mm up to 180 mm side length with different detectors. A simple linear correction for the energy response of solid state detectors is proposed. This led to identical values down to 8 mm field size, as long as the size of the detector is small against the field size. The correction was of the order of a few percent. For a shielded silicon diode it was well below 1%. A physically meaningful function is proposed in order to calculate output factors for arbitrary field sizes with high accuracy.

Precision of image-guided radiotherapy (IGRT) in six degrees of freedom and limitations in clinical practice.

Purpose:To evaluate the precision of image-guided radiotherapy (IGRT) using cone-beam computed tomography (CB-CT) for volume imaging and a robotic couch for correcting setup errors in six degrees of freedom.Patients and Methods:47 consecutive patients with 372 fractions were classified according to whether a patient fixation device was used (patfix: n = 28) or not (patnon-fix: n = 19). Prior to treatment a CB-CT was acquired and translational and rotational setup errors were corrected online without an action level using a robotic couch (HexaPOD). A second CB-CT was acquired after the correction process and after treatment in 134 and 238 fractions, respectively.Results:In 17 fractions (4.6%) rotational errors > 3° exceeded the motion range of the HexaPOD. Errors (3D vector) after the correction process were significantly smaller for patfix compared to patnon-fix (p < 0.001): 0.9 mm ± 0.5 mm and 1.6 mm ± 0.8 mm, respectively. For patnon-fix the correction of rotational errors resulted in displacements of the patients on the angled couch of 0.6 mm/1°. Intrafractional motion further decreased precision in patnon-fix but not in patfix.Conclusion:Very high precision in cranial and extracranial treatment of immobilized patients was demonstrated. Without application of adequate immobilization the correction of rotational errors and intrafractional patient motion significantly decreased the accuracy of the online correction protocol.Ziel:Untersucht wurde die Präzision eines bildgestützten Bestrahlungsprotokolls, basierend auf Volumenbildgebung mittels Cone-Beam-Computertomographie (CB-CT) und Korrektur von Lagerungsfehlern in sechs Freiheitsgraden.Patienten und Methodik:47 Patienten mit 372 Behandlungsfraktionen wurden ausgewertet: Differenziert wurde zwischen Patienten, die auf dem Behandlungstisch fixiert wurden (patfix: n = 28) oder nicht (patnon-fix: n = 19). Vor der Behandlung wurde ein CB-CT angefertigt, und translatorische und rotatorische Lagerungsfehler wurden vollständig mittels eines robotischen Behandlungstisches (HexaPOD) korrigiert. Bildgebung nach der Fehlerkorrektur und nach der Behandlung wurde in jeweils 134 und 238 Fraktionen durchgeführt.Ergebnisse:Bei 17 Fraktionen (4,6%) überschritten die Rotationsfehler die Reichweite des HexaPOD von 3°. Fehler nach der Korrektur (3D-Vektor) waren bei patnon-fix im Vergleich zu patfix signifikant größer (p < 0,001): 1,6 mm ± 0,8 mm versus 0,9 mm ± 0,5 mm. Bei patnon-fix führte die Korrektur von Rotationsfehlern zu einer Verlagerung der Patienten auf dem abgewinkelten HexaPOD von 0,6 mm/1°. Intrafraktionelle Patientenbewegung resultierte in weiteren Fehlern bei patnon-fix, jedoch nicht bei patfix.Schlussfolgerung:Bei Immobilisierung der Patienten wurde eine Präzision mittels bildgestützter Radiotherapie von 1 mm und 1° erreicht. Ohne ausreichende Immobilisation reduzieren Verlagerungen nach Korrektur von Rotationsfehlern und intrafraktionelle Patientenbewegungen den Nutzen eines solches Behandlungsprotokolls erheblich.

Monte Carlo- versus pencil-beam-/collapsed-cone-dose calculation in a heterogeneous multi-layer phantom.

The aim of this work was to evaluate the accuracy of dose predicted in heterogeneous media by a pencil beam (PB), a collapsed cone (CC) and a Monte Carlo (MC) algorithm. For this purpose, a simple multi-layer phantom composed of Styrofoam and white polystyrene was irradiated with 10 x 10 cm2 as well as 20 x 20 cm2 open 6 MV photon fields. The beam axis was aligned parallel to the layers and various field offsets were applied. Thereby, the amount of lateral scatter was controlled. Dose measurements were performed with an ionization chamber positioned both in the central layer of white polystyrene and the adjacent layers of Styrofoam. It was found that, in white polystyrene, both MC and CC calculations agreed satisfactorily with the measurements whereas the PB algorithm calculated 12% higher doses on average. By studying off-axis dose profiles the observed differences in the calculation results increased dramatically for the three algorithms. In the regions of low density CC calculated 10% (8%) lower doses for the 10 x 10 cm2 (20 x 20 cm2) fields than MC. The MC data on the other hand agreed well with the measurements, presuming that proper replacement correction for the ionization chamber embedded in Styrofoam was performed. PB results evidently did not account for the scattering geometry and were therefore not really comparable. Our investigations showed that the PB algorithm generates very large errors for the dose in the vicinity of interfaces and within low-density regions. We also found that for the used CC algorithm large deviations for the absolute dose (dose/monitor unit) occur in regions of electronic disequilibrium. The performance might be improved by better adapted parameters. Therefore, we recommend a careful investigation of the accuracy for dose calculations in heterogeneous media for each beam data set and algorithm.

Image-Guided Radiotherapy for Liver Cancer Using Respiratory-Correlated Computed Tomography and Cone-Beam Computed Tomography

PURPOSE To evaluate a novel four-dimensional (4D) image-guided radiotherapy (IGRT) technique in stereotactic body RT for liver tumors. METHODS AND MATERIALS For 11 patients with 13 intrahepatic tumors, a respiratory-correlated 4D computed tomography (CT) scan was acquired at treatment planning. The target was defined using CT series reconstructed at end-inhalation and end-exhalation. The liver was delineated on these two CT series and served as a reference for image guidance. A cone-beam CT scan was acquired after patient positioning; the blurred diaphragm dome was interpreted as a probability density function showing the motion range of the liver. Manual contour matching of the liver structures from the planning 4D CT scan with the cone-beam CT scan was performed. Inter- and intrafractional uncertainties of target position and motion range were evaluated, and interobserver variability of the 4D-IGRT technique was tested. RESULTS The workflow of 4D-IGRT was successfully practiced in all patients. The absolute error in the liver position and error in relation to the bony anatomy was 8 +/- 4 mm and 5 +/- 2 mm (three-dimensional vector), respectively. Margins of 4-6 mm were calculated for compensation of the intrafractional drifts of the liver. The motion range of the diaphragm dome was reproducible within 5 mm for 11 of 13 lesions, and the interobserver variability of the 4D-IGRT technique was small (standard deviation, 1.5 mm). In 4 patients, the position of the intrahepatic lesion was directly verified using a mobile in-room CT scanner after application of intravenous contrast. CONCLUSION The results of our study have shown that 4D image guidance using liver contour matching between respiratory-correlated CT and cone-beam CT scans increased the accuracy compared with stereotactic positioning and compared with IGRT without consideration of breathing motion.

Definition of stereotactic body radiotherapy

This report from the Stereotactic Radiotherapy Working Group of the German Society of Radiation Oncology (Deutschen Gesellschaft für Radioonkologie, DEGRO) provides a definition of stereotactic body radiotherapy (SBRT) that agrees with that of other international societies. SBRT is defined as a method of external beam radiotherapy (EBRT) that accurately delivers a high irradiation dose to an extracranial target in one or few treatment fractions. Detailed recommendations concerning the principles and practice of SBRT for early stage non-small cell lung cancer (NSCLC) are given. These cover the entire treatment process; from patient selection, staging, treatment planning and delivery to follow-up. SBRT was identified as the method of choice when compared to best supportive care (BSC), conventionally fractionated radiotherapy and radiofrequency ablation. Based on current evidence, SBRT appears to be on a par with sublobar resection and is an effective treatment option in operable patients who refuse lobectomy.ZusammenfassungDie Arbeitsgruppe „Stereotaktische Radiotherapie“ der Deutschen Gesellschaft für Radioonkologie (DEGRO) erarbeitete eine Definition der Körperstereotaxie (SBRT), die sich an vorhandene internationale Definitionen anlehnt: Die SBRT ist eine Form der perkutanen Strahlentherapie, die mit hoher Präzision eine hohe Bestrahlungsdosis in einer oder wenigen Bestrahlungsfraktionen in einem extrakraniellen Zielvolumen appliziert. Zur Praxis der SBRT beim nichtkleinzelligen Bronchialkarzinom (NSCLC) im frühen Stadium werden detaillierte Empfehlungen gegeben, die den gesamten Ablauf der Behandlung von der Indikationsstellung, Staging, Behandlungsplanung und Applikation sowie Nachsorge umfassen. Die Körperstereotaxie wurde als Methode der Wahl im Vergleich zu Best Supportive Care, zur konventionell fraktionierten Strahlentherapie sowie zur Radiofrequenzablation identifiziert. Die Ergebnisse nach SBRT und sublobärer Resektion erscheinen auf aktueller Datenbasis ebenbürtig. Die SBRT ist die Methode der Wahl, wenn Patienten einen operativen Eingriff in Form der Lappenresektion ablehnen.

Definition of stereotactic body radiotherapy: principles and practice for the treatment of stage I non-small cell lung cancer.

This report from the Stereotactic Radiotherapy Working Group of the German Society of Radiation Oncology (Deutschen Gesellschaft für Radioonkologie, DEGRO) provides a definition of stereotactic body radiotherapy (SBRT) that agrees with that of other international societies. SBRT is defined as a method of external beam radiotherapy (EBRT) that accurately delivers a high irradiation dose to an extracranial target in one or few treatment fractions. Detailed recommendations concerning the principles and practice of SBRT for early stage non-small cell lung cancer (NSCLC) are given. These cover the entire treatment process; from patient selection, staging, treatment planning and delivery to follow-up. SBRT was identified as the method of choice when compared to best supportive care (BSC), conventionally fractionated radiotherapy and radiofrequency ablation. Based on current evidence, SBRT appears to be on a par with sublobar resection and is an effective treatment option in operable patients who refuse lobectomy.ZusammenfassungDie Arbeitsgruppe „Stereotaktische Radiotherapie“ der Deutschen Gesellschaft für Radioonkologie (DEGRO) erarbeitete eine Definition der Körperstereotaxie (SBRT), die sich an vorhandene internationale Definitionen anlehnt: Die SBRT ist eine Form der perkutanen Strahlentherapie, die mit hoher Präzision eine hohe Bestrahlungsdosis in einer oder wenigen Bestrahlungsfraktionen in einem extrakraniellen Zielvolumen appliziert. Zur Praxis der SBRT beim nichtkleinzelligen Bronchialkarzinom (NSCLC) im frühen Stadium werden detaillierte Empfehlungen gegeben, die den gesamten Ablauf der Behandlung von der Indikationsstellung, Staging, Behandlungsplanung und Applikation sowie Nachsorge umfassen. Die Körperstereotaxie wurde als Methode der Wahl im Vergleich zu Best Supportive Care, zur konventionell fraktionierten Strahlentherapie sowie zur Radiofrequenzablation identifiziert. Die Ergebnisse nach SBRT und sublobärer Resektion erscheinen auf aktueller Datenbasis ebenbürtig. Die SBRT ist die Methode der Wahl, wenn Patienten einen operativen Eingriff in Form der Lappenresektion ablehnen.

Application of constrained optimization to radiotherapy planning

Essential for the calculation of photon fluence distributions for intensity modulated radiotherapy (IMRT) is the use of a suitable objective function. The objective function should reflect the clinical aims of tumor control and low side effect probability. Individual radiobiological parameters for patient organs are not yet available with sufficient accuracy. Some of the major drawbacks of some current optimization methods include an inability to converge to a solution for arbitrary input parameters, and/or a need for intensive user input in order to guide the optimization. In this work, a constrained optimization method was implemented and tested. It is closely related to the demanded clinical aims, avoiding the drawbacks mentioned above. In a prototype treatment planning system for IMRT, tumor control was guaranteed by setting a lower boundary for target dose. The aim of low complication is fulfilled by minimizing the dose to organs at risk. If only one type of tissue is involved, there is no absolute need for radiobiological parameters. For different organs, threshold dose, relative seriality of the organs or an upper dose limit could be set. All parameters, however, were optional, and could be omitted. Dose-volume constraints were not used, avoiding the possibility of local minima in the objective function. The approach was benchmarked through the simulation of both a head and neck and a lung case. A cylinder phantom with precalculated dose distributions of individual pencil beams was used. The dose to regions at risk could be significantly reduced using at least seven ports of beam incidence. Increasing the number of ports beyond seven produced only minor further gain. The relative seriality of organs was modeled through the use of an added exponent to the dose. This approach however increased calculation time significantly. The alternative of setting an upper limit is much faster and allows direct control of the maximum dose. Constrained optimization guarantees high tumor control probability, it is computationally more efficient than adding penalty terms to the objective function, and the input parameters are dose limits known in clinical practice.

Determination of the quality index (Q) for photon beams at arbitrary field sizes.

PURPOSE A commonly used beam quality index (Q) for high-energy photon beams is the tissue phantom ratio (TPR20,10) for a square field of 10 x 10 cm2 and SDD of 100 cm. On some specialized radiotherapy treatment equipment such a reference collimator setting is not achievable. Likewise a flat beam profile, not explicitly required in dosimetry protocols, but certainly influences the measurement of Q, is not always produced. In this work, a method was developed in order to determine Q at any field size, especially for small and nonflattened beams. METHODS An analytical relationship was derived between TPR20,10 for arbitrary field sizes and Q [the TPR20,10 (10 x 10 cm2)] as quality index. The proposed model equation was fitted to the measured and published data in order to achieve three general fit parameters. The procedure was then tested with published data from TomoTherapy and CyperKnife treatment devices. RESULTS For standard flattened photon fields, the uncertainty in Q measured at any field size using the parameters derived from this study is better than 1%. In flattening-filter free beams, the proposed procedure results in a reliable Q for any field size setting. CONCLUSIONS A method is introduced and successfully tested in order to measure the beam quality under nonstandard conditions. It can be used, e.g., to get energy dependent correction factors as tabulated in dosimetry codes of practice even if standard conditions are not adjustable.

Calculation of dose distributions in the vicinity of high‐Z interfaces for photon beams

In the vicinity of interfaces between materials of different atomic number Z, extremes in absorbed dose occur for high-energy photon irradiations. The spatial extension of the effects is within the range of 1 cm, which may not be ignorable from the radiobiological point of view. At the front side of a high-Z slab a maximum is observed, whereas at the exit side a small buildup zone of the dose occurs, e.g., for a 5 MV beam, in front of a water/iron interface, the enhancement is about 30% of that to the homogeneous medium. The reduction at the back of the iron slab is about 16% for this energy, but vanishes with increasing energy. For high-energy photons this effect is mainly caused by the strong atomic number dependence of the scattering power for secondary electrons. The amount and extent of the scattering effects have been measured for aluminum and for iron slabs embedded in water or PMMA. The experimental data are in good agreement with Monte Carlo calculated values. Therefore the data form a reliable base to test the performance of commonly used treatment planning algorithms. The convolution or superposition method is used to calculate dose distributions. To account for the Z dependence of the scattering and the stopping power of the secondary electrons, corrections are applied to the energy deposition kernels. The boundary crossing of energy deposition kernels can be treated only in an approximate manner. However, the algorithm developed improves the accuracy of the dose calculation in the vicinity of interfaces significantly.

Influence of calculation algorithm on dose distribution in irradiation of non-small cell lung cancer (NSCLC) collapsed cone versus pencil beam.

Purpose:The influence of two different calculation algorithms (“pencil beam” [PB] versus “collapsed cone” [CC]) on dose distribution, as well as the dose-volume histograms (DVHs) of the planning target volume (PTV) and the organs at risk was analyzed for irradiation of lung cancer.Material and Methods:Between 10/2001 and 02/2002 three-dimensional treatment planning was done in ten patients with lung cancer (Helax, TMS®, V.6.01). The PTV, the ipsilateral lung (IL) and the contralateral lung (CL) were defined in each axial CT slice (slice thickness 1 cm). Dose distributions for three-dimensional multiple-field technique were calculated using a PB and a CC algorithm, respectively. Normalization was in accordance with ICRU 50. The DVHs were analyzed relating the minimum, maximum, median and mean dose to the volumes of interest (VOI).Results:Median PTV amounted to 774 cm3. Minimum dose within the PTV was 67.4% for CC and 75.6% for PB algorithm (p = 0.04). Using the CC algorithm, only 76.5% of the PTV was included by the 95% isodose, whereas 90.1% was included when the PB algorithm (p = 0.01) was used. Median volume of IL amounted to 1 953 cm3. Mean dose to IL was 43.0% for CC and 44.0% for PB algorithm (p = 0.02). Median volume of IL within the 80% isodose was 19.6% for CC and 24.1% for PB algorithm (p < 0.01). Median volume of CL amounted to 1 847 cm3. Mean dose to CL was 17.4% for CC and 18.1% for PB algorithm (p < 0.01). Volume of CL within the 80% isodose was 3.3% for CC and 4.1% for PB algorithm (p = 0.03).Conclusion:The CC and PB calculation algorithms result in different dose distributions in case of lung tumors. Particularly the minimum dose to the PTV, which may be relevant for tumor control, is significantly lower for CC. Since it is generally accepted that the CC algorithm describes secondary particle transport more exactly than PB models, the use of the latter should be critically evaluated in the treatment planning of lung cancer.Ziel:Der Einfluss zweier unterschiedlicher Rechenalgorithmen (“pencil beam” [PB] versus “collapsed cone” [CC]) auf die Dosisverteilung sowie die Dosis-Volumen-Histogramme (DVH) des Planungszielvolumens (PTV) und der Risikoorgane wird für die Bestrahlung des Lungenkarzinoms untersucht.Material und Methodik:Zwischen 10/2001 und 02/2002 wurde bei zehn Patienten mit Bronchialkarzinom eine dreidimensionale Bestrahlungsplanung durchgeführt (Helax, TMS®, V.6.01). Das PTV, die ipsilaterale Lunge (IL) und die kontralaterale Lunge (CL) wurden in jeder axialen CT-Schicht definiert (Schichtdicke 1 cm). Die Dosisverteilung für eine Mehrfeldertechnik wurde zunächst unter Verwendung des PB-Algorithmus optimiert. Anschließend wurde die Dosisverteilung der sich dabei ergebenden Bestrahlungspläne unter Beibehaltung der Feldparameter mittels des CC-Algorithmus erneut berechnet. Die Dosis wurde gemäß ICRU 50 normiert. Die DVH von PTV, IL und CL wurden analysiert.Ergebnisse:Das PTV betrug im Median 774 cm3. Die minimale Dosis im PTV war 67,4% für den CC- und 75,6% für den PB-Algorithmus (p = 0,04). Unter Verwendung von CC wurden lediglich 76,5% des PTV von der 95%-Isodose umschlossen, während dies unter Verwendung des PB bei 90,1% der Fall war (p = 0,01). Das mediane Volumen der IL war 1 953 cm3. Die mittlere Dosis in der IL betrug für den CC-Algorithmus 43,0% bzw. für den PB-Algorithmus 44,0% (p = 0,02). Das Volumen der IL innerhalb der 80%-Isodose betrug 19,6% für den CC- und 24,1% für den PB-Algorithmus (p < 0,01). Das mediane Volumen der CL lag bei 1 847 cm3. Die mittlere Dosis im Bereich der CL betrug 17,4% für den CC- und 18,1% für den PB-Algorithmus (p < 0,01). Das Volumen der CL innerhalb der 80%-Isodose war 3,3% für den CC- und 4,1% für den PB-Algorithmus (p = 0,03).Schlussfolgerung:Die Berechnung der Dosisverteilung mit dem CC- bzw. dem PB-Algorithmus führt bei gleicher Feldkonfiguration zu erheblich unterschiedlichen Ergebnissen. Insbesondere die sich dabei ergebende Minimaldosis im Bereich des PTV, welche für die Tumorkontrolle relevant sein kann, ist beim CC-Algorithmus signifikant niedriger. Da der CC-Algorithmus die tatsächlichen Streuungsverhältnisse im Gewebe unterschiedlicher Dichte genauer berücksichtigt als der PB-Algorithmus, sollte die Verwendung des PB-Algorithmus für die Bestrahlungsplanung des Bronchialkarzinoms sehr kritisch beurteilt werden.