B. Knäusl

Dosimetric characteristics of 6 and 10 MV unflattened photon beams

PURPOSE To determine dosimetric properties of unflattened megavoltage photon beams. MATERIALS AND METHODS Dosimetric data including depth dose, profiles, output factors and phantom scatter factors from three different beam qualities provided by Elekta Precise linacs, operated with and without flattening filter were examined. Additional measurements of leaf transmission, leakage radiation and surface dose were performed. In flattening filter free (FFF) mode a 6-mm thick copper filter was placed into the beam to stabilize it. RESULTS Depths of dose maxima for flattened and unflattened beams did not deviate by more than 2mm and penumbral widths agreed within 1mm. In FFF mode the collimator exchange effect was found to be on average 0.3% for rectangular fields. Between maximum and minimum field size head scatter factors of unflattened beams showed on average 40% and 56% less variation for 6 and 10MV beams than conventional beams. Phantom scatter factors for FFF beams differed up to 4% from the published reference data. For field sizes smaller than 15cm, surface doses relative to the dose at d(max) increased for unflattened beams with maximum differences of 7% at 6MV and 25% at 10MV for a 5x5cm(2) field. For a 30x30cm(2) field, relative surface dose decreased by about 10% for FFF beams. Leaf transmission on the central axis was 0.3% and 0.4% lower for unflattened 6 and 10MV beams, respectively. Leakage radiation was reduced by 52% for 6MV and by 65% for 10MV unflattened beams. CONCLUSIONS The results of the study were independently confirmed at two radiotherapy centres. Phantom scatter reference data need to be reconsidered for medical accelerators operated without a flattening filter.

PET based volume segmentation with emphasis on the iterative TrueX algorithm.

PURPOSE To assess the influence of reconstruction algorithms for positron emission tomography (PET) based volume quantification. The specifically detected activity in the threshold defined volume was investigated for different reconstruction algorithms as a function of volume size and signal to background ratio (SBR), especially for volumes smaller than 1ml. Special attention was given to the Siemens specific iterative reconstruction algorithm TrueX. METHODS Measurements were performed with a modified in-house produced IEC body phantom on a Siemens Biograph 64 True Point PET/CT scanner (Siemens, Medical Systems) for six different SBRs (2.1, 3.8, 4.9, 6.7, 8.9, 9.4 and without active background (BG)). The phantom consisted of a water-filled cavity with built-in plastic spheres (0.27, 0.52, 1.15, 2.57, 5.58 and 11.49ml). The following reconstruction algorithms available on the Siemens Syngo workstation were evaluated: Iterative OSEM (OSEM) (4 iterations, 21 subsets), iterative TrueX (TrueX) (4 iterations, 21 subsets) and filtered backprojection (FBP). For the threshold based volume segmentation the software Rover (ABX, Dresden) was used. RESULTS For spheres larger than 2.5ml a constant threshold (standard deviation (SD) 10%) level was found for a given SBR and reconstruction algorithm and therefore a mean threshold for the largest three spheres was calculated. This threshold could be approximated by a function inversely proportional to the SBR. The threshold decreased with increasing SBR for all sphere sizes. For the OSEM algorithm the threshold for small spheres with 0.27, 0.52 and 1.15ml varied between 17% and 44% (depending on sphere size). The threshold for the TrueX algorithm was substantially lower (up to 17%) than for the OSEM algorithm for all sphere sizes. The maximum activity in a specific volume yielded the true activity for the OSEM algorithm when using a SBR independent correction factor C, which depended on sphere size. For the largest three volumes a constant factor C=1.10±0.03 was found. For smaller volumes, C increased exponentially due to the partial volume effect. For the TrueX algorithm the maximum activity overestimated the true activity. CONCLUSION The threshold values for PET based target volume segmentation increased with increasing sphere size for all tested algorithms. True activity values of spheres in the phantom could be extracted using experimentally determined correction factors C. The TrueX algorithm has to be used carefully for quantitative comparison (e.g. follow-up) and multicenter studies.

ART for head and neck patients: On the difference between VMAT and IMPT.

Abstract Anatomical changes in the head-and-neck (H&N) region during the course of treatment can cause deteriorated dose distributions. Different replanning strategies were investigated for volumetric modulated arc therapy (VMAT) and intensity-modulated proton therapy (IMPT). Material and methods. For six H&N patients two repeated computed tomography (CT) and magnetic resonance (MR) (CT1/MR1 at week 2 and CT2/MR2 at week 4) scans were acquired additionally to the initial planning CT/MR. Organs-at-risk (OARs) and three targets (CTV70Gy, CTV63Gy, CTV56Gy) were delineated on MRs and transferred to respective CT data set. Simultaneously integrated boost plans were created using VMAT (two arcs) and IMPT (four beams). To assess the need of replanning the initial VMAT and IMPT plans were recalculated on repeated CTs. Furthermore, VMAT and IMPT plans were replanned on the repeated CTs. A Demon algorithm was used for deformable registration of the repeated CTs with the initial CT and utilized for dose accumulation. Total dose estimations were performed to compare ART versus standard treatment strategies. Results. Dosimetric evaluation of recalculated plans on CT1 and CT2 showed increasing OAR doses for both, VMAT and IMPT. The target coverage of recalculated VMAT plans was considered acceptable in three cases, while for all IMPT plans it dropped. Adaptation of the treatment reduced D2% for brainstem by 6.7 Gy for VMAT and by 8 Gy for IMPT, for particular patients. These D2% reductions were reaching 9 Gy and 14 Gy for the spinal cord. ART improved target dose homogeneity, especially for protons, i.e. D2% decreased by up to 8 Gy while D98% increased by 1.2 Gy. Conclusion. ART showed benefits for both modalities. However, as IMPT is more conformal, the magnitude of dosimetric changes was more pronounced compared to VMAT. Large anatomic variations had a severe impact on treatment plan quality for both VMAT and IMPT. ART is justified in those cases irrespective of treatment modalities.

PET image segmentation using a Gaussian mixture model and Markov random fields

BackgroundClassification algorithms for positron emission tomography (PET) images support computational treatment planning in radiotherapy. Common clinical practice is based on manual delineation and fixed or iterative threshold methods, the latter of which requires regression curves dependent on many parameters.MethodsAn improved statistical approach using a Gaussian mixture model (GMM) is proposed to obtain initial estimates of a target volume, followed by a correction step based on a Markov random field (MRF) and a Gibbs distribution to account for dependencies among neighboring voxels. In order to evaluate the proposed algorithm, phantom measurements of spherical and non-spherical objects with the smallest diameter being 8 mm were performed at signal-to-background ratios (SBRs) between 2.06 and 9.39. Additionally 68Ga-PET data from patients with lesions in the liver and lymph nodes were evaluated.ResultsThe proposed algorithm produces stable results for different reconstruction algorithms and different lesion shapes. Furthermore, it outperforms all threshold methods regarding detection rate, determines the spheres’ volumes more accurately than fixed threshold methods, and produces similar values as iterative thresholding. In a comparison with other statistical approaches, the algorithm performs equally well for larger volumes and even shows improvements for small volumes and SBRs. The comparison with experts’ manual delineations on the clinical data shows the same qualitative behavior as for the phantom measurements.ConclusionsIn conclusion, a generic probabilistic approach that does not require data measured beforehand is presented whose performance, robustness, and swiftness make it a feasible choice for PET segmentation.

Robustness of IMPT treatment plans with respect to inter-fractional set-up uncertainties: Impact of various beam arrangements for cranial targets

Abstract In the current study IMPT plan robustness was evaluated with respect to inter-fractional patient positioning for various beam arrangements and two tumor indications in the cranial region. Material and methods. For 14 patients suffering from tumors in the cranial region [skull base (SB; n = 7) and paranasal sinus (PS; n = 7)] the CTV and OARs were delineated. A safety margin of 3 mm was applied to the CTV. A prescribed dose of 2 GyE was planned via three beam arrangements (α, β, γ). Beam arrangement α consisted of lateral opposed fields for both tumor groups while beam arrangement β was optimized according to respective tumor and OAR locations, using two beams only. Beam arrangement γ applied four beams in the SB group and three beams in the PS group. Dose distributions were recalculated subjected to virtual patient translations along the major anatomical axes. The following dosimetric indices were evaluated and compared to original plans: target coverage (TC), target dose homogeneity (HI), CTV median and average dose (Dmedian, Dmean). For OARs near maximum dose and average dose (D2%, Dmean) were evaluated. Results. Dose distributions were distorted after introducing shifts. In the SB group, TC and HI were significantly different for caudal, cranial and anterior shifts for all beam arrangements. For PS patients, all but right shifts differed significantly from the original plans for all beam arrangements, although clinical relevance was not reached for arrangement γ (ΔTC < 1.5%). In general, beam arrangement γ exhibited the least spread of data regarding target indices and was consequently considered the most robust. Dosimetric parameters regarding the brainstem were mostly influenced by shifts along the anterio-posterior axis. Conclusion. For cranial IMPT, set-up uncertainties may lead to pronounced deterioration of dose distributions. According to our investigations, multi-beam arrangements were dosimetrically more robust and hence preferable over two beam arrangements.

Novel radiotherapy techniques for involved-field and involved-node treatment of mediastinal Hodgkin lymphoma

PurposeHodgkin lymphoma (HL) is a highly curable disease. Reducing late complications and second malignancies has become increasingly important. Radiotherapy target paradigms are currently changing and radiotherapy techniques are evolving rapidly.DesignThis overview reports to what extent target volume reduction in involved-node (IN) and advanced radiotherapy techniques, such as intensity-modulated radiotherapy (IMRT) and proton therapy–compared with involved-field (IF) and 3D radiotherapy (3D-RT)– can reduce high doses to organs at risk (OAR) and examines the issues that still remain open.ResultsAlthough no comparison of all available techniques on identical patient datasets exists, clear patterns emerge. Advanced dose-calculation algorithms (e.g., convolution-superposition/Monte Carlo) should be used in mediastinal HL. INRT consistently reduces treated volumes when compared with IFRT with the exact amount depending on the INRT definition. The number of patients that might significantly benefit from highly conformal techniques such as IMRT over 3D-RT regarding high-dose exposure to organs at risk (OAR) is smaller with INRT. The impact of larger volumes treated with low doses in advanced techniques is unclear. The type of IMRT used (static/rotational) is of minor importance. All advanced photon techniques result in similar potential benefits and disadvantages, therefore only the degree-of-modulation should be chosen based on individual treatment goals. Treatment in deep inspiration breath hold is being evaluated. Protons theoretically provide both excellent high-dose conformality and reduced integral dose.ConclusionFurther reduction of treated volumes most effectively reduces OAR dose, most likely without disadvantages if the excellent control rates achieved currently are maintained. For both IFRT and INRT, the benefits of advanced radiotherapy techniques depend on the individual patient/target geometry. Their use should therefore be decided case by case with comparative treatment planning.ZusammenfassungHintergrund und ZielDas Hodgkin-Lymphom (HL) ist eine Erkrankung mit hohen Heilungsraten. Die Verringerung von Spätkomplikationen und Zweittumoren wird daher immer wichtiger. Zielvolumenkonzepte der Strahlentherapie (RT) verändern sich gegenwärtig und Strahlentherapietechniken entwickeln sich sehr schnell weiter.MethodenDiese Übersichtsarbeit stellt dar, inwiefern die Zielvolumenreduktion hin zum Involved-node(IN)-Konzept und hochentwickelte Strahlentherapietechniken wie die intensitätsmodulierte Strahlentherapie (IMRT) und Protonentherapie, im Vergleich zu Involved-field(IF)-Konzept und 3-D-konformaler Strahlentherapie (3D-RT), die Belastung von Risikoorganen (OAR) mit hohen Dosen reduzieren können und welche Fragen in diesem Kontext noch geklärt werden müssen.ErgebnisseObwohl kein Vergleich aller verfügbaren Techniken auf identischen Patientendatensätzen existiert, entsteht folgendes Bild: Fortgeschrittene Dosisberechnungsalgorithmen (z. B. convolution-superposition/Monte Carlo) sollten im Rahmen der Behandlung des mediastinalen HL zur Anwendung kommen. INRT reduziert unter allen Bedingungen die behandelten Volumina im Vergleich zur IFRT, wobei die Höhe des Vorteils von der jeweiligen INRT-Definition abhängt. Die Anzahl der Patienten, die deutlich von hochkonformalen Techniken wie IMRT gegenüber der 3D-RT hinsichtlich der OAR-Belastung profitiert, ist bei INRT geringer. Die Konsequenz größerer Volumina, die bei modernen Techniken mit eher isotroper Strahlanordnung mit niedrigen Dosen belastet werden, ist unklar. Die Art der verwendeten IMRT-Technik (statisch/Rotation) ist von geringer Relevanz. Alle fortgeschrittenen Photonentechniken resultieren in den gleichen Vorteilen und Nachteilen. Daher muss nur die Modulationstiefe abhängig von den individuellen Behandlungszielen gewählt werden. Die Bestrahlung in tiefer Inspiration wird gegenwärtig evaluiert. Protonentherapie kann theoretisch bei hervorragender Hochdosiskonformalität die applizierte Integraldosis reduzieren.SchlussfolgerungDie weitere Verkleinerung der behandelten Volumina reduziert die Risikoorganbelastung am effektivsten und ohne Nachteile, wenn die gegenwärtig exzellenten Kontrollraten weiterhin erreicht werden können. Sowohl für IFRT als auch INRT hängen Vor- und Nachteile der modernen Strahlentherapietechniken von der individuellen Patientengeometrie ab. Die Entscheidung für die jeweils anzuwendende Technik sollte daher Fall für Fall auf Basis einer vergleichenden Bestrahlungsplanung getroffen werden.

Can particle beam therapy be improved using helium ions? - a planning study focusing on pediatric patients.

Abstract Aim To explore the potential of scanned helium ion beam therapy (4He) compared to proton therapy in a comparative planning study focusing on pediatric patients. This was motivated by the superior biological and physical characteristics of 4He. Material and methods For eleven neuroblastoma (NB), nine Hodgkin lymphoma (HL), five Wilms tumor (WT), five ependymoma (EP) and four Ewing sarcoma (EW) patients, treatment plans were created for protons and 4He. Dose prescription to the planning target volume (PTV) was 21 Gy [relative biological effectiveness (RBE)] (NB), 19.8 Gy (RBE) (HL), 25.2 Gy (RBE) for the WT boost volume and 54 Gy (RBE) for EP and EW patients. A pencil beam algorithm for protons (constant RBE = 1.1) and 4He was implemented in the treatment planning system Hyperion. For 4He the relative biological effectiveness (RBE) was calculated with a ‘zonal’ model based on different linear energy transfer regions. Results Target constraints were fulfilled for all indications. For NB patients differences for kidneys and liver were observed for all dose-volume areas, except the high-dose volume. The body volume receiving up to 12.6 Gy (RBE) was reduced by up to 10% with 4He. For WT patients the mean and high-dose volume for the liver was improved when using 4He. For EP normal tissue dose was reduced using 4He with 12.7% of the voxels receiving higher doses using protons. For HL and EW sarcoma patients the combination of large PTV volumes with the position of the organs at risk (OARs) obliterated the differences between the two particle species, while patients with the heart close to the PTV could benefit from 4He. Conclusion Treatment plan quality improved with 4He compared to proton plans, but advantages in OAR sparing were depending on indication and tumor geometries. These first results of scanned 4He therapy motivate comprehensive research on 4He, including acquisition of experimental data to improve modeling of 4He.

Influence of PET reconstruction parameters on the TrueX algorithm. A combined phantom and patient study.

UNLABELLED With the increasing use of functional imaging in modern radiotherapy (RT) and the envisaged automated integration of PET into target definition, the need for reliable quantification of PET is growing. Reconstruction algorithms in new PET scanners employ point-spread-function (PSF) based resolution recovery, however, their impact on PET quantification still requires thorough investigation. PATIENTS, MATERIAL, METHODS Measurements were performed on a Siemens PET/CT using an IEC phantom filled with varying activity. Data were reconstructed using the OSEM (Gauss filter) and the PSF TrueX (Gauss and Allpass filter) algorithm with all available products of iterations (i) and subsets (ss). The recovery coeffcient (RC) and threshold defining the real sphere volume were determined for all settings and compared to the clinical standard (4i21ss). PET acquisitions of eight lung patients were reconstructed using all algorithms with 4i21ss. Volume size and tracer uptake were determined with different segmentation methods. RESULTS The threshold for the TrueX was lower (up to 40%) than for the OSEM. The RC for the different algorithms and filters varied. TrueX was more sensitive to permutations of i and ss and only the RC of the OSEM stabilised with increasing number. For patient scans the difference of the volume and activity between TrueX and OSEM could be reduced by applying an adapted threshold and activity correction. CONCLUSION The TrueX algorithm results in excellent diagnostic image quality, however, guidelines for native algorithms have to be extended for PSF based reconstruction methods. For appropriate tumour delineation, for the TrueX a lower threshold than the 42% recommended for the OSEM is necessary. These filter dependent thresholds have to be verified for different scanners prior to using them in multicenter trials.

Feasibility of dominant intraprostatic lesion boosting using advanced photon-, proton- or brachytherapy

BACKGROUND AND PURPOSE Advancements in imaging and dose delivery enable boosting of the dominant intraprostatic lesions (DIL), while maintaining organs-at-risk (OAR) tolerances. This study aimed to assess the feasibility of DIL boosting for volumetric modulated arc therapy (VMAT), intensity modulated proton therapy (IMPT) and high dose rate brachytherapy (HDR-BT). MATERIAL AND METHODS DILs were defined on multiparametric magnetic resonance imaging and fused with planning CT for twelve patients. VMAT, IMPT and HDR-BT plans were created for each patient with an EQD2(α/β) DIL aimed at 111.6 Gy, PTV(initial) D(pres) was 80.9 Gy (EBRT) with CTV D90%=81.9 Gy (HDR-BT). Hard dose constraints were applied for OARs. RESULTS Higher boost doses were achieved with IMPT compared to VMAT, keeping major OAR doses at similar levels. Patient averaged EQD2(α/β) D50% to DIL were 110.7, 114.2 and 150.1 Gy(IsoE) for VMAT, IMPT and HDR-BT, respectively. Respective rectal wall D(mean) were 30.5±5.0, 16.7±3.6, 9.5±2.5 Gy(IsoE) and bladder wall D(mean) were 21.0±5.5, 15.6±4.3 and 6.3±2.2 Gy(IsoE). CONCLUSIONS DIL boosting was found to be feasible with all investigated techniques. Although OAR doses were higher than for standard treatment approach, the risk levels were reasonably low. HDR-BT was superior to VMAT and IMPT, both in terms of OAR sparing and DIL boosting.

Novel radiotherapy techniques for involved-field and involved-node treatment of mediastinal Hodgkin lymphoma: when should they be considered and which questions remain open?

PurposeHodgkin lymphoma (HL) is a highly curable disease. Reducing late complications and second malignancies has become increasingly important. Radiotherapy target paradigms are currently changing and radiotherapy techniques are evolving rapidly.DesignThis overview reports to what extent target volume reduction in involved-node (IN) and advanced radiotherapy techniques, such as intensity-modulated radiotherapy (IMRT) and proton therapy–compared with involved-field (IF) and 3D radiotherapy (3D-RT)– can reduce high doses to organs at risk (OAR) and examines the issues that still remain open.ResultsAlthough no comparison of all available techniques on identical patient datasets exists, clear patterns emerge. Advanced dose-calculation algorithms (e.g., convolution-superposition/Monte Carlo) should be used in mediastinal HL. INRT consistently reduces treated volumes when compared with IFRT with the exact amount depending on the INRT definition. The number of patients that might significantly benefit from highly conformal techniques such as IMRT over 3D-RT regarding high-dose exposure to organs at risk (OAR) is smaller with INRT. The impact of larger volumes treated with low doses in advanced techniques is unclear. The type of IMRT used (static/rotational) is of minor importance. All advanced photon techniques result in similar potential benefits and disadvantages, therefore only the degree-of-modulation should be chosen based on individual treatment goals. Treatment in deep inspiration breath hold is being evaluated. Protons theoretically provide both excellent high-dose conformality and reduced integral dose.ConclusionFurther reduction of treated volumes most effectively reduces OAR dose, most likely without disadvantages if the excellent control rates achieved currently are maintained. For both IFRT and INRT, the benefits of advanced radiotherapy techniques depend on the individual patient/target geometry. Their use should therefore be decided case by case with comparative treatment planning.ZusammenfassungHintergrund und ZielDas Hodgkin-Lymphom (HL) ist eine Erkrankung mit hohen Heilungsraten. Die Verringerung von Spätkomplikationen und Zweittumoren wird daher immer wichtiger. Zielvolumenkonzepte der Strahlentherapie (RT) verändern sich gegenwärtig und Strahlentherapietechniken entwickeln sich sehr schnell weiter.MethodenDiese Übersichtsarbeit stellt dar, inwiefern die Zielvolumenreduktion hin zum Involved-node(IN)-Konzept und hochentwickelte Strahlentherapietechniken wie die intensitätsmodulierte Strahlentherapie (IMRT) und Protonentherapie, im Vergleich zu Involved-field(IF)-Konzept und 3-D-konformaler Strahlentherapie (3D-RT), die Belastung von Risikoorganen (OAR) mit hohen Dosen reduzieren können und welche Fragen in diesem Kontext noch geklärt werden müssen.ErgebnisseObwohl kein Vergleich aller verfügbaren Techniken auf identischen Patientendatensätzen existiert, entsteht folgendes Bild: Fortgeschrittene Dosisberechnungsalgorithmen (z. B. convolution-superposition/Monte Carlo) sollten im Rahmen der Behandlung des mediastinalen HL zur Anwendung kommen. INRT reduziert unter allen Bedingungen die behandelten Volumina im Vergleich zur IFRT, wobei die Höhe des Vorteils von der jeweiligen INRT-Definition abhängt. Die Anzahl der Patienten, die deutlich von hochkonformalen Techniken wie IMRT gegenüber der 3D-RT hinsichtlich der OAR-Belastung profitiert, ist bei INRT geringer. Die Konsequenz größerer Volumina, die bei modernen Techniken mit eher isotroper Strahlanordnung mit niedrigen Dosen belastet werden, ist unklar. Die Art der verwendeten IMRT-Technik (statisch/Rotation) ist von geringer Relevanz. Alle fortgeschrittenen Photonentechniken resultieren in den gleichen Vorteilen und Nachteilen. Daher muss nur die Modulationstiefe abhängig von den individuellen Behandlungszielen gewählt werden. Die Bestrahlung in tiefer Inspiration wird gegenwärtig evaluiert. Protonentherapie kann theoretisch bei hervorragender Hochdosiskonformalität die applizierte Integraldosis reduzieren.SchlussfolgerungDie weitere Verkleinerung der behandelten Volumina reduziert die Risikoorganbelastung am effektivsten und ohne Nachteile, wenn die gegenwärtig exzellenten Kontrollraten weiterhin erreicht werden können. Sowohl für IFRT als auch INRT hängen Vor- und Nachteile der modernen Strahlentherapietechniken von der individuellen Patientengeometrie ab. Die Entscheidung für die jeweils anzuwendende Technik sollte daher Fall für Fall auf Basis einer vergleichenden Bestrahlungsplanung getroffen werden.