Die vorliegende retrospektive Studie zur postoperativen Radiochemothera-pie bei fortgeschrittenen Kopf-Hals-Tumoren sollte die eigenen Ergebnisse mit den prospektiv-randomisierten Studien vergleichend darlegen und dabei den Nutzen einer Radiochemotherapie überprüfen. Insgesamt wurden 155 Patienten in der retrospektiven Analyse eingeschlossen. Die Überlebens- und Rezidivraten des Patientengutes konnten anlehnend zu den publizier-ten Studien reproduziert werden. Ein Vorteil der Radiochemotherapie in Bezug nehmend auf den posttherapeutischen Verlauf konnte hierbei nicht festgestellt werden. Es traten signifikant vermehrt höhergradige Akutne-benwirkungen nach Applizierung der simultanen, systemischen Therapie auf. Die Arbeit konnte zeigen, dass sich durch die Reduzierung der Gesamt-behandungszeit als auch des Intervalls zwischen Operation und Beginn der adjuvanten Therapie das Gesamtüberleben sowie die lokoregionäre Rezidiv-rate signifikant verbessern ließen. Insgesamt scheinen die Fernmetastasie-rungen und die lokoregionären Rezidive maßgebend für die immer noch un-befriedigenden Überlebensraten zu sein. Gegenstand weiterer Untersu-chungen sollte daher die Optimierung der prätherapeutischen Diagnostik sowie der adjuvanten Therapie sein.

Treatment delivery verification is an essential step of radiotherapy. The purpose of this thesis is to develop new methods to improve the verification of photon and electron beam radiotherapy treatments. This is achieved through developing and testing (1) a way to acquire portal images during electron beam treatments, (2) a method to reconstruct the dose delivered to patients during photon beam treatments and (3) a technique to improve image quality in kilovoltage (kV) cone beam computed tomography (CBCT) by correcting for scattered radiation. The portal images were acquired using the Varian CL21EX linac and the Varian aS500 electronic portal imaging device (EPID). The EGSnrc code was used to model fully the CL21EX, the aS500 and the kV CBCT system. / We demonstrate that portal images of electron beam treatments with adequate contrast and resolution can be produced using the bremsstrahlung photons portion of the electron beam. Monte Carlo (MC) calculations were used to characterize the bremsstrahlung photons and to obtain predicted images of various phantoms. The technique was applied on a head and neck patient. / An algorithm to reconstruct the dose given to patients during photon beam radiotherapy was developed and validated. The algorithm uses portal images and MC simulations. The primary fluence at the detector is back-projected through the patient. CT geometry to obtain a reconstructed phase space file. The reconstructed phase space file is used to calculate the reconstructed dose to the patient using MC simulations. The reconstruction method was validated in homogeneous and heterogeneous phantoms for conventional and IMRT fields. / The scattered radiation present in kV CBCT images was evaluated using MC simulations. Simulated predictions of the scatter distribution were subtracted from CBCT projection images prior to the reconstruction to improve the reconstructed image quality. Reducing the scattered radiation was found to improve contrast and reduce shading artifacts. / MC simulations, in combination with experimental techniques, have been shown to be valuable tools in the development of treatment verification methods. The three novel methods presented in this thesis contribute to the improvement of radiotherapy treatment verification. They can potentially improve treatment outcome by ensuring a better target coverage.

Chemical Vapour Deposition (CVD) diamond detectors were modelled for dosimetry of radiotherapy beams. This was achieved by employing the EGSnrc Monte Carlo (MC) method to investigate certain properties of the detector, such as size, shape and electrode materials. Simulations were carried out for a broad 6 MV photon beam, and water phantoms with both uniform and non-uniform voxel dimensions. A number of critical MC parameters were investigated for the development of a model that can simulate very small voxels. For a given number of histories (100 million), combinations of the following parameters were analyzed: cross section data, boundary crossing algorithm and the HOWFARLESS option, with the rest of the transport parameters being kept at default values. The MC model obtained with the optimized parameters was successfully validated against published data for a 1.25 MeV photon beam and CVD diamond detector with silver/carbon/silver structure with thicknesses of 0.07/0.2/0.07 cm for the electrode/detector/electrode, respectively. The interface phenomena were investigated for a 6 MV beam by simulating different electrode materials: aluminium, silver, copper and gold for perpendicular and parallel detector orientation with regards to the beam. The smallest interface phenomena were observed for parallel detector orientation with electrodes made of the lowest atomic number material, which was aluminium. The simulated percentage depth dose and beam profiles were compared with experimental data. The best agreement between simulation and measurement was achieved for the detector in parallel orientation and aluminium electrodes, with differences of approximately 1%. In summary, investigations related to the CVD diamond detector modelling revealed that the EGSnrc MC code is suitable for simulation of small size detectors. The simulation results are in good agreement with experimental data and the model can now be used to assist with the design and construction of prototype diamond detectors for clinical dosimetry. Future work will include investigating the detector response for different energies, small field sizes, different orientations other than perpendicular and parallel to the beam, and the influence of each electrode on the absorbed dose.

This thesis details an investigation of the suitability of commercially-available single crystal and polycrystalline diamond films made via chemical vapor deposition (CVD) that were not studied previously for use in radiotherapy dosimetry. Novel sandwich-type detectors were designed and constructed to investigate the dosimetric response of diamond films under clinical conditions. Relatively inexpensive diamond films were obtained from three manufacturers: Diamonex, Diamond Materials GmbH and Element Six. Spectrophotometry, Raman spectroscopy and bulk conductivity studies were used to characterize these films and correlate crystalline quality with detector performance. Novel detectors were designed and constructed to investigate detectors under clinical conditions, including Perspex encapsulations and PCBs to minimize fluence perturbations. The dosimetric response of these diamond detectors was examined using a 6 MV beam from a Varian Clinac 600C linear accelerator. Diamond detectors were evaluated by measuring a number of response characteristics. Polycrystalline CVD diamond films from Diamonex (100, 200, 400-μm thicknesses) were considered unsuitable for dosimetric applications due to their lack of stability, low sensitivity, high leakage currents, high priming dose and dependence on dose rate. High-quality polycrystalline diamond films from Diamond Materials (100, 200, 400-μm thicknesses) displayed characteristics that varied with film thickness. A 100-μm film featured slow response dynamics and high priming doses. Thicker films featured suitable dosimetric characteristics, e.g. negligible leakage currents, low priming doses, fast response dynamics and good sensitivity with small sensitive volumes. Element Six single crystal CVD diamond films (500-μm thicknesses) with small sensitive volumes (0.39 mm³) exhibited suitable characteristics for dosimetry. These films showed negligible leakage currents (< 1.25 pA), low priming doses (1–10 Gy), quick response dynamics, high sensitivity (47–230 nC Gy⁻¹) and were weakly dependent on dose rate and directional dependence (±1%). A relatively inexpensive single crystal CVD diamond film from Element Six that exhibited high sensitivity (230 nC Gy⁻¹ at 0.5 V μm⁻¹), amongst other favourable characteristics, was selected for further analyses. An appropriate operating voltage was determined before further clinically relevant measurements could be conducted. This included how changes in an applied electric field affected detector response, and determined whether an optimal operating voltage could be realized within the parameters of conventional instrumentation used in radiation therapy. The results of this study indicated a preference towards using 62.5 V (at ~0.13 V μm⁻¹) out of a range of 30.8–248.0 V for temporal response as required for modulated beams due to its minimal rise time (2 s) and fall time (2 s) yet sufficient sensitivity (37 nC Gy⁻¹) and weak dependence on polarity (±1.5%). Investigations were then performed on the same diamond detector to evaluate its performance under more clinically relevant conditions. Repeatability experiments revealed a temporary loss in sensitivity due to charge detrapping effects following irradiation, which was modelled to make corrections that improved short-term precision. It was shown that this detector could statistically distinguish between dose values separated by a single Monitor Unit, which corresponded to 0.77 cGy. Dose rate dependence was observed when using low, fixed doses in contrast to using stabilized currents and higher doses. Depth dose measurements using this detector compared well with ion chambers and diode dosimeters. Comparisons of initial measurements with values in the literature indicate encouraging results for fields sizes < 4 x 4 cm², but further measurements and comparisons with Monte Carlo calculations are required. Using this detector to make off-axis measurements in the edge-on orientation reduced perturbation of the beam due to its sandwich configuration and thin 150 nm Ag contacts. This diamond detector was found to be suitable for routine dosimetry with conventional radiotherapy instrumentation with a materials cost of < NZ$200.

An approach to quantitatively determine patient surface contours as part of an augmented reality (AR) system for patient position and posture correction was developed. Quantitative evaluation of the accuracy of patient positioning and posture correction requires the knowledge of coordinates of the patient contour. The system developed uses the surface contours from the planning CT data as the reference surface coordinates. The corresponding reference point cloud is displayed on screen to enable AR assisted patient positioning. A 3D computer vision system using structured light then captures the current 3D surface of the patient. The offset between the acquired surface and the reference surface, representing the desired patient position, is the alignment error. Two codification strategies, spatial encoding, and temporal encoding, were examined. Spatial encoding methods require a single static pattern to work, thus enabling dynamic scenes to be captured. Temporal encoding methods require a set of patterns to be successively projected onto the object, the encoding for each pixel is only complete when the entire series of patterns has been projected. The system was tested on a camera tracking object. The structured light reconstruction was accurate to within ±1 mm, ±1.5 mm, and ±4 mm in x, y, and z-directions (camera optical axis) respectively. The method was integrated into a simplified AR system and a visualization scheme based on z-direction offset was developed. A demonstration of how the final AR-3D vision hybrid system can be used in a clinical situation was given using an anatomical teaching phantom. The system and visualisation worked well and demonstrated the proof of principal of the approach. It was found that the achieved accuracy was not yet sufficient for clinical use. Further work on improving the projector calibration accuracy is required. Both the camera registration process and 3D computer vision using structured light have been shown to be capable of sub-millimeter accuracy on their own. If that level of accuracy can be reproduced in this system, the concept presented can potentially be used in Oncology departments as a cost-effective patient setup guidance system for external beam radiotherapy, used in addition to current laser/portal imaging/cone beam CT based setup procedures.

The dosimetric properties of fast-neutron beams with energies ≤80 MeV were explored using Monte Carlo techniques. Taking into account transport of all relevant types of released charged particles (electrons, protons, deuterons, tritons, 3He and α particles) pencil-beam dose distributions were derived and used to calculate absorbed dose distributions. Broad-beam depth doses in phantoms of different materials were calculated and compared and the scaling factors required for converting absorbed dose in one material to absorbed dose in another derived. The scaling factors were in good agreement with available published data and show that water is a good substitute for soft tissue even at neutron energies as high as 80 MeV. The inherent penumbra and the fraction of absorbed dose due to photon interactions were also studied, and found to be consistent with measured values reported in the literature. Treatment planning in fast-neutron therapy is commonly performed using dose calculation algorithms designed for photon beam therapy. When applied to neutron beams, these algorithms have limitations arising from the physical models used. Monte Carlo derived neutron pencil-beam kernels were parameterized and implemented in the photon dose calculation algorithms of the TMS (MDS Nordion) treatment planning system. It was shown that these algorithms yield good results in homogeneous water media. However, the method used to calculate heterogeneity corrections in the photon dose calculation algorithm did not yield correct results for neutron beams in heterogeneous media. To achieve results with adequate accuracy using Monte Carlo simulations, fundamental cross-section data are needed. Neutron cross-sections are still not sufficiently well known. At the The Svedberg Laboratory in Uppsala, Sweden, an experimental facility has been designed to measure neutron-induced charged-particle production cross-sections for (n,xp), (n,xd), (n,xt), (n,x3He) and (n,xα) reactions at neutron energies up to 100 MeV. Depending on neutron energy, these generated particles account for up to 90% of the absorbed dose. In experimental determination of the cross-sections, measured data have to be corrected for the energies lost by the charged particles before leaving the target in which they were generated. To correct for the energy-losses, a computational code (CRAWL) was developed. It uses a stripping method. With the limitation of reduced energy resolution, spectra derived using CRAWL compares well with those derived using other methods. In fast-neutron therapy, the relative biological effectiveness (RBE) varies from 1.5 to 5, depending on neutron energy, dose level and biological end-point. LET and other physical quantities, developed within the field of microdosimetry over the past couple of decades, have been used to describe RBE variations between different fast-neutron beams as well as within a neutron irradiated body. In this work, a Monte Carlo code (SHIELD-HIT) capable of transporting all charged particles contributing to absorbed dose, was used to calculate energy-differential charged particle spectra. Using these spectra, values of the RBE related quantities LD, γD, γ* and R were derived and studied as function of neutron energy, phantom material and position in a phantom. Reasonable agreement with measured data in the literature was found and indicates that the quantities may be used to predict RBE variations in an arbitrary fast-neutron beam.

Purpose: At the Wellington Blood and Cancer Centre (WBCC), Volumetric Modulated Arc Therapy (VMAT) is used to treat a variety of head and neck (H&N) cancers. Presently, the complexity of plans is limited to ensure the accuracy of patient treatment within the range of the departmental experience. The complexity limitation is applied through use of a monitor unit (MU) constraint during plan optimisation. Plans of higher complexity can be obtained by loosening the MU constraint, and setting more stringent optimisation objectives on organs at risk (OAR) and target volumes (PTV). This could potentially yield higher quality treatment plans but may also degrade the accuracy of the TPS calculation or the plan delivery at the treatment machine. The aim of this study is to investigate the level of plan complexity that results in accurate treatment plan calculation and delivery, and quantify the corresponding gain in plan quality. Methods: Five previously treated H&N patients were selected for the study. Each patient’s clinical plan was used as the lowest complexity level and labelled C1. Subsequently, an approximate pareto-optimal plan (C3) was created that focused equally on sparing spinal cord, brain stem and parotid gland while maintaining, or improving on, the previously obtained target coverage. Next, a C2 plan was created such that the plan quality was in between C1 and C3. Plan quality of each complexity level was assessed in terms of OAR sparing and PTV coverage. The average leaf pair opening (LPO), critical leaf pair opening (%LPO<1cm) and mean leaf travel were used as plan complexity metrics. The calculation and delivery accuracy of each complexity level using Varian TrueBeam LINAC/Eclipse TPS was verified using time resolved point dose measurements (TRPD), EBT film measurements (Ashland Inc.) and ArcCheck measurements (Sun Nuclear Corp.). A comprehensive uncertainty analysis was carried out including a quantification of the measurement and delivery reproducibility. Results: Increasing plan complexity from C1 to C3 reduced the Spinal Cord D1cc, Brain Stem D1 and Parotid Gland Dmean up to 14.7 Gy, 7.1 Gy and 7.8 Gy, respectively. In addition, C3 plans improved the target coverage compared to C1 plans, with the PTV66 and PTV54 D98 increasing up to 1.0 Gy and 0.6 Gy, respectively. The verification measurements showed that the plan calculation and delivery for all complexity levels was well within clinical acceptance levels (Table 1). TRPD showed that VMAT dose delivery itself was repeatable within 0.1% (1 S.D.) over 10 consecutive deliveries for both C1 and C3 complexity levels. Discussion & Conclusions: This study has shown that increasing the plan complexity can provide significant dosimetric advantages for the treatment of H&N cancer. Verification measurement results indicated that this did not noticeably degrade the calculation and delivery accuracy of VMAT using a Varian TrueBeam LINAC and our Eclipse TPS beam model. H&N VMAT at the WBCC can now be developed further with greater confidence in the dosimetric accuracy of higher complexity plans.

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Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

An uncommon case with semi-spheric perivesical calcification after pelvic radiotherapy is reported and the possible pathogenesis of this phenomenon is discussed. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.