Ludwig Bogner

Verification of IMRT: Techniques and Problems

Purpose:IMRT (intensity-modulated radiotherapy) verification techniques are reviewed together with investigations demonstrating the intrinsic verification problems.Material and Methods:Different IMRT verification procedures for either class solutions or individual patients are demonstrated. Among the latter are techniques like fluence or three-dimensional (3-D) dose distribution verification within a transfer phantom. Different radiographic films and absolute dose probes are investigated for their suitability. Finally, Monte Carlo techniques (XVMC/VEF) are used for error detection and IMRT verification.Results:During introduction of clinical IMRT for head and neck (H&N) tumors, we concurrently applied fluence, relative, and absolute dose measurement. While fluence and relative dose are in rather good agreement with calculations, absolute dose is always low when compared to the TPS (TMS 6.1A, Nucletron B.V.) by 5–7%. This deviation seems to depend not on the number of segments, but can strongly depend on MLC misalignment. Further investigations have revealed the importance of a detailed commissioning of the TPS down to the small-field range using diamond or diode probes and its detailed verification. In addition, simple tests have shown that dose calculation approximations in the IMRT option of TMS are one major source of the dose deviation. XVMC/VEF does not use such approximations.Conclusion:The procedure starts with a detailed TPS commissioning and verification process. Different verification methods are recommended during clinical IMRT implementation phase, in order to locate sources of error. Later on, a minimal program could consist of a fluence or relative dose verification procedure with few films and absolute dose measurement, followed by an intensive MLC quality assurance (QA). Inverse Monte Carlo systems, like IMCO++/IKO or Hyperion, seem to be able to reduce the effort.Ziel:Neben einem Überblick über Verifikationstechniken der IMRT (intensitätsmodulierten Radiotherapie) werden Untersuchungen zu intrinsischen Verifikationsproblemen dargestellt.Material und Methodik:Verschiedene Verifikationsmethoden für Klassenlösungen sowie für individuelle Patientenpläne werden demonstriert, wie etwa Fluenz- und dreidimensionale Dosisverteilungen in einem Ersatzphantom. Dazu werden verschiedene Radiographiefilme und Absolutdosissonden auf ihre Eignung untersucht. Monte-Carlo-Techniken (XVMC/VEF) werden zur Fehleranalyse eingesetzt und auf ihre Tauglichkeit zur Verifikation untersucht.Ergebnisse:Bei der klinischen Einführung der IMRT bei Kopf-Hals-Tumoren wurden parallel Fluenz-, relativ- und absolutdosimetrische Verfahren zur Verifikation angewandt. Während die Fluenz- und Dosisverteilungen gut mit den Berechnungen übereinstimmen, sind die Absolutdosen systematisch um 5–7% niedriger als die mit dem TPS (TMS 6.1A) berechneten. Die Abweichung scheint nicht von der Gesamtzahl der Segmente des Plans abzuhängen, kann aber relativ stark von einer geringen Abweichung der MLC-Leaves vom Sollwert abhängen. Weitere Untersuchungen zeigen auf, wie wichtig eine detaillierte TPS-Kommissionierung bis in den Bereich kleiner Feldgrößen mit umfangreichen Verifikationen und einfachen Tests ist. Damit konnte ein wichtiger Beitrag zum Dosisfehler, nämlich zusätzliche Näherungen bei der Dosisberechnung von IMRT-Plänen, verantwortlich gemacht werden. XVMC/VEF verwendet keine derartigen Näherungen.Schlussfolgerung:Die Prozedur beginnt mit einer detaillierten TPS-Kommisionierung und Verifikation. Während der klinischen Einführungsphase sollten unterschiedliche Verifikationsverfahren eingesetzt werden, um etwaige Fehler aufzuspüren. Später kann ein Minimalprogramm verwendet werden, das entweder aus Fluenz- oder Relativdosismessungen in Verbindung mit einer Absolutdosisbestimmung besteht. Unabhängig davon sollte eine intensive Qualitätssicherung (QS) des MLCs erfolgen. Inverse Monte- Carlo-Systeme wie IMCO++/IKO oder Hyperion sind vermutlich geeignet, den Aufwand beträchtlich zu reduzieren.

Preliminary study on the use of an inhomogeneous anthropomorphic Fricke gel phantom and 3D magnetic resonance dosimetry for verification of IMRT treatment plans

An inhomogeneous anthropomorphic phantom of the human thorax including lungs and spine was developed for verification of three-dimensional (3D) intensity-modulated radiotherapy (IMRT). The phantom and spinal cord were filled with undiluted Fricke gel, whereas the lungs were filled with a special low-density Fricke gel. Based on a computed tomography scan of the phantom, an intensity-modulated stereotactic radiotherapy plan for a bronchial carcinoma was calculated using an inverse planning system (KonRad, DKFZ Heidelberg, Germany). The plan consisted of seven beams delivered in a step and shoot technique out of 67 sub-fields. Immediately after irradiation 3D magnetic resonance (MR) imaging of the phantom was performed using a special pulse sequence for T1 relaxometry. From the MR image data maps of the longitudinal relaxation rate R1 = 1/T1 were calculated. The R1 maps were converted to dose-proportional image data and compared to planning data. Measurement and planning show good agreement in regions of standard Fricke gel with an average deviation below 5%. In regions of the low-density Fricke gel, deviations are higher due to a decreased signal-to-noise ratio in the MR measurement. In these areas also a different sensitivity of the dose response was observed as compared to standard Fricke gel. The inhomogeneous thorax phantom has proven to be a useful pre-clinical tool for 3D methodical verifications.

A biologically adapted dose-escalation approach, demonstrated for 18F-FET-PET in brain tumors

Purpose:To demonstrate the feasibility of a biologically adapted dose-escalation approach to brain tumors.Material and Methods:Due to the specific accumulation of fluoroethyltyrosine (FET) in brain tumors, 18F-FET-PET imaging is used to derive a voxel-by-voxel dose distribution. Although the kinetics of 18F-FET are not completely understood, the authors regard regions with high tracer uptake as vital and aggressive tumor and use a linear dose-escalation function between SUV (standard uptake value) 3 and SUV 5. The resulting dose distribution is then planned using the inverse Monte Carlo treatment- planning system IKO. In a theoretical study, the dose range is clinically adapted from 1.8 Gy to 2.68 Gy per fraction (with a total of 30 fractions). In a second study, the maximum dose of the model is increased step by step from 2.5 Gy to 3.4 Gy to investigate whether a significant dose escalation to tracer-accumulating subvolumes is possible without affecting the shell-shaped organ at risk (OAR). For all dose-escalation levels the dose difference ΔD of each voxel inside the target volume is calculated and the mean dose difference

Quasi-IMAT Study with Conventional Equipment to Show High Plan Quality with a Single Gantry Arc

\overline{{\Delta D}}

Direct machine parameter optimization for intensity modulated radiation therapy (IMRT) of oropharyngeal cancer – a planning study

and their standard deviation σΔD are determined. The dose to the OAR is evaluated by the dose values

IMRT of prostate cancer: a comparison of fluence optimization with sequential segmentation and direct step-and-shoot optimization.

D^{{OAR}}_{{50\% }}