Construction of a biomechanical head and neck motion model as a guide to evaluation of deformable image registration.

The use of deformable image registration methods in the context of adaptive radiotherapy leads to uncertainties in the simulation of the administered dose distributions during the treatment course. Evaluation of these methods is a prerequisite to decide if a plan adaptation will improve the individual treatment. Current approaches using manual references limit the validity of evaluation, especially for low-contrast regions. In particular, for the head and neck region, the highly flexible anatomy and low soft tissue contrast in control images pose a challenge to image registration and its evaluation. Biomechanical models promise to overcome this issue by providing anthropomorphic motion modelling of the patient. We introduce a novel biomechanical motion model for the generation and sampling of different postures of the head and neck anatomy. Motion propagation behaviour of the individual bones is defined by an underlying kinematic model. This model interconnects the bones by joints and thus is capable of providing a wide range of motion. Triggered by the motion of the individual bones, soft tissue deformation is described by an extended heterogeneous tissue model based on the chainmail approach. This extension, for the first time, allows the propagation of decaying rotations within soft tissue without the necessity for explicit tissue segmentation. Overall motion simulation and sampling of deformed CT scans including a basic noise model is achieved within 30 s. The proposed biomechanical motion model for the head and neck site generates displacement vector fields on a voxel basis, approximating arbitrary anthropomorphic postures of the patient. It was developed with the intention of providing input data for the evaluation of deformable image registration.

Accuracy quantification of a deformable image registration tool applied in a clinical setting.

The purpose of this study was to test the accuracy of a commercially available deformable image registration tool in a clinical situation. In addition, to demonstrate a method to evaluate the resulting transformation of such a tool to a reference defined by multiple experts. For 16 patients (seven head and neck, four thoracic, five abdominal), 30-50 anatomical landmarks were defined on recognizable spots of a planning CT and a corresponding fraction CT. A commercially available deformable image registration tool, Velocity AI, was used to align all fraction CTs with the respective planning CTs. The registration accuracy was quantified by means of the target registration error in respect to expert-defined landmarks, considering the interobserver variation of five observers. The interobserver uncertainty of the landmark definition in our data sets is found to be 1.2 ± 1.1 mm. In general the deformable image registration tool decreases the extent of observable misalignments from 4-8 mm to 1-4 mm for nearly 50% of the landmarks (to 77% in sum). Only small differences are observed in the alignment quality of scans with different tumor location. Smallest residual deviations were achieved in scans of the head and neck region (79%, ≤ 4 mm) and the thoracic cases (79%, ≤ 4 mm), followed by the abdominal cases (59%, ≤ 4 mm). No difference is observed in the alignment quality of different tissue types (bony vs. soft tissue). The investigated commercially available deformable image registration tool is capable of reducing a mean target registration error to a level that is clinically acceptable for the evaluation of retreatment plans and replanning in case of gross tumor change during treatment. Yet, since the alignment quality needs to be improved further, the individual result of the deformable image registration tool has still to be judged by the physician prior to application.

Regional cumulative maximum dose to the spinal cord in head-and-neck cancer: considerations for re-irradiation.

PURPOSE: To present a new method that assesses the delivered maximum dose of different spinal cord sections in head-and-neck cancer treated with intensity-modulated radiation therapy (IMRT). This allows a more accurate estimation of the remaining cord dose tolerance in case of a later re-irradiation treatment planning. MATERIALS AND METHODS: The suggested workflow is demonstrated using daily acquired kilo-voltage control-CTs of four head-and-neck cancer patients (118 control-CTs). The local maximum dose inside different cord levels is determined and accumulated for the planning situation and over the treatment course for an IGRT and a non-IGRT approach. RESULTS: The approach is suitable to accurately detect and document the delivered maximum dose dependent on the cord levels. The delivered maximum dose differed up to 13% from the planned one in all sections due to setup uncertainties and the applied correction strategy. CONCLUSION: The presented approach facilitates later re-irradiation treatment planning due to detailed documentation of the delivered maximum dose to the spinal cord levels in the primary IMRT. The method also facilitates the interpretation of complex 3D dose information by reducing it to its essentials. This 2D illustration is an aid to orientation for the physician in the re-irradiation planning process.

IGRT versus non-IGRT for postoperative head-and-neck IMRT patients: dosimetric consequences arising from a PTV margin reduction.

BACKGROUND: To evaluate the impact of image-guided radiation therapy (IGRT) versus non-image-guided radiation therapy (non-IGRT) on the dose to the clinical target volume (CTV) and the cervical spinal cord during fractionated intensity-modulated radiation therapy (IMRT) for head-and-neck cancer (HNC) patients. MATERIAL AND METHODS: For detailed investigation, 4 exemplary patients with daily control-CT scans (total 118 CT scans) were analyzed. For the IGRT approach a target point correction (TPC) derived from a rigid registration focused to the high-dose region was used. In the non-IGRT setting, instead of a TPC, an additional cohort-based safety margin was applied. The dose distributions of the CTV and spinal cord were calculated on each control-CT and the resulting dose volume histograms (DVHs) were compared with the planned ones fraction by fraction. The D50 and D98 values for the CTV and the D5 values of the spinal cord were additionally reported. RESULTS: In general, the D50 and D98 histograms show no remarkable difference between both strategies. Yet, our detailed analysis also reveals differences in individual dose coverage worth inspection. Using IGRT, the D5 histograms show that the spinal cord less frequently receives a higher dose than planned compared to the non-IGRT setting. This effect is even more pronounced when looking at the curve progressions of the respective DVHs. CONCLUSIONS: Both approaches are equally effective in maintaining CTV coverage. However, IGRT is beneficial in spinal cord sparing. The use of an additional margin in the non-IGRT approach frequently results in a higher dose to the spinal cord than originally planned. This implies that a margin reduction combined with an IGRT correction helps to maintain spinal cord dose sparing best as possible. Yet, a detailed analysis of the dosimetric consequences dependent on the used strategy is required, to detect single fractions with unacceptable dosimetric deviations.

Local setup errors in image-guided radiotherapy for head and neck cancer patients immobilized with a custom-made device.

PURPOSE: To evaluate the local positioning uncertainties during fractionated radiotherapy of head-and-neck cancer patients immobilized using a custom-made fixation device and discuss the effect of possible patient correction strategies for these uncertainties. METHODS AND MATERIALS: A total of 45 head-and-neck patients underwent regular control computed tomography scanning using an in-room computed tomography scanner. The local and global positioning variations of all patients were evaluated by applying a rigid registration algorithm. One bounding box around the complete target volume and nine local registration boxes containing relevant anatomic structures were introduced. The resulting uncertainties for a stereotactic setup and the deformations referenced to one anatomic local registration box were determined. Local deformations of the patients immobilized using our custom-made device were compared with previously published results. Several patient positioning correction strategies were simulated, and the residual local uncertainties were calculated. RESULTS: The patient anatomy in the stereotactic setup showed local systematic positioning deviations of 1-4 mm. The deformations referenced to a particular anatomic local registration box were similar to the reported deformations assessed from patients immobilized with commercially available Aquaplast masks. A global correction, including the rotational error compensation, decreased the remaining local translational errors. Depending on the chosen patient positioning strategy, the remaining local uncertainties varied considerably. CONCLUSIONS: Local deformations in head-and-neck patients occur even if an elaborate, custom-made patient fixation method is used. A rotational error correction decreased the required margins considerably. None of the considered correction strategies achieved perfect alignment. Therefore, weighting of anatomic subregions to obtain the optimal correction vector should be investigated in the future.

Intensity modulated radiotherapy (IMRT) in the treatment of children and adolescents--a single institution's experience and a review of the literature.

BACKGROUND: While IMRT is widely used in treating complex oncological cases in adults, it is not commonly used in pediatric radiation oncology for a variety of reasons. This report evaluates our 9 year experience using stereotactic-guided, inverse planned intensity-modulated radiotherapy (IMRT) in children and adolescents in the context of the current literature. METHODS: Between 1999 and 2008 thirty-one children and adolescents with a mean age of 14.2 years (1.5 - 20.5) were treated with IMRT in our department. This heterogeneous group of patients consisted of 20 different tumor entities, with Ewing's sarcoma being the largest (5 patients), followed by juvenile nasopharyngeal fibroma, esthesioneuroblastoma and rhabdomyosarcoma (3 patients each). In addition a review of the available literature reporting on technology, quality, toxicity, outcome and concerns of IMRT was performed. RESULTS: With IMRT individualized dose distributions and excellent sparing of organs at risk were obtained in the most challenging cases. This was achieved at the cost of an increased volume of normal tissue receiving low radiation doses. Local control was achieved in 21 patients. 5 patients died due to progressive distant metastases. No severe acute or chronic toxicity was observed. CONCLUSION: IMRT in the treatment of children and adolescents is feasible and was applied safely within the last 9 years at our institution. Several reports in literature show the excellent possibilities of IMRT in selective sparing of organs at risk and achieving local control. In selected cases the quality of IMRT plans increases the therapeutic ratio and outweighs the risk of potentially increased rates of secondary malignancies by the augmented low dose exposure.

Quantitative assessment of image-guided radiotherapy for paraspinal tumors.

PURPOSE: To evaluate stereotactic positioning uncertainties of patients with paraspinal tumors treated with fractionated intensity-modulated radiotherapy; and to determine whether target-point correction via rigid registration is sufficient for daily patient positioning. PATIENTS AND METHODS: Forty-five patients with tumors at the cervical, thoracic, and lumbar spine received regular control computed-tomography (CT) scans using an in-room CT scanner. All patients were immobilized with the combination of Scotch cast torso and head masks. The positioning was evaluated regarding translational and rotational errors by applying a rigid registration algorithm based on mutual information. The registration box was fitted to the target volume for optimal registration in the high-dose area. To evaluate the suitability of the rigid registration result for correcting the target volume position we subsequently registered three small subsections of the upper, middle, and lower target volume. The resulting residual deviations reflect the extent of the elastic deformations, which cannot be covered by the rigid-body registration procedure. RESULTS: A total of 321 control CT scans were evaluated. The rotational errors were negligible. Translational errors were smallest for cervical tumors (-0.1 +/- 1.1, 0.3 +/- 0.8, and 0.1 +/- 0.9 mm along left-right, anterior-posterior, and superior-inferior axes), followed by thoracic (0.8 +/- 1.1, 0.3 +/- 0.8, and 1.1 +/- 1.3 mm) and lumbar tumors (-0.7 +/- 1.3, 0.0 +/- 0.9, and 0.5 +/- 1.6 mm). The residual deviations of the three subsections were <1 mm. CONCLUSIONS: The applied stereotactic patient setup resulted in small rotational errors. However, considerable translational positioning errors may occur; thus, on the basis of these data daily control CT scans are recommended. Rigid transformation is adequate for correcting the target volume position.