Anne Richter

Is a single arc sufficient in volumetric-modulated arc therapy (VMAT) for complex-shaped target volumes?

PURPOSE To compare step-and-shoot intensity-modulated radiotherapy (ss-IMRT) with volumetric-modulated arc therapy (VMAT) for complex-shaped target volumes with a simultaneous integrated boost (SIB). MATERIALS AND METHODS This retrospective planning study was based on 20 patients composed of prostate cancer (n=5), postoperative (n=5) or primary (n=5) radiotherapy for pharyngeal cancer and for cancer of the paranasal sinuses (n=5); a SIB with two or three dose levels was planned in all patients. For each patient, one ss-IMRT plan with direct-machine-parameter optimization (DMPO) and VMAT plans with one to three arcs (SmartArc technique) were generated in the Pinnacle planning system. RESULTS Single arc VMAT improved target coverage and dose homogeneity in radiotherapy for prostate cancer. Two and three VMAT arcs were required to achieve equivalent results compared to ss-IMRT in postoperative and primary radiotherapy for pharyngeal cancer, respectively. In radiotherapy for cancer of the paranasal sinuses, multiarc VMAT resulted in increased spread of low doses to the lenses and decreased target coverage in the region between the orbits. CONCLUSIONS The complexity of the target volume determined whether single arc VMAT was equivalent to ss-IMRT. Multiple arc VMAT improved results compared to single arc VMAT at cost of increased delivery times, increased monitor unites and increased spread of low doses.

Dose–Response Relationship for Image-Guided Stereotactic Body Radiotherapy of Pulmonary Tumors: Relevance of 4D Dose Calculation

PURPOSE To evaluate outcome after image-guided stereotactic body radiotherapy (SBRT) for early-stage non-small-cell lung cancer (NSCLC) and pulmonary metastases. METHODS AND MATERIALS A total of 124 patients with 159 pulmonary lesions (metastases n = 118; NSCLC, n = 41; Stage IA, n = 13; Stage IB, n = 19; T3N0, n = 9) were treated with SBRT. Patients were treated with hypofractionated schemata (one to eight fractions of 6-26 Gy); biologic effective doses (BED) to the clinical target volume (CTV) were calculated based on four-dimensional (4D) dose calculation. The position of the pulmonary target was verified using volume imaging before all treatments. RESULTS With mean/median follow-up of 18/14 months, actuarial local control was 83% at 36 months with no difference between NSCLC and metastases. The dose to the CTV based on 4D dose calculation was closely correlated with local control: local control rates were 89% and 62% at 36 months for >100 Gy and <100 Gy BED (p = 0.0001), respectively. Actuarial freedom from regional and systemic progression was 34% at 36 months for primary NSCLC group; crude rate of regional failure was 15%. Three-year overall survival was 37% for primary NSCLC and 16% for metastases; no dose-response relationship for survival was observed. Exacerbation of comorbidities was the most frequent cause of death for primary NSCLC. CONCLUSIONS Doses of >100 Gy BED to the CTV based on 4D dose calculation resulted in excellent local control rates. This cutoff dose is not specific to the treatment technique and protocol of our study and may serve as a general recommendation.

Potential of image-guidance, gating and real-time tracking to improve accuracy in pulmonary stereotactic body radiotherapy

PURPOSE To evaluate the potential of image-guidance, gating and real-time tumor tracking to improve accuracy in pulmonary stereotactic body radiotherapy (SBRT). MATERIALS AND METHODS Safety margins for compensation of inter- and intra-fractional uncertainties of the target position were calculated based on SBRT treatments of 43 patients with pre- and post-treatment cone-beam CT imaging. Safety margins for compensation of breathing motion were evaluated for 17 pulmonary tumors using respiratory correlated CT, model-based segmentation of 4D-CT images and voxel-based dose accumulation; the target in the mid-ventilation position was the reference. RESULTS Because of large inter-fractional base-line shifts of the tumor, stereotactic patient positioning and image-guidance based on the bony anatomy required safety margins of 12 mm and 9 mm, respectively. Four-dimensional image-guidance targeting the tumor itself and intra-fractional tumor tracking reduced margins to <5 mm and <3 mm, respectively. Additional safety margins are required to compensate for breathing motion. A quadratic relationship between tumor motion and margins for motion compensation was observed: safety margins of 2.4mm and 6mm were calculated for compensation of 10 mm and 20 mm motion amplitudes in cranio-caudal direction, respectively. CONCLUSION Four-dimensional image-guidance with pre-treatment verification of the target position and online correction of errors reduced safety margins most effectively in pulmonary SBRT.

Investigation of the usability of conebeam CT data sets for dose calculation

BackgroundTo investigate the feasibility and accuracy of dose calculation in cone beam CT (CBCT) data sets.MethodsKilovoltage CBCT images were acquired with the Elekta XVI system, CT studies generated with a conventional multi-slice CT scanner (Siemens Somatom Sensation Open) served as reference images. Material specific volumes of interest (VOI) were defined for commercial CT Phantoms (CATPhan® and Gammex RMI®) and CT values were evaluated in CT and CBCT images. For CBCT imaging, the influence of image acquisition parameters such as tube voltage, with or without filter (F1 or F0) and collimation on the CT values was investigated. CBCT images of 33 patients (pelvis n = 11, thorax n = 11, head n = 11) were compared with corresponding planning CT studies. Dose distributions for three different treatment plans were calculated in CT and CBCT images and differences were evaluated. Four different correction strategies to match CT values (HU) and density (D) in CBCT images were analysed: standard CT HU-D table without adjustment for CBCT; phantom based HU-D tables; patient group based HU-D tables (pelvis, thorax, head); and patient specific HU-D tables.ResultsCT values in the CBCT images of the CATPhan® were highly variable depending on the image acquisition parameters: a mean difference of 564 HU ± 377 HU was calculated between CT values determined from the planning CT and CBCT images. Hence, two protocols were selected for CBCT imaging in the further part of the study and HU-D tables were always specific for these protocols (pelvis and thorax with M20F1 filter, 120 kV; head S10F0 no filter, 100 kV). For dose calculation in real patient CBCT images, the largest differences between CT and CBCT were observed for the standard CT HU-D table: differences were 8.0% ± 5.7%, 10.9% ± 6.8% and 14.5% ± 10.4% respectively for pelvis, thorax and head patients using clinical treatment plans. The use of patient and group based HU-D tables resulted in small dose differences between planning CT and CBCT: 0.9% ± 0.9%, 1.8% ± 1.6%, 1.5% ± 2.5% for pelvis, thorax and head patients, respectively. The application of the phantom based HU-D table was acceptable for the head patients but larger deviations were determined for the pelvis and thorax patient populations.ConclusionThe generation of three HU-D tables specific for the anatomical regions pelvis, thorax and head and specific for the corresponding CBCT image acquisition parameters resulted in accurate dose calculation in CBCT images. Once these HU-D tables are created, direct dose calculation on CBCT datasets is possible without the need of a reference CT images for pixel value calibration.

Dose–response relationship for radiation-induced pneumonitis after pulmonary stereotactic body radiotherapy

PURPOSE To evaluate dosimetric factors predictive for radiation-induced pneumonitis (RP) after pulmonary stereotactic body radiotherapy (SBRT). MATERIALS AND METHODS A retrospective analysis was performed based on 59 consecutive patients treated with cone-beam CT-based image-guided SBRT for primary NSCLC (n=21) or pulmonary metastases (n=54). The majority of patients were treated with radiosurgery of 26 Gy to 80% (n=29) or three fractions of 12.5 Gy to 65% (n=40). To correct for different single fraction doses, local doses were converted to 2 Gy equivalent normalized total doses (NTDs) using α/β ratio of 3 Gy for RP. Dose-volume parameters and incidences of RP ≥ grade II SWOG were fitted using NTCP models. RESULTS Eleven patients developed RP grade II. With an average MLD of 10.3±5.6 Gy to the ipsilateral lung, a significant dose-response relationship was observed: the MLD was 12.5±4.3 Gy and 9.9±5.8 Gy for patients with and without development of RP, respectively. Additionally, volumes of the lung exposed to minimum doses between 2.5 and 50 Gy (V(2.5)-V(50)) were correlated with incidences of RP with a continuous decrease of the goodness of fit for higher doses. CONCLUSIONS The MLD and V(2.5)-V(50) of the ipsilateral lung were correlated with incidences of RP after pulmonary SBRT.

Tumor tracking and motion compensation with an adaptive tumor tracking system (ATTS): System description and prototype testing

A novel system for real-time tumor tracking and motion compensation with a robotic HexaPOD treatment couch is described. The approach is based on continuous tracking of the tumor motion in portal images without implanted fiducial markers, using the therapeutic megavoltage beam, and tracking of abdominal breathing motion with optical markers. Based on the two independently acquired data sets the table movements for motion compensation are calculated. The principle of operation of the entire prototype system is detailed first. In the second part the performance of the HexaPOD couch was investigated with a robotic four-dimensional-phantom capable of simulating real patient tumor trajectories in three-dimensional space. The performance and limitations of the HexaPOD table and the control system were characterized in terms of its dynamic behavior. The maximum speed and acceleration of the HexaPOD were 8mm∕s and 34.5mm∕s2 in the lateral direction, and 9.5mm∕s and 29.5mm∕s2 in longitudinal and anterior-posterior direction, respectively. Base line drifts of the mean tumor position of realistic lung tumor trajectories could be fully compensated. For continuous tumor tracking and motion compensation a reduction of tumor motion up to 68% of the original amplitude was achieved. In conclusion, this study demonstrated that it is technically feasible to compensate breathing induced tumor motion in the lung with the adaptive tumor tracking system.

Potential of Adaptive Radiotherapy to Escalate the Radiation Dose in Combined Radiochemotherapy for Locally Advanced Non–Small Cell Lung Cancer

PURPOSE To evaluate the potential of adaptive radiotherapy (ART) for advanced-stage non-small cell lung cancer (NSCLC) in terms of lung sparing and dose escalation. METHODS AND MATERIALS In 13 patients with locally advanced NSCLC, weekly CT images were acquired during radio- (n=1) or radiochemotherapy (n=12) for simulation of ART. Three-dimensional (3D) conformal treatment plans were generated: conventionally fractionated doses of 66 Gy were prescribed to the planning target volume without elective lymph node irradiation (Plan_3D). Using a surface-based algorithm of deformable image registration, accumulated doses were calculated in the CT images acquired during the treatment course (Plan_4D). Field sizes were adapted to tumor shrinkage once in week 3 or 5 and twice in weeks 3 and 5. RESULTS A continuous tumor regression of 1.2% per day resulted in a residual gross tumor volume (GTV) of 49%±15% after six weeks of treatment. No systematic differences between Plan_3D and Plan_4D were observed regarding doses to the GTV, lung, and spinal cord. Plan adaptation to tumor shrinkage resulted in significantly decreased lung doses without compromising GTV coverage: single-plan adaptation in Week 3 or 5 and twice-plan adaptation in Weeks 3 and 5 reduced the mean lung dose by 5.0%±4.4%, 5.6%±2.9% and 7.9%±4.8%, respectively. This lung sparing with twice ART allowed an iso-mean lung dose escalation of the GTV dose from 66.8 Gy±0.8 Gy to 73.6 Gy±3.8 Gy. CONCLUSIONS Adaptation of radiotherapy to continuous tumor shrinkage during the treatment course reduced doses to the lung, allowed significant dose escalation and has the potential of increased local control.

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.

Feasibility Study for Markerless Tracking of Lung Tumors in Stereotactic Body Radiotherapy

PURPOSE To evaluate the feasibility and accuracy of a method for markerless tracking of lung tumors in electronic portal imaging device (EPID) movies and to analyze intra- and interfractional variations in tumor motion. METHODS AND MATERIALS EPID movies were acquired during stereotactic body radiotherapy (SBRT) given to 40 patients with 49 pulmonary targets and retrospectively analyzed. Tumor visibility and tracking accuracy were determined by three observers. Tumor motion of 30 targets was analyzed in detail via four-dimensional computed tomography (4DCT) and EPID in the superior-inferior direction for intra- and interfractional variations. RESULTS Tumor visibility was sufficient for markerless tracking in 47% of the EPID movies. Tumor size and visibility in the DRR were correlated with visibility in the EPID images. The difference between automatic and manual tracking was a maximum of 2 mm for 98.3% in the x direction and 89.4% in the y direction. Motion amplitudes in 4DCT images (range, 0.7-17.9 mm; median, 4.9 mm) were closely correlated with amplitudes in the EPID movies. Intrafractional and interfractional variability of tumor motion amplitude were of similar magnitude: 1 mm on average to a maximum of 4 mm. A change in moving average of more than ±1 mm, ±2 mm, and ±4 mm were observed in 47.1%, 17.1%, and 4.5% of treatment time for all trajectories, respectively. Mean tumor velocity was 3.4 mm/sec, to a maximum 61 mm/sec. CONCLUSIONS Tracking of pulmonary tumors in EPID images without implanted markers was feasible in 47% of all treatment beams. 4DCT is representative of the evaluation of mean breathing motion on average, but larger deviations occurred in target motion between treatment planning and delivery effort a monitoring during delivery.

A multi-institution evaluation of deformable image registration algorithms for automatic organ delineation in adaptive head and neck radiotherapy

BackgroundAdaptive Radiotherapy aims to identify anatomical deviations during a radiotherapy course and modify the treatment plan to maintain treatment objectives. This requires regions of interest (ROIs) to be defined using the most recent imaging data. This study investigates the clinical utility of using deformable image registration (DIR) to automatically propagate ROIs.MethodsTarget (GTV) and organ-at-risk (OAR) ROIs were non-rigidly propagated from a planning CT scan to a per-treatment CT scan for 22 patients. Propagated ROIs were quantitatively compared with expert physician-drawn ROIs on the per-treatment scan using Dice scores and mean slicewise Hausdorff distances, and center of mass distances for GTVs. The propagated ROIs were qualitatively examined by experts and scored based on their clinical utility.ResultsGood agreement between the DIR-propagated ROIs and expert-drawn ROIs was observed based on the metrics used. 94% of all ROIs generated using DIR were scored as being clinically useful, requiring minimal or no edits. However, 27% (12/44) of the GTVs required major edits.ConclusionDIR was successfully used on 22 patients to propagate target and OAR structures for ART with good anatomical agreement for OARs. It is recommended that propagated target structures be thoroughly reviewed by the treating physician.