Dosimetric effects of respiratory motion during stereotactic body radiation therapy of lung tumors.

BACKGROUND: Respiratory-induced lung tumor motion may affect the delivered dose in stereotactic body radiation therapy (SBRT). Previous studies are often based on phantom studies for one specific treatment technique. In this study, the dosimetric effect of tumor motion was quantified in real patient geometries for different modulated treatments and tumor motion amplitudes for lung-SBRT. MATERIAL AND METHODS: A simulation method using deformable image registrations and 4-dimensional computed tomographies (4DCT) was developed to assess the dosimetric effects of tumor motion. The method was evaluated with ionization chamber and Gafchromic film measurements in a thorax phantom and used to simulate the effect for 15 patients with lung tumors moving 7.3-27.4 mm. Four treatment plans with different complexities were created for each patient and the motion-induced dosimetric effect to the gross tumor volume (GTV) was simulated. The difference between the planned dose to the static tumor and the simulated delivered dose to the moving tumor was quantified for the near minimum (D98%), near maximum (D2%) and mean dose (Dmean) to the GTV as well as the largest observed local difference within the GTV (Maxdiff). RESULTS: No correlation was found between the dose differences and the tumor motion amplitude or plan complexity. However, the largest deviations were observed for tumors moving >15.0 mm. The simulated delivered dose was within 2.5% from the planned dose for D98% (tumors moving <15 mm) and within 3.3% (tumors moving >15 mm). The corresponding values were 1.7% vs. 6.4% (D2%); 1.7% vs. 2.4% (Dmean) and 8.9% vs. 35.2% (Maxdiff). Using less complex treatment techniques minimized Maxdiff for tumors moving >15.0 mm. CONCLUSION: The dosimetric effects of respiratory-induced motion during lung SBRT are patient and plan specific. The magnitude of the dosimetric effect cannot be assessed solely based upon tumor motion amplitude or plan complexity.

Surface guided frameless positioning for lung stereotactic body radiation therapy.

BACKGROUND AND PURPOSE: When treating lung tumors with stereotactic body radiation therapy (SBRT), patient immobilization is of outmost importance. In this study, the intra-fractional shifts of the patient (based on bony anatomy) and the tumor (based on the visible target volume) are quantified, and the associated impact on the delivered dose is estimated for a frameless immobilization approach in combination with surface guided radiation therapy (SGRT) monitoring. METHODS: Cone beam computed tomographies (CBCT) were collected in free breathing prior and after each treatment for 25 patients with lung tumors, in total 137 fractions. The CBCT collected after each treatment was registered to the CBCT collected before each treatment with focus on bony anatomy to determine the shift of the patient, and with focus on the visible target volume to determine the shift of the tumor. Rigid registrations with 6 degrees of freedom were used. The patients were positioned in frameless immobilizations with their position and respiration continuously monitored by a commercial SGRT system. The patients were breathing freely within a preset gating window during treatment delivery. The beam was automatically interrupted if isocenter shifts >4 mm or breathing amplitudes outside the gating window were detected by the SGRT system. The time between the acquisition of the CBCTs was registered for each fraction to examine correlations between treatment time and patient shift. The impact of the observed shifts on the dose to organs at risk (OAR) and the gross tumor volume (GTV) was assessed. RESULTS: The shift of the patient in the CBCTs was ≤2 mm for 132/137 fractions in the vertical (vrt) and lateral (lat) directions, and 134/137 fractions in the longitudinal (lng) direction and ≤4 mm in 134/137 (vrt) and 137/137 (lat, lng) of the fractions. The shift of the tumor was ≤2 mm in 116/137 (vrt), 123/137 (lat) and 115/137 (lng) fractions and ≤4 mm in 136/137 (vrt), 137/137 (lat), and 135/137 (lng) fractions. The maximal observed shift in the evaluated CBCT data was 4.6 mm for the patient and 7.2 mm for the tumor. Rotations were ≤3.3ᵒ for all fractions and the mean/standard deviation were 0.2/1.0ᵒ (roll), 0.1/0.8ᵒ (yaw), and 0.3/1.0ᵒ (pitch). The SGRT system interrupted the beam due to intra-fractional isocenter shifts >4 mm for 21% of the fractions, but the patients always returned within tolerance without the need of repositioning. The maximal observed isocenter shift by the SGRT system during the beam holds was 8 mm. For the respiration monitoring, the beam was interrupted at least one time for 54% of the fractions. The visual tumor was within the planned internal target volume (ITV) for 136/137 fractions in the evaluated CBCT data collected at the end of each fraction. For the fraction where the tumor was outside the ITV, the D98% for the GTV decreased with 0.4 Gy. For the OARs, the difference between planned and estimated dose from the CBCT data (D2% or Dmean ) was ≤2.6% of the prescribed PTV dose. No correlation was found between treatment time and the magnitude of the patient shift. CONCLUSIONS: Using SGRT for motion management and respiration monitoring in combination with a frameless immobilization is a feasible approach for lung SBRT.

AlignRT® and Catalyst™ in whole-breast radiotherapy with DIBH: Is IGRT still needed?

PURPOSE: Surface guided radiotherapy (SGRT) is reported as a feasible setup technique for whole-breast radiotherapy in deep inspiration breath hold (DIBH), but position errors of bony structures related to deeper parts of the target are not fully known. The aim of this study was to estimate patient setup accuracy and margins obtained with two different SGRT workflows with and without daily kV- and/or MV-based image guidance (IGRT). METHODS: A total of 50 breast cancer patients were treated in DIBH, using SGRT for the patient setup, and IGRT for isocenter corrections. The patients were treated at two different departments, one using AlignRT® (25 patients) and the other using Catalyst™ (25 patients). Inter-fractional position errors were analyzed retrospectively in orthogonal and tangential setup images, and analyzed with and without IGRT. RESULTS: In the orthogonal kV-kV images, the systematic residual errors of the bony structures were ≤ 3 mm in both groups with SGRT-only. When fine-adjusted by daily IGRT, the errors decreased to ≤ 2 mm; except for the shoulder joint. The residual errors of the ribs in tangential images were between 1 and 2 mm with both workflows. The heart planning margins were between 3 and 7 mm. CONCLUSIONS: The frequency of IGRT may be considerably reduced with a well-planned SGRT-workflow for whole-breast DIBH with residual errors ≤ 3 mm. This accuracy can be further improved with an IGRT scheme.

Evaluation of deformable image registration accuracy for CT images of the thorax region.

PURPOSE: Evaluate the performance of three commercial deformable image registration (DIR) solutions on computed tomography (CT) image-series of the thorax. METHODS: DIRs were performed on CT image-series of a thorax phantom with tumor inserts and on six 4-dimensional patient CT image-series of the thorax. The center of mass shift (CMS), dice similarity coefficient (DSC) and dose-volume-histogram (DVH) parameters were used to evaluate the accuracy. Dose calculations on deformed patient images were compared to calculations on un-deformed images for the gross tumor volume (GTV) (Dmean, D98%), lung (V20Gy, V12Gy), heart and spinal cord (D2%). RESULTS: Phantom structures with constant volume and shifts ≤30 mm were reproduced with visually acceptable accuracy (DSC ≥ 0.91, CMS ≤ 0.9 mm) for all software solutions. Deformations including volume changes were less accurate with 9/12 DIRs considered visually unacceptable. In patients, organs were reproduced with DSC ≥ 0.83. GTV shifts ≤1.6 cm were reproduced with visually acceptable accuracy by all software while larger shifts resulted in failures for at least one of the software. In total, the best software succeeded in 18/25 DIRs while the worst succeeded in 12/25 DIRs. Visually acceptable DIRs resulted in deviations ≤3.0% of the prescribed dose and ≤3.6% of the total structure volume in the evaluated DVH-parameters. CONCLUSIONS: The take home message from the results of this study is the importance to have a visually acceptable registration. DSC and CMS are not predictive of the associated dose deviation. Visually acceptable DIRs implied dose deviations ≤3.0%.

Systematic evaluation of lung tumor motion using four-dimensional computed tomography.

BACKGROUND: Respiratory-induced lung tumor motion may decrease robustness and outcome of radiation therapy (RT) if not accounted for. This study provides detailed information on the motion distribution of lung tumors for a group of 126 patients treated with stereotactic body RT. MATERIAL AND METHODS: Four-dimensional computed tomography scans were reviewed to assess lung tumor motion. The tumor motion was determined by the center of mass shift based on a rigid registration of the breathing phases containing the largest positional differences in the inferior-superior (IS), left-right (LR), and anterior-posterior (AP) directions. The patients were divided into subgroups depending on tumor diameter (φ < 2.0 cm, 2.0 ≤ φ ≤ 5.0 cm, φ > 5.0 cm) and tumor location within the lung (upper, middle, or lower lobe). The observed motion distributions were evaluated for each group separately to assess the dependence on tumor size and location. For each tumor size, the motion pattern in each direction (IS, LR, and AP) was analyzed for every tumor moving >5 mm. Sinusoidal trigonometric functions were fitted to the measured data using the least mean square method to determine which type of function best describes the motion pattern. Tumor volumes between 1.6 and 52.3 cm3 were evaluated. Mann-Whitney statistical tests were used for statistical analyses. RESULTS: The mean amplitude for the tumors in this study was 1.5 mm (LR), 2.5 mm (AP), and 6.9 mm (IS) while the maximum amplitude was 11.0 mm (LR), 9.0 mm (AP), and 53.0 mm (IS). In total, 95% of the tumors moved ≤20 mm in the IS direction, ≤3 mm in the LR direction, and ≤6 mm in the AP direction. The observed motion distributions showed no statistically significant correlation with tumor size or location within the lung except for motion in the IS direction, where the mean and maximum amplitudes significantly increased for tumors located in the middle and lower parts of the lung. The motion pattern of a tumor in any direction was best described using a squared trigonometric function of the type [Formula: see text], where A is the maximum amplitude of the motion in the current direction, t is the time of measurement, T is the total time of the breathing cycle and B is a constant used to synchronize the starting point of the breathing cycle. CONCLUSION: Lung tumor movements were generally larger in the IS direction and the motion amplitude in this direction increased for tumors located in the middle and lower parts of the lungs. Motions in LR or AP showed no such relation. Tumor size was not found to have any correlation with the motion amplitude in any direction. The motion pattern of a lung tumor in any direction is best described with a squared sinusoidal function independently of the tumor size or tumor location.