Intrafraction patient motion is much more likely in intensity-modulated radiation therapy (IMRT) than in conventional radiotherapy primarily due to longer beam delivery times in IMRT treatment. In this study, we evaluated the uncertainty of intrafraction patient displacement in CNS and head and neck IMRT patients. Immobilization is performed in three steps: (1) the patient is immobilized with thermoplastic facemask, (2) the patient displacement is monitored using a commercial stereotactic infrared IR camera (ExacTrac, BrainLab) during treatment, and (3) repositioning is carried out as needed. The displacement data were recorded during beam-on time for the entire treatment duration for 5 patients using the camera system. We used the concept of cumulative time versus patient position uncertainty, referred to as an uncertainty time histogram (UTH), to analyze the data. UTH is a plot of the accumulated time during which a patient stays within the corresponding movement uncertainty. The University of Florida immobilization procedure showed an effective immobilization capability for CNS and head and neck IMRT patients by keeping the patient displacement less than 1.5 mm for 95% of treatment time (1.43 mm for 1, and 1.02 mm for 1, and less than 1.0 mm for 3 patients). The maximum displacement was 2.0 mm.
We have formulated a unified dosimetry index (UDI) that computes, for any given treatment plan, its deviations in terms of dose coverage, conformity, homogeneity, and dose gradient vis‐à‐vis an ideal plan (which we define as a dosimetry plan of perfect dose coverage, conformity, homogeneity, and step‐wise fall‐off to zero dose outside the planning target volume). In order to validate the UDI scoring system, 21 stereotactic cranial radiosurgery cases were evaluated retrospectively. The cases were planned on the BrainSCAN treatment planning system (BrainLAB, Feldkirchen, Germany) using 6 to 8 non‐coplanar static beams collimated with the micro multi‐leaf collimator (mMLC). We suggest a technique for creating a ranking system that can be utilized for plan evaluation and comparison between multiple plans. Under this system treatment plans are classified as “excellent”, “good”, “average”, or “poor”. The proposed ranking system can be utilized as a general guide for generating an optimal dosimetry plan for external beam radiation therapy.
Purpose: To present a quantitative evaluation of the impact of scan length, monitor unit (MU)/projection, and total MU on the image quality in mega‐voltage cone‐beam computed tomography (MV‐CBCT). We present results of a systematic comparison of contrast‐to‐noise ratio and modulation transfer function obtained for 9 varying acquisition protocols with the MVision system (Siemens Medical Solutions, Concord, CA). Method and Materials: Image quality is characterized by contrast, noise and spatial resolution. Utilizing the manufacturer.s image quality phantom, we obtained sets of images from 9 different acquisition protocols of varying scan length, MU/projection, and total MU. The images are analyzed in terms of contrast‐to‐noise ratio (CNR) and modulation transfer function (MTF), which quantifies spatial resolution. Results: The CNR data suggest that the image contrast is not enhanced by increasing the MU/projection beyond 0.0675, but rather may be diminished. However, for comparable MU/projection the CNR is reduced from 7.25 to 4.8 by lowering the total MU from 15 to 8. For the same MU/projection the CNR is slightly better for the 360° scan compared to the 200° scan. On the other hand, the MTF analysis shows that neither the scan length, nor the sampling rate, nor the total MU have an effect on spatial resolution. Furthermore, the MTF analysis indicates that objects of spatial frequency larger than 0.3 line pair/mm cannot be resolved adequately with current MV‐CBCT imaging irrespective of the scanning protocol utilized. Conclusion: The study shows that spatial resolution is not affected by acquisition or reconstruction parameters in MV‐CBCT. However, the system is optimally used in terms of contrast‐to‐noise ratio with the default setting of 1 projection every degree. Conflict of Interest: This research was partially supported by Siemens Medical Solutions.
Purpose: To demonstrate the effectiveness of a new pitch, roll and yaw device for helical Tomotherapy head and neck and brain treatment. Methods & Materials: The pitch, roll and yaw device is a headrest frame with a mechanism for adjusting angular orientation. It is fabricated using two overlaying Lucite panels mounted on an adjustable wheel base. With the use of the pitch, roll and yaw headrest device three MVCT image sets are obtained, and two displacement adjustments (first angular, second translational) are made. Since the axis of rotation may not be located in the center of the image volume, adjustments for the rotational and translational displacements have to be made independently. The rotational adjustments are made after the first scan, and the translational adjustment is made after the second scan. A third MVCT scan is obtained for final verification prior to treatment. Results: Using the headrest device, on average the pitch, roll and yaw angular displacements can be reduced to less than 0.2°, 0.3°, and 0.25°, respectively, after three MVCT scans. The final translational x, y, z, displacements are still be minimized to less than 1mm after rotational adjustments. Conclusion: As a mechanism for adjusting angular displacements, the pitch, roll and yaw device provides additional degrees of freedom for target localization. However, one additional MVCT image set is needed in order to independently adjust the rotational and translational displacements.
Purpose: To demonstrate a new technique for non‐coplanar helical Tomotherapy cranial radiosurgery treatment. Evaluation of the improvement, in terms of minimizing the dose to healthy tissue, of the non‐coplanar technique over the conventional coplanar treatment. Methods & Materials: The proposed technique for non‐coplanar helical Tomotherapy cranial radiosurgery is demonstrated using an anthropomorphic phantom (Rando phantom). Obtaining a composite dose distribution of the optimized non‐coplanar plans is not possible with the current helical Tomotherapy planning system. For comparison of the dose coverage, conformality and dose volumes between the non‐coplanar and the conventional coplanar treatment techniques, film dosimetry is used to obtain isodose distributions. A calibrated Welhofer scanner was utilized for the film dosimetry. Results: The results show significant reduction in the volumes receiving 60% or less of the prescribed dose. There is no appreciable change in volumes receiving 70% or more of the prescribed dose. Dose volume ratios, DV20/80 and DV30/70 (defined as the ratio of the volumes receiving 20% and 80%, and the ratio of volumes receiving 30% and 70% of the prescribed dose, respectively) are used to quantify the improvement of a non‐coplanar helical Tomotherapy cranial radiosurgery technique. The DV20/80 and DV30/70 dose volume ratios obtained for the non‐coplanar technique are 9.33 and 4.57, respectively, whereas ratios obtained for the standard coplanar technique are 11.33 and 5.01, respectively. Conclusion: The results of the study clearly demonstrate that the dose to healthy tissue can be minimized by using three non‐coplanar angular orientations for helical Tomotherapy cranial radiosurgery. More marked improvements can be obtained by utilizing more than three non‐coplanar angles, but at a high cost in terms of planning and treatment times. Balancing the improvements in DV20/80 and DV30/70 ratios and the required treatment and planning times entails making proper clinical judgment for each case.
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