Purpose: Image guided radiation therapy (IGRT) is becoming the standard of practice for many treatment sites. Our purpose in this study was to quantify the improvement in setup accuracy with the use of IGRT in TomoTherapy. We have analyzed shifts in the X,Y,Z directions for patients undergoing daily IGRT and treatment with a TomoTherapy Hi‐Art unit. Total vector shifts were also calculated. Four distinct patient groups were analyzed: cranial, head and neck, prostate with implanted fiducials, and prostatectomy patients. Method and Materials: Megavoltage computed tomography (MVCT) was carried out at each treatment fraction. The MVCT was fused to the planning KVCT and the resultant X,Y,Z offsets determined. A total of 1,303 X,Y,Z measurements from 48 patients were collected for this study. The data is presented separately for each of the four groups. Results: Prostate patients with implanted gold markers had the largest vector shifts with a mean of 9.0 mm. The magnitude of this vector was largely due to vertical shifts (Z direction) with a mean of 7.1 mm. Prostatectomy patients were set up based on bone fusion and had an average vector shift of 7.8 mm, while head and neck and cranial patients had an average shift of 5.0 mm and 4.7 mm respectively. Data for the displacement in each of the X,Y,Z directions is also presented. The variation in shifts between different patients within a group was significant and varied by at least a factor of four. Conclusion: This study demonstrates quantitatively the potential improvement in radiotherapy accuracy with the use of IGRT in TomoTherapy. Corrections in setup in the Z (vertical) direction account for the largest contribution to the vectors calculated. The use of extremely high dose gradients in IMRT has made patient positioning accuracy even more critical than in previous modalities.
Abstract-We have used TomoTherapy for the treatment of cranial lesions with single fraction radiosurgery (SRS) as well as fractionated radiotherapy (SRT). TomoTherapy image guided radiation therapy (IGRT) allows very good imaging of bony anatomy, however regions where subtle contour changes occur may be difficult to match and are prone to subjective interpretation. The purpose of this study is to use highcontrast gold fiducials, permanently implanted into the skull, as objective markers to aid in determining the accuracy of cranial setup, and autofusion in TomoTherapy. At each treatment a TomoTherapy scan (MV-CT) was carried out and fused to the conventional KV-CT to visualize the position of bony anatomy, and the position of the implanted fiducials. The fiducials were clearly seen on the MV-CT and were used to determine shifts in the longitudinal, lateral and vertical directions as well as roll, pitch and yaw. Vertical shifts were most significant ranging from 2-6 mm. Lateral and longitudinal shifts were typically 0-3 mm. Vector analyses of daily shifts indicated that the displacement from initial setup position was an average of 4.1 mm, but that displacements of up to 6 mm can occur in some fractions. This demonstrates quantitatively the importance of using IGRT in cranial treatments. When autofusion was activated using all six degrees of freedom (linear plus rotational corrections) the displacement was typically 2 mm. The displacement is unlikely to exceed 1.0 mm when fiducials are used for fusion. The fiducials are a significant aid in determining setup accuracy for the treatment of brain lesions. Advantages for this method of stereotactic localization include patient comfort, absence of external devices allowing fractionated treatment courses, and a high degree of accuracy.
Purpose: TomoTherapy MVCT can be carried out using three distinct imaging modes: Fine, Normal and Coarse. These correspond to a CT slice thickness of 2 mm, 4 mm and 6 mm. In this study we evaluate a new Ultrafine mode allowing slice thickness of 1 mm. Measurements in a phantom are carried out to estimate typical patient dose for all 4 modes. Image quality for the various modes is compared. Method and Materials: A standard TomoTherapy solid water “cheese” phantom was used for all dosimetric analyses. Measurements were made at superficial (0.5 cm) and deep (15.0 cm) regions using ionization chambers. Image quality was evaluated using a megavoltage CT phantom containing 0.067–1.0 Lp/mm. In addition, clinical images of a cranium containing 2 mm gold markers were also evaluated for quality. The collimator width was maintained at standard settings for Fine, Normal and Coarse modes, and reduced to finer settings for the new Ultrafine mode. Results: The collimator opening on our unit was measured at isocenter and was found to be 0.69 cm in the three standard modes, and 0.39 cm in the new Ultrafine mode. The measured imaging dose was: Ultrafine 2.07 cGy, Fine 2.44 cGy, Normal 1.30 cGy and Coarse 0.90 cGy. Dose at superficial and deep regions was similar. The axial views for all four modes show similar resolution as expected since a 512×512 matrix is used regardless of mode. However, the sagittal images show considerable improvement, in both phantom and clinical studies, when imaged in Ultrafine mode. Conclusion: A new Ultrafine collimator setting for TomoTherapy, allowing a resolution of 1 mm in the sagittal plane, is characterized. This improvement is gained without increased patient exposure compared to the standard 2 mm Fine mode. This new imaging mode is expected to be particularly useful in radiosurgery.
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