TomoTherapy for Cranial Radiosurgery/Radiotherapy

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 high-contrast 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.