Jeffrey M. Kapatoes

Limited-data image registration for radiotherapy positioning and verification.

PURPOSE One benefit to having on-line CT imaging integrated into a radiotherapy system is that images can be collected with the patient in the treatment position. These images can then be automatically registered to planning images for improved positioning and verification. However, many such on-line imaging systems have a limited field of view (LFOV) that could potentially impair registration. Thus, the viability of automatic registration was investigated in the context of collecting on-line LFOV and also limited-slice CT images for radiotherapy. METHODS AND MATERIALS Mutual information and two new voxel-based registration algorithms were tested to align LFOV and limited-slice prostate and breast images given known displacements. Success rates were tallied for different field-of-view sizes, slice distributions, and initial displacements. RESULTS Most of the automatic registration algorithms tested were useful for solving these LFOV and limited-slice problems. Registration of LFOV images was generally successful, especially for fields of view of at least half the patients size. For limited-slice images, success was more closely correlated to the slice spacing than to the number of slices used, with sparse slice spacing being preferable. CONCLUSIONS Mutual information and other automatic registration algorithms have been identified as useful methods for registering LFOV and limited-slice radiotherapy images with planning CT images.

Methods for improving limited field-of-view radiotherapy reconstructions using imperfect a priori images

There are many benefits to having an online CT imaging system for radiotherapy, as it helps identify changes in the patients position and anatomy between the time of planning and treatment. However, many current online CT systems suffer from a limited field-of-view (LFOV) in that collected data do not encompass the patients complete cross section. Reconstruction of these data sets can quantitatively distort the image values and introduce artifacts. This work explores the use of planning CT data as a priori information for improving these reconstructions. Methods are presented to incorporate this data by aligning the LFOV with the planning images and then merging the data sets in sinogram space. One alignment option is explicit fusion, producing fusion-aligned reprojection (FAR) images. For cases where explicit fusion is not viable, FAR can be implemented using the implicit fusion of normal setup error, referred to as normal-error-aligned reprojection (NEAR). These methods are evaluated for multiday patient images showing both internal and skin-surface anatomical variation. The iterative use of NEAR and FAR is also investigated, as are applications of NEAR and FAR to dose calculations and the compensation of LFOV online MVCT images with kVCT planning images. Results indicate that NEAR and FAR can utilize planning CT data as imperfect a priori information to reduce artifacts and quantitatively improve images. These benefits can also increase the accuracy of dose calculations and be used for augmenting CT images (e.g., MVCT) acquired at different energies than the planning CT.

SU‐EE‐A1‐01: Acceleration of Tomotherapy Treatment Delivery by Increasing Pitch and Decreasing Modulation

Purpose: To determine the feasibility of reducing Tomotherapy treatment delivery time by planning with increased pitch and reduced modulation factor (MF). Method and Materials: Twelve patients with Tomotherapy treatment sites (head/neck, prostate, prostate/nodes, lung) and fraction size of 1.8–2.5 Gy originally planned with a low pitch (< 0.3) and fast gantry period (< 20 s) were replanned. The first replan increased the pitch to 0.43 and used the original MF (MF0). If the first replan was acceptable and the gantry period increased, the patient was replanned again with pitch = 0.43 and a lower MF to reduce treatment time. This was repeated until the plan (DVH) became unacceptable or the gantry period reached its minimum (15 s). If the first replan was not satisfactory or if the gantry period did not increase, the MF was increased by 0.5 to improve the plan and to test if the treatment time was still reduced. Results: For all patients, we were able to produce a DVH comparable to the original DVH using pitch = 0.43 and MF0. When comparing the original DVH to the accelerated plan DVH that most closely matched it, treatment times were reduced by 13–66%. Comparing these accelerated plans to the original plans showed the PTV median dose increase 0.4±0.5 Gy, insignificant changes in target coverage and homogeneity index, an average change in critical structure median dose of −0.8±1.3 Gy, and a maximum critical structure median dose increase of 1.3 Gy. Conclusion: Faster treatment times can be achieved by planning with increased pitch and reduced modulation. Treatments planned with pitch = 0.43 and MF0 showed little or no plan degradation and most had faster treatment times. Further reduction in treatment time was possible by reducing the MF. Conflict of Interest: Some of the authors have financial interest in TomoTherapy, Inc.

Gating-by-rotation: a solution to the problem of intratreatment motion in helical tomotherapy

Purpose: To assess the feasibility of addressing intratreatment motion issues in helical tomotherapy by gating the treatments by rotation. Intratreatment motion is a problem common to all IMRT techniques. Traditional methods of gating in conformal radiotherapy and some forms of IMRT are not applicable to helical tomotherapy due to the continuous rotation of the gantry. An alternative method is presented. Materials and Methods: Rotation-gating in helical tomotherapy is the process in which one rotation of treatment is immediately followed by a rotation of non-treatment. This on-off strategy is repeated for the full treatment volume. During the treatment rotations, the patient is required to hold their breath while the intensity-modulated fan beam deposits dose. For the non-treatment rotations, the patient is allowed to breathe freely as all leaves of the MLC will be closed, the accelerator disabled, or both. The couch indexes normally for treatment rotations and holds the patient stationary during non-treatment rotations. An investigation was conducted to assess the feasibility of rotation-gating. Film was placed between two hemispheres of a water phantom and a continuous helical delivery was carried out with all leaves opened. The film was replaced and another treatment was performed employing rotation-gating. The two films were compared to assess the process. The films were irradiated to dose levels within the linear region of the film response curve (maximum film dose ∼ 35 cGy). Films were also acquired with all leaves closed to quantify leakage dose through the collimation systems. Results: Central profiles for the inferior-superior direction (parallel to the direction of translation) for both films are displayed in Figure 1. The profiles agree very well, illustrating that a rotation-gated treatment closely mimics a continuous helical delivery. The only significant discrepancy lay in the tails of the profiles: a higher film dose is seen for the rotation-gated treatment as the leakage dose is higher for this delivery strategy. There are two possibilities for addressing this leakage dose: 1. it can be used to image the patient and thus provide information (albeit motion-blurred) regarding the patient position during the non-treatment rotations, 2. it can be completely removed by disabling the beam pulses during these rotations using the accelerator’s gridded-gun feature. Conclusion: Rotation-gating is a feasible solution for solving the problem of intratreatment motion in helical tomotherapy. Further studies are planned on the helical tomotherapy prototype at the University of Wisconsin. Download full-size image

Adaptive Radiation Therapy (ART) Strategies Using Helical Tomotherapy

Tomotherapy is intensity-modulated rotational radiotherapy utilizing a photon fan beam [1–6]. In helical tomotherapy the gantry and couch are in continuous motion and, from the patient’s point of view, the source describes a helical trajectory. Helical tomotherapy was designed around a ring gantry similar to a helical CT scanner. The constraints of a ring gantry are minimal since few patients are treated with non-coplanar radiation fields and IMRT diminishes the need for these types of field arrangements. Most importantly, a ring gantry is a very stable platform for CT scanning. The original helical tomotherapy concept had kV and MV beams for imaging and treatment respectively [1]. On the current version megavoltage photons from the treatment linac are used to generate the CT scans [7,8]. The same detector can also be used to detect the exiting treatment beam in order to provide the data for dose reconstruction [9]. Figure 1 is a labeled photograph of the TomoTherapy (Madison WI) Hi-Art tomotherapy unit with its covers off. The helical tomotherapy beam line was designed for image-guided IMRT treatments. The linac and gantry systems of the tomotherapy system are highly favorable for CT. The gantry sag of the tomotherapy system is negligible so that no sag corrections are required. The size of the electron beam on the target is about 1 mm so that the resolution is about 1.2 mm to 1.6 mm which is comparable to a conventional CT scanner for high contrast objects. Operating with typical patient doses of 1 cGy, the soft tissue contrast is about 2–3% which is higher than a modern CT scanner. Nevertheless, the images have sufficient quality for adaptive radiotherapy processes. The tomotherapy unit’s xenon gas detector elements have tungsten septa separating ionizationcavities. Inaddition to the ionizationcollectors, the tungstenplates are embeddedphotonconverters intercepting themegavoltage photons and yet are thin enough to let an appreciable fraction of the electrons set in motion to deposit energy in the xenon gas. The interception of the beam by the tungsten means that the quantum efficiency of the system is about 25%, which is much more than the few percent collection efficiency of modern portal imaging systems, and decreases necessary imaging dose proportionally.