Technical note: deformable image registration on partially matched images for radiotherapy applications.

In radiation therapy applications, deformable image registrations (DIRs) are often carried out between two images that only partially match. Image mismatching could present as superior-inferior coverage differences, field-of-view (FOV) cutoffs, or motion crossing the image boundaries. In this study, the authors propose a method to improve the existing DIR algorithms so that DIR can be carried out in such situations. The basic idea is to extend the image volumes and define the extension voxels (outside the FOV or outside the original image volume) as NaN (not-a-number) values that are transparent to all floating-point computations in the DIR algorithms. Registrations are then carried out with one additional rule that NaN voxels can match any voxels. In this way, the matched sections of the images are registered properly, and the mismatched sections of the images are registered to NaN voxels. This method makes it possible to perform DIR on partially matched images that otherwise are difficult to register. It may also improve DIR accuracy, especially near or in the mismatched image regions.

The validation of tomotherapy dose calculations in low-density lung media.

The dose-calculation accuracy of the tomotherapy Hi-Art II(R) (Tomotherapy, Inc., Madison, WI) treatment planning system (TPS) in the presence of low-density lung media was investigated. In this evaluation, a custom-designed heterogeneous phantom mimicking the mediastinum geometry was used. Gammex LN300 and balsa wood were selected as two lung-equivalent materials with different densities. Film analysis and ionization chamber measurements were performed. Treatment plans for esophageal cancers were used in the evaluation. The agreement between the dose calculated by the TPS and the dose measured via ionization chambers was, in most cases, within 0.8%. Gamma analysis using 3% and 3 mm criteria for radiochromic film dosimetry showed that 98% and 95% of the measured dose distribution had passing gamma values < or =1 for LN300 and balsa wood, respectively. For a homogeneous water-equivalent phantom, 95% of the points passed the gamma test. It was found that for the interface between the low-density medium and water-equivalent medium, the TPS calculated the dose distribution within acceptable limits. The phantom developed for this work enabled detailed quality-assurance testing under realistic conditions with heterogeneous media.

Helical tomotherapy planning for left-sided breast cancer patients with positive lymph nodes: comparison to conventional multiport breast technique.

PURPOSE: To evaluate the feasibility of using helical tomotherapy for locally advanced left-sided breast cancer. METHODS AND MATERIALS: Treatment plans were generated for 10 left-sided breast cancer patients with positive lymph nodes comparing a multiport breast (three-dimensional) technique with the tomotherapy treatment planning system. The planning target volumes, including the chest wall/breast, supraclavicular, axillary, and internal mammary lymph nodes, were contoured. The treatment plans were generated on the tomotherapy treatment planning system to deliver 50.4 Gy to the planning target volume. To spare the contralateral tissues, directional blocking was applied to the right breast and right lung. The optimization goals were to protect the lungs, heart, and right breast. RESULTS: The tomotherapy plans increased the minimal dose to the planning target volume (minimal dose received by 99% of target volume = 46.2 +/- 1.3 Gy vs. 27.9 +/- 17.1 Gy) while improving the dose homogeneity (dose difference between the minimal dose received by 5% and 95% of the planning target volume = 7.5 +/- 1.8 Gy vs. 37.5 +/- 26.9 Gy). The mean percentage of the left lung volume receiving >or=20 Gy in the tomotherapy plans decreased from 32.6% +/- 4.1% to 17.6% +/- 3.5%, while restricting the right-lung mean dose to <5 Gy. However, the mean percentage of volume receiving >or=5 Gy for the total lung increased from 25.2% +/- 4.2% for the three-dimensional technique to 46.9% +/- 8.4% for the tomotherapy plan. The mean volume receiving >or=35 Gy for the heart decreased from 5.6% +/- 4.8% to 2.2% +/- 1.5% in the tomotherapy plans. However, the mean heart dose for tomotherapy delivery increased from 7.5 +/- 3.4 Gy to 12.2 +/- 1.8 Gy. CONCLUSION: The tomotherapy plans provided better dose conformity and homogeneity than did the three-dimensional plans for treatment of left-sided breast tumors with regional lymph node involvement, while allowing greater sparing of the heart and left lung from doses associated with increased complications.

Enhanced efficiency in helical tomotherapy quality assurance using a custom-designed water-equivalent phantom.

Tomotherapy is an image-guided, intensity-modulated radiation therapy system that delivers highly conformal dose distributions in a helical fashion. This system is also capable of acquiring megavoltage computed-tomography images and registering them to the planning kVCT images for accurate target localization. Quality assurance (QA) of this device is time intensive, but can be expedited by improved QA tools and procedures. A custom-designed phantom was fabricated to improve the efficiency of daily QA of our Tomotherapy machine. The phantom incorporates ionization chamber measurement points, plugs of different densities and slide-out film cartridges. The QA procedure was designed to verify in less than 30 min the vital components of the tomotherapy system: static beam quality and output, image quality, correctness of image registration and energy of the helical dose delivery. Machine output, percent depth dose and off-axis factors are simultaneously evaluated using a static 5 x 40 cm(2) open field. A single phantom scan is used to evaluate image quality and registration accuracy. The phantom can also be used for patient plan-specific QA. The QA results over a period of 6 months are reported in this paper. The QA process was found to be simple, efficient and capable of simultaneously verifying several important parameters.

Dosimetric consequences of uncorrected setup errors in helical Tomotherapy treatments of breast-cancer patients.

BACKGROUND AND PURPOSE: The Tomotherapy Hi-Art II system allows acquisition of pre-treatment MVCT images to correct patient position. This work evaluates the dosimetric impact of uncorrected setup errors in breast-cancer radiation therapy. MATERIALS AND METHODS: Breast-cancer patient-positioning errors were simulated by shifting the patient computed-tomography (CT) dataset relative to the planned photon fluence and re-computing the dose distributions. To properly evaluate the superficial region, film measurements were compared against the Tomotherapy treatment planning system (TPS) calculations. A simulation of the integrated dose distribution was performed to evaluate the setup error impact over the course of treatment. RESULTS: Significant dose differences were observed for 11-mm shifts in the anterolateral and 3-mm shifts in the posteromedial directions. The results of film measurements in the superficial region showed that the TPS overestimated the dose by 14% at a 1-mm depth, improving to 3% at depths >or=5mm. Significant dose reductions in PTV were observed in the dose distributions simulated over the course of treatment. CONCLUSIONS: Tomotherapy's rotational delivery provides sufficient photon fluence extending beyond the skin surface to allow an up to 7-mm uncorrected setup error in the anterolateral direction. However, the steep dose falloff that conforms to the lung surface leads to compromised dose distributions with uncorrected posteromedial shifts. Therefore, daily image guidance and consequent patient repositioning is warranted for breast-cancer patients.