Optimization of megavoltage CT scan registration settings for thoracic cases on helical tomotherapy

Abstract

This study aims to investigate the settings that provide optimum registration accuracy when registering megavoltage CT (MVCT) studies acquired on tomotherapy with planning kilovoltage CT (kVCT) studies of patients with lung cancer. For each experiment, the systematic difference between the actual and planned positions of the thorax phantom was determined by setting the phantom up at the planning isocenter, generating and registering an MVCT study. The phantom was translated by 5 or 10 mm, MVCT scanned, and registration was performed again. A root-mean-square equation that calculated the residual error of the registration based on the known shift and systematic difference was used to assess the accuracy of the registration process. The phantom study results for 18 combinations of different MVCT/kVCT registration options are presented and compared to clinical registration data from 17 lung cancer patients. MVCT studies acquired with coarse (6 mm), normal (4 mm) and fine (2 mm) slice spacings could all be registered with similar residual errors. No specific combination of resolution and fusion selection technique resulted in a lower residual error. A scan length of 6 cm with any slice spacing registered with the full image fusion selection technique and fine resolution will result in a low residual error most of the time. On average, large corrections made manually by clinicians to the automatic registration values are infrequent. Small manual corrections within the residual error averages of the registration process occur, but their impact on the average patient position is small. Registrations using the full image fusion selection technique and fine resolution of 6 cm MVCT scans with coarse slices have a low residual error, and this strategy can be clinically used for lung cancer patients treated on tomotherapy. Automatic registration values are accurate on average, and a quick verification on a sagittal MVCT slice should be enough to detect registration outliers.

A retrospective tomotherapy image‐guidance study: analysis of more than 9,000 MVCT scans for ten different tumor sites

“…Usually, image registration was based on bony or bony and soft tissue anatomy. The accuracy and precision of the MVCT‐kVCT registration is lower than 1 mm, as has been proved by other authors 10 , 11 . As pitch and yaw deviations cannot be corrected by gantry or automatic couch offsets, the patient was repositioned if these deviations were greater than 3°.…”

“…This effect, may be due to several reasons: the couch top is being dragged into the bore by the patient's weight against the longitudinal drive mechanism; (22) there is a mismatch between the CT laser and tomotherapy red lasers, and/or the MVCT slice resolution, as suggested by Hui et al (20) Woodford et al (11) observed that this scan resolution leads to large longitudinal shifts, independently of the combination of fusion technique and registration resolution selected.…”

QA for helical tomotherapy: Report of the AAPM Task Group 148a)

“…Similarly, the scan range and parameters influence the available information and this can affect the registration precision. 26,27 It should further be understood that registering patient images can be more subjective depending on the anatomical site because anatomical changes in the patient can add a level of complexity and subjectivity that is absent from rigid phantom alignments. The clinical registration precision can be determined using actual patient images, clinical operators, and clinical alignment techniques.…”

Toxicity and Outcome Results of a Class Solution With Moderately Hypofractionated Radiotherapy in Inoperable Stage III Non–Small Cell Lung Cancer Using Helical Tomotherapy

“…The tumor region was scanned daily, and positioning was done using the integrated registration with the planning CT (19)(20)(21). The megavoltage computed tomography (MVCT) images were extracted from the TomoTherapy data server for every fraction and were imported into iPlan (BrainLAB AG, Feldkirchen, Germany) for review and analysis of primary tumor volume regression.…”