Fast, Low-Dose Megavoltage-Topogram Localization on TomoTherapy: Initial Clinical Experience With Mesothelioma...

This study aimed to evaluate the potential of megavoltage-topogram (MV-topogram)-based alignment as an alternative to megavoltage computed tomography (MVCT) in reducing setup time and imaging dose for patients with malignant pleural mesothelioma who are receiving TomoTherapy.

Twelve patients were enrolled in an ongoing institutional review board approved clinical trial at our institute. Patients were set up with a clinical protocol using red lasers. Anteroposterior (AP) and lateral (LAT) MV-topograms were acquired using gantry angles of 0°/90° with a 1 mm collimator opening, all multileaf collimator leaves open, a couch speed of 4 cm/s, and a 12.5-second scanning time. Routine MVCT scans were performed immediately afterward. The MV-topograms were reconstructed and enhanced using contrast-limited adaptive histogram equalization. Anteroposterior and LAT kilovoltage digital reconstructed topogram images were reconstructed based on TomoTherapy geometry from computed tomography simulation scans. Registrations between MV-topograms and kilovoltage-digital reconstructed topogram images were performed manually, and patients' daily shifts were recorded. Results were compared against the corresponding daily MVCT shifts. MV-topogram and MVCT doses were measured and recorded using an ion chamber on a cheese phantom with depths between 1 and 14 cm, as well as the times required to acquire the 2 image modalities.

The mean and standard deviation of shift discrepancies between MV-topogram and MVCT were 0.74 ± 2.08, -0.09 ± 4.46, and 0.45 ± 3.57 mm in the LAT, longitudinal, and vertical directions, respectively. The MVCT imaging doses measured were 14.74 to 26.92 times higher than the MV-topogram doses, depending on depth. On average, MV-topograms with a mean scan length of 50 cm achieved a 5-fold image acquisition time savings over MVCT, with a mean scan length of 38 cm.

MV-topograms has the potential to provide alignment performance equivalent to that of MVCT for patients with mesothelioma, with a significant reduction in imaging dose and acquisition time.