Megavoltage 2D topographic imaging: An attractive alternative to megavoltage CT for the localization of breast cancer patients treated with TomoDirect

Highlights

• A software interface was developed to generate topographic Tomotherapy images.

• A TomoDirect treatment was simulated on a female chest phantom.

• MVCT and MV2D images were compared for 7 different shifted positions of the phantom.

• Topographic image may be an alternative to MVCT for TomoDirect treatments.

Abstract

Purpose

To show the usefulness of topographic 2D megavoltage images (MV2D) for the localization of breast cancer patients treated with TomoDirect (TD), a radiotherapy treatment technique with fixed-angle beams performed on a TomoTherapy system.

Methods

A method was developed to quickly localize breast cancer patients treated with TD by registering the MV2D images produced before a TD treatment with reference images reconstructed from a kilovoltage CT simulation scanner and by using the projection of the beam-eye-view TD treatment field. Dose and image quality measurements were performed to determine the optimal parameters for acquiring MV2D images. A TD treatment was simulated on a chest phantom equipped with a breast attachment. MVCT and MV2D images were performed for 7 different shifted positions of the phantom and registered by 10 different operators with the simulation kilovoltage CT images.

Results

Compared to MVCT, MV2D imaging reduces the dose by a factor of up to 45 and the acquisition time by a factor of up to 49. Comparing the registration shift values obtained for the phantom images obtained with MVCT in the coarse mode to those obtained with MV2D, the mean difference is 1.0 ± 1.1 mm, −1.1 mm ± 1.1, and −0.1 ± 2.2 mm, respectively, in the lateral, longitudinal, and vertical directions.

Conclusions

With dual advantages (very fast imaging and a potentially reduced dose to the heart and contralateral organs), MV2D topographic images may be an attractive alternative to MVCT for the localization of breast cancer patients treated with TomoDirect.

Introduction

The TomoTherapy unit (Accuray, Madison, WI, USA) is a radiation therapy treatment system consisting of a 6 MeV linear accelerator that continually rotates around a patient who translates through the beam, resulting in helical fan beam delivery. TomoDirect is a radiotherapy treatment technique with fixed-angle beams performed on the TomoTherapy system [1]. Up to 12 beams can be set up at different fixed gantry angles. Each beam is delivered as the patient advances through the ring and the beam fluence is modulated by the binary multileaf collimator of the TomoTherapy system. This intensity-modulated radiation therapy (IMRT) modality has shown to be a promising treatment of the single breast, allowing better PTV dose coverage and helping to spare organs at risk compared to 3D techniques and linac-based IMRT [2], [3].

The localization of patients receiving radiation therapy for breast cancer is challenging because of the mobility of the treated area. Even with thoracic thermoplastic immobilization systems, inter-fraction positioning errors remain significant [4]. To prevent a degradation of the PTV coverage during a breast treatment with TomoTherapy, a daily pre-treatment imaging control is mandatory [5].

The TomoTherapy imaging system has been widely described in the literature [6]. It uses megavoltage-computed tomography (MVCT) and allows a 3D reconstruction of the patient's anatomy from a helical acquisition. The dose delivered to the patient during image acquisition depends on several parameters [7] and is less than 1 cGy per image if considering the coarse irradiating acquisition mode [8]. Although the doses to the organs at risk (OAR) are low in absolute value, they generate an additional risk to the patient: daily MVCT imaging would increase the risk of developing a second cancer by 33% and 63%, respectively, in the contralateral breast and lungs [9].

The theory of the TomoTherapy topographic images was presented for the first time in 2010 by Moore et al., who showed the results of using 2D topographic megavoltage images (MV2D) generated from topographic data of the TomoTherapy imager [10]. The term topographic commonly refers to scout images in conventional CT imaging: the TomoTherapy MV radiation source is placed at a fixed angulation, the patient translates with the movement of the table, and the acquisition is carried out using the TomoTherapy imager. A 2D image can then be reconstructed from the data retrieved by the imager. Kiely et al. designed a post-image process to improve the quality of MV topographic images by reducing noise and enhancing the bony anatomy [11]. Besides these theoretical studies, Takahashi studied the clinical relevance of MV2D imaging applied to a specific indication with a very long target size, total marrow irradiation (TMI) [12]. The image acquisition is much faster for MV2D than for MVCT, so MV2D can be very interesting for TMI, for which MVCT acquisition time takes ≥600 s [12]. The usefulness of MV2D imaging has never been studied in the clinical context of breast treatments: however, even though imaging acquisition time is not as decisive since the breast target volume is not so large, the dose delivered by MVCT imaging may have a significant impact, as shown previously.

The purpose of this work is to present pre-clinical characterization of MV2D imaging using phantom simulating localization of breast cancer patients treated with TomoDirect.