Introduction

Since the advent of computed tomography (CT), sophisticated techniques in radiation treatment, such as three-dimensional conformal radiotherapy, stereotactic radiotherapy, and intensity-modulated radiotherapy, have been developed in order to focus and escalate the radiation dose to the tumor while sparing normal tissues. With these techniques, it is important to precisely determine the tumor volume. With their high anatomic resolution, CT and magnetic resonance imaging (MRI) have been primarily used for target volume delineation in radiotherapy treatment planning. However, when delineating the target volume, it is sometimes difficult to distinguish between tumor and nontumor tissues using anatomical imaging alone. In the past 10 years, positron emission tomography (PET) with [F-18] fluorodeoxy-glucose (18F-FDG), which is able to visualize molecular information for the tumor, has been widely used in oncology for the diagnosis and staging of various cancers. This functional imaging has been adopted in radiotherapy, and several studies have examined the clinical impact of PET on radiotherapy planning [1-5]. However, because PET is not an inherently accurate test, with a spatial resolution of approximately 4 to 7 mm [6], it is difficult to determine tumor boundaries on conventional scintillator PET images. In 2007, a novel PET scanner with semiconductor detectors, the first in the world, was developed with Hitachi (Hitachi, Japan), and installed at our institute. Phantom studies have revealed that this new device has the potential to give high-quality images with lower scatter noise and a spatial resolution of 2.3 mm [7]. The difference in the signal-to-noise ratio between the new semiconductor PET scanner and a conventional scintillator PET scanner in a phantom was impressive; allowing radiation oncologists to expect precise delineation with the new machine (Fig. 1). When we used the two PETs for 18F-FDG examinations of the same patient with nasopharyngeal carcinoma (NPC), it was apparent that the semiconductor PET image showed a different image from that shown by the scintillator PET scanner (Fig. 2). However, there are many questions to find the reason for the difference behind the images. Is the difference due to

Fig. 1a,b. Illustration of the difference in delineation of a tumor (yellow outlines) in a radiotherapy treatment plan based on a semiconductor positron emission tomography (PET) images (gross tumor volume [GTV] new), and b a radiotherapy treatment plan based on conventional scintillator PET images (GTVconv) for phantom data. It is apparent that it is better to use the semiconductor PET scanner for tumor delineation

Fig. 1a,b. Illustration of the difference in delineation of a tumor (yellow outlines) in a radiotherapy treatment plan based on a semiconductor positron emission tomography (PET) images (gross tumor volume [GTV] new), and b a radiotherapy treatment plan based on conventional scintillator PET images (GTVconv) for phantom data. It is apparent that it is better to use the semiconductor PET scanner for tumor delineation b a

Fig. 2a,b. The two PET examinations of the same nasopharyngeal carcinoma (NPC) patient with T1N2 disease. a Semiconductor PET image; b scintillator PET image

b a time of examination? Or is it due to fluctuation of the uptake of the FDG? Or is it because of the better signal-to-noise ratio of the semiconductor PET scanner? The purpose of this study was to compare the new semiconductor PET scanner and a conventional scintillator PET scanner with regard to tumor volume delineation, using treatment planning for NPC to find the answers to these questions.

Was this article helpful?

0 0

Post a comment