The consequence of these revolutions is that the current state of the art is the fact that highly conformal radiation dose distributions (using IMRT) can be designed to an image dataset that is a snapshot of the patient geometry. While this has led to great improvements in the quality of treatment plans, the GTV is designed using bulky disease imaged using a single image dataset. These limitations are being overcome in the next generation of radiation therapy technology.

Improvements in Tumor Imaging

Improving tumor imaging is being actively investigated by radiology and radiation oncology departments and includes the development of molecular imaging, functional imaging, and biological imaging.The specific concepts behind these imaging categories overlap, but essentially, are that the imaging modalities will exploit highly sensitive imaging techniques to differentiate between tissue types. Some of these differentials are due to tumor physiology. For example, many tumors are more physiologically active, and therefore process more glucose, than the surrounding tissues. Positron emission tomography (PET) using 18F-fluorodeoxyglucose, a glucose analog, provides highly sensitive images of active glucose uptake within rapidly growing tumors. It is expected that imaging advances will include the ability to determine aspects of the tumor environment, such as hypoxic state, and tumor dose response.

Removing the Limitations Associated With the Useofa Single Image Dataset

The use of the PTV concept allows the radiation dose distribution to encompass the tumor within the expected tumor positioning errors. An important technology, onboard imaging (OBI), is being developed that may allow monitoring of the tumor position for each treatment. OBI technology consists of imaging hardware attached to the linear accelerator that allows the acquisition of a 3-D CT image dataset. These 3-D CT images can be used to identify misalignments of the tumor, allowing the patient to be repositioned, placing the tumor in the planned position. In principal, this may allow for reduced PTV margins, leading to smaller high-dose volumes and correspondingly reduced normal tissue irradiation.

While OBI seems to provide the ideal tool to significantly reduce the PTV margins, there are some challenges that need to be met before it comes into widespread use. In order for therapists to use the image data to reposition the patient, they need to have efficient and effective tools to evaluate the tumor position relative to the planned position. The current commercial tools are relatively crude and need significant improvements.