Physicians also need to access the registration results at convenient locations (as opposed to requiring them to travel to the treatment unit for the review process).This access requires a database system that can deal with the large image datasets and will transfer the image information efficiently to the physician.

The use of OBI promises to be a definite advance in patient positioning but it has some significant technical limitations.The image acquisition process uses the linear accelerator gantry rotating around the patient.The time of acquisition is typically one minute; longer than a patient can hold their breath and long enough for other physiologic processes (e.g. digestion) to significantly move internal organs.This motion results in image artifacts that can disturb or completely destroy the image quality.The number of tumor sites is limited where OBI imaging will guarantee high-quality images that can be relied on for daily positioning. Research is beingconducted to analyze the raw image data to reduce motion artifacts.

Another technical limitation is that the imaging modality uses kilovolt or megavolt X-rays and therefore the softtissue contrast is limited. Many tumors in the abdomen or pelvis cannot be visualized using these modalities.

Adaptive Radiation Therapy

An important potential advance that can take place with the implementation of OBI involves the periodic review of the image datasets to determine if the tumor has regressed significantly. Some tumors, such as cervical cancer and Hodgkin’s lymphoma, are very radiosensitive and regress dramatically throughout therapy.The initial treatment plan may be inappropriate part way through the course of therapy. In the past, review of the relationship between the treatment plan and the tumor was limited to monitoring the patient’s weight or how well they fit into custom-fabricated immobilization devices. With OBI, there is the potential for adjusting the treatment plan to changes in the tumor size or position based on 3-D image datasets. This process is termed adaptive radiation therapy (ART). ART includes replanning for geometric changes, but can also be used to modify the patient’s treatment plan based on the actual radiation response as compared against the expected response. For example, if the tumor is expected to regress at a specific rate and the tumor regresses slower, increasing the dose fractionation may be appropriate. Conversely, if the tumor regresses more rapidly than expected, the dose fractionation or total dose could be reduced.

The customization of the treatment plan during therapy is an exciting potential advance in radiation therapy practice.There is anecdotal evidence that some tumor regression may cause the tumor to leave the high-dose region. Having the capability to vary the total dose or dose fractionation with respect to variations in tumor response rates may improve the local control and complication rates for many sites.

Breathing Motion

As mentioned earlier, one of the challenges to accurate imaging is the management of breathing motion. Breathing motion also causes other difficulties for conformal therapy or IMRT. The motion of the tumor during the initial CT imaging will cause artifacts, leading to errors in determining the shape of the tumor. Breathing motion of the tumor during treatment causes the radiation beam to be larger than the tumor cross section.Techniques are being developed to quantify the breathing motion to allow for a more accurately designed radiation beam.These techniques are also being used to gate the linear accelerator to effectively limit breathing motion, as well as investigating the option of tracking the tumor motion with the radiation beam.

Conclusions

This is an exciting time in radiation therapy. The industry has the ability to deliver customized, highly conformal dose distributions and is implementing the capability of collecting 3-D images prior to each treatment. Advances in radiology imaging techniques will be used to improve the ability to identify otherwise microscopic disease, aid in the definition of the GTV, and ultimately improve local control and cure rates.