Correlation Between External and Internal Markers for Abdominal Tumors: Implications for Respiratory Gating

Reviewer: John Wilson, MD
Abramson Cancer Center of the University of Pennsylvania
Last Modified: October 21, 2003

Presenter: D. Gierga
Presenter's Affiliation: Massachusetts General Hospital
Type of Session: Scientific

Background

Treating tumors in the abdomen with radiation can be challenging because patient respirations can cause tumor movement, thus making tumor targeting difficult
Many radiation centers are beginning to use gated therapy, which turns the radiation beam on and off or moves the radiation field to coincide with respiratory variation
Gated therapy often uses markers on the skin to approximate tumor motion
Some studies (Vedan, 2003 and Mageras 2001) have correlated external markers with the diaphragm for treating lung cancers, and one study (Ozhasoglu, 2002) evaluated this in a single patient with pancreatic cancer
This study was designed to evaluate the effect of motion of abdominal tumors comparing external and internal markers.

Materials and Methods

Five abdominal cancer patients (4 with liver cancer and 1 with pancreatic cancer) with surgically implanted metallic clips (internal markers) and external radio-opaque markers were observed from a lateral view under fluoroscopy
The motion of the internal and external markers were then correlated using computer software

Results

There was a strong linear correlation between the motion of the internal and external markers
The internal markers usually showed more movement than the external markers, with up to a five-fold difference
Internal motion varied by as much as 14 mm in the cranio-caudal direction and 10 mm in the anterior-posterior direction

Author's Conclusions

Internal and external marker motion is generally consistent, but marker placement affects the magnitude of external marker motion
External markers can be used for respiratory gating of abdominal tumors
Further studies are being performed to look at variations in correlation of internal and external markers

Clinical/Scientific Implications

This study showed that the motion of internal and external markers correlated well with each other. However, there was a large variation in the slope of this correlation, with internal motion usually being much larger than external motion. The slope could be as steep as -5.2, so that for every millimeter of external marker motion there was 5.2 mm of internal motion. The speaker did not address how to predict what the slope would be without having internal markers. Therefore, it would be premature to say that external markers can be used for respiratory gating of abdominal tumors, and more research would be needed to use external markers alone for gating purposes. Also, there are centers that use internal markers directly for gating, so if surgical clips are in place, this would be more preferable than using surrogate external markers. If surgical clips are not in place, they can potentially be placed under radiologic guidance.

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