Virtual patient model monitors organ movement in real time

Gwynne D. Koch

Breathing causes the lungs to expand and contract, making small tumors in the chest cavity a moving target and affecting the quality of radiation treatment. Therefore, medical physicists either expand the treatment area, knowing that healthy tissue also will be damaged, or they deliver radiation bursts timed to coincide with the beginning or end of the breathing cycle, a procedure that requires complicated treatment.

In 2000, a research team led by Xie George Xu, a professor at Rensselaer Polytechnic Institute in Troy, N.Y., created Visible Photographic Man, an advanced computer model that simulates in three dimensions how radiation affects the organs and tissues in the human body. However, this model could not simulate organ deformation in real time.

Researchers are developing a virtual model of the chest cavity that can be used to predict and monitor in real time organ movement caused by breathing (left). Designed to improve radiation treatment of cancer, this project is an extension of the team’s previous work on Visible Photographic Man, an advanced 3-D computer model of the human body (inset).

With a recently awarded four-year $2 million grant from the National Institutes of Health, Xu and his team will build on their previous work, developing a physics-based virtual model of the lungs and surrounding tissues that moves realistically in real time. When the revised model is used in conjunction with existing 3-D models, its added ability to predict and monitor anatomical changes resulting from respiration could significantly improve the accuracy and effectiveness of radiation treatment for lung and liver cancers.

A patient would receive a CT scan before treatment to record his or her breathing cycle and the shape and location of structures in the body. That information would be fed into the computer model, enabling radiation to be focused on the moving tumor during several phases of the breathing cycle, shortening treatment time and preventing irradiation of surrounding healthy tissues.

The project involves anatomical modeling, radiation transport and biomechanical computation, and medical imaging and clinical radiation oncology. Most of the computational research will be performed by Xu and Suvranu De, also a professor at Rensselaer Polytechnic. Chengyu Shi and Dr. Martin Fuss of the Cancer Therapy and Research Center in San Antonio will conduct clinical evaluations.

The team has developed preliminary surface geometric modeling for a few organs, which they will combine with physics-based modeling to improve the realism of tissue deformation in real time. They will use a computational tool known as the Monte Carlo method to calculate radiation doses; they also will use a “mesh free” technology known as the point-associated finite field to simulate organ deformation and a multidetector CT to validate the new model for patient-specific treatment planning.

This computational technology also can help minimize motion artifacts in diagnostic imaging devices such as CT and single-photon-emission computed tomography. The researchers anticipate that the model eventually will benefit surgical procedures, drug development and the study of respiratory diseases.