Treating Lung Cancer with 4D Protons

Compared to the x rays traditionally used in radiation therapy, protons offer the potential to destroy lung tumors just as competently while inflicting less damage to surrounding healthy tissue. At next week's meeting of the American Association of Physicists in Medicine (AAPM) in Seattle, a research team (including Martijn Engelsman, now at MAASTRO clinic, Netherlands, martijn.engelsman@maastro.nl) will describe a method for increasing the effectiveness of using protons to treat lung tumors in a small experimental study of four patients.

In traditional radiation therapy, one must use multiple beams of x rays to deliver a uniform dose to a lung tumor; often at least one of the x-ray beams will exit from the healthy (non-tumor-containing) lung and potentially damage it. On the other hand, positively charged, subatomic protons only travel a limited distance through the body; they never make it to the other lung, and they also are more likely to spare nearby organs such as the esophagus and heart.

However, the protons' finite range makes their trajectories particularly sensitive to density changes in the lung, caused, for example, by the expansion of the lung during inhalation. For that reason, if the proton treatment is not carefully planned, there is the chance of missing the tumor, thus decreasing the chance of curing the patient. So in planning the treatment of lung cancer patients, the researchers adopted the 4D approach, which is already used in traditional x-ray cancer therapy.

In the 4D approach, one takes into account how the patient's breathing moves the lung back and forth over time (the fourth dimension) so that the radiation hits the tumor precisely over all phases of a patient's breathing cycle. In a study of four patients at Massachusetts General Hospital, the researchers have found that planning and carrying out 4D proton therapy delivers excellent dose levels to lung tumors in all cases. The only thing preventing this technique from wider use is the need to develop an algorithm that cuts down the currently lengthy time it takes to calculate and plan the proton beam's direction and intensity for each breathing phase.

The 4D approach is also applicable to radiation therapy using carbon ions, which is currently being used to help defeat lung cancer in a couple of centers in Japan. (Paper WE-E-J-6C-7; for more information on the meeting; go to http://www.aapm.org/meetings/05AM/)

While currently small, the numbers of proton therapy centers are expected to grow exponentially over the next 20 years; for example, a major proton therapy center is scheduled to open at the University of Texas's M.D. Anderson Cancer Center in Spring 2006.