Potential of In-room PET for Image Guided Proton Therapy

6.30.2009

SPEAKER: Georges El Fakhri, PhD, DABR, DABR, MGH and HMS

MODERATOR: Thomas Brady, MD: MGH, HMS, CIMIT

Video not available.

Forum Summary

Proton beam therapy is used to irradiate tumors, and although the technique is useful in many situations, it can be difficult to calibrate the beam so that the dose of radiation is deposited at the target site.  When a proton beam passes through tissue, it produces a number of radioactive nuclides such as carbon-11 and oxygen-15, and these nuclides gradually decay, emitting positrons that can be detected using positron emission tomography (PET).  If PET images are to be useful, however, they must be acquired soon after proton beam therapy is administered, for the radioactivity produced by the proton beam decays within minutes.  Researchers led by Georges El Fakhri, PhD, are designing a mobile full-ring PET scanner so that patients can be imaged immediately after irradiation without having to change beds.
The imaging system being designed by Fakhri’s team combines desirable features from two existing imaging techniques.  Currently, doctors can acquire PET images related to proton beam therapy using expensive in-beam scanners or using normal PET facilities.  In-beam scanners image the proton beam in real time but are expensive and do not provide tomographic information.  Off-line PET facilities provide tomographic information but invariably introduce delays and errors related to repositioning the patient.  The in-room, full-ring system being developed by Dr. Fakhri’s group involves a delay of approximately one minute, requires no patient repositioning, and provides tomographic information.

Such a PET scanner promises to make it easier for doctors to verify the range of a proton beam and could also make it possible to adaptively monitor the beam.  Adaptive monitoring involves depositing a small dose of radiation, verifying the range, and then depositing the rest of the dose.  Depositing the dose in this manner would prevent dangerous radiation from being deposited in healthy parts of the body. 

Fakhri’s team has built a prototype to test their imaging system, and initial results indicate that their system could be useful in clinical settings.  They found that a five-minute in-room scan produced a better image than did a thirty-minute off-line scan.  One drawback of their prototype is that it is currently only large enough for brain scans or for pediatric scans.

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