Imaging Needs for Proton Therapy
J Kruse1*, H Kooy2*, L Dong3*, (1) Mayo Clinic, Rochester, MN, (2) Massachusetts General Hospital, Boston, MA, (3) Scripps Proton Therapy Center, San Diego, CAWE-C-500-1 Wednesday 10:30AM - 12:30PM Room: 500 Ballroom
Proton therapy is a rapidly growing treatment modality. Superficially, proton therapy looks identical to x-ray therapy – the patient receives a treatment planning CT followed by daily image-guided treatment sessions. However, the specifics of proton therapy impose unique demands on the imaging procedures at each phase of the treatment. In x-ray therapy the Hounsfield Units (HU) of the CT are converted to relative electron density for megavoltage x-ray dose calculations. In proton therapy the pertinent value to be determined from HU is proton stopping power for human tissue, usually different from stopping power in the phantom used for measurement. Accurate determination of stopping power from HU requires a unique calibration process and tight controls on CT imaging technique. A number of approaches have emerged in x-ray therapy for daily localization of soft tissues – the most popular involving placement of fiducial markers or Cone Beam CT (CBCT). However, dose shadowing downstream of fiducials may disallow them from clinical application for proton therapy. CBCT is not yet used for proton therapy, but for some treatment sites daily localization of the tumor may not guarantee dosimetric coverage as variations in the relative positions of tumor and normal tissues in the beam path perturb the proton dose distribution. Direct proton beam imaging – radiography and tomography - are in development. Nuclear activation of tissues by the proton beam presents a unique quality assurance measure in which imaging of the activation could verify the range of the protons in the patient.
1. Present the unique demands of the treatment simulation process for proton therapy, including HU to stopping power calibration, technique standardization, and artifact reduction.
2. Discuss the proton specific challenges in daily target localization including dose shadowing from fiducials, dose perturbation by anatomical changes, and the need for respiratory correlated IGRT.
3. Introduce the possibility of proton beam range verification by direct proton beam imaging and post-treatment activation imaging.
Funding Support, Disclosures, and Conflict of Interest: Research grant from Varian Medical Systems