Article

Experimental characterization of the low-dose envelope of spot scanning proton beams

Department of Radiation Physics, The University of Texas M D Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030, USA.
Physics in Medicine and Biology (Impact Factor: 2.92). 06/2010; 55(12):3467-78. DOI: 10.1088/0031-9155/55/12/013
Source: PubMed

ABSTRACT In scanned proton beam radiotherapy, multiple pencil beams are used to deliver the total dose to the target volume. Because the number of such beams can be very large, an accurate dosimetric characterization of every single pencil beam is important to provide adequate input data for the configuration of the treatment planning system. In this work, we present a method to measure the low-dose envelope of single pencil beams, known to play a meaningful role in the dose computation for scanned proton beams. We measured the low-dose proton beam envelope, which extends several centimeters outwards from the center of each single pencil beam, by acquiring lateral dose profile data, down to relative dose levels that were a factor of 10(4) lower than the central axis dose. The overall effect of the low-dose envelope on the total dose delivered by multiple pencil beams was determined by measuring the dose output as a function of field size. We determined that the low-dose envelope can be influential even for fields as large as 20 cm x 20 cm.

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    • "Thus an alternative definition of the halo might be 'any dose from charged particles outside the Fermi-Eyges Gaussian', and we consider all such particles, including elastically scattered protons, to be secondaries. We exclude from the halo any beam contamination such as degraded and scattered protons from beam profile monitor wires [6]. Such contamination can be avoided by careful beam line design and, if present, is more properly treated as a secondary core [7]. "
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