Medical Physics Seminar – Friday, March 22, 2013
SPECIAL SEMINAR: Experimental characterization of two-dimensional pencil beam scanning proton spot profiles
Liyong Lin, Ph.D.
Assistant Professor, University of Pennsylvania, Department of Radiation Oncology, 3400 Civic Boulevard, Philadelphia, PA 19104
Dose calculation of pencil-beam scanning treatment plans relies on the accuracy of proton spot profiles not only the primary component but also the broad tail components. Pair-magnification method is applied to EBT3 films to generate primary and tail components of two-dimensional spot profiles. Four films are taken for each location in air and each depth for six selected energies. The films used for primary component are exposed to 50-200 MU, depending on proton energy and location in air. The films used for tail components are exposed to 800, 8000 and 80000 MU. By pairing two films used for primary and tail or tail and tail components, dose kernels down to 10-4 of central spot dose can be generated. Calculation of output versus field size agrees with measurement within 1% using dose kernels. The asymmetric comet-shaped profile tails has bigger impacts at superficial depths of low energies. This output difference between two orientations at the surface of rectangle field of 40 mm by 200 mm is about 2%. Dose deficit is calculated to be less than 5.5% for all depths and all energies of Bragg peaks collected by 40-mm radius PTW chamber. The paper that has a positive review from PMB is: "A novel technique for measuring the low-dose envelope of pencil beam scanning spot profiles" Abstract: Well-characterized spot profiles are known to be important for pencil beam scanning treatment planning system dose calculation accuracy. To investigate the profile measurement capabilities of an IBA-Dosimetry scintillation detector and to assess its feasibility for determining the low-intensity tails of these spots, the responses of the scintillation detector and Gafchromic EBT2 film to a 115-MeV proton spot were measured in-air at isocenter. Pairs of irradiations were made: one lower-level irradiation (5 MU and 160 MU for the scintillation detector and film, respectively) insufficient to cause saturation, and one higher-level irradiation (640 MU and 32,000 MU for the scintillation detector and film, respectively) which deliberately saturated the central region of the spot, but provided magnification of the tails. By employing the pair/magnification technique, agreement between the film and scintillation detector measurements of the spot profile can be extended from 4% of the central spot dose down to 0.01%. Gamma analysis between these measurements in the region from 0.4% to 0.01% of the peak dose shows 95% agreement using 3 mm/3% criteria and 99% agreement using 5 mm/5% within a ±9 cm bound. Above 4%, our 115 MeV proton spot can be well-described by Gaussian function; below 4%, non-Gaussian, diamond-shaped tails predominate.