SU‐E‐T‐299: Proton Pencil Beam Spot Scanning Phase Space in the IBA System and the Clinical Implications for Superficial Targets

PURPOSE: In the treatment of superficial lesions with proton Pencil Beam Scanning (PBS), spot size is dominated by the nozzle contribution. Accuracy of phase-space modeling is therefore paramount. IBA's Dedicated (DN) and Universal Nozzles (UN) have different designs and, consequently, characteristics. Here we report the phase spaces of these two nozzles, without and with a range shifter (RS). METHODS: In-air spot fluence measurements were made for five proton energies: 225, 210, 180, 150 and 115 MeV and at five distances from isocenter pertinent to SAD-type treatments: +33, +20, +10, 0 and -10 cm ('+' implies upstream), without and with a 7.5 cm water-equivalently-thick RS (sufficient to pull back the lowest energy Bragg peak to patient surface), fixed with its upstream side 41 cm from isocenter. Data collected on a fixed horizontal beam-line with a DN and a gantry-mounted UN were compared. The full-width-at-half-maximum (FWHM) of a Gaussian fit to each spot fluence profile was extracted along the two principal axes. RESULTS: With no RS, the proton spots are ∼20-70% larger at isocenter in the UN than in the DN. Spots are less asymmetric, and eccentricity increases more slowly with energy, in the UN than in the DN. Over the 33 cm in-air travel upstream of isocenter, the spot FWHM varies by less than ∼2 mm. However, spot asymmetry becomes more severe upstream (for 115 MeV spots, 30-40% compared to <20% at isocenter for DN, but similar and <10% for UN). With an RS, spot FWHM at isocenter increases by 12.7 mm from 8.3 mm (DN) and 10.7 mm from 13 mm (UN) for 150 MeV protons (typical for brain treatments). With no RS, relatively distance-independent spot size facilitates SAD-type treatments. For patients with superficial lesions, where an RS is required and the phase space varies rapidly with distance, the RS should be permitted at two additional locations. US Army Medical Research and Materiel Command under Contact Agreement No. DAMD17-W81XWH-04-2-0022.