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

• Published in: Medical Physics 2012-06, Vol.39 (6), p.3772-3772
• Main Authors: Lin, L, Ainsley, C, Dolney, D, Closset, M, Dessy, F, De Wilde, O, Maughan, R, McDonough, J
• Format: Article
• Language: English
• Subjects: Brain; Phase space methods; Protons; Surface treatments; Ultrasonography
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.4735385

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