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SU‐E‐T‐496: Dosimetric Comparison between Protons and Photons in the Field Junction in Craniospinal Irradiation (CSI)
Author(s) -
Cheng C,
Srivastava S,
Buchsbaum J,
Zhao L,
Wolanski M,
Simmons J,
Johnstone P,
Das I
Publication year - 2011
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3612449
Subject(s) - sobp , proton , dosimetry , proton therapy , photon , irradiation , physics , optics , penumbra , materials science , atomic physics , nuclear medicine , nuclear physics , medicine , ischemia , cardiology
Purpose: In this study, we investigated the dosimetry in the moving gap region in proton CSI for various combinations of field junction widths and feathering step sizes. We have also compared the dosimetry in the field junction between proton and photon beams. Methods: Dose profiles for two proton ranges, 11.6cm and 16cm, both with 10cm SOBP for the 30cm snout are used. Feathering of the junction is simulated by shifting the profiles by two successive steps. Three junction widths (0, 2, 4mm) and two feathering step sizes (5, 10 mm) are investigated. Similar simulations (but also include larger gap widths due to larger penumbra) are performed for 6 and 15 MV x‐ rays. Dose profiles at the field junctions are then compared for the proton and photon fields for various gaps and feathering step sizes. Results: For protons, full dose is achieved in the field junction for 0mm gap for both 5mm and 10mm featherings. However, with even a 2mm gap, the dose falls to below 90% in the moving gap due to the steep proton penumbra. For photons, even 4mm gap still results in >90% dose in the moving gap due to the large penumbra and the ‘tail’ in the profiles. Despite the different dose characteristics, it is possible to produce similar dose profiles in the moving gap for proton and photon fields by varying the gap and the feathering sizes for photon fields. Conclusion: Full dose can be safely achieved in the junction by ‘hot‐matching’ the spine and the brain fields in proton CSI and feathering the junctions. The steep proton penumbra results in a large dose gradient even with 2mm gap between fields. It is possible to produce similar dosimetry in the junction with photon CSI. The difference is that there is no exit dose with protons.