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A comparison of air‐cavity inhomogeneity effects for cobalt‐60, 6‐, and 10‐MV x‐ray beams
Author(s) -
Beach J. L.,
Mendiondo M. S.,
Mendiondo O. A.
Publication year - 1987
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 - 10.1118/1.596101
Subject(s) - materials science , cobalt 60 , cobalt , optics , x ray , dosimetry , physics , nuclear medicine , nuclear physics , medicine , irradiation , metallurgy
The inclusion of air‐filled spaces in treatment fields creates a potential dosimetric problem due to the loss of charged particle equilibrium near the air–tissue interface. We have used a simulated larynx phantom and a small buildup/extrapolation chamber to compare the magnitude and spatial extent of underdosing and overdosing at the distal surface for two linear accelerators (10‐ and 6‐MV x rays) and a cobalt‐60 machine. Surface doses were compared to doses measured in a similar but homogeneous phantom to give observed/expected ratios (O/E), which were greater than 1.0 for large field sizes and less than 1.0 for small field sizes on all machines. The minimum field sizes which produce no surface underdosing for a simulated 2‐cm‐diam larynx are roughly 7×7 cm for 10‐MV x rays, 6×6 cm for 6‐MV x rays, and 5×5 cm for cobalt‐60. In addition, the depth over which underdosing occurs is seen to increase with increasing energy.