title: A comparison of air-cavity inhomogeneity effects for cobalt-60, 6-, and 10-MV x-ray beams.
abstract: 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 X 7 cm for 10-MV x-rays, 6 X 6 cm for 6-MV x-rays, and 5 X 5 cm for cobalt-60. In addition, the depth over which underdosing occurs is seen to increase with increasing energy.
mesh_terms: D000388:Air; D003037:Cobalt Radioisotopes / Q000627:therapeutic use; D006801:Humans; D007830:Larynx; D008953:Models, Anatomic; D010315:Particle Accelerators; D011867:Radioisotope Teletherapy; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy
chemical_list: D003037:Cobalt Radioisotopes