Mantle field dosimetry comparing 4 MV with cobalt 60.

Three off-axis dosimetric parameters were evaluted for identical mantle fields using the Varian 4 MV linear accelerator and a Picker C8M cobalt-60 unit. These parameters were dose distribution, skin dose and integral dose. Comparisons showed higher doses in all three categories with the 4 MV accelerator when the tumor dose is specified at the central axis midline. These results emphasize the importance of off-axis dosimetry with both machines.