Measurement of superficial dose from a static tomotherapy beam

Superficial doses were measured for static TomoTherapy Hi-Art beams for normal and oblique incidence. Dose was measured at depths < or = 2 cm along the central axis of 40 x 5 cm2 and 40 x 2.5 cm2 beams at normal incidence for source to detector distances (SDDs) of 55, 70, and 85 cm. Measurements were also made at depths normal to the phantom surface for the same beams at oblique angles of 30 degrees, 45 degrees, 60 degrees, 75 degrees, and 83 degrees from the normal. Data were collected with a Gammex/RMI model 449 parallel-plate chamber embedded in a solid water phantom and with LiF thermoluminescent dosimeters (TLDs) in the form of powder. For comparison, measurements were made on a conventional 6 MV beam (Varian Clinac 2100C) at normal incidence and at an oblique angle of 60 degrees from the normal. TomoTherapy surface dose varied with the distance from the source and the angle of incidence. For normal incidence, surface dose increased from 0.16 to 0.43 cGy/MU as the distance from the source decreased from 85 to 55 cm for the 40 x 5 cm2 field and increased from 0.12 to 0.32 cGy/MU for the 40 x 2.5 cm2 field. As the angle of incidence increased from 0 degrees to 83 degrees, surface dose increased from 0.24 to 0.63 cGy/MU for the 40 x 5 cm2 field and from 0.18 to 0.58 cGy/MU for the 40 x 2.5 cm2 field. For normal incidence at 55 cm SDD, the surface dose relative to the dose at d(max) for the 40 x 5 cm2 TomoTherapy Hi-Art beam was 31% less than that from a conventional, flattening filter based linear accelerator. These data should prove useful in accessing the accuracy of the TomoTherapy treatment planning system to predict the dose at superficial depths for a static beam delivery.