WE‐D‐224C‐05: Investigation of Superficial Dose From a Static TomoTherapy Beam

Purpose: To determine the magnitude of superficial doses produced from a static TomoTherapy beam directed at varying SSDs and angles of incidence. Method and Materials: Measurements of superficial doses have been made along the central axis of a static TomoTherapy beam at normal incidence for SSDs of 55, 70, and 85 cm for typical TomoTherapy jaw sizes (40×2.5 cm 2 and 40×5 cm 2 ). Measurements have also been made along the central axis of a TomoTherapy beam at oblique angles of 30°, 45°, 60°, 75°, and 85°. Data were collected with a Gammex Model 449 parallel‐plate chamber embedded in a solid water phantom and LiF TLD powder. For comparison, measurements have been made on a 6X Varian 2100C accelerator with the same jaw width (5 cm at 85 SSD) and at the same SSDs. Percentage depth dose (PDD) profiles for depths ⩽2 cm have been obtained from the data. Results: TomoTherapy surface dose measurements vary weakly with SSD, ranging from 16%–18% for the 40×5 cm2 field, and from 12%–14% for the 40×2.5 cm 2 field. The measured doses increase rapidly with depth, with PDD>90% obtained at depths < 0.6 cm. Surface dose ranges from 17%–26% on the Varian 2100C for the same SSDs and 5cm jaw width. TomoTherapy surface dose increases from 16% to 43% as the angle of incidence increases from 0° to 85° for the 40×5 cm 2 field and increases from 12% to 40% for the 40×2.5 cm 2 field. Conclusion: TomoTherapy surface doses do not vary significantly with SSD but increase dramatically with increasing angle of incidence. Generally, the magnitude is less than that measured from a conventional, flattening filter‐based linac. These data should assist in assessing the accuracy of the TomoTherapy planning system in the calculation of superficial doses. Supported in part by a research agreement with TomoTherapy, Inc.