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WE‐E‐M100F‐04: Commissioning and Validation of a Beam Model for Calculating Megavoltage CT Dose From Imaging with a Helical Tomotherapy Unit
Author(s) -
Shah A,
Langen K,
Meeks S,
Kupelian P,
Staton R,
Cox A,
Ruchala K
Publication year - 2007
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 - 10.1118/1.2761586
Subject(s) - tomotherapy , imaging phantom , ionization chamber , nuclear medicine , dosimetry , collimator , percentage depth dose curve , dose profile , beam (structure) , medical imaging , materials science , medicine , physics , optics , radiation therapy , radiology , ionization , ion , quantum mechanics
Purpose: To commission and validate an MVCT beam model that allows for the calculation of dose received by patients due to megavoltage imaging on a helical tomotherapy unit (Tomotherapy, Inc., Madison, WI). Method and Materials: Percent depth dose and profile data were collected in order to commission a new MVCT beam model. The fluence output for the beam model was adjusted to match the measured dose in phantom. The model was then verified through a series of absorbed dose measurements in three phantoms (20‐cm cylindrical phantom, CIRS anthropomorphic phantom, and 30‐cm “cheese” phantom). The multiple scan average dose was recorded for all three phantoms with various changes to CT collimator pitch and ion chamber location (central versus peripheral points). Results: The delivered doses and the computed doses were on average within 1.5% for all three phantoms, when the ion chamber was centrally located; and within 3.5%, when the chamber was located on the peripheral edge of the phantoms. The measured dose in the anthropomorphic phantom was 2.3 cGy with a pitch of 1.0 (4 mm couch movement per gantry rotation), 1.4 cGy with a pitch 2.0, and 0.90 cGy with a pitch of 3.0, these matched within 1% to the calculated dose. The computed versus measured dose was also within 1% when calculating dose in different tissue densities (lung and bone). Conclusion: This study has shown that with the development of a new MVCT beam model, dose delivered from MVCT imaging can be calculated. Validation measurements, in phantom, have verified that the computed dose can be reported to within 1.5% of the measured dose. The rationale for implementing this MVCT beam model is to provide a future method for calculating patient‐specific MVCT dose. Conflict of Interest: Co‐authors are either funded by a research grant or employed by TomoTherapy, Inc.