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Dosimetry validation of treatment room shielding design
Author(s) -
Stathakis Sotirios,
Price Robert,
Ma C.M.
Publication year - 2005
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 - 10.1118/1.1853632
Subject(s) - linear particle accelerator , dosimetry , collimator , electromagnetic shielding , nuclear medicine , beam (structure) , leakage (economics) , materials science , optics , physics , medicine , economics , composite material , macroeconomics
Intensity‐modulated radiation therapy (IMRT) can lead to an increase in leakage radiation. The total number of monitor units (MUs) for IMRT is typically 2–5 times that for conventional treatments [the ratio of the two is used to derive the effective modulation scaling factor ( MSF eff ) ]. Shielding calculations for IMRT can be done by applying theMSF effto measured exposures under conservative conditions (standard beam setup 40 cm × 40 cm field, 45° collimator angle) to account for the increased leakage. In this work, we verified this approach for two existing vaults housing a Siemens Primart 6 MV linac and a Varian 21Ex 10 MV linac. We measured the cumulative exposures at various locations around the vaults for typical IMRT cases and for the standard beam setup using the same MUs. For the standard beam setup, the IMRT gantry angles and eight equally spaced angles were used. Estimations of weekly exposures for IMRT were carried out using exposure rates measured under standard beam setup and theMSF effaveraged over 20 treatment cases. The accumulated exposures under realistic IMRT conditions were 30%–50% lower than the estimated values using equally spaced gantry angles except for two locations where the real IMRT leakage was higher than the estimated value by approximately 10%. Measurements using the same gantry angles yielded similar results. Our results indicate that it is adequate to use theMSF effand previously measured exposures to estimate the leakage increase due to IMRT for an existing vault. Different approaches should be followed when considering primary or secondary barriers since the standard beam setup is overestimating the exposures behind primary barriers compared to IMRT. In such cases, a 10 cm × 10 cm field can be used for more accurate shielding evaluation.