Abstract
The additional intensity modulated radiation therapy (IMRT) and total body irradiation (TBI) to conventional treatment clinical treatment procedures can significantly increase the contribution of accelerator head leakage radiation. Previously recommended procedures by the National Council on Radiation Protection and Measurements (NCRP) for vault design, specifically calculations of primary and secondary barrier thicknesses, are not valid when leakage radiation significantly exceeds direct radiation. Use factor distributions are also influenced by IMRT and TBI procedures. Methods are proposed to extend the NCRP barrier design formulas to resolve these problems. The medical accelerator (weekly) workload is separately determined for the direct, leakage, and scatter radiation components. Applications of the formulas to the calculation of primary and secondary barriers are discussed. The addition of IMRT to the shielding design is explored as a function of the fraction patients receiving IMRT and the MU to dose ratio. Secondary barrier thicknesses could be increased by as much as 1 TVL.PACS number(s): 87.52.–g, 87.53.–j
Highlights
In the last 15 years, the technology of treatment delivery for radiation oncology patients has undergone tremendous change
The ratio, C, of the number of MU to dosenumber of cGyat the isocenter has been found to range[1,2,3,4,5] from 2 to 10, with some proposed methods going even higher.[6,7]. This increased MU load means the leakage radiation from the accelerator assembly has increased by a factor of approximately C and raises concern about the adequacy of radiation protection afforded by existing facilities embarking on IMRT as well as how to design new shielded vaults for IMRT
In order to handle the problem, increased leakage radiation produced by IMRT and TBI treatments the calculational will use three types of workloads–one for direct, one for leakage, and one for scatter radiation, respectively
Summary
In the last 15 years, the technology of treatment delivery for radiation oncology patients has undergone tremendous change. Multileaf collimator based intensity modulated radiation therapyIMRT, in its various versions, has been found relatively inefficient compared to conventional treatment methods. The nature of the inefficiency is that more accelerator monitor unitsMUare required per unit dose to the target volume. The reason is that only a fraction, often small, of each treatment field is being irradiated at any instance of delivery. The ratio, C, of the number of MU to dosenumber of cGyat the isocenter has been found to range[1,2,3,4,5] from 2 to 10, with some proposed methods going even higher.[6,7] This increased MU load means the leakage radiation from the accelerator assembly has increased by a factor of approximately C and raises concern about the adequacy of radiation protection afforded by existing facilities embarking on IMRT as well as how to design new shielded vaults for IMRT
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