Abstract
The formalism used for barriers calculations is based on a conservative estimation of workload, use factor, and occupancy factor. IMRT techniques (Intensity Modulated Radiation Therapy) and VMAT (Volumetric Modulated Arc Therapy) are known for being superior to conventional techniques, but costly from the shielding standpoint, as they increase the number of monitor units used to deliver the same dose to the patient, increasing the leakage radiation produced and, consequently, the thickness of the secondary barriers. At InRad (Radiology Institute of HC-FMUSP) a 2100CD LINAC already installed was upgraded to perform IMRT/VMAT techniques, and the existing barrier was reassessed. The present study proposes a methodology for acquiring real workload data from the institution's management software (MOSAIQ®) to replace the initially estimated data and recalculate the thickness of the barriers, assessing the impact of the introduction of these techniques and understanding the profile of the treatments carried out at the institution over the years of 2010 to 2020. Through this methodology, a decrease in the workload of 15 MV was observed as the technique of modulated intensity with 6 MV was introduced, reducing the thicknesses calculated for primary barriers. However, no significant changes were observed in the thicknesses calculated for the secondary barriers, because despite the increase in the leakage workload of 6 MV, the total workload of 15 MV decreased. There was also a trend towards an increase in the number of patients treated with modulated intensity year after year, which went from 5% in 2016 to 67% in 2020.
Highlights
The concern with the damage caused by ionizing radiation in the human body is one of the main pillars of radioprotection and maximum dose values are established, both for occupationally exposed individuals (OEI) and for the general public, in order to limit exposure to levels as low as reasonably achievable
Data Acquisition Clinical workload data was acquired through automatically generated reports by the MOSAIQ® (Elekta, Stockholm-Sweden) management software, making it possible to extract the technique and energy used to deliver the treatment, in addition to the dose received at the isocenter and the monitor units delivered to each patient in each treatment session
Through the workload data in terms of dose (WD) and in terms of monitor units (WMU) extracted from the system, it is possible to determine the “C factor”, updated for the casuistry of the techniques performed in the institution, which in this article was used to consider the contribution of the total monitor units delivered in increasing the leakage workload
Summary
The concern with the damage caused by ionizing radiation in the human body is one of the main pillars of radioprotection and maximum dose values are established, both for occupationally exposed individuals (OEI) and for the general public, in order to limit exposure to levels as low as reasonably achievable. Practices that use ionizing radiation, such as radiotherapy, must ensure compliance with the limits authorized by the responsible organization CNEN (National Nuclear Energy Commission) in Brazil and with the optimization principle To achieve those goals shielding is fundamental so, the thickness of the walls, ceiling, floor, and doors, in addition to the labyrinth extension of a radiotherapy room are calculated based on international recommendations (reports 49 and 151 of the NCRP [1,2]). Superior in terms of conformity, IMRT (Intensity Modulated Radiation Therapy) technique is considered costly from the shielding standpoint since more monitor units are used to deliver the same dose to the patient This happens because only a small fraction of each treatment field is irradiated at every moment, which promotes an increase in leakage radiation and an increase in the thickness of secondary barriers. The thickness of the primary barrier should not change if the average doses administered per patient are the same. [2,3,4,5,6,7]
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