Métodos de cálculo de blindagem considerando IMRT e VMAT para radioterapia moderna

Abstract

Title: Shielding calculation methods considering IMRT and VMAT for modern radiotherapy Radiotherapy has advanced significantly in the last 10 years, the new technologies of Linear Accelerators (AL) and modern treatment planning systems, among them, Intensity Modulated Radiotherapy (IMRT) and Intensity Modulated Arc Radiotherapy (VMAT) have been predominant. Use of IMRT and VMAT techniques requires an increase in the number of Monitor Units (UM) to deliver the prescribed radiation dose when compared to three-dimensional conformational radiotherapy (3DCRT). The shielding calculation for radiotherapy rooms is mandatory to safety and radiological protection of patients, employees and individuals from the public. However, the main references used, despite addressing the dependence of the IMRT technique in shielding calculations, are from almost 2 decades ago. The objective of the work is to find the C factors of IMRT and VMAT for the calculation of shielding in modern radiotherapy, to compare the thickness of the secondary barriers with the C factor of IMRT and VMAT found in the work and those currently used, as well as the costs involved. Finally, carry out measurements of scattered radiation and head leakage of the 4 AL included in the work. The study included 187 patients following anatomical sites: 77 breasts, 41 head and neck, 41 prostates and 28 lungs that were planned for 4 different AL and 3 dose delivery techniques: 3DCRT, IMRT and VMAT totaling 2244 planning. Shielding calculations to compare the thickness of secondary barriers were performed for each wall defined for a standard room using the method according to international protocols and with the recommendations of this study. Leakage and scattered radiation measurements in the 4 AL were made using 3 different dosimeters comparing the values found with the protocols. The C factor found for VMAT was 19% lower when compared to IMRT (p < 0.001) evidencing the need to differentiate between the IMRT and VMAT techniques. When individualized the analysis of C factor by AL and by technique, the TrueBeam presented the highest value, which was 3.00 for the IMRT technique and the Halcyon the lowest value for the VMAT technique, which was 0.94. For VMAT, a reduction of up to 20% in the thickness of concrete required for the secondary barrier was found when compared to the one currently used and for the IMRT technique of 11%. The work identified a reduction of at least 7 m3 of concrete when using the C factors found and savings of up to 66 thousand dollars in secondary barriers. The leakage radiation measurements of the 4 AL did not have values greater than 0.9 mSv, an excess of conservatism is evident when compared to the recommended one, which is 2.5 mSv.