Purpose: Intracavitary accelerated partial breast irradiation ͑APBI͒ has become a popular treatment for early stage breast cancer in recent years due to its shortened course of treatment and simplified treatment planning compared to traditional external beam breast conservation therapy. However, the exit dose to the skin is a major concern and can be a limiting factor for these treatments. Most treatment planning systems ͑TPSs͒ currently used for high dose-rate ͑HDR͒ 192Ir brachytherapy overestimate the exit skin dose because they assume a homogeneous water medium and do not account for finite patient dimensions. The purpose of this work was to quantify the TPS overestimation of the exit skin dose for a group of patients and several phantom configurations. Methods: The TPS calculated skin dose for 59 HDR 192 Ir APBI patients was compared to the skin dose measured with LiF:Mg,Ti thermoluminescent dosimeters ͑TLDs͒. Additionally, the TPS calculated dose was compared to the TLD measured dose and the Monte Carlo ͑MC͒ calculated dose for eight phantom configurations. Four of the phantom configurations simulated treatment conditions with no scattering material beyond the point of measurement and the other four configurations simulated the homogeneous scattering conditions assumed by the TPS. Since the calibration TLDs for this work were irradiated with 137 Cs and the experimental irradiations were performed with 192 Ir, experiments were performed to determine the intrinsic energy dependence of the TLDs. Correction factors that relate the dose at the point of measurement ͑center of TLD͒ to the dose at the point of interest ͑basal skin layer͒ were also determined and applied for each irradiation geometry. Results: The TLD intrinsic energy dependence for 192 Ir relative to 137 Cs was 1.041Ϯ 1.78%. The TPS overestimated the exit skin dose by an average of 16% for the group of 59 patients studied, and by 9%-15% for the four phantom setups simulating treatment conditions. For the four phantom setups simulating the conditions assumed by the TPS, the TPS calculated dose agreed well with the TLD and MC results ͑within 3% and 1%, respectively͒. The inverse square geometry correction factor ranged from 1.023 to 1.042, and an additional correction factor of 0.978 was applied to account for the lack of charged particle equilibrium in the TLD and basal skin layer. Conclusions: TPS calculations that assume a homogeneous water medium overestimate the exit skin dose for intracavitary APBI treatments. It is important to determine the actual skin dose received during intracavitary APBI to determine the skin dose-response relationship and establish dose limits for optimal skin sparing. This study has demonstrated that TLDs can measure the skin dose with an expanded uncertainty ͑k =2͒ of 5.6% when the proper corrections are applied.
Purpose: To accurately determine the exit skin dose from MammoSite® Radiation Therapy System treatments using thermoluminescent dosimeters (TLDs) placed on the surface of the skin during each treatment fraction. Methods and Materials: Well‐characterized TLD‐100 chips were calibrated using 137Cs to establish an energy response correction factor for 192Ir measurements. TLD response vs. material thickness was measured and compared for thicknesses of Virtual Water™ and breast‐equivalent material from 2 to 10cm. Monte Carlo simulations were performed to determine the relationship between the dose to TLD and dose to basal skin layer for a range of treatment parameters (size and content of balloon, distance from skin, etc). Treatment planning system (TPS) predicted skin doses were also compared to the TLD measured skin dose for 29 patients and two phantom setups. Results: The TLD‐100 energy response for 192Ir relative to 137Cs was determined to be 1.045. The TL vs. thickness curves for Virtual Water™ and breast‐equivalent material were found to be within 2.5% for all thicknesses studied for 192Ir, and can be used interchangeably within the 2.5% limit, which falls within our measurement uncertainty of 3%. The Monte Carlo calculated dose to TLD agreed with the 1/r2 corrected basal skin dose to within 0.5% for the entire range of treatment parameters studied when the dose to TLD vs. dose to water correction factor of 1.2 was applied. TPS predicted doses overestimated the TLD measured skin dose by an average of 26% for the group of 29 patients studied, and by an average of 40% for the two phantom setups. Conclusions: TLDs placed on the surface of the breast during MammoSite RTS treatments can accurately measure the skin dose when the proper correction factors are applied. The TLD measured dose is much more accurate than the TPS predicted dose.
Purpose: To quantify the difference between inhomogeneity‐corrected brachytherapy treatment planning dose calculations and regular homogeneous (TG‐43) dose calculations and to compare the results from both calculation methods with TLD measurements for well‐defined phantom geometries. Methods and Materials: Well‐characterized TLD‐100 chips were used to measure the dose at multiple reference locations on a torso phantom with gel breasts attached simulating a HDR 192Ir intracavitary APBI treatment. A CT scan of the phantom was obtained and the dose to each reference location was calculated in the treatment planning software, both with the standard TG‐43 dosimetry formalism and with a grid‐based Boltzmann solver (GBBS) that calculates the dose distribution for the actual patient dimensions and tissue compositions. Results: The TLD measured doses ranged from approximately 40 cGy to 300 cGy and were 5% lower, on average, than the GBBS calculated doses and 17% lower, on average, than the TG‐43 calculated doses. The GBBS doses were always lower than the TG‐43 doses, with the discrepancy becoming more pronounced at lower doses (further from the source) and at the surface of the phantom. The discrepancy between the TLD measured doses and the GBBS doses did not appear to trend with dose. Conclusion: The application of inhomogeneity corrections reduces the discrepancy between the TPS calculated and TLD measured doses. When inhomogeneity corrections were not applied, the TPS overestimated the dose at each reference location.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.