Abstract P1-10-11: Optical stimulated luminescence dosimeters for skin dose measurements during accelerated partial breast brachytherapy: In vivo dosimetric validation and clinical outcomes
Authors: Gutschenritter Tyler , Yorke Afua , Polissar Nayak , Miljacic Nirnaya , Kim Janice , Young Lori 
Published: March 2023

Abstract
BACKGROUND: The role of multi-lumen catheter devices in the delivery of high dose rate (HDR) brachytherapy for adjuvant accelerated partial breast irradiation (APBI) for women with DCIS or early-stage breast cancer has been well established. In vivo dosimetry (IVD) using optically stimulating luminescence dosimeters (OSLD) is a feasible way to detect applicator instability that may cause translational and rotational inaccuracies during HDR brachytherapy delivery. Herein, we evaluate the accuracy and effectiveness of IVD to improve patient outcomes following APBI brachytherapy using the Strut Adjusted Volume Irradiation (SAVI) device.
METHODS: Single-institution cohort study piloting the use of OSLD on the patient’s skin during HDR brachytherapy APBI to assess the estimated maximum skin dose (skin Dmax) and the achievable accuracy of OSLD for skin Dmax measurements. Women treated with SAVI-based APBI between November 1, 2018 and October 1, 2021 were eligible to enroll in our study. All patients met the American Society for Radiation Oncology “suitable” or “cautionary” criteria and were treated according to the American Brachytherapy Society consensus treatment planning guidelines. A SAVI preplan was created to estimate the probable point of skin Dmax. During CT simulation, a radiopaque marker was placed on the skin for visualization during treatment planning. Correction factors were applied to account for the OSLD tissue-air interface that is not present in the TG-43 expected dose calculations. Measured and calculated IVD doses were analyzed to assess agreement and cause for discrepancies found. Breast phantom IVD measurements repeated seven times with two independent dose points were analyzed to assess the best accuracy achievable under ideal conditions.
RESULTS: We enrolled 41 patients with an average age of 68 years. Table 1 details the brachytherapy treatment delivery and dosimetry data. Higher skin Dmax as a percentage of the radiation prescription was associated with the occurrence of breast volume loss noted on subsequent clinical breast exams (OR 1.84 per each 5% increase in skin Dmax; 95% CI 1.21 – 3.32; p=0.016). The graphically estimated skin Dmax threshold was 80% with all cases of noted breast volume loss occurring above this threshold. The overall level of achievable IVD accuracy was ±3.32% based on breast phantom measurements. Under ideal conditions, all measurements were in the ±7% range. Three of 14 phantom measurement exceeded the 6% discrepancy range. Clinical IVD measurements detected 5 true positive cases (12.2% of cohort) that required intervention due to rotations in the SAVI applicator. For a ±7% tolerance criteria, 90.2% of the patients tested passed IVD. Of the eight patients that failed at the 7% level, IVD detected 3 OSLD placement errors, 2 rotational discrepancy cases, and 1 large air-gap at the point of measurement.
CONCLUSIONS: Accuracy and reproducibility are imperative tenets of HDR brachytherapy APBI. IVD utilizing OSLD is an effective method for detecting treatment set-up and delivery errors for patients receiving SAVI-based APBI with dose accuracy within ±3.32%. Errors requiring intervention due to SAVI device rotation were detected in 12.2% of our cohort, and these errors would have likely gone undetected without IVD. Using OSLD as IVD to measure skin Dmax doses that may increase the risk of breast cosmetic defects should be investigated further.
Table 1. Brachytherapy treatment delivery and dosimetry data. *PTV-Eval is a uniform 10 mm expansion on the CTV with exclusion of PTV within 5 mm of skin or chest wall. +Percent dose discrepancy refers to difference in dose between APBI plan calculations and IVD measurements.
Citation Format: Tyler Gutschenritter, Afua Yorke, Nayak Polissar, Nirnaya Miljacic, Janice Kim, Lori Young. Optical stimulated luminescence dosimeters for skin dose measurements during accelerated partial breast brachytherapy: In vivo dosimetric validation and clinical outcomes [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P1-10-11.