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Date of Award
This study aims to investigate the dosimetry of the INTRABEAM x-ray source (XRS) used in the delivery of breast intraoperative radiotherapy (BIORT) for patients enrolled in clinical trials (TARGIT-US, TARGIT-B, TARGIT-E). Building on the existing dosimetry studies, we perform a dosimetric comparison of the INTRABEAM and the Axxent system to show differences in dosimetric coverage because of the treatment delivery technique. We observed that both devices provide excellent dosimetric coverage of the proximal cavity tissue. However, the extended ‘effective source’ created by the multiple dwell positions for the Axxent system increases the dose to tissues ≥ 0.5-cm from the applicator surface.
BIORT is performed within the operating room without the use of CT-based planning, which limits our ability to produce a DVH for the lumpectomy cavity. We were the first to produce a DVH and report a dosimetric analysis of the lumpectomy cavity using V90, V80, V50, Dmin, D0.1, and homogeneity index (HI) dose-volume parameters and to assess the change in R80 and R90 therapeutic treatment depths (TTDs) because of spherical applicator size and filter design. For spherical applicator diameters 1.5- to 5.0-cm, DVPs ranged from: 2.6-6.1% for V90, 5.5-13.5% for V80, 21.1-53.5% for V50, and 6.4-2.9 for HI. The presence of the aluminum filter for applicators ≤ 3.0-cm increases the R80 and R90 TTDs but reduces the dose rate at the surface of the applicator.
Although each INTRABEAM XRS has the same design, the output and the shape of the depth-dose curve (DDC) for each XRS can vary because of manufacturing variances in the thickness of the gold x-ray target and electron source. Thus, we performed an inter-comparison of 15 INTRABEAM XRSs, to understand output variations of the manufactured product and to dosimetrically characterize the impact these variations have on the dose delivered to the tumor bed. The results indicate that the output characteristics of the XRS have a unimodal distribution with up to a 4.4% coefficient of variation (CV) for 40-kVp and up to a 4.2% CV for 50-kVp. We have demonstrated the variability in output characteristics of an XRS, and the maximum dosimetric effect of these variations within 10 mm of the applicator surface is ≤ 2.5%.
The factory-calibrated XRS is delivered with two DDCs named: “TARGIT,” and “V4.0.” Research has recently shown that the that V4.0 DDC is a more accurate representation of the dose received by the tumor bed. Thus, treatments delivered with the TARGIT DDC were re-calculated with the V4.0 DDC. The results presented in this work provide evidence that the dose to the tumor bed is higher than previously estimated.
We have quantified the impact of output, calibration, and technique on the dose delivered to the tumor bed to assist clinical trials comparing INTRABEAM intraoperative radiotherapy with other radiation modalities. The completion of this work supports a deeper dosimetric understanding of the INTRABEAM system and ultimately improves the quality of patient care.
Shaikh, Mubin Yasin, "Characterization Of A Miniature X-Ray Source And Evaluation Of The Clinical Implications For Intraoperative Breast Radiotherapy" (2020). Wayne State University Dissertations. 2398.