2260 - Tailoring Treatment Margins: A Study of PTV Reduction Using X-Ray Based Adaptive Radiotherapy for Stereotactic Partial Breast Irradiation (A-SPBI) Treatments
University of Texas Southwestern Medical Center Dallas, TX
S. J. Domal, J. Visak, M. L. Hsieh, G. Gard, A. He, M. Arbab, N. Wandrey, D. Li, P. G. Alluri, D. D. M. Parsons, A. R. Godley, M. H. Lin, and A. S. Rahimi; Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX
Purpose/Objective(s): This study aimed to empirically evaluate our current clinical practice of CTV to PTV 3 mm margin expansion by accurately measuring fiducial marker motion for patients undergoing adaptive radiotherapy stereotactic partial breast irradiation (A-SPBI) on x-ray based adaptive radiotherapy platform. We hypothesize that a 3 mm margin sufficiently accommodates fiducial marker motion, representing a significant reduction from the conventional 1.0 cm expansion utilized in PBI with IMRT or 3D treatments. Materials/
Methods: To ensure a more objective measure of motion compared to the manual contouring of the breast in daily ART sessions, we opted to employ fiducial markers in this study. We leveraged a Python-based Graphical User Interface (GUI) application, designed to adhere to DICOM standards, to analyze fiducial marker movements in 162 CBCT images from 12 A-SPBI patients. During adaptive session patients typically receive 2-3 cone-beam-CT (CBCT) to assess intrafraction motion. Movement of fiducials was measured using these CBCT as a surrogate for cavity motion. The average absolute motion of 3 cardinal directions was recorded. Results: The fiducial marker movements had a mean of 2.18 mm laterally, 1.86 mm anterior/posteriorly, and 1.94 mm superior/inferiorly, with standard deviations of 0.84 mm, 0.93 mm, and 0.94 mm, respectively. Gaussian-like distribution patterns emerged from both histogram and boxplot analyses, indicating consistent containment of movements within the pre-specified margin. One-sample t-tests were conducted to compare the sample means against predefined thresholds of 3 mm, 2.5 mm, and 2 mm. The tests revealed that the lateral, anterior/posterior, and superior/inferior movements were significantly less than the 3 mm and 2.5 mm thresholds (p < 0.001), but not significantly different from the 2 mm threshold (lateral: p = 0.110, anterior/posterior: p = 0.272, superior/inferior: p = 0.655). Conclusion: Our analysis, utilizing one-sample t-tests, confirms the adequacy of the 3 mm PTV margin clinically employed in daily adaptive SBPI treatments. The statistically significant results (p < 0.001) for the marker movements compared to the 3 mm and 2.5 mm thresholds reinforce the suitability of this margin. In contrast, the lack of statistical significance when comparing the movements to a 2 mm threshold (p-values > 0.05) does not challenge the 3 mm margins clinical utility. Abstract 2260 – Table 1
