PV QA 4 - Poster Viewing Q&A 4
Purpose/Objective(s): Accurate localization of the lumpectomy cavity is essential during daily setup to ensure that the prescribed dose encompasses the target and avoids unnecessary irradiation to surrounding normal tissues. This is particularly important during accelerated partial breast radiation (APBR) as a higher dose of radiation is delivered over a shorter period of time. APBR has become increasingly popular and is now frequently utilized for early stage breast cancer. Currently, portal films are most commonly used for localization, despite the lack of direct visualization of the lumpectomy cavity. However, 3-dimensional ultrasound (3D-US) allows direct visualization of the lumpectomy cavity without additional radiation exposure. While a limited number of studies reported the use of 3D-US imaging during boost simulations, none have studied its use for daily positioning during APBR. The purpose of this study was to evaluate the feasibility of 3D-US in daily target localization for APBR.
Materials/Methods: 47 patients with Stage I breast cancer who underwent breast conserving surgery were treated with a 2 week course of APBR. Patients with visible lumpectomy cavities on high quality 3D-US images were included in this analysis. The patients had portal films and 3D-US images obtained at the day of simulation. Prior to each treatment, portals and 3D-US were acquired and compared to images from simulation to determine shifts. A total of 118 images of each modality from 12 eligible patients were analyzed. Volume change of the lumpectomy cavity was determined daily with 3D-US. The mean (median, interquartile range) of the shifts in all three dimensions were calculated: anterior/posterior (A/P), left/right (L/R) and superior/inferior (S/I). The Wilcoxon Rank Sum test was used to determine significant differences (p<0.05) between the modalities.
Results: The average decrease in cavity volume was 41.7% on 3D-US from simulation to end of treatment. 3D-US demonstrated significantly larger shifts than portal films in all 3 dimensions: A/P: p=7E-11, L/R: p=0.002, and S/I p=0.004. The mean (median; interquartile range) values for each dimension and modality were A/P 3D-US: 0.38cm (0.39; 0.7 – 0) & portals: 0.05cm (0; 0.1 – 0), L/R 3D-US: 0.19cm (0.07; 0.2 – 0.6) & portals: 0.03cm (0; 0 – 0.2), and S/I 3D-US: 0.10cm (0.12; 0.3 – 0.1) & portals: 0.02cm (0; 0 – 0.2).
Conclusion: In this prospective study, we demonstrated feasibility of using 3D-US for daily target localization and observed significant differences between shifts when compared to portal films. Given that the lumpectomy cavity is not directly visible via portal films, accurate positioning may not be fully achieved by portal films. Therefore, 3D-US can be considered as an alternative to portal films during daily positioning for selected patients treated with APBR thus avoiding additional exposure to ionizing radiation.
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