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Impact of Air Cavity to Critical Structures for Accelerated Partial Breast Irradiation (APBI) Brachytherapy


M Ashenafi

M Ashenafi*, J Peng, J Harper, J Jenrette, K Vanek, Medical University of South Carolina, Charleston, SC

PO-BPC-Exhibit Hall-20 Saturday  Room: Exhibit Hall

Purpose:To investigate the feasibility of re-planning due to air cavity effect for accelerated partial breast irradiation (APBI) brachytherapy.

Methods:Dosimetric data of six patients treated with Contura multi-lumen balloon was retrospectively assessed for adaptive planning. Two axial CT image data-sets were acquired; one was CT simulation used for planning and the other was prior to the first treatment. Ribs, skin (2mm skin rendering), air cavity and PTV_EVAL following NSABP B-39 protocol were contoured on both CT data-sets. Plans were generated using criteria including, for PTV_EVAL, V90PD (volume received 90% of prescribed dose (PD)) ≥90%; V150PD <50cc; V200PD <10cc. For skin & rib, Dmax(0.1cc) (maximum dose) <125%PD. Dosimetric effect due to air cavity was evaluated by re-computing dose distributions on the second CT data set by applying the same dwell planning time and comparing to the original plan. Differences were assessed for statistical significance (p<0.05) using Student's t-test.

Results:50% of patients had an average 1.4cc air volume decrease between planning and treatment day. As the results, the average maximum skin dose increased by 18.7%. Similar dosimetry effect was discovered as the air cavity shown near the rib. The air volume decreased by 1.7cc causing the maximums rib dose change by 15.5%. Overall, there was no statically significant difference in target dosimetery (V90PD, V150PD, and V200PD) due to change in air volume.

Conclusion:Our study demonstrated a correlation between air volume change with skin and rib dose. Consideration of pre-treatment 3D CT image acquisition and re-planning is suggested when a significant change in air volume detected near critical structures.

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